EPA-600/4-75-003
DECEMBER 1975
Environmental Monitoring Series
TECHNICAL ASSISTANCE DOCUMENT FOR THE
CHEMILUMINESCENCE MEASUREMENT OF
NITROGEN DIOXIDE
Environmental Monitoring and Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL MONITORING series.
This series describes research conducted to develop new or improved methods
and instrumentation for the identification and quantification of environmental
pollutants at the lowest conceivably significant concentrations. It also includes
studies to determine the ambient concentrations of pollutants in the environment
and/or the variance of pollutants as a function of time or meteorological factors.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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TECHNICAL ASSISTANCE DOCUMENT
FOR THE CHEMILUMINESCENCE MEASUREMENT
OF NITROGEN DIOXIDE
by
Elizabeth Carol Ellis
Quality Assurance Branch
Environmental Monitoring and Support Laboratory
Research Triangle Park, North Carolina 27711
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental Monitoring and Support
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
ii
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PREFACE
The Environmental Protection Agency has replaced the original reference
method for nitrogen dioxide with automated reference methods based on the gas
phase chemiluminescence measurement principle and prescribed calibration pro-
cedures. This document provides technical information and illustrative examples
to aid in the understanding of the measurement principle and particularly of
the two specified calibration procedures. It should be used as a complement to
both the regulatory specifications as well as individual analyzer instruction
manuals and should serve to provide practical guidance to the analyst on the
use and calibration of reference method analyzers -— the end result being NO^
measurements of quality. For easy reference, the regulatory specifications
(prescribed in Title 40 of the Code of Federal Regulations, Part 50, Appendix F)
are included as an addendum.
The author gratefully acknowledges the assistance of other members of
the Environmental Monitoring and Support Laboratory who helped make this
document possible. Michael E. Beard, Robert G. Fuerst, and John H. Margeson
of the Quality Assurance Branch and Kenneth A. Rehme, Frank F. McElroy, and
Larry J. Purdue of the Environmental Monitoring Branch each contributed
through discussions and critical evaluation of the manuscript.
m
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ABSTRACT
Gas phase chemiluminescence has been designated as the reference measure-
ment principle for the measurement of nitrogen dioxide (N02) in the ambient
atmosphere. Continuous analyzers based on this measurement principle may be
calibrated with N02 either from the gas phase titration of nitric oxide (NO)
with ozone (03) or from an N02 permeation device. This document presents
pertinent technical information to aid in the understanding of the measurement
principle and the prescribed calibration procedures and also includes illus-
trative examples on how to implement the calibration procedures. The discussion
includes recommendations on how to recognize and eliminate potential errors in
the individual calibration procedures as well as with the use of N02 chemilumi-
nescence analyzers. Suggestions on the design and construction of calibration
apparatus and procedures for handling and certifying both NO and N02 calibration
standards are included also.
1v
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CONTENTS
Preface iii
Abstract iy
Figures vi
1. Introduction 1
2. Chemiluminescence Measurement Principle 3
Chemiluminescent Reaction of NO with (L 3
Interferences 4
Analyzer designs 5
3. Calibration Procedures 8
General descriptions 8
Alternative A: Gas phase titration of an NO
standard with 03 9
Preliminary GPT design considerations 10
Zero air source and flow control 12
NO in N2 standard 14
Reaction, mixing and sampling chambers 17
N0-03 reaction requirements 17
Application of dynamic parameter 19
Completeness of N0-03 reaction 23
N0? analyzer calibration 23
Certification of NO standard 26
Alternative B: N02 permeation device 28
Components of calibration system 29
Standard N02 permeation device 32
Design of calibration system 33
N02 analyzer calibration 34
Certification of N02 or NO standard 37
Intercomparison of N02 and NO standards 38
References 39
Addendum 41
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FIGURES
Number Page
1 Schematic diagram of a typical GPT calibration system 11
2 Schematic diagram of a typical calibration apparatus
using an N02 permeation device 30
vl
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SECTION 1
INTRODUCTION
In April 1971 the Environmental Protection Agency (EPA) promulgated
national primary and secondary ambient air quality standards for six air
pollutants, including nitrogen dioxide (N02). At the same time EPA published
reference methods that were to be used by EPA and by state and local air
pollution agencies in measuring ambient concentrations of the six pollutants.
2
Subsequent investigations convinced EPA that the reference method for N02 was
severely deficient and the Administrator, in June 1973, announced his intention
to propose amendments to Federal regulations which would w.thdraw the original
N02 reference method and designate a new one. Extensive laboratory and field
evaluation of three manual and two automated methods for the measurement of
N02 ensued. When those evaluations were completed, EPA proposed that the
original N02 reference method be replaced by "automated reference methods"
based on the gas phase chemiluminescence measurement principle and associated
calibration procedures. In December 1976, this proposal became a Federal
4
regulation.
In accordance with EPA's Equivalency Regulations, an N02 analyzer based
on the gas phase chemiluminescence measurement principle could be designated
as a "reference method" provided it is calibrated by specified procedures and
also conforms to prescribed performance specifications. A description of the
measurement principle, two alternative calibration procedures and N0« analyzer
performance specifications are published in the Federal Register.4 However,
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there are several aspects of the measurement principle and calibration
procedures that warrant more detailed explanations than are possible in the
Federal Register description. The basis for the following discussion is
the results of laboratory evaluations and field monitoring studies by the
Environmental Monitoring and Support Laboratory (EMSL) of EPA. These comments
are directed toward persons having a need to make N02 measurements with
chemiluminescence analyzers and are offered primarily to assist the analyst
in obtaining N02 measurements of quality.
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SECTION 2
CHEMILUMINESCENCE MEASUREMENT PRINCIPLE
CHEMILUMINESCENT REACTION OF NO WITH Og
In the reaction of nitric oxide (NO) with ozone (Oo), some of the resultant
N02 is produced in an electronically excited state; the excited N02 immediately
decays to the ground state or normal N02 while emitting light in the spectral
region from about 600 nm to 2400 nm with a peak at about 1200 nm. This phenom-
enon is known as chemiluminescence. The intensity of the light generated in the
N0-03 reaction is proportional to the reactant concentraticn of NO and the re-
action is applicable to the direct measurement of atmospheric concentrations
of NO.7
Atmospheric concentrations of N02 are measured indirectly by chemilumi-
nescence by first reducing the N02 to NO, then reacting the resultant NO with
ozone and measuring the light intensity from the reaction. " In practice,
in chemiluminescence N02 analyzers, the N02 in a sample of air is first re-
11 12
duced to NO ' by means of a converter; any NO, which is normally present
in ambient air, passes through the converter unchanged causing a resultant
total NO concentration equal to NO + N0?. A portion of the air sample is
« ^
also reacted with 0, without having passed through the converter. This
latter NO measurement is subtracted from the former measurement (NO + N02)
to yield the final N02 measurement.
Since the detection of N02 by this chemiluminescence technique is
directly dependent on the analyzer's capability to reduce N02 to NO, it is
3
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important that the conversion be essentially quantitative over a wide range
of N02 concentrations. Accordingly, the determination of the converter
efficiency of an analyzer is an integral part of both calibration procedures
with the additional requirement that the converter be replaced or serviced
if its demonstrated efficiency should be less than 96 percent.
INTERFERENCES
The chemiluminescence detection of NO based upon the reaction of NO
with Og is not subject to interference from any of the common air pollutants
such as 03> N02, carbon monoxide, ammonia and sulfur dioxide. However, some
unsaturated hydrocarbons react with 03 to luminesce in the visible region of
the spectrum; a red sharp-cut optical filter is used to eliminate this possible
interference by absoring emissions below 600 nm.
Any compounds other than N02 that will be converted to NO in the converter
will interfere with the measurement of the NO (NO + N0?) —and hence the
A £
N02 — concentration. There are two basic types of converters in current
use —thermal and chemical. Thermal converters, which are made of metallic
materials such as stainless steel, operate at temperatures between 600° and
800°C and "thermally" reduce N02, N02 + NO + 1/2 02- At lower temperatures,
N02 conversion is not quantitative. Ammonia is the principal interferent of
concern with thermal converters since ammonia is oxidized to NO above about
600°C on a variety of metallic surfaces. Ammonia is generally not an inter-
ferent with chemical converters which operate at temperatures as low as 200°C
and generally no higher than 400°C. Chemical converters are made from a
variety of materials: pure metals like molybdenum, tungsten or platinum;
various alloys; spectroscopic carbon and some nonabsorptive charcoals; and
carbon impregnated with various metals. Although of differing material com-
4
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position, chemical converters reduce N02 to NO by forming an oxide of the
converter material, e.g., C + NO^ -> CO + NO. By the nature of the
conversion process chemical converters will eventually expend themselves. This
is no great disadvantage, however, since the carbon type converters are inex-
pensive to replace and the metal based converters are easily reactivated by
exposing the converter surface to a reducing gas such as hydrogen. Although
ammonia is not oxidized to NO by chemical converters at normal operating
temperatures, other unstable nitrogen compounds such as peroxyacetyl nitrate
(PAN), some amines and certain organic nitrites and nitrates will decompose
quantitatively to form NO. The ambient concentration of these compounds
is usually so low in most areas of the country that this interference can be
disregarded. However, when the concentrations of interferent compounds
are significant relative to the NOp concentration, the interference should
be taken into account. If possible, the magnitude of the ir.terferent com-
pounds should be determined by an independent method and the N02 concentrations
adjusted accordingly. Alternatively, it may be necessary to determine the
5
N02 concentrations by an equivalent method that is not affected by the
interferents of concern.
ANALYZER DESIGNS
Chemiluminescence NOo analyzers employ one of two basic configurations
for the NO and NO measurements —dual or cyclic. In dual type analyzers,
/\
the air sample is divided at the analyzer inlet and half passes continuously
through a converter to one reaction chamber while the other half passes
continuously (through an equivalent converter volume) to a second reaction
chamber. The NO and NO concentrations are measured continuously with either
n
a single detector time shared between the two reaction cells or a pair of
5
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matched detectors — one for each reaction cell. In contrast, cyclic analyzers
have a single reaction chamber and detector and must alternate between the NO
and NOX measurements, i.e., the air sample alternately by-passes or passes
through the converter. With this cyclic operation, NO and NO readings are
n
taken one after the other and short term changes in inlet concentration can
lead to skewing of resultant N02 values. This is not a serious problem
provided the cycle time is held to a minimum and any negative N02 values
are included in the data averaging process.
It should be noted that the use of an integrating volume on the intake
of an analyzer to average any short term changes is discouraged since it
introduces a distortion in the N0/N02 ratio due to the perturbation of a
photostationary state between NO, 0^ and N02- This distortion develops
because in the atmosphere the two reactions,
NO + 0, ->• N09 + 09
«j £ £
and
N02 su^ght N0 + 0>
maintain an equilibrium. When there is no sunlight, such as in an inte-
grating volume, the dissociation of N02 does not take place but the oxidation
of NO still occurs. Thus, after a short time, the NO measured will be below
the true value and N02 will be higher than the true value.
Separate range selectors for NO and N02 are a useful feature of an
analyzer especially when it is located in a geographical area where the NO
concentration is high relative to the N02 concentration. Independent zero
and span controls and range selectors for NO, N02 and NOX are often a common
feature of cyclic type analyzers. For dual channel analyzers with a shared
detector, only one zero and one span control are provided; the NO and NO
/\
measurements must be made on the same concentration range. The two detector
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design provides zero and span controls for both NO and NO with range
J\
selectors which may or may not operate independently depending on the
electronic design of the detector circuitry.
14
In a recent collaborative test ten participants used ten chemilumi-
nescence N0« analyzers (both cyclic and dual channel designs representative
of three manufacturers of N02 analyzers) to monitor the same N02 concentrations
over a range of 0.03 to 0.16 ppm.* The results of the test for the particular
analyzers involved indicate an average bias of -5% and a relative standard
deviation in the measurements of 6% and 14% for within- and between-laboratory
variation, respectively. A lower detectable limit of approximately 0.01 ppm
(22 yg/m ) when using a 0.0 to 0.5 ppm range is also reported.
*ppm = part per million.
ug/m = microgram per cubic meter at 25°C and 760 mm Hg.
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SECTION 3
CALIBRATION PROCEDURES
GENERAL DESCRIPTIONS
There are two allowable N02 calibration procedures (see addendum): gas
phase titration (GPT) of an NO standard with 03 to produce N02 or alternatively
the dilution of NOp emitted from a permeation device. Both calibration pro-
cedures are applicable to many calibration ranges; for ambient air measurements
the normal working range is 0.0 to 0.5 ppm N02 or less. The particular N02
concentrations used for calibration should cover tfe working range and should
include values that are representative of those normally encountered at the
location of interest as well as concentrations below and near the National
Ambient Air Quality Standard of 0.05 ppm N02-
The accuracy of the NOo calibration is dependent on the analyzer's converter
efficiency; therefore, a determination of converter efficiency is a required part
of the calibration procedures. The measurement of converter efficiency requires
a source of NO as well as N02- In essence, calibration consists of the following:
1. Calibration of NO and NO responses of the analyzer using an NO
standard.
2. Calibration of N02 response with concurrent determination of
converter efficiency using standard N02 from GPT or from a
permeation device.
It appears that GPT may be the preferred N02 calibration scheme since both
NO and N0« are so readily available by this technique. On the other hand, some
users may find it more convenient to couple an N02 permeation device with an NO
8
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source rather than work with ozone titrations. Given that an NO source is
required in both calibration procedures, the user has a choice of adding an
ozone source or an N02 permeation device to complete the calibration system.
One important feature of the two prescribed calibration procedures is
the requirement that working calibration standards be traceable to National
Bureau of Standards (NBS) Standard Reference Materials (SRM). For the GPT
calibration option, commercially available mixtures (pressurized cylinders
of NO in Np) should be used for routine calibration but the NO content of
such mixtures must be periodically assayed against an NBS traceable NO in N2
or N02 permeation device standard. It is also necessary to determine any
trace N02 that may be present in the commercial NO calibration mixtures. In
the alternative calibration procedure using an N02 permeation device plus an
NO cylinder, only one of the two standards need be periodically assayed against
an NBS traceable standard. The remaining standard is the.i periodically
referenced to the first for consistency. Procedures for certifying an NO
cylinder or N02 permeation device against either an NO or N02 NBS traceable
standard are included below.
Details of some reliable analytical techniques as well as the apparatus
for both calibration schemes are described in the sections below. The suggested
component parts have been used extensively and have been found to perform
satisfactorily; however, these suggestions in no way assert that other apparatus
can not be used effectively.
ALTERNATIVE A: GAS PHASE TITRATION OF AN NO STANDARD WITH 03
This calibration technique is based upon the rapid gas phase reaction
between NO and 03 to produce N02 as described in Equation i.6'15'16
NO + 03 ->- N02 + 02; k = 1.0 x 107 liter mole"1 sec"1 (1)
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The quantitative nature of the reaction is used in a manner such that, once the
concentration of reacted NO is known, the concentration of N0« is determined.
Ozone is added to excess NO in a dynamic calibration system, and a chemilumi-
nescence NO analyzer is used to measure changes in NO concentration. Upon the
addition of 0^, the decrease in NO concentration observed on the calibrated
NO analyzer is equivalent to the concentration of N02 produced. The amount
of N02 generated is varied by changing the concentration of 0, added.
Figure 1 shows the suggested placement of the component parts of a typical
gas phase titration apparatus. Such systems are also available commercially.
All connections between components in the system should be made with glass or
(R)
Teflon or other nonreactive material. The discussion below is restricted
to apparatus capable of producing sample flows between one and ten liters per
minute (1/min) at the manifold. This is the flow range over which gas phase
titration of excess NO with 03 has been most widely used and investigated.
Preliminary GPT Design Consideration
In setting up the apparatus some general considerations are important.
Firs-.;, determine the minimum total flow required at the sample manifold. This
flow is controlled by the number and sample flow rate demand of the individual
analyzers to be connected to the manifold at the same time. Allow at least
10 to 50% flow in excess of the required total flow. The operational
characteristics of the ozone source delimit the maximum flow of the calibration
system. One ozone source that has been used extensively for gas phase titration
consists of a quartz tube fixed adjacent to a low pressure mercury vapor lamp.
Ozone-free air is passed through the tube and is irradiated with 185-nm light
17 18
from the mercury lamp. ' The level of irradiation is controlled by an
adjustable opaque sleeve that fits around the lamp. Ozone concentrations are
10
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o
VENT
OUTPUT
MANIFOLD
NO
STD.
VENT
r1"
EXTRA OUTLETS CAPPED
WHEN NOT IN USE
TO INLET OF ANALYZER
UNDER CALIBRATION
Figure 1. Schematic diagram of a typical GPT calibration system.
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varied by adjustment of the sleeve to expose the air in the quartz tube to
various levels of ultraviolet (UV) radiation. At a fixed temperature, pressure,
air flow and level of irradiation, ozone is produced at a constant rate. A
change in air flow causes an inverse change in the ozone concentration when
all other variables are held constant. This type of ozone source can generally
supply up to 3 ppm 03 at air flows in the range of 1 to 10 1/min, depending on
the size of the generator.
To determine the operational characteristics of a particular ozone
generator, adjust the ozone source to near maximum irradiation then measure
the 0, produced at different levels of air flew through the generator, e.g.,
1 to 10 1/min. (A calibrated ozone monitor or other means of measuring Og
concentrations is necessary.) A plot of the 0, concentration versus the
reciprocal air flow should be linear. The air flow that gives the desired
maximum 0, concentration, as determined by the maximum concentration of NOg
needed for calibration, represents the maximum total flow for a calibration
system using the generator. Of course, lower air flows can be used to generate
the required 03 concentrations by simply reducing the level of irradiation of
the UV lamp. If the air flow characteristics of the ozone generator do not
meet the minimum total flow requirements of the analyzer under calibration,
then either the generator must be replaced or the number of analyzers to be
calibrated simultaneously must be reduced.
Zero Air Source and Flow Control
Purified cylinder or compressed air is suitable for the zero air; however,
if large volumes of zero air are required for the calibration or especially if
continuous operation is desired, purified compressed air would be preferred.
The zero air must be free of contaminants, such as NO, NOg. 0- or reactive
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hydrocarbons, that will cause a detectable response on the NO or N02 analyzer
or that might react with either NO or N02 in the calibration system. To meet
these specifications, the air can be purified by passing it through silica
gel for drying, treating it with ozone to convert any NO to H^2> and passing
it through a mixture of activated charcoal (6-14 mesh) and molecular sieve
(6-16 mesh,type 4A) to remove any NOo, excess 03 or hydrocarbons.
Silica gel maintains its drying efficiency until it has absorbed 20% of
its weight and it can be regenerated indefinitely at 120°C. The addition of
cobalt chloride to the surface of the gel provides an indicating ability; this
type of gel contained in a transparent drying column is recommended. The mixture
of activated charcoal and molecular sieve also has a finite absorption capability.
Since it is difficult to determine when the mixture's absorption capacity has been
exceeded, it is recommended that the mixture be replaced at regular intervals —
at least every three months for an absorption volume of about 100 cm .
To control and measure the air flow to an accuracy of +_ 2%, as required by
the calibration procedure, the following apparatus and procedures have been used
successfully. Maintain the air source at a constant pressure between about 140
and 210 kPa* (20-30 psig ) using a single stage or two stage gas pressure
regulator; general purpose gas regulators work satisfactorily. When a constant
air pressure is maintained upstream, fine metering needle valves can be used to
maintain constant air flows in the calibration system. Volumetric flowmeters
such as rotameters are an inexpensive means of measuring and monitoring the air
flows. Mass flowmeters can also be used for this purpose. Either type of flow-
meter must be calibrated, the former under the actual conditions of use, against
*kPa = kilopascal (pascal = 1 newton/m )
psig = pound per square inch gauge.
13
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a primary standard volume meter, e.g., a spirometer or a soap bubble meter,
or against an intermediate standard volume meter, e.g., a wet test meter,
•I g
that is traceable to a primary standard meter. (Nelson discusses flowmeter
calibration in Chapter 3 of his book.) As depicted in Figure 1, the zero
air is split into two streams to allow only a portion of the total air flow
to pass through the ozone generator. (Guidelines on splitting the air
stream are discussed below.) The calibration range of the flowmeters used
on each stream should reflect the respective air flow through that stream.
In an alternative apparatus design, all cf the zero air passes through
a single flow controller and flowmeter; the stream then splits with a portion
of the air passing through a capillary restrictive orifice to the entrance of
the ozone generator and the remainder flowing directly to the mixing chamber.
When the total air flow is held constant, the proportion of air flow through
the orifice and the generator remains constant. The capillary orifice should
be of the proper length and internal diameter to allow the desired portion of
the total air flow to pass through the ozone generator. •
NO n N2 Standard, Associated Delivery Apparatus and Handling Procedures
Pressurized cylinders of NO in N~ at levels between 50 and 100 ppm are
available commercially as working calibration standards. The buyer should
specify that oxygen-free nitrogen be used as the diluent gas for the standard
mixture to minimize the problem of N02 formation within the cylinder. In any
case, the standard NO mixture must contain no more than 1.0 ppm NOo as impurity.
Since the manufacturer's certification of the NO content of NO in N2 mixtures
has sometimes been found to be unreliable, the calibration procedure requires
that the NO content of such mixtures be assayed initially and periodically
thereafter against an NBS traceable NO or N02 standard. Traceability may be
14
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made to NO SRM 1683 or 1684 or to N02 SRM 1629. (The certification procedure
is discussed below.) It is suggested that the recertification of working NO
standards be done on a quarterly basis since the long term stability of NO
mixtures has not been firmly established.
Special apparatus and procedures apply when handling a reactive, toxic
gas like NO even at concentrations of 50 to 100 ppm. It is imperative that
the integrity of the NO standard be maintained when the gas is transferred
from the pressurized cylinder to the reaction chamber. In addition, pre-
cautions must be taken to assure that the gas is not allowed to leak to the
surroundings during the transfer. All materials and surfaces that the NO gas
(B)
contacts must be clean and of an unreactive material such as glass, Teflon
or stainless steel. The cleanliness of the NO pressure regulator and asso-
ciated gas delivery system can not be overemphasized. Sone of the problems
of NOg impurity in the calibration system have been traced to the conversion
of the standard NO to NOo by oxygen or other contaminants trapped within the
pressure regulator and gas delivery system rather than N02 impurity within the
standard cylinder. Small amounts of NOp formed within the pressure regulator
have been found to be especially persistent.
One Useful, optional feature for the NO regulator is a purge port or
purge assembly accessory. With purge capabilities, the regulator as well
as the delivery system can be easily evacuated or purged with an inert gas
such as nitrogen after the regulator is connected to the NO cylinder but
before the cylinder control valve has been opened. Even if the NO regu-
lator has no purge port, regulator and delivery system contamination can
be minimized and eliminated by using the following procedure:
1. Connect the pressure regulator and delivery system to
the NO cylinder and evacuate the entire system before
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opening the cylinder control valve.
2. Open the cylinder control valve and flush the system with NO
for about 15 seconds at a delivery pressure of about 415 kPa
(60 psig).
3. Close the cylinder control valve and re-evacuate the system.
Repeat the flushing and evacuation procedures, alternately,
about five times.
When flushing is complete, reduce the delivery pressure to the normal value
between 140 and 240 kPa (20 and 35 psig), close the NO flow control valve,
and also close the cylinder control valve until NO is required for calibrations.
The regulator should be kept pressurized and attached to the NO cylinder except
when the cylinder is being transported to a different location. Any time that
the pressure regulator must be removed from the NO cylinder, the decontamination
procedures must be repeated before reuse.
The pressure regulator for the NO cylinder must be constructed of non-
reactive materials; a two stage regulator (for safety precautions as well as
more accurate pressure regulation) that has internal parts and diaphram of
stainless steel and a Teflon or Kel-F® seat with the capability to
accurately deliver a pressure of 210 kPa (30 psig) is recommended. (NOTE: All
NO cylinders require a regulator with a size 660 C6A connection fitting.) A
fine metering stainless steel needle valve can be used to control the NO flow
to the required accuracy of +_ 2%. Since the accuracy of the NO flow measurement
is so important to the overall accuracy of the GPT calibration procedure, special
attention and due care should be given to this measurement. The recommended
procedure for measuring the NO flow is to measure it directly with a soap bubble
19
meter each time the flow requires alteration. A calibrated rotameter may
serve as an "in-line" monitor of the NO flow, but it is not recommended for
absolute measurement of the NO flow. Alternatively, a calibrated mass flow
meter19 may be used to both measure and monitor the NO flow accurately.
16
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Reaction, Mixing and Sampling Chambers
The NO stream combines with the 03 stream at the exit of the ozone
generator (See Figure 1). Immediately upon mixing of the two gas streams,
the N0-03 reaction begins and it continues to completion in the reaction
chamber, provided the reaction chamber is of adequate volume. The final
mixture of NOg and excess NO mixes with the bulk of the zero air and flows
to the manifold for sampling. Glass Kjeldahl connecting bulbs make satisfactory
reaction and mixing chambers. It is important in the design of these chambers
that their entrance and exit ports be located at a maximum separation so that
the bulk of the chamber volume is utilized for reaction or mixing. A mixing
3
chamber volume of approximately 150 to 250 cm is adequate for thorough mixing
of the calibration gases and diluent air; however, the volume requirements of
the reaction chamber are more critical and these will be discussed in detail
(R)
below. The sample manifold may be constructed of glass or Teflon (or other
nonreactive material) with enough ports to accomodate the maximum number of
analyzers to be calibrated simultaneously. In addition, the manifold should
have a vent of sufficient diameter to assure atmospheric pressure at the
sampling ports and sufficient length to prevent ambient air from entering the
manifold.
NO-O^ Reaction Requirements and Dynamic Parameter Specification
The key to a quantitative reaction between NO and 0, in gas phase
titration is to provide a reaction chamber of sufficient volume to allow the
reactants to remain in close proximity for a minimum time such that the
reaction goes to completion. But how does one know when this condition has
been met without performing an involved calculation of reaction times from
17
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the rate equation and the initial NO and 03 concentrations in the reaction
chamber? As discussed earlier, the zero air stream is split such that only
a portion of the air passes through the ozone generator, increasing the
concentration of CL at the generator exit even though the Og mass flow is
constant. Thus, locally high concentrations of 03 and NO are created in the
reaction chamber, which in turn provides for quantitative reaction in a much
shorter time — and therefore within a much smaller volume — than would
otherwise be possible. For example, if only ten percent of the total flow
(at the manifold) is passed through the ozone generator, then the initial
concentrations of NO and Og in the reaction chamber are ten times greater
than what their respective concentrations would be if all of the zero air
was passed through the generator; thus, the N0-03 reaction time decreases
20
by an order of magnitude. It has been determined empirically that the
NO-Og reaction goes to completion (less than 1% residual 03) if the following
criterion is met: The product of the concentration of NO in the reaction
chamber, [NO]Rp, (in ppm) times the residence time of the reactants in the
reaction chamber, tR, (in minutes) must be at least 2.75 ppm-minutes or greater.
This product is called the dynamic parameter specification, PR. Expressed
algebraically, the specified condition is
[NO]RC x tR = PR (£2.75 ppm-min) (2)
where
[NO]RC = [NO]STD(F fp ) (3)
KL DIU I- + C
and t0 = c "c < 2 minutes. (4)
K hO hNO
18
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In the above equations
[NO]STD = concentration of the undiluted NO standard, ppm
3
VRC = vo''ume °f reaction chamber, cm
3
FO = air flow through 03 generator, cm /min
3
FNO = NO flow, cm /min
3
FT = FO + FNQ + FD = total flow at manifold, cm /min
F = diluent air flow, cm /min.
Application of Dynamic Parameter Specification
As the specification is written, a wide range of combinations of reactant
NO concentrations and residence times is possible, giving the analyst broad
latitude in designing a 6PT calibration system to meet individual requirements.
For rapid calibration, it is suggested that the residence time be restricted to
times shorter than 2 minutes. Now the question arises as to how the dynamic
parameter specification is used in actual practice to set up a 6PT dynamic
calibration system. The following approach is recommended.
1. Select the total flow, FT, for the calibration system as
measured at the sampling manifold, the recommended range for
3
FT is 1000 to 10,000 cm /min. For a particular system the
minimum value for FT is determined from the sample flow
requirements of the analyzer(s) under calibration with pro-
vision made for a suitable excess flow. (An excess flow of
at least 10 to 50% is suggested.) The maximum value for FT
is determined by the operation characteristics of the particular
ozone source. Considering the restraints on FT, the analyst
should select a suitable value for FT>
2. Select a suitable volume, VR-, for the reaction chamber. This
volume will be fixed (and can be estimated) if a commercial
calibration system is used. The recommended range for VRC is
100 to 500 cm3.
19
-------
3. Select an NO cylinder to be used for 6PT that has a nominal
concentration in the range of 50 to 100 ppm NO. The exact
cylinder concentration, [NO]STD, is determined by referencing
the cylinder against an NBS traceable NO or N02 standard.
(This procedure is discussed below.)
4. Once Fy, VR~ and [NO]STD are determined, calculate the flow
of NO, FNQ, required to generate an NO concentration at the
manifold, [NO]QUT, of 90% of the upper range limit (URL) of
the NO range. For example, if the URL for NO is 0.5 ppm,
then the required NO concentration is 0.45 ppm.
[NO]n||T x FT
F = OUT T
hNO LNO]ST[) '
5. Calculate the flow required through the 0., generator, FQ,
which results in the product of the reactant NO concentration
and the residence time being equal to 2.75, i.e., set the left
hand side of Equation 2 equal to 2.75 and solve for FQ using
Equations 3 and 4. The resulting expression is
, . / [NO]STD X FNO X
'"O "V 2775
F
" rNO
NOTE: The value of FQ determined by Equation 6 is the maximum
value for FQ. Lower values of FQ may be used.
6. Calculate the diluent air flow, FD<
FD ' FT - F0 - FNO '
7. Calculate the reactant NO concentration from Equation 3.
8. Calculate the residence time in the reaction chamber from
Equation 4. For a rapid calibration, the residence time
should be less than 2 minutes.
9. As a final check, calculate the dynamic parameter, PR, for
the reactant NO concentration and the residence time as
determined in steps 7 and 8 above,
PR = [NO]RC x tR = [NO]STD (F F .) (F ) . (2')
K Kl K ilu ho NO 0 NO
20
-------
Varying any single parameter on the right-hand side of Equation 2' affects
PR as follows:
a. Decrease in FQ -»- increase in PR.
b. Increase in VRC ->• increase in PR.
c. Increase in F.|Q + increase in PR.
Example. Calibrate two N09 analyzers, each requiring a sample flow of
3
250 cm /min. The calibration range for each is 0 to 0.5 ppm N02. Set up a GPT
dynamic calibration system using an available ozone generator that will produce
about 0.5 ppm 03 at a total air flow of about 5 1/min.
1. Select the total flow, FT<
FT (minimum) = 2(250) + 500 (excess)
3
= 1000 cm /min
FT (maximum) = 5000 cm /min
3
Let FT = 3000 cm /min.
2. Select a reaction chamber volume, VRC. A Kjeldahl connecting
bulb of about 300 cm in volume is available.
3
VRC = 300 cm.
3. An NO cylinder containing 80.0 ppm NO in N2 is available.
[NO]STD = 80.0 ppm.
4. Calculate FNQ. The required NO concentration is 0.45 ppm
(90% of URL of 0.5 ppm).
x FT _ (0.45 ppm) (3000 cm3/min.)
_
[NO]$TD 80 ppm
= 16.9 cm /min.
5. Calculate
Fo=V
•/
[NO]STD x FNQ x VRC
2.75 " ""NO
(80.0 ppm)(16.9 cm3/min)(300 cm3)
)(
75 ppm-min
3 3
= 384.0 cm /min - 16.9 cm /min
3
= 367 cm /min.
21
-------
6. Calculate
FD = FT " F0 " FNO
= (3000 - 367 - 16.9) cm3/min
= 2616 cm /min.
7- [NO] - [NO]
RC
KL
16'9
ppm
(367 + 16.9) cnT/min
= 3.52 ppm.
84. _ l\U
tp ~ p T—F
R F0 + FNO
(300 cm3)
367 cm /min + 16.9 cm /min
= 0.781 min.
9. PR = [NO]RC x tR
= (3.52 ppm)(.781 min)
= 2.75 ppm-min.
A GPT system with the following operating conditions will be suitable to
perform the calibration:
FT = 3000 cm /min
VRC = 300 cm3
FJ.Q =16.9 cm /min
3
FQ = 367.1 cm /min
FD = 2616 cm /min.
Changes in the above conditions are possible as long as the dynamic
parameter £ 2.75 is maintained.
22
-------
heck for Completeness of NO-P, Reaction
After the gas phase titration apparatus has been assembled, the completeness
f the N0-03 reaction may be verified before proceeding with the calibrations.
his verification must be made if the dynamic parameter for the GPT system is
ess than 2.75. A chemiluminescence 03 analyzer (calibrated to within +_ 50%)
annected to the manifold is required for this experiment. Generate an NO
oncentration near 90% of the upper range limit of the desired NO range; for
to 1.0 ppm and 0 to 0.5 ppm ranges, the required NO concentration is about
.9 and 0.45 ppm NO, respectively. Next, adjust the ozone source to generate
nough Og to produce an N02 concentration of approximately 80% of the upper
ange limit of the N02 range. For an NOo range of 0 to 0.5 ppm, the required
3 and N02 concentrations would be about 0.4 ppm. (The suggested upper range
imit for N02 analyzers that are calibrated for ambient air nonitoring purposes
s 0.5 ppm or less.) This is the most critical point in the gas phase titration
ince about 90% of the available NO must be reacted for the reaction to be
amplete. Note the response of the ozone monitor. There should be no
stectable Og response measured by the Og analyzer if the NO-Og reaction goes
a completion in the reaction chamber. An 0^ response greater than 1% of the
i/ailable 0^ concentration indicates an incomplete NO-Og reaction.
3p Analyzer Calibration
Once the gas phase titration system has been assembled and is operative
id the NO working standard has been certified (see the following section), the
:tual calibration of an NOp chemiluminescence analyzer is straightforward.
le object of the calibration is to determine the NO, N0? and NO responses
£ X
F the analyzer as a function of known NO and N02 concentrations. This requires
23
-------
that adjustments be made to the zero and span controls of the analyzer; the
number and function of these controls will vary according to analyzer design.
For analyzers with only one zero or span control, the respective zero or span
adjustments are made with respect to the NO response of the analyzer. Once
these adjustments are made the N09 and NO responses are then exact, or else,
b A
reduced by a factor equivalent to the converter efficiency of the analyzer.
Adjustments to analyzers with two zero or two span controls are made with
respect to the NO and NO responses — both span adjustments are made using
A
an NO source; the NO? response is again fixed and reflects any inefficiency
of the converter. Zero and span adjustments to analyzers with three separate
zero or span controls must be made for all three analyzer responses — NO, NO
n
and NO? . For these analyzers, the NO and NO span adjustments are made using
£ J\
a known NO concentration and the NO? adjustment is made using a known NO?
concentration. By adjusting the NO span control with essentially NO only,
A
the converter efficiency can be determined. For a particular analyzer, the
instruction manual will provide an in-depth discussion of its design and
operational controls.
A brief outline of the 6PT calibration procedure follows:
1. Select the analyzer calibration ranges for NO, NO and N09.
X b
2. Connect strip chart recorders to the analyzer recorder
terminals. Consult the recorder instruction manual for
procedures on making zero, gain, span, damping and other
operational adjustments.
3. Allow the analyzer to sample zero air. (The NO source
should be vented to exhaust.) Make adjustements to the
analyzer zero control(s) such that the analyzer and
recorder responses are offset by + 5% of the full scale
range. (For a calibration range of 0 to 0.5 ppm, the zero
offset would be equivalent to 0.025 ppm.)
24
-------
Adjust the NO flow to generate an NO concentration of
about 80% of the upper range limit of the NO range. After
the analyzer's responses stabilize, adjust the NO span
control such that the NO recorder response reflects the
exact NO concentration generated plus the NO zero offset.
Also, adjust the NO span control (for analyzers equipped
A
with two or three span controls) such that the NO recorder
A
response reflects the sum of the following: the NO con-
centration, any N02 impurity concentration in the NO working
standard and the NO zero offset. No NOV span adjustment
A A
is necessary for analyzers with only one span control.
NOTE: If the analyzer NO and NO responses fail to stabilize
(all flows constant) with the NO response gradually increasing
and the NO response gradually decreasing, check for contami-
A
nation of the NO pressure regulator and delivery system.
Generate about five additional NO concentrations over the
NO/NO range and record the analyzer NO and NOV responses
A A
(taken from the strip chart recorders) to each concentration.
Plot the NO and NOV calibration curves as explained in the
4
calibration procedure.
Generate an NO concentration of about 90% of the NO range.
Using the NO and NO calibration curves, measure and record
A
the NO concentration as [N0]rtv,^ and the NOU concentration
«
Adjust the 03 generator to generate sufficient 03 to produce
an N02 concentration of about 80% of the selected N02 range.
When the analyzer responses have stabilized, measure (from
calibration curves) and record the resultant NO and NO re-
A
sponses as [N0]..am and [NOv!LQm, respectively. No N09 span
i Gill X rcHi t
adjustement is necessary for analyzers with one or two span
controls. For analyzers with three span controls, adjust the
N02 span control so that the N02 recorder response reflects
the sum of the following: the N02 concentration generated by
GPT ([NO]Q . - [NO]reJ, any N02 impurity in the NO standard
and the N02 zero offset. Record the stable N02 response.
Adjust the ozone generator to obtain at least five other N02
concentrations over the N02 range and record the analyzer's
stable NO, NOX and N02 responses (strip chart readings) to
each concentration. Plot the NO, calibration curve as ex-
* 4
plained in the calibration procedure.
25
-------
9. Determine the converter efficiency. The total N02 concentration
generated at the manifold [N02]out, during the GPT is given by
the sum of the N02 concentration from GPT plus any N09 impurity
from the NO cylinder. i
* [NO]rem> + N02 ™purity. (8)
The total N0~ concentration converted to NO in the analyzer,
[N0]conv, is'given by
- [N0x]rem). (9)
The slope of a plot of CN02]conv versus [N02]Q t is the average
converter efficiency of the analyzer. If the converter efficiency
is less than 96%, replace or service the converter.
Certification of NO in NO Working Standard Against NBS Traceable Standards
The NO content of the NO working standard must be periodically assayed
against NBS traceable NO or N02 standards. Any N02 impurity in the cylinder must
also be assayed. Certification of the NO working standard should be made on a
quarterly basis or more frequently as required. Procedures are outlined below for
certification against either an NO or N02 NBS traceable standard. The simplest
and most straightforward procedure is to certify against an NO standard. NOTE:
If the assayed N02 impurity concentration, [N02]IMp, is greater than the 1 ppm
4
value allowed in the calibration procedure, make certain that the NO delivery
system is not the source of contamination before discarding the NO standard.
Certification of NO Working Standard Against an NBS Traceable NO Standard.
Use the NBS traceable NO standard and the GPT calibration procedure to
calibrate the NO, NO and N02 responses of a chemiluminescence analyzer. Also
determine the converter efficiency of the analyzer. Refer to the calibration
procedure4 for exact details; ignore the recommended zero offset adjustments.
26
-------
Generate several NO concentrations by dilution of the NO working standard.
Use the nominal NO concentration, [NO]no|T), to calculate the diluted concen-
trations. Plot the analyzer NO response (in ppm) versus the nominal diluted NO
concentration and determine the slope, SNQM. Calculate the NO concentration
of the working standard, [NO]STD, from
[NO]STD = [NO]NQM x SNQM. (10)
If the nominal NO concentration of the working standard is unknown, generate
several NO concentrations to give on-scale NO responses. Measure and record
FjYQ and FT for each NO concentration generated. Plot the analyzer NO response
versus f^Q/Fj and determine the slope which gives [NO]STD directly.
The analyzer NO responses to the generated NO concentrations reflect any
A
NOp impurity in the NO working standard. Plot the difference between the
analyzer NO and NO responses versus FMQ/FT- The slope of this plot is [NOp
In the procedure above it is possible to assay the NO content of the working
standard without first calibrating the NO and NO responses of the analyzer. This
A
is done by simply comparing relative NO responses of the working NO standard to
the NBS traceable NO standard. The NOp impurity can be determined from the
analyzer NO responses provided the converter efficiency is known.
A
Certification of NO Working Standard Against an NBS Traceable NO,, Standard.
Use the NO working standard and the GPT calibration procedure to "calibrate"
the NO, NO and N09 responses of a chemi luminescence analyzer. Refer to the
A £
calibration procedure for exact details; ignore the recommended zero offset
adjustements. For this pseudo-calibration use the nominal NO cylinder value
and assume no N02 impurity is in the cylinder. For an analyzer with dual detectors,
27
-------
the NO span adjustment must be made by diverting the sample flow around the
A
converter and routing it directly to the NO detector. This operation
A
electronically balances the two detectors.
From the GPT data, plot the analyzer NO,, response versus the N0« concen-
tration generated by GPT. Determine the slope, SNQM, and the X-intercept of
the curve. Generate several N0« concentrations by dilution of the NBS
traceable N02 standard. Plot the analyzer N02 response versus N02 concen-
tration. Determine the slope, SNB<.. Calculate the NO concentration of the
working standard, [NO]<.jD, from
[NO]STD - [NO]NOH x . (11)
Calculate the N02 impurity from
(X-intercept) FT S
~ X
F
hNO
ALTERNATIVE B: N02 PERMEATION DEVICE
21
In a permeation device, an easily liquifiable gas such as N0» is con-
densed inside an inert container, all or part of which is constructed from a
(R)
polymeric material (often Teflon^ ). Gas escapes from the container by
dissolving in and permeating through the polymer walls at a temperature
dependent rate. The rate of gas effusion (in yg/min) at a constant temperature
can be established by gravimetric determination of the weight loss of the
permeation device over a known period of time. In this calibration procedure,
the NO and NO responses of a chemi luminescence analyzer are first calibrated
A
with an NO standard; next accurately known concentrations of N02 are produced
dynamically by diluting the effusion from an N02 permeation device with various
28
-------
flows of clean air to obtain a calibration for NOp. Either the NOp permeation
device or the NO source may be chosen as the reference standard for calibration.
The remaining standard must be assayed against the reference standard for
consistency.
Components of a Permeation Device Calibration System
Figure 2 shows a diagram of a typical permeation device calibration system.
Such systems have been described in the literature ' and they are also
commercially available. All connections between components in the system should
(B)
be glass or Teflon or other nonreactive material. The system consists of
four functional sections:
1. A controlled-temperature section that houses the NOo permeation
device and is flushed continuously with purified, dry zero air
or nitrogen.
2. A regulated source of clean, dry zero air for dilution of the
NOp gas effluent from the permeation device. The source should
be capable of providing air flows up to about 20 1/min.
3. An NO standard and delivery system.
4. A dilution-mixing, sampling and exhaust section.
The suggestions for preparing, regulating and measuring zero air flows discussed
in connection with gas phase titration are applicable to this calibration system
also. In addition, an NO standard with delivery system and a suitable dilution-
mixing, sampling and exhaust assembly were also discussed above. Therefore, the
latter three sections of the permeation device calibration system do not warrant
further discussion. A description of the constant temperature section follows.
Constant Temperature Section. Temperature control is the primary concern
in using an N02 permeation device as a standard N02 source. For example, a
change in temperature of about 0.5°C effects a change in the permeation rate
29
-------
CO
o
o
VENT
EXTRA OUTLETS CAPPED
WHEN NOT IN USE
TO INLET OF ANALYZER
UNDER CALIBRATION
THERMOMETER
PERMEATION
DEVICE /
I
CONSTANT TEMPERATURE
CHAMBER
Figure 2. Schematic diagram of a typical calibration apparatus using an NO2 permeation device.
-------
of the device of about four percent (4%). For this reason, it is important
that the temperature of the device be maintained at a constant value within
+ 0.1°C and that it be closely monitored when the device is in use.
Generally, the NOg permeation device is housed in a temperature-controlled
glass container that has an entrance and exit port at opposite ends; a glass
thermometer accurate to + 0.05°C may be placed beside the device to monitor its
3
temperature. A small fixed zero air or nitrogen flow (about 100 cm /min) that
is maintained at the same temperature as the permeation device flushes the N02
out of the device housing into a mixing chamber where the N02 is diluted with
clean dry zero air. A valve, e.g., a three-way stopcock, placed at the exit of
the device housing may be used to divert the NOo stream to a vent when clean
air is required at the manifold for making the necessary z»ro adjustments to the
analyzer.
To maintain the temperature of the permeation device to within +_ 0.1°C of
the desired value, the device and housing may be either placed physically inside
a constant temperature chamber as depicted in Figure 2 or they can be located
external to the constant temperature chamber with the heat transfer medium
circulated around the device housing, e.g., a jacketed condenser (West or
Liebig type). The flushing zero air or nitrogen passes through a heat exchanger,
e.g., a coil of copper tubing, contained in the constant temperature chamber
before passing over the device to adjust its temperature to that of the device.
For a calibration system to be used in a laboratory or other permanent location,
a circulating water bath makes an excellent constant temperature chamber. Many
circulating water baths are available that are capable of temperature control to
i0.1°C over a suitable temperature range (usually 15°C to 35°C for most cali-
bration work). Commercial calibration systems often use circulating air in
31
-------
the constant temperature chamber; such a chamber has the advantage of being
more portable than a water bath.
Flush Gas for Permeation Device. In Figure 2 the zero air stream is
split to allow a small air flow to pass continuously over the permeation
device. Alternatively, the flush gas could be supplied from a cylinder of
prepurified dry air or nitrogen. Whatever its source is extremely important
that the flushing stream be extra dry so that moisture does not condense on
the surface of the device. Water condensate could react with the effusing
N02 to form an acid mist thus changing the N02 concentration. A transparent
drying column containing a mixture of molecular sieve (e.g., 6-16 mesh, type 4A)
and indicating calcium sulfate (e.g., Drierite) has been used effectively as a
moisture scrubber on the flush gas line.
Standard NOp Permeation Device
The diffusion properties of NOo has made the construction of stable, accurate
N02 permeation devices no easy feat. For this reason due care must be given to
their handling for reliable use. Permeation devices are available from commercial
sources and from NBS as a Standard Reference Material (SRM 1629). The NBS device
has a certified permeation rate of approximately 1 ^g/min at about 25°C.
Permeation rates of commercial devices vary according to size and recommended
operating temperature. Both NBS and commercial manufacturers provide explicit
instructions on the use of their respective devices which the user should follow
for accurate measurements.
Most permeation devices must equilibrate for at least 24 hours at the
certified or operating temperature before the permeation rate stabilizes.
32
-------
Equilibration times may be longer and the permeation rate may be erratic if
the device is subjected to extreme temperature variations when not in use.
It was mentioned above that the flush gas over the permeation device must be
extra dry. This is especially true of the NBS device and many others which
have a large surface area for N02 permeation. Some commercial devices which
have a very small permeating area and are designed to operate at elevated
temperatures (40 to 60°C) may not be as susceptible to trace moisture in the
flush gas. Additional information regarding the use of permeation devices
22-24
for calibration purposes is documented elsewhere.
If the NOp permeation device is to be used as the reference standard for
calibration, then the permeation rate of the device must be traceable to an
NBS NO in NZ standard (SRM 1683 or 1684) or N02 standard (SRM 1629). Otherwise,
the permeation device need only be periodically assayed against the reference
NO standard to assure consistency between the two working standards. Procedures
for certifying the reference standard against NBS traceable NOo or NO in N2
standards and for intercomparing the N02 and NO working standards are discussed
below.
Basic Design Considerations for a Calibration System
When designing a calibration system, the analyst should first determine
the relevant operational criteria that the system must meet. The calibration
range(s) that the system must accommodate should be considered along with the
corresponding total air flow that will be required. For maximum flexibility,
the system should be designed for use with the widest applicable range (normally
0 to 0.5 ppm N02 for ambient air measurements); it will serve more sensitive
ranges when necessary. Since the N02 concentration is inversely proportional to
33
-------
the total flow at the manifold, the minimum required N0? concentration sets
the upper limit of the dilution air flow. For example, using one NBS permeation
device that generates about 1 yg N02/min, a total air flow of about 18 1/min is
required to generate about 0.03 ppm N02- Lower concentrations would, of course,
require higher dilution air flows. A second consideration is the number of NOp
analyzers that can be calibrated simultaneously with the calibration system.
This is controlled not only by the sum of the respective analyzer sample flow
rates but also and most importantly, by the minimum total flow of the cali-
bration system at the manifold. Air flow is a minimum when the NOp concentration
is a maximum. As specified in the calibration procedure, the maximum required
N02 concentration is about 80% of the calibration range. For example, again
using one NBS permeation device and specifying an NOp analyzer range of 0 to
0.5 ppm N02, a total flow of about 1.3 1/min is required to generate about 0.4
ppm NOp. Allowing about 0.3 1/min as excess flow, only about 1.0 1/min flow is
available at the manifold for calibration of the N02 analyzer(s). If the lower
limit of the total flow of the N02 calibration gas is insufficient to meet the
flow demand of the NOp analyzer(s), then the problem could be solved by cali-
brating and using the analyzer(s) on a more sensitive range whenever possible
and appropriate. Alternatively, two or more permeation devices could be used
in parallel to generate NOp concentrations at the upper end of the calibration
range. For example, two NBS devices would permit the doubling of the total
flow at the manifold. By venting the effluent of all but one of the devices,
N02 concentrations in the lower portion of the range could be easily provided.
NOp Analyzer Calibration
Ont_e L!ic permeation device system has been assembled and is operative and
tuc NO and NCp Corking standards have been intercompared with rccpcct to
34
-------
the certified standard (see the following section), the actual calibration of
an N02
chemiluminescence analyzer is straightforward. The object of the
calibration is to determine the NO, N0~ and NO responses of the analyzer as
£- X
a function of known NO and NOp concentrations. This requires that adjustments
be made to the zero and span controls of the analyzer; the number and function
of these controls will vary according to anlyzer design. For analyzers with
only one zero or span control, the respective zero or span adjustments are
made with respect to the NO response of the analyzer. Once these adjustments
are made the N09 and NO responses are then exact, or else, reduced by a factor
L- A
equivalent to the converter efficiency of the analyzer. Adjustments to analyzers
with two zero or two span controls are made with respect to the NO and NO
/\
responses — both span adjustments are made us'ny an NO source, the NOo response
is again fixed and reflects any inefficiency of the converter. Zero and span
adjustments to analyzers with three separate zero or span controls must be made
for all three analyzer responses — NO, NO and NO?. For these analyzers, the
X C-
NO and NO span adjustments are made using a known NO concentration and the N0~
X L-
adjustment is made using a known N0~ concentration. By adjusting the NO span
£ /\
control with essentially NO only, the converter efficiency can be determined.
For a particular analyzer, the instruction manual will provide an in-depth
discussion of its design and operational controls.
A brief outline of the calibration procedure follows:
1. Select the analyzer calibration ranges for NO, NO and N09.
X C.
2. Connect strip chart recorders to the analyzer recorder terminals.
Consult the recorder instruction manual for procedures on making
zero, gain, span, damping, and other operational adjustments.
3. Allow the analyzer to sample zero air. (The NO and N02 sources
Should be w°.r>ted tO «?vhaijst.) Matp ari-in<;tmpr)t<; tn thp ana1v7Pr
zero control(s) such that the analyzer and recorder responses
are offset by + 5% of thp full ?cale vannp. (For a calibration
range of 0 to 0.5 ppm, the zero offset would be equivalent to
0.025 ppm.)
35
-------
4. Ad.iust the NO flow to generate an NO concentration of about
80% of the upper ranqe limit of the NO range. After the
analyzer's responses stabilize, ad.iust the NO span control
such that the NO recorder response reflects the exact NO
concentration generated plus the NO zero offset. Also ad.iust
the NO span control (for analyzers eguioped with two or three
y\
span controls) such that the NO recorder response reflects
y\
the sum of the following: the NO concentration, any NOp im-
purity concentration in the NO working standard and the NO
y\
zero offset. No NO span adjustment is necessary for analyzers
J\
with only one span control.
NOTE: If the analyzer NO and NOx responses fail to stabilize
(all flows constant) with the NO response gradually increasing
and the NO response gradually decreasing, check for contami-
A
nation of the NO pressure regulator and delivery system.
5. Generate about five additional NO concentrations over the NO/NO
A
range and record the analyzer NO and NOV responses (taken from
/\
the strip chart recorders) to each concentration. Plot the NO and
NO calibration curves as explained in the calibration procedure.
A
6. Divert the NO flow to exhaust and the NOp flow to the manifold.
Ad.iust the dilution air flow, FQ, to generate an N02 concentration
of about 80% of the upper range limit of the NOp range. After the
analyzer response stabilizes, ad.iust the N02 span control (only
for analyzers with three span controls) so that the NOp recorder
response reflects the sum of the NOp concentration from the
permeation device plus the NOp zero offset. Record the stable
NOp and NO responses.
£ «
7. Generate at least five additional NOp concentrations by varying
the dilution air flow; record the stable N02 and NOX responses
to each concentration. An equilibration time of at least ten
minutes is suggested between concentration changes. Plot the 4
NOp calibration curve as explained in the calibration procedure.
8. Determine the converter efficiency. The N02 concentration at the
manifold, [N^out* ls emulated from the permeation rate and
the total flow at the manifold. The N02 concentration converted
to NO by the analyzer is given by the NO response to the
n
generated N02 concentrations. The slope of a plot of the NO
response versus [NOp] . gives the average converter efficiency
of the analyzer. If the converter efficiency is less than 96%,
replace or service the converter.
36
-------
Certification of Working N02 or NO in N2 Standard Against NBS Traceable Standard
Either the NOo permeation device or the NO source may be chosen as the
reference standard for calibration. The reference standard must be certified
aqainst an NBS traceable standard. The remaining standard must then be assayed
against the reference standard for consistency. To show consistency, the NOo
generated by a permeation device is compared to the N02 generated by gas
phase titration of the NO standard. Certifications and intercomDarisons
should be done quarterly or more frequently as required.
If the NO standard is chosen as the reference standard, it may be certified
aqainst an NBS traceable N02 or NO standard. These certification procedures
were outlined above under the discussion of gas phase titration and need not be
repeated here. Certification of an N02 standard against NBS traceable standards
and the intercomparison of the NO and N02 standards are discussed below.
Certification of N02 Working Standard Against ar NBS Traceable NOp Standard.
The N02 chemiluminescence analyzer need not be in calibration for these
measurements. Generate several N02 concentrations by dilution of the NBS
traceable NOo standard. Plot the analyzer N02 response versus N02 concentration
and determine the slope, SNB<-. Generate several N02 concentrations by dilution
of the working N02 standard to give on-scale N02 responses. Measure the total
flow at the manifold, F,., for each N02 concentration generated. Plot the
analyzer N02 response versus 1/F,. and determine the slope, SSTD. Calculate the
permeation rate, R, from
(13)
where
K = 0.532 yl N02/ug N02 (at 25°C and 760 mm Hg)
37
-------
Certification of NOo Working Standard Against an NBS Traceable NO Standard.
Use the NBS traceable NO standard and the GPT calibration procedure to calibrate
the NO, NO and N09 responses of a chemiluminescence analyzer. Refer to the
A L.
GPT calibration procedure for exact details; ignore the recommended zero
offset adjustments. Generate several N02 concentrations by dilution of the
working N02 standard to give on-scale N02 responses. Measure the total flow
at the manifold, FT, for each N02 concentration generated. Plot the analyzer
N02 response versus 1/Fj and determine the slope, SST[). Calculate the per-
meation rate, R, from
. 04)
Intercomparison of NOo and NO Working Standards.
To compare the working N02 standard to a certified NO working standard,
simply follow the same procedure as outlined above for "certifying an N02
working standard against an NBS traceable NO standard". The N02 and NOX span
adjustments must take into account any N02 impurity in the NO working standard.
To make comparison between a working NO standard and a certified N02 standard,
follow the same procedure as outlined for "certifying a working NO standard
against an NBS traceable N02 standard". This procedure was discussed in the
GPT section.
38
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REFERENCES
1. Environmental Protection Agency, Title 40, Code of Federal Regulations,
"Part 50 - National Primary and Secondary Ambient Air Quality Standards,"
Federal Register, 36, 8186, April 30, 1971.
2. T. R. Hauser and C. M. Shy, Envir. Sci. Tech.. 6_, 890 (1972).
3. Environmental Protection Agency, Title 40, Code of Federal Regulations,
"Part 50 - Reference Method for Determination of Nitrogen Dioxide,"
Federal Register. 38_, 15174, June 8, 1973.
4. Environmental Protection Agency, Title 40, Code of Federal Regulations,
"Part 50 - Measurement Principle and Calibration Procedure for the
Measurement of Nitrogen Dioxide in the Atmosphere (Gas Phase
Chemiluminescence)," Federal Register, 41, 52688, December 1, 1976.
5. Environmental Protection Agency, Title 40, Code of Federal Regulations,
"Part 53 - Ambient Air Monitoring Reference and Equivalent Methods,"
Federal Register. 40_(33), 7044, February 18, 1975 as amended in
Federal Register, 41_, 52692, December 1, 1976.
6. E. C. Ellis and J. H. Margeson, "Evaluation of Gas Phase Titration
Technique as Used for Calibration of Nitrogen Dioxide Chemilumi-
nescence Analyzers." Environmental Protection Agency, Research
Triangle Park, N. C. EPA Publication No. EPA-650/4-75-021,
April 1975.
7. A. Fontijn, A. J. Sabadell, and R. J. Ronco, "Homogeneous
Chemiluminescent Measurement of Nitric Oxide with Ozone,"
Anal. Chem., 42_, 575 (1970).
8. D. H. Stedman, E. E. Daby, F. Stuhl, and H. Niki, "Analysis of
Ozone and Nitric Oxide by a Chemiluminescent Method in Laboratory
and Atmospheric Studies of Photochemical Smog," J_. Air Poll.
Control Assoc.. 22_, 260 (1972).
9. B. E. Martin, J. A. Hodgeson and R. K. Stevens, "Detection of
Nitric Oxide Chemiluminescence at Atmospheric Pressure," Presented
at 164th National American Chemical Society Meeting, New York City,
August 1972.
10. R. K. Stevens and J. A. Hodgeson, "Application of Chemiluminescent
Reactions to the Measurement of Air Pollutants, " Anal. Chem.. 45,
443A (1973).
39
-------
11. J. A. Hodgeson, K. A. Rehme, B. E. Martin, and R. K. Stevens,
"Measurement for Atmospheric Oxides of Nitrogen and Ammonia by
Chemiluminescence," Preprint, Presented at 1972 Air Pollution
Control Association Meeting, Miami, Florida, June 1972, Paper
No. 72-12.
12. L. P. Breitenbach and M. Shelef, "Development of a Method for
the Analysis of N0? and NH~ by NO-Measuring Instruments,"
J.. Air Poll. Control AssocT. 23, 128 (1973).
13. A. M. Winer, J. W. Peters, J. P. Smith and J. N. Pitts, Jr.,
"Response of Commercial Chemiluminescent N0-N0« Analyzers to
Other Nitrogen-Containing Compounds," Envir. Sci. Tech.. 8,
1118 (1974).
14. P. C. Constant, M. C. Sharp and G. W. Scheil, "Collaborative
Testing of Methods for Measurement of NO, in Ambient Air,"
EPA Report No. 650/4-75-013, February 1975.
15. K. A. Rehme, B. E. Martin, and J. A Hodgeson, "Tentative Method
for the Calibration of Nitric Oxide, Nitrogen Dioxide and Ozone
Analyzers by Gas Phase Titration," Environmental Protection Agency,
Research Triangle Park, N.C., EPA Publication No. EPA-R2-73-246,
March 1974.
16. M. A. A. Clyne, B. A. Thrush and R. P. Wayne, Trans. Faraday
Soc.. 60, 359 (1964).
17. J. A. Hodgeson, R. K. Stevens, and B. E. Martin, "A Stable Ozone
Source Applicable as a Secondary Standard for Calibration of
Atmospheric Monitors," ISA Transactions. Jl, 161 (1972).
18. NBS Technical Note, No. 585, pp. 11-25 (January 1972). Available
through: Superintendent of Documents, Government Printing Office,
Washington, D. C. 20402. Price 70$.
19. G. 0. Nelson, Controlled Test Atmospheres, (Ann Arbor Science
Publishers, Inc., Ann Arbor, 1971), Chapter 3.
20. K. A. Rehme and C. F. Smith, Environmental Monitoring and Support
Laboratory, Environmental Protection Agency, private communication.
21. A. E. O'Keeffe and G. C. Ortman, "Primary Standards for Trace Gas
Analysis." Anal. Chem.. 38, 760 (1966).
22. F. P. Scaringelli, A. E. O'Keeffe, E. Rosenberg, and J. B. Bell,
"Preparation of Known Concentrations of Gases and Vapors with
Permeation Devices Calibrated Gravimetrically," Anal. Chem., 42,
871 (1970).
23. H. L. Rook, E. E. Hughes, R. G. Fuerst and J. H. Margeson,
Operation Characteristics of NOo Permeation Devices,"
Anal. Chem., In press.
24. Calibration in Air Monitoring. ASTM STP 598, (American Society for
Testing and Materials, Philadelphia, 1976).
40
-------
ADDENDUM
41
-------
Part 50 of Chapter I, Title 40, Cc.-le of Foderel Regulations,
is {••r.:'!i'/iG;1 by -revising Appendix F to rr.cri as follows:
APj'tN'Jix F - KEAV.JREMENT PRINCIPLE A?ID CALIBRATION PROCEDURE FOR
THE MEASUREMENT Of NITROGEN DIOXIDE IN THE ATMOSPHERE
(GAS PHASE CHEMJLUHlNtSCEiiCE)
Pr'ij^"i].-j_c end Ajp.pl "I Cubi_li_ty_
1. Atmospheric concentrations of nitrogen dioxide (N02)
are measured indirectly by photometrically measuring the light
intensity, at wavelengths greater than 600 nanometers, resulting
from the che.Tiiluminescent reaction of nitric oxide (NO) with
ozone (03)."*2*3' N02 is first quantitatively reduced to
Mr''0"'' by means of a converter. NO, which commonly exists in
ambient
-------
generally low relative to K02 and valid N02 measurements may be
obtained. In certain geographical Erear, where the concentration
of these potential interferences is known or suspected to be high
relative to N0?, the use of an equivalent method for the measure-
ment of NOp is reccrrraundec!.
2.2 The use of integrating flasks on the snmrile inlet line
of chemiluminescence NO/NO /NCk analyzers is stroiioly discouraged.
The sample residence time between the sampling point and the
analyzer should be kept to n minimum to avoid erroneous NCL
measurements resulting from the reaction of ambient levels of NO'
and 0^ in the sampling system.
2.3 The use of participate filters on the sample inlet line
of chemiluminescence NO/NO /NOp analyzers is optional and left to
the discretion of the user or the manufacturer. Use of the
filter should depend on tho analyzer's susceptibility to inter-
ference, malfunction, or damage due to parti oilates. Users ore
cautioned that participate matter concentrated on a filter may
cause erroneous N0? measurements and therefore filters should be
changed frequently.
3i An analyzer based on this principle will ba considered
a reference method only if it has been designated as a reference
method in accordance with Part S3 of this chapter.
Calibration
!• AI1^.r!i>iLLvJ?..A - Gas Phase ti (.ration (GPT) of an NO
standard with CL.
44
-------
ggui pmgn t r equ i red : Stable 0-j generator.
Chenii luminescence NO/NC^/NOg analyzer
with strip chart recorder (s).
KO concentration standard.
1.1 Principle. Tin's calibration tcichniqi:? is based upon
the rapid gas phase reaction between NO and CL to produce stoichio-
/0\
metric quantities of N(L in accordance with the following equation:v '
NO + 03 + N02 + 02 (1)
The quantitative nature of this reaction is such that when the NO
concentration is known » the concentration of NOp can be determined.
Ozone is added to excess NO in a dynamic calibration system, and
the NO channel of the chemi luminescence NO/NO /NOp analyser is
used as an indicator of changes in NO concentration. Upon the
addition of 03> the decrease in NO concentration observed on the
calibrated NO channel is equivalent to the concentration of NOp
produced. The amount of NOp generated may be varied by adding
variable amounts of 0~ from a stable uncalibrated Q~ genera tor.
1.2 Apparatus. Figure 1, a schematic of a typical GPT
apparatus, shows the suggested configuration of the components
listed below. All connections between components in the calibration
system downstream from the 0.. generator should be of glass,
©
Teflon , or other non-reactive material.
1.2.1 Air flow controllers. Devices capable of maintaining
constant air flows within .J2" of the required flowrate.
45
-------
1.2.2 NO flow controller. A device capable of maintaining
constant !!0 flows within ±23 of the required flov.rate. Component
parts in contact with the NO should be of a non-reactive material.
1.2,3 Air flowmeters. Calibrated flowmeters capable of
measuring an.d monitoring air flowrates with an accuracy of ±2% of
the measured flowrate.
1.2.4 NO flowineter. A calibrated flowmeter capable of
measuring and monitoring NO flowrates with an accuracy of ±2% of
the measured flowrate. (Rotameters have been reported to operate
unreliably when measuring low NO flows and are not recommended.)
1.2.5 Pressure regulator for stanoard NO cylinder. This
regulator must have a non-reactive diaphragm and internal parts
and a suitable delivery pressure.
1.2.6 Ozone generator. The generator must be capable of
generating sufficient and stable.levels of 03 for reaction with
NO to generate N02 concentrations in the range required. Ozone
generators of the electric discharge type may produce NO and NOg
and are not recommended.
1.2.7 Valve. A valve may be used as shown in Figure 1 to
divert the 110 flow when zero air is required at the manifold.
The valve should be constructed of glass, Teflon , or other non-
reactive material.
1.2.8 Reaction chamber. A chamber, constructed of glass,
Teflon", or other non-reactive material, for the quantitative
reaction of ()-, with excess NO. T!;e chamber should bo of sufficient
46
-------
volume (VL,.) such that the residence time (tR) mests the require-
ments specified in 1.4. For practical reasons, tR should be less
than 2 minutes.
1.2.9 Mixing chamber. A chamber constructed of glass,
Teflon', or other non-reactive material and designed to provide
thorough mixing of reaction products and diluent air. The residence
time is not critical when the dynamic parameter specification
given in 1.4 is met.
1.2.10 Output manifold. The output manifold should bo
constructed of glass, Teflon , or other non-reactive material and
should be of sufficient diameter to insure an insignificant
pressure drop at the analyzer connection. The system must: have a
vent designed to insure atmospheric pressure at the manifo'id and
to prevent ambient a.ir from entering the manifold.
1.3 Reagents.
1.3.1 NO concentration standard. Cylinder containing 50 to
100 ppm NO in N2 with less than 1 ppm N02. The cylinder must be
traceable to a National Bureau of Standards NO in N2 Standard
Reference Material (SRM 1683 or SRM 1684) or N02 Standard Reference
Material (SRM 1629). Procedures for certifying the NO cylinder
(working standard) against an NBS traceable JJO or N02 standard
and for determining the amount of N02 impurity are given in
reference 13. The cylinder should be recertified on a regular
busis as determined by the local quality control program.
47
-------
1.3;2 Zero air. Air, free of contaminants which will cause
a detectable response on the NO/NO/NO,, analyzer or which might
rk £
react with either NO, 03, or NOo in the gas phase titration. A
procedure for generating zero air is given in reference 13.
1.4 Dynamic parameter specification.
• •
1.4.1 The 03 generator air flowrate (FQ) and NO flowrate
(FNQ) (see Figure 1) must be adjusted such that the following
relationship holds:
x *R >. 2-75 ppm-mirutes (2)
CNO]RC = [NO] (--) (3)
KU -
DP
tD = r . c < 2 minutes (4)
R F0 * FNO
where: PR = dynamic parameter specification, determined
empirically, to insure complete reaction of the
available 03> ppm-minute
[NO]RC = NO concentration in the reaction chamber, ppm
tp = residence time of the reactant gases in the
reaction chamber, minute
[NO]STD= concentration of the undiluted NO standard, ppm
3
^NO = NO flowrate, scm /min
o
FQ = 03 generator air flowrate, scm /min
o
V, - volume of the reaction chamber, scm
48
-------
1.4.2 The flow conditions to be used in the GPT system are
determined by the following procedure:
(a) Determine Fy, the total flow required at the
output manifold (F-r = analyzer demand plus 10 to 50% excess).
(b) Establish [NOJQ,.,. as the highest NO concentration
(ppm) which will be. required at the output manifold. [NOjnijT
should be approximately equivalent to 90% of the upper range
limit (URL) of the N02 concentration range to be covered.
(c) Determine FNQ as
MOUT^T
•NO TNCOSTD
(d) Select a convenient or available reaction chamber
volume. Initially, a trial VRC may be selected to be in the
3
range of approximately 200 to 500 son .
(e) Compute F0 as
'.-V
[NO]STD x FNQ x VRC
T.75 FNO
(f) . Compute tR as
NO
Verify that tR < 2 minutes. If not, select a reaction chamber
with a smaller VRC-
-------
• (g) Comput3 the diluent air flowrate as
FD - FT-F0-FNO
3
where: FD = diluent air flowrate, son /min
(h) If FQ'turns out to be impractical for the desired
system, select a reaction chamber having a different VRC and
recompute FQ and FQ.
NOTE: A dynamic parameter lower than 2.75 ppm-minutes may be
used if it can be determined empirically that quantitative reaction
of (L with NO occurs. A procedure for making this determination
as well as a more detailed discussion of the above requirements
and other related considerations is given In reference 13.
1.5 Procedure.
1.5.1 Assemble a dynamic calibration system such as the one
shown in Figure 1.
1.5.2 Insure that all flowmeters are calibrated under the
conditions of use against a reliable standard such as a soap-
bubble meter or wet-test meter. All volumetric flev/rates should
be corrected to 25°C and 760 mm Hg. A discussion on the cali-
bration of flowineters is given in reference 13.
1.5.3 Precautions mi'st be taken to remove Op and other
contaminants from the NO pressure regulator and delivery system
prior to t\,c start of calibration to avoid any conversion of the
standard NO to NO.,. Failure to do so can cause significant
50
-------
errors in calibration. This problem may be minimized by
(1) carefully evacuating the regulator, when possible, after tha
regulator has been connected to the cylinder and before opening
the cylinder valve; (2) thoroughly flushing the regulator and
delivery system with NO after opening the cylinder valve; (3) not
removing the regulator from the cylinder between calibrations
unless absolutely necessary. Further discussion of these pro-
cedures is given in reference 13.
1.5.4 Select the operating range of the NO/NO /NCL analyzer
« k
to be calibrated. In order to obtain maximum precision and
accuracy for NCL calibration, all three channels of the analyzer
should be set to the same range. If operation of the NO and NO
A
channels on higher ranges is desired, subsequent recalibration of
the NO and NO channels on the higher ranges is recommended.
4\
NOTE: Some analyzer designs may require identical ranges for NO,
NOX, and NOp during operation of the analyzer.
1.5.5 Connect the recorder output cable(s) of the NO/NO /N02
analyzer to the input terminals of the strip chart recorder(s).
All adjustments to the analyzer should be performed based on the
appropriate strip chart readings. References to analyzer responses
in the procedures given below refer to recorder responses.
1.5.6 Determine the GPT flow conditions required to meet
the dynamic parameter specification as indicated in 1.4.
51
-------
1.5.7 Adjust the diluent sir ?nd 03 operator air flows to
obtain the flows determined in 1.4.2. The total air flow must
exceed the total demand of the cinaiyz^r(s) connected to the
output manifold to insure that no ambient air is pulled into the
manifold vent. Allow the analyzer to sample zero air until
stable NO, NO , and NO^ responses are obtained. After the
responses have stabilized, adjust the analyzer .zero control(s).
NOTE: Some analyzers may have separata zero controls for NO,
NO , and N0~. Other analyzers may have separate zero controls
A £
only for NO and NO , while still others may have only one zero
• "
control common to all three channels.
Offsetting the analyzer zero adjustments to +5% of scale is
recommended to facilitate observing negative zero drift. Record
the stable zero air responses as Z^g, ZMQ , and ZNQ .
1.5.8 Preparation of NO and NOV calibration curves.
A
1.5.8.1 Adjustment of NO span control. Adjust the NO flow
from the standard NO cylinder to generate an NO concentration of
approximately 80% of the upper range limit (URL) of the NO range.
The exact f!0 concentration is calculated from:
Fwn x [NO]
. r .
0
where: [N°"JniJT ~ diluted NO concentration at the output
manifold, pp.-n
52
-------
Sample this NO concentration until the HO and NOX responses have
stabilized. Adjust the NO span control to obtain a recorder
..response as indicated below:
[NO]OUT
recorder response (% scale) = ( .. " x 100) + 1 (10)
UKL INU
where: URL = nominal upper range limit of the NO channel, ppm
NOTE: Some analyzers may have separate span controls for NO,
NO , and NOp. Other analyzers may have separate span controls
only for NO and NO , while still others may have only one span
A
control common to all three channels. When only one span control
is available, the span adjustment is made on the NO channel of
the analyzer.
If substantial adjustment of the NO span control is necessary, it
may be necessary to recheck the zero and span adjustments by
repeating steps 1.5.7 and 1.5.8.1. Record the NO concentration
o
•3
and the analyzer's NO response.
1.5.8.2 Adjustment of NO span control. When adjusting the
A
analyzer's NO span control, the presence of any N09 impurity in
A *•
the standard NO cylinder must be taken into account. Procedures
for determining the amount of NOp impurity in the standard NO
cylinder are given in reference 13. The exact NO concentration
xv
is calculated from:
53
-------
where: [KOx]0iJT = diluted NOX concentration at the output
manifold, ppm
[NOgljjjp = concentration of N00 impurity in the standard
NO cylinder, ppm
Adjust the NO span control to obtain a recorder response as
indicated below:
[NO ]ou,
recorder response (% scale) = (—jjR||u' x 100) + ZNQ (12)
n
NOTE: If the analyzer has only one span control, the span adjust-
ment is made on the NO channel and no further adjustment is made
here for NO .
A
If substantial adjustment of the NO span control is necessary,
A
it may be necessary to recheck the zero and span adjustments by
repeating steps 1.5.7 and 1.5.8.2. Record the NO concentration
A
and the analyzer's NO response.
1.5.8.3 Generate several additional concentrations (at
least five evenly spaced points across the remaining scale are
suggested to verify linearity) by decreasing F..Q or increasing
Fp. For each concentration generated, calculate the exact NO and
NO concentrations using equations (9) and (11) respectively.
54
-------
Record the analyzer's NO and NO responses for each concentration.
Plot the analyzer responses versus the respective calculated NO
and NO concentrations and draw or calculate the NO and NO
A s*
calibration curves. For subsequent calibrations where linearity
can be assumed, these curves may be checked with a two-point
calibration consisting of a zero air point and NO and NO., concen-
A
trations of approximately 80% of the URL.
1.5.9 Preparation of N02 calibration curve.
1.5.9.1 Assuming the NO^ zero has been properly adjusted
while sampling zero air in step 1.5.7, adjust FQ and FQ as deter-
mined in 1.4.2. Adjust F.,0 to generate an NO concentration near
90% of the URL of the NO range. Sample this NO concentration
until the NO and NO responses have stabilized. Using the NO
calibration curve obtained in 1.5.8, measure and record the NO
concentration as [NO] . . Using the NO calibration curve
obtained in 1.5.8, measure and record the NO concentration as
1.5.9.2 Adjust the 0., generator to generate sufficient 0^
to produce a decrease in the NO concentration equivalent to
approximately 80% of the URL of the NOp range. The decrease must
not exceed 90% of the NO concentration determined in step 1.5.9.1.
After the analyzer responses have stabilized, record the resultant
NO and NOV concentrations as [NO!, , and [N0lom.
A i ClU /> I t,fli
1.5.9.3 Calculate the resulting NO- concentration from:
55
-------
NO 0 D
where: C^JnUT = Diluted ^2 concen-trati°n at tne output
nanifold, ppm
[N0]o . = original NO concentration, prior to addition
of Og, ppm
[NO] = NO concentration remaining after addition of
03, ppm
Adjust the NO,, span control to obtain a recorder response as
indicated below:
recorder response (% scale) = (—x TOO) •» ZNQ
NOTE: If the analyzer has only one or two span controls, the
span adjustments are mads on the NO channel or NO and NO channels
and no further adjustment is made here for NO.
If substantial adjustment of the N02 span control is necessary,
it may.be necessary to rccheck the zero arid span adjustments by
repeating steps 1.5.7 and 1.5.9.3. Record the N02 concentration
and the corresponding analyzer NOg and NOX responses.
1.5.9.4 Maintaining the same Fj,,g, FQ, and FD as in 1.5.9.1,
adjust the ozone generator to obtain several other concentrations
of NOp over- the NOp range (at least five evenly spaced points
56
-------
across the remaining scale are suggested). Calculate esch NC^
concentration using equation (13) and record the corresponding
analyzer N0~ and NO responses. Plot the analyzer's NOp responses
versus the corresponding calculated NOp concentrations and draw
or calculate the NCL calibration curve.
1.5.10 Determination of converter efficiency.
1.5.10.1 For each NOp concentration generated during the
preparation of the N02 calibration curve (see 1.5.9) calculate
the concentration of NOp converted from:
(15)
where: C^2^CONV= concentrai;ion °f NO- converted, ppni
[NO ] . - original NO concentration prior to addition
x OPT 9 x
of 00, ppm
j
[NO ] = NO concentration remaining after addition of
X i CHI X
03, ppm
Plot [NOg^CONV ^'"ax1's^ versus CN02^0UT (x~axi's) anci draw or
calculate the converter efficiency curve. The slope of the curve
times 100 is the average converter efficiency, E£. The average
converter efficiency must be greater than 96%; if it is less than
9G%, replace or service the converter.
NOTE: Supplemental information on calibration and other procedures
in this method are given in reference 13.
57
-------
2. Alternative B - N02 permeation device.
Major equipment required: Stable (L generator.
Chemiluminescence NO/NO /N0? analyzer
n £
with strip chart recorder(s).
NO concentration standard.
N02 concentration standard.
2.1 Principle. Atmospheres containing accurately known
concentrations of nitrogen dioxide are generated by means of a
permeation device.* ' The permeation device emits NCL at a
known constant rate provided the temperature of the device is
held constant (±0.1°C) and the device has been accurately cali-
brated at the temperature of use. The NCL emitted from the
device is diluted with zero air to produce N02 concentrations
•suitable for calibration of the N02 channel of the NO/NOX/N02
analyzer. An NO concentration standard is used for calibration
of the NO and NO channels of the analyzer.
A
2.2 Apparatus. A typical system suitable for generating
f
the required NO and N02 concentrations is shown in Figure 2. All
connections between components downstream from the permeation
device should be of glass, Teflon®, or other non-reactive material.
2.2.1 Air flow controllers. Devices capable of maintaining
constant air flows within ±2% of the required flowrate.
2.2.2 NO flow controller. A device capable of maintaining
constant NO flows within ±2% of the required flowrate. Component
parts in contact with the NO must be of a non-reactive material.
58
-------
2.2.3 Air flowmeters. Calibrated flownetcrs capable of
measuring and monitoring air flowratcs with an accuracy of ±2% of
the measured flowrate.
2.2.4 NO flowrneter. A calibrated flowmeter capable of
measuring and monitoring NO flowratss with an accuracy of ±2% of
the measured flowrate. (Rotameters have been reported to operate
unreliably when measuring low NO flows and are not recommended.)
2.2.5 Pressure regulator for standard NO cylinder. This
regulator must have a non-reactive diaphragm and internal parts
and a suitable delivery pressure.
2.2.6 Drier. Scrubber to remove moisture from tiie permeation
device air stream. The use of the drier is optional with N02
permeation devices not sensitive to moisture. (Refer to the
supplier's instructions for use of the permeation device.)
2.2.7 Constant temperature.chamber. Chamber capable of
housing the NO^ permeation device and maintaining its temperature
to within ±0.1°C.
2.2.8 Temperature measuring device. Device capable of
measuring and monitoring the temperature of the N02 permeation
device'with an accuracy of ±0.05°C.
2.2.9 Valves. A valve may be used as shown in Figure 2 to
divert the NO- from the permeation device when zero air or NO is
required at the manifold. A second valve may be used to divert
the NO flow when zero air or N0~ is required at the manifold.
59
-------
The valves should be constructed of glass, Teflon®, or other non-
reactive material.
2.2.1C Mixing chamber. A chamber constructed of glass.
Teflon , or other non-reactive material and designed to provide
thorough mixing of pollutant gas streams and diluent air.
2.2.11 Output manifold. The output manifold should be
constructed of glass, Teflon , or other non-reactive material and
should be of sufficient diameter to insure an insignificant
pressure drop at the analyzer connection. The system must have a
vent designed to insure atmospheric pressure at the manifold and
to prevent ambient air from entering the manifold.
2.3 Reagents.
2.3.1 Calibration standards. Calibration standards are
required for both NO and NO,. The reference standard for the
calibration may be either an NO or N02 standard. The reference
standard must be used to certify the other standard to ensure
consistency between the two standards.
2.3.1.1 NOp concentration standard. A permeation device
suitable for generating N02 concentrations at the required
flowrates over the required concentration range. If the permeation
device is used as the reference standard, it must be traceable to
a National Bureau of Standards NO,, Standard Reference Material
(SRM 1629) or NO in N2 Standard Reference Material (SRM 1683 or
SRM 1684). If an NO cylinder is used as the reference standard,
the N02 permeation device must be certified against the NO
60
-------
standard according to the procedure given in reference 13. The
use of the permeation device should be in strict accordance with
the instructions supplied with the device. Additional infor-
mation regarding the use of permeation devices is given by
Scaringelli et al/11* and Rook et al.^12^
2.3.1.2 NO concentration standard. Cylinder containing 50
to 100 ppm NO in Np with less than 1 ppm NOp. If the cylinder is
used as the reference standard, it must be traceable to a National
Bureau of Standards NO in N2 Standard Reference Material (SRM
1683 or SRM 1684) or N02 Standard Reference Material (SRM 1629).
If an NOp permeation device is used as the reference standard,
the NO cylinder must be certified against the NOp standard
according to the procedure given in reference 13. The cylinder
should be recertified on a regular basis as determined by the
local quality control program. A procedure for determining the
amount of NOp impurity in the NO cylinder is also given in
reference 13.
2.3.3 Zero air. Air, free of contaminants which might
react with NO or NO, or cause a detectable response on the
1*
NO/NO /NOp analyzer. When using permeation devices that are
A £,
sensitive to moisture, the zero air passing across the permeation
device must be dry to avoid surface reactions on the device.
(Refer to the supplier's instruction?; for use of the permeation
device.). A procedure for generating zero air is given in
reference 13.
61
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2.4 Procedure.
2.4.1 Assemble the calibration apparatus such as the typical
.one shown in Figure 2.
2.4.2 Insure that all flowmet.ers are calibrated under the
conditions of use against a reliable standard such as a soap-
bubble meter or wet-test meter. All volumetric flowrates should
be corrected to 25°C and 760 mm Hg. A discussion on the calibration
of flowmeters is given in reference 13.
2.4.3 Install the permeation device in the constant tempera-
o
ture chamber. Provide a small fixed air flow (200-400 son /min)
across the device. The permeation device should always have a
continuous air flow across it to prevent large buildup of NOp in
the system and a consequent restabilization period. Record the
flowrate as Fp. Allow the device to stabilize at the calibration
temperature for at least" 24 hours. The temperature must be
adjusted and controlled to within ±0.1°C or less of the calibration
temperature as monitored with the temperature measuring device.
2.4.4 Precautions must be taken to remove Og and other
contaminants from the NO pressure regulator and delivery system
prior to the start of calibration to avoid any conversion of the
standard NO to NOp. Failure to do so can cause significant
errors in calibration. This problem may be minimized by
(1) carefully evacuating the regulator, when possible, after the
regulator has been connected to the cylinder and before opening
the cylinder valve; (2) thoroughly flushing the regulator and
62
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delivery system with NO after opening the cylinder valve; (3) not
removing the regulator from the cylinder between calibrations
..unless absolutely necessary. Further discussion of these procedures
is given in reference 13.
2.4.5 Select the operating range of the NO/NO/NO,, analyzer
/» £
to be calibrated. In order to obtain maximum precision and
accuracy for N02 calibration, all three channels of the analyzer
should be set to the same range. If operation of the NO and NO
n
channels on higher ranges is desired, subsequent recalibration of
the NO and NO channels on the higher ranges is recomn.ended.
A -
NOTE: Some analyzer designs may require identical ranges for NO,
N0x, and NGp during operation of the analyzer.
2.4.6 Connect the recorder output cable(s) of the NO/NOV/N09
X £
analyzer to the input terminals of the strip chart recorder(s).
All adjustments to the analyzer should be performed based on the
appropriate strip chart readings. References to analyzer responses
in the procedures given below refer to recorder responses.
2.4.7 Switch the valve to vent the flow from the permeation
device and adjust the diluent air flowrate, FD> to provide zero
air at the output manifold. The total air flow must exceed the
total demand of the analyzer(s) connected to the output manifold to
insure that no ambient air is pulled into the manifold vent.
Allow the analyzer to sample zero air until stable NO, NO , and
J\
NOp responses are obtained. After the responses have stabilized,
adjust the analyzer zero control(s).
63
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NOTE: Some analyzers may have separate zero controls for
NO, NOX, and NO^. Other analyzers may have separate zero controls
.only for NO and NO while still others may have only one zero
X
control conmon to all three channels.
Offsetting the analyzer zero adjustments to +5% of scale is
recornnsnded to facilitate observing negative zero drift. Record
the stable zero air responses as ZNO, ZNQ , and Z..Q .
A £
2.4.8 Preparation of NO and NO calibration curves.
A
2.4.8.1 Adjustment of NO span control. Adjust the NO flow
from the standard NO cylinder to generate an NO concentration of
approximately 80% of the upper range limit (URL) of the NO range.
The exact NO concentration is calculated from:
FMn x [NO]QTn
[NO]OUT = N? tF (16)
OUT h * F
where: [NO]nilT = diluted NO concentration at the output manifold,
UU I
r>
ppm
3
F.,n = NO flowrate, scm /mm
NO
[NO]SyD = concentration of the undiluted NO standard,
ppm
F_ = diluent air flowrate, scm /min
Sample tiiis NO concentration until the NO and NO responses have
A
stabilized. Adjust the NO span control to obtain a recorder
response as indicated below:
64
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[NO]OUT
recorder response (X scale) = ( URj; x TOO) + ZNQ (17)
where: URL = nominal upper range limit of the NO channel, ppm
NOTE: Some analyzers may have separate span controls for NO,
NO , and N02* Other analyzers may have separate span controls
only for NO and NO , while still others may have only one span
A
control common to all three channels. When only one span control
Is available, the span adjustment is made on the NO channel of
the analyzer.
If substantial adjustment of the NO span control is necessary, it
may be necessary to recheck the zero and span adjustments by
repeating steps 2.4.7 and 2.4.8.1. Record the NO concentration
and the analyzer's NO response.
2.4.8.2 Adjustment of NO span control. When adjusting the
A
analyzer's NO span control, the presence of any N0~ impurity in
A t
the standard NO cylinder must be taken into account. Procedures
for determining the amount of NOo impurity in the standard NO
cylinder are given in reference 13. The exact NO concentration
A
is calculated from:
nn - FNO
["
65
-------
where: [NOX]OUT = diluted NOX concentration at the output
" manifold, ppm
[NC^lj^p = concentration of NOp impurity in the standard
NO cylinder, ppm
Adjust the NOX span control to obtain a convenient recorder
response as indicated below:
recorder response (% scale) = (—g^- x 100) + ZNO (19)
NOTE: If the analyzer has only one span control, the span adjust-
ment is made on the NO channel and no further adjustment is made
here for NO
If substantial adjustment of the NO span control is necessary,
rt
it may be necessary to recheck the zero and span adjustments by
repeating steps 2.4.7 and 2.4.8.2. Record the NO concentration
and the analyzer's NO response.
A
2.4.8.3 Generate several additional concentrations (at
least five evenly spaced points across the remaining scale are
suggested to verify linearity) by decreasing F^Q or increasing
FD. For each concentration generated, calculate the exact NO and
NO concentrations using equations (16) and (18) respectively.
J\
Record the analyzer's NO and NO responses for each concentration.
Plot the analyzer responses versus the respective calculated NO
and NO concentrations and draw or calculate the NO and NO^
66
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calibration curves. For subsequent calibrations where linearity
can be assumed, these curves may be checked with a two-point
calibration consisting of a zero air point and MO and NO
A
concentrations of approximately 80% of the URL.
2.4.9 Preparation of N02 calibration curve.
2.4.9.1 Remove the NO flow. Assuming the N02 zero has been
properly adjusted while sampling zero air in step 2.4.7, switch
the valve to provide N02 at the output manifold.
2.4.9.2 Adjust F^ to generate an N0~ concentration of
approximately 80% of the URL of the N02 range. The total air
flow must exceed the demand of the analyzer(s) under calibration.
The actual concentration of N02 is calculated from:
-- <20>
where: [NOolnn-r = diluted N02 concentration at the output
manifold, ppm
R = permeation rate, yg/min
K = 0.532 yl N02/yg N02 (at 25°C and 760 m Hg)
. Fp B air flowrate across permeation device,
3. .
scm /nun
3
FD - diluent air flowrate, scm /min
Sample this N02 concentration until the NOX and NOj, responses
have stabilised. Adjust the NO- span control to obtain a recorder
response as indicated below:
67
-------
recorder response (% scale) = (—' x 100) + ZNQ (21)
NOTE: If the analyzer has only one or two span controls, the
span adjustments are made on the NO channel or NO and NO channels
• "
and no further adjustment is made here for N02-
If substantial adjustment of the NCL span control is necessary,
it may be necessary to recheck the zero and span adjustments by
repeating steps 2.4.7 and 2.4.9.2. itecord the N02 concentration
and the analyzer's N0? response. Usi ic; the T'O calibration curve
£ A
obtained in step 2.4.8, measure and record the NO concentration
A
as [KOX]M.
2.4.9.3 Adjust FQ to obtain several other concentrations of
WL over the N02 range (at least five evenly spaced points across
the remaining scale are suggested). Calculate each N02 concen-
tration using equation (20) and record the corresponding analyzer
N0p and NOX responses. Plot the analyzer's N02 responses versus
the corresponding calculated NOp concentrations and draw or
calculate the N02 calibration curve.
2.4.10 Determination of converter efficiency.
2.4.10.1 Plot [NOXJM (y-axis) versus [K02]QUT (x-axis) and
draw or calculate the converter efficiency curve. The slope of
the curve times 100 is the average converter efficiency, EC> The
average converter efficiency must be greater than 96«; if it is
less than %*', replace or service the converter.
68
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NOTE: Supplemental information on calibration and other pro-
cedures 1n this method are given in reference 13.
3. Frequency of calibration. The frequency of calibration,
as well as the number of points necessary to establish the cali-
bration curve and the frequency .of other performance checks, will
vary from one analyzer to another. The user's quality control
program should provide guidelines for initial establishment of
these variables and for subsequent alteration as operational
experience is accumulated. Manufacturers of analyzers should
include in their instruction/operation manuals information and
guidance as to these variables and on other matters of operation,
calibration, and quality control.
-------
REFERENCES
1. A. Fontijn, A. 0. Sabadell, and R. J. Ronco, "Homogeneous
Chemiluminescent Measurement of Nitric Oxide with Ozone," Anal.
Chem., 42_, 575 (1970).
2. D. H. Stedman, E. E. Daby, F. Stub], and H. Niki, "Analysis
of Ozone and Nitric Oxide by a Chemiluminescent Method in Laboratory
and Atmospheric Studies of Photochemical Smog," J. Air Poll.
Control Assoc., 22, 260 (1972).
3. B. E. Martin, J. A. Hodgeson, and R. K. Stevens, "Detection
of Nitric Oxide Chemiluminescence at Atmospheric Pressure,"
Presented at 164th National ACS Meeting, New York City, August
1972.
4. J. A. Hodgeson, K. A. Rehme, B. E. Martin, and R. K. Stevens,
"Measurements for Atmopsheric Oxides of Nitrogen and Armenia by
Chemiluminescence," Presented at 1972.APCA Meeting, Miami, Florida,
June 1972.
5. R. K. Stevens and J. A. Hodgeson, "Applications of Chemi-
luminescence Reactions to the Measurement of Air Pollutants,"
Anal. Chem., 45_, 443A (1973).
6. L. P. Breitenbach and M. Shelef, "Development of a Method
for the Analysis of N09 and NH7 by NO-Measuring Instruments," J_._
Air Poll. Control As sot.. 23., T28 (1973).
7. A. H. Winer, J. W. Peters, J. P. Smith, and J. N. Pitts,
Jr., "Response of Commercial Cherniluminescent NO-NOp Analyzers to
Other Nitrogen-Containing Compounds," Environ. Sci.Techno!., 8_,
1118 (1974).
8. K. A. Rehme, B. E. Martin, and J. A. Hodgeson, "Tentative
Method for the Calibration of Nitric Oxide, Nitrogen Dioxide, and
Ozone Analyzers by Gas Phase Titration," EPA-R2-73-246, March
1974. -
9. J. A. Hodgeson, R. K. Stevens, and B. E. Martin, "A Stable
Ozone Source Applicable as a Secondary Standard for Calibration
of Atmospheric Monitors," ISA Transactions. 1J_, 161 (1972).
10. A. E. O'Keeffe and G. C. Ortman, "Primary Standards for
Trace Gas Analysis," Anal. Chem., 38, 760 (1956).
70
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11. F. P. Scaringelli, A. E. O'Keeffe, E. Rosenberg, and J. P.
Bell, "Preparation of Known Concentrations of Gases and Vapors
with Permeation Devices Calibrated Gravimetrically," Anal. Chem.,
42, 871 (1970).
12. H. L. Rook, E. E. Hughes, R. S. Fuerst, and J. H. Margeson,
"Operation Characteristics of N0? Permeation Devices," Presented
at 167th National ACS Meeting, Los Angeles, California, April
1974.
13. E. C. Ellis, "Technical Assistance Document for the Chemi-
luminescence Measurement of Nitrogen Dioxide," EPA-E600/4-75-003
(Available in draft form from the United States Environmental
Protection Agency, Department E (MD-76), Environmental Monitoring
and Support Laboratory, Research Triangle Park, North Carolina
27711).
(Sec. 4, Pub. L. 91-604, 84 Stat. 1678 (42 U.S.C. 1857c-4))
71
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o
03
GENERATOR
REACTION
CHAMBER
VENT
NO
STO.
OUTPUT
MANIFOLD
VENT
IT U I,
EXTRA OUTLETS CAPPED
WHEN NOT IN USE
MIXING
CHAMBER
TO INLET OF ANALYZER
UNDER CALIBRATION
Figure 1. Schematic diagram of a typical GPT calibration system.
-------
o
VENT
EXTRA OUTLETS CAPPED
WHEN NOT IN USE
TO INLET OF ANALYZER
UNDER CALIBRATION
THERMOMETER
PERMEATION
DEVICE /
I
CONSTANT TEMPERATURE
CHAMBER
Figure 2. Schematic diagram of a typical calibration apparatus using an NO2 permeation device.
-------
TECHNICAL REPORT DATA
(Please read Inunctions on the reverse before completing}
1. REPORT NO.
EPA-600/4-75-003
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
TECHNICAL ASSISTANCE DOCUMENT FOR THE CHEMILUMINESCENCE
MEASUREMENT OF NITROGEN DIOXIDE
REPORT DATE
October 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)'
8. PERFORMING ORGANIZATION REPORT NO
Elizabeth Carol Ellis, Ph.D.
I. PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
Environmental Monitoring and Support Laboratory
Quality Assurance Branch
Research Triangle Park, NC 27711
10. PROGRAM ELEMENT NO.
1HA327
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Monitoring and Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Gas phase chemiluminescence has been designated as the reference measure-
ment principle for the measurement of nitrogen dioxide (N02) in the ambient
atmosphere. Continuous analyzers based on this measurement principle may be
calibrated with N0? either from the gas phase titration of nitric oxide (NO)
with ozone (OJ or from an NO,, permeation device. This document presents
pertinent technical information to aid in the understanding of the measurement
principle and the prescribed calibration procedures and also includes illus-
trative examples on how to implement the calibration procedures. The discussion
includes recommendations on how to recognize and eliminate potential errors in
the individual calibration procedures as well as with the use of N02 chemilumi-
nescence analyzers. Suggestions on the design and construction of calibration
apparatus and procedures for handling and certifying both NO and N02 calibration
standards are included also.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Nitrogen Dioxide
Calibration
Chemiluminescence
Air Pollution
N09 Measurement
N0« Calibration
Gas Phase Titration
N02 Permeation Device
13B
8. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
79
20. SECURITY CL.ASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
70.
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