PB-204 877
INSTRUMENTATION FOR THE DETERMINATION OF NITROGEN OXIDES
CONTENT OF STATIONARY SOURCE EMISSIONS VOLUME 1
Leo P. Parts, et al
Monsanto Research Corporation
Dayton, Ohio
October 1971
DISTRIBUTED BY:
KTLTl
National Technical Information Service
U. S. DEPARTMENT OF COMMERCE
5285 Port Royal Road, Springfield Va. 22151
This document has been approved for pubfic release and sale.
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MAC-DA-307
PB 204 877
INSTRUMENTATION FOR THE DETERMINATION
OF NITROGEN OXIDES CONTENT
OF STATIONARY SOURCE EMISSIONS
Br
Lao P. Parts
Paul L. Sherman
Arthur Q'Snyder
Contract No EHSO 71—30
For
Environmental PrOtectnn A^sncr
Office of Research land Mon/t bring
Durham, North Carolina
MONSANTO RESEARCH CORPORATION
A SUBSIDIARY O V HONS ANfrd COUPaMV
DAYTON
LABORATORY
flAYTOJI, OHIO 4&40T
I««<«Cw«d Err
NATIONAL TECHNICAL
INFORMATION 5ERVICE
-------
When U. S. Government drawings, specifications, or other data
are used for any purpose other than a definitely related
Government procurement operatloa, tbe Goveraaeat thereby
incurs ao responsibility nor any obligation whatsoever, and
the fact that the Government may have formulated, furnished,
or in ao-y way supplied the said. drawings, specifications,
or other data, is not to be regarded by implication or other-
wise, or in any manner licensing the holder or any other
person or corporation, or conveying any rights or permission
to manufacture, use, or sell any patented Invention that may
in any way be related thereto.
References to named commercial products in this report are
not to be considered in any senae as an endorsement of the
product by tbe Government.
-------
urn* - o s-^?7
MRC-DA-307
PB 204 877
INSTRUMENTATION FOR THE DETERMINATION
OF NITROGEN OXIDES CONTENT
OF STATIONARY SOURCE EMISSIONS
By
Leo P. Parts
Paul L. Sherman
Arthur 0 Snyder
Contract No EHSD 71-30
For
Environmental Protection Agency
Office of Research and Monitoring
Durham. North Carolina
MONHANTO RES3ARCII CORPOIiATION
A SUBSIDIARY of M O n 8 a .nto companv
, DAYTON
j. , ¦ ij LABORATORY
3 .J
• »N,r»wiu i* »oT
NATIO ML TECHMlCAl
INFORMATION SEPVICE
V, lit l
t\5
(f
When U. S. Government drawings, specif lea clone, or other data
are used for any purpose other than a definitely related
Government procurement operation, the Government thereby
incurs no responsibility nor any obligation whatsoever, and
the fact that the Government may have formulated, furnished,
or In any way supplied the said drawings, specifications,
or other data. Is not to be regarded by implication or other*
vise, or In any manner licensing the holder or any other
person or corporation, or conveying any rights or permission
to manufacture, use, or sell any patented invention that may
In any way be related thereto.
References to named commercial products
not to be considered in any sense as an
product by the Government.
In this report
endorsement of
are
the
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[4. Title toJ Sstxirle
INSTRUMENTATION FOR THE DETERMINATION OF NITROGEN OXIDES
CONTENT OF STATIOHARt SOURCE EMISSIONS
QiaUOGAAPtllC OATA
SHEET
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3> Recipi«ctf*« Acr««aioo No*
» Report Dim
October 1971
7 Aubor(a)
Leo P. Parts, Paul
Sherman, Arthur D. Snyder
,l> PcrigreiM Orguiucioo Rep
C S/i~3fi 7
Perlorcaiog Orgaaizacipo V*»* and Addrcts
Monsanto Research Corporation
Oiytotv Laboratory
Dayton, Ohio 45407
10 P!OKcl/T*Bk/^0tl> Uou No
11 ComractArtaat No
EHSD 71-30
11 Sponsoring Orguiiwiofl Sow tod U^ess
Environmental Protection Agency
Office of Research and Monitoring
Durham, N C.
IX Tvm of Rco.ur Ncc. DISCLAIMER: This report was furnished to tfie Office of Air Programs
by Monsanto Research Corporation, Dayton Laboratory, Dayton, Ohio 45407 1n
fnlf111 mpnt nf rnntrart FHSD-71-30
16 Abitncii
* -Information regarding the state-of-the-art of (x = 1 to 2) monitoring instrumenta-
tion has been a^seobled and evaluated The evaluation was based cm the present and
projected require merits in stationary source emissions monitoring, the operational
characteristics and performance capabilities of the instruments, and on the cost/per-
formance criteria. Commercially available and prototype instruments, and laboratory-
stage instrumental methods are covered- Instruments based upon wet chemical aethods
were excluded. The design and operational Features of seven instrunerrts which are
undergoing laboratory and cti-site evaluation at a fossil fuel burning installation are
described Sixteen other UC^c analysis concepts were also evaluated. Cherailuninescencet
correlation spectrometry, mass spectrometry and selective pTiotoionizatioo have been
identified as the preferred analysis methods upon which new emissions Monitoring
instruments can be based, using currently available technology, to meet the performance
requirements* Novel and potentially useful monitoring techniques, based on the evolving
17 key a ad Document Anlysii J7
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FOREWORD
In December of 1970, the Environmental Protection Agency
contracted with Monsanto Research Corporation to conduct a
laboratory and field evaluation of commercially available con-
tinuous monitors for nitrogen oxides emitted from stationary
sources.
This report, Volume I, details the survey of instrumenta-
tion and techniques capable of measuring NO emissions which
preceded the start of laboratory and field testing of the
commercially available units.
The literature search conducted for this report encom-
passed. the following Journals and Government abstracting
publications:
Chemical Abstracts (1906-1970)
Chemical Titles (last two years)
Analytical Abstracts (1969-1970)
Analytical Chemistry (April issues from odd-numbered years)
Environmental Science and Technology (1967-1970)
Journal of Air Pollution Control Association (1958-1970)
NAPCA Abstract Bulletin (1970)
Technical Abstract Bulletin (196lt-1970)
Scientific ana Technical Aerospace Reports (1963-1970)
U.S. Government Research and Development Reports (196B-197D)
Approximately 300 abstracts and papers were identified as those
most relevant for Establishing the methods which have been
developed, proposed, or which appear applicable for continuous
monitoring of nitrogen oxides.
Information regarding commercial instruments was obtained
from the manufacturers in the form of technical sales brochures
and operating manuals. The information collected by Stevenson,
Jordan and Harrison, Management Consultants, Inc., In interviews
of manufacturers was also used.
A computer-search of Information pertaining to nitrogen
oxides measurement and measurement methods was conducted by the
Air Pollution Technical Information Center. This search has
been supplemented by monthly updates through June 1971, ab-
stracted primarily from recent acquisitions
11
Volume II, which will-contain the results of the laboratory
and field tests, will be available on or about 1 February 1972.
This report Is based on work accomplished under Contract
EHSD 71-30, with the Environmental Protection Agency, Division
of Chemistry and Physics, Durham, N.C.
Frederic C. Jaye
Project Officer
Stationary Sources Emission Measurement Methods Section
ill
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ABSTRACT
Information regarding the state-of-the art of NO
(x = 1 to 2) monitoring Instrumentation has been assembled
and evaluated. The evaluation was based on the present
and projected requirements In stationary source emissions
monitoring, the operational characteristics and performance
capabilities of the instruments, and on the cost/performance
criteria
The report encompasses commercially available and proto-
type instruments, and laboratory-stage Instrumental methods,^*:
Instruments based upon wet chemical methods were excluded from
consideration.
Pour commercial Instruments based on the measurement
of nondlspersed infrared radiation absorption, one Instrument
based on visible radiation absorption, and two Instruments
based on voltarrcmetric principles^&i^e un3ergolng laboratory
and on-site evaluation at a fossil fuel-burning installation
in the "ppesaB^-prograai^> The design and operational features
of instrujnents^are aes"cflBe"d.
The evaluation encompassed sixteen other NO analysis
concept^ Chenflumi'nescence, correlation spectrometry, mass
spectrometry and selective photolonlzation have been identified
as the preferred analysis methods upon which new NO emissions
monitoring Instruments can be based, using currently available
technology, to meet the performance requirements.
Novel and potentially useful monitoring techniques,
based on the evolving laser technology, were identified
Materials, devices, and information needed to establish the
practical usefulness of these techniques In source monitoring
are discussed.
lv
TABLE OF CONTENTS
SECTION Page
FOREWORD 11
ABSTRACT lv
1 INTRODUCTION 1
2 DISCUSSION 3
2.1 Methods Applied In Commercially Available 3
Instruments
2.2 NO Monitoring Methods Used in Prototype 5
Systems
2.3 Brief Discussion of Potential New 7
Approaches to the Monitoring of NO
Emissions *
3 ABSTRACT DESCRIPTIONS OP N0X MONITORING METHODS 1^4
Methods Applied in Commercially Available lJJ
Instruments
Nondlsperslve Infrared 15
Beckman Instruments, Inc., Model 315 15
InTrared Analyzer
The Bendlx Corporation, UNOR 2 16
Intertech Corporation, Uras 2 18
Mine Safety Appliances Company, 20
LIRA Model 200
NorvJlspersive Vi3ib\e and Ultraviolet 22
Analyzer
duPont Company, Model ^61 Photometric 22
Analyzer
Electrochemical Analyzers 2«
Dynasclences Corporation, Air Pollution 2H
Monitor Model NX 130
Envlrometrlcs, Inc., Series NS-200A 26
v
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Table of Concents - Cont'd
SECTION
3 (cont'd)
NO Monitoring Methods Currently In 28
Prototype Systems
Chemiluroinescence (Vslng Atomic Oxygen) 29
Chemllumlnescenee {Using Ozone) 31
Gas Chromatography 33
Condensation Nucleatlon 35
Correlation Spectrometry 37
Dielectric Constant Measurement 39
Electron-Excitation
Gas Phase Ionic Conductance I12
Inverse Radioactive Tracing 4t|
Interference Spectrometry lig
Laser Radiation Absorption 1(8
Laser Raman Scattering 50
Mass Spectrometry 52
Paramagnetism 55
P ho t-o Ionization 57
Vibrational Fluorescence 59
REFERENCES
1. INTRODUCTION
The purpose of the effort summarized in this report was to
assemble and evaluate information regarding the state-of-tfie-ai-t
of continuous NO (x = 1 to 2) monitoring instrumentation.
Reliable instrumentation will be required for the development
of effective air pollution control technology. The present
report encompasses commercially available instruments, proto-
types, and laboratory-stags Instrumental methods. The instru-
ments and methods have been evaluated for their present or
potential applicability to the monitoring of NO emissions
which emanate from stationary installations.
Host of the presently used source monitoring instruments
represent adaptations of laboratory and process control Instru-
mentation. The instruments most widely used for N0X monitoring
are based on the absorption of nondispersed infrared radiation
(ref. 1,2), on the oxidation of NO to NOj and subsequent mea-
surement of absorption In the visible spectral range (i-ef. 3),
and on electrochemical principles {ref. ^,5).
The need for pollution monitoring imstrunents of Improved
specificity an^ sensitivity has been discussed in recent publi-
cations (ref. 6-11] . To reduce the maintenance requirements
and complexity or Instruments, it is highly desirable that the
MO mor.lt orir.g instruments vould be tasec or
-------
Attainable accuracy and repeatability ±2J of maximum
range during seven days of continuous operation.
Minimal Interference by water vapor, carbon dioxide,
carbon monoxide, and sulfur dioxide.
Response time less than 2 minutes.
Capability for multifunctional operation (NO,
NOi, S02. CO, and 03).
Sufficiently rugged to require minimal repair while
used In fuel-burning Industrial Installations.
Simplicity of design and operation
Minimal sample conditioning required.
Low cost.
The sources of information, upon which the present evalua-
tion and report are based, are listed in the Foreword. In
Section 2 of this report, three categories of N0X monitoring
methods are discussed. Those which are applied in the presently
available commercial instruments are described in Section 2.1.
Five methods of demonstrated feasibility, that could be used
as the basis of new source monitoring Instruments, are described
In Section 2.2. Three additional methods of potential utility,
which are based on recent technology and require further
development and evaluation to establish their practical utility,
are discussed In Section 2.3.
Abstract descriptions of NO* monitoring methods of the
above-listed three categories with relevant references are pre-
sented in Section 3* This section contains also the abstract
descriptions of eight additional N0X analysis methods that have
been described In the literature. The latter are not believed
to represent the basis for reliable, economically competitive
monitoring instruments.
2
2. DISCUSSIOH
2.1 Methods Applied In Commercially Available Instruments
The instruments for the nonitorlng of N0X emissions from
all types of stationary sources, that were commercially available
at the time when work on this program was Initiated, are based
on three detection concepts
Absorption of nondispersed Infrared radiation
Photometric measurement of visible and ultraviolet
radiation absorption In selected wavelength ranges.
Voltammetrlc measurement.
Nondisperslve infrared radiation measurement instruments utilize
a broad-spectrum emission source in conjunction with a spectro-
phonic detector. This detector Is responsive only to radiation
at the NO absorption wavelength. Since emission gases other
than NO (e.g., HjO) absorb to some extent at this wavelength,
the nondisperslve infrared Instruments are subject to inter-
ference. To eliminate or minimize Interference, auxiliary
absorption cells or optical niters are Incorporated that remove
most of the radiation at wavelengths at which the Interfering
substances absorb. The presently available nondisperslve infra-
red Instruments do not monitor the NOa component of N0X which
constitutes approximately 5* of the total nitrogen oxides.
The commercially available photometric analyzer for N0X
represents an adaptation of a multipurpose Instrument, based
on dual-wavelength monitoring, to pollution control applications.
Monitoring for NOx Is based on the measurement of light trans-
mission at a wavelength (4360 ft) In the visible spectral region
at which N02 absorbs radiation. No other gaseous component,
emitted by fossil fuel burning installations, absorbs at this
wavelength. Therefore, no interference would be caused by
other emission gas components.
Since NO2 is a minor nitrogen oxide component in emission
gases, the gas sanple Is oxidized with oxygen at 5 atmospheres
pressure. The 10-mlnute analysis cycle, necessary for complet-
ing the oxidation, introduces a time delay Into the analytical
data acquisition process
The voltammetrlc Instruments have been developed within
the past five yearc Their design entails new concept-,, combin-
ing diffusion of i;i:eous pollutants through a permtdM e menr,rane
Into an electro 1/te wltn electrochemical oxidation or reduction.
The diffusion 1'; the rate-controlling process. The current
generated In the. electrochemical oxidation at a fixed potential
3
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is proportional to the concentration of the electro-reactive
species in the gas. It appears that the attainment or selective
oxidation, alteration of electrode surfaces, temperature-bensi-
tlvlty of output, and evaporation of water from the sensor
co.npartnrent may represent problem areas that will require
Investigation and remedial measures. It should also be noted
that the voltajmr.etrlc instruments ara very compact and therefore
convenient to use for nonccntinuous multi-site monitoring
at a fuel-ournlng Installation.
The following monitors which were commercially available
In January 1971 are being, evaluated in our laboratory and field
test program.
Nondlspersive infrared instruments
Model 315 Infrared Analyzer by Beckman
Instruments, Inc.
UNOR-2 by the Bendlx Corporation
Uras-2 by Intertech Corporation
LIRA Model 200 by the Mine Safety Appliances
Company
Photometric analyzer
Model ^61 Photometric Analyzer by the duPont Company
Voltammetric instruments.
Air Pollution Monitor Model NX 130 by the
Dynasciences Corporation
Series NS-200A Monitor with Type N7£>H2 Faristor
sensor by EnviroMetrlcs, Inc.
A third instrument, based on the voltammetric technique,
has recently become available from Theta Sources, Inc (ref 12)
Section 3 of this report provides specific information
regarding each of the instruments that is undergoing testing in
the present program The selection was based on a preliminary
user survey, conducted under a subcontract by Stevenson, Jordan,
and Harrison, Management Consultants, Inc., In which the currently
more widely used instruments were established
The following novel types of instruments have either very
recently been introduced or the market, or they are known to
be under development In industry
An on-stack optical absorption measurement instrument
of proprietary design, offered by the Envlromental
Data Corporation (ref. 13).
An emission spectrcpTiatometer for muIt 1 component analysis
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1 . Paramagnetism measurement
15. Photoionlzation
16. Vibrational fluorescence
Abstract descriptions of the principles Involved In the
utilization of each listed method are presented in Section 3.
References to pertinent publications, comments regarding the
operational principles and features, and information regarding
the commercial developmental status can also be found in that
section of trie report
Eight methods merit further consideration. The following
five are at the developmental stage at which prototype source
monitoring Instruments can be designed, constructed and
evaluated.
Chemiluminescence (N(3X + 0)
Chemilumlnescence (NO + converter + 0i)
Correlation spectrometry
Mass spectrometry
Selective photolonlzation
Three additional methods may prove to be of practical
utility at some time within the next ten years-
Laser radiation absorption
Laser Raman scattering
Vibrational fluorescence
The latter three approaches to N0X monitoring are based on
the evolving laser technology. Conceptual feasibility has been
demonstrated. However, exploratory research, laser materials
development, and equipment design are required before these
methods can be reliably evaluated for their practical utility
in monitoring N0x emissions in plant environment.
It should be noted that two of the methods tnat are recom-
mended for immediate consideration (chemllumlnescent emission
from the reaction with atomic oxygen and correlation spec-
trometry) are applicable for simultaneous N0X, SOi, and CO
monitoring. Mass spectrometry can be applied for simultaneous
monitoring of N0X, S02, and O2. All eight methods are based
on the measurement of some specific structure-related property
of the pollutants contained in the emission gases
6
2 3 Brief Discussion of Potential New Approaches to the
Monitoring of NO^ Emissions
It has recently been demonstrated that the measurement of
chemi luminescent emission can be utilized for the monitoring of
atr.ospheric NO content at concentrations ranging to the low
value of 5 ppb (ref. 17). This method is based upon the mea-
surement of chemllumlnescent emission intensity associated with
tne reaction of nitric oxide with atomic oxygen
NO + 0 N0j» (l)
N0i» ~ NO* + h\i (2)
The method is applicable for the monitoring of total nitrogen
oxides, since the NOj present in the gaseous sample yields NO
upon reaction with atomic oxygen'
N02 +0 - NO + 02 (3)
Nitric oxide formed from N02 by reaction (3) is reoxldlzed by
atomic oxygen, with concomitant emission of radiation around
6300 A
The system developed for nitric oxide monitoring in ambient
atmosphere has been demonstrated to be even more sensitive in
the monitoring of SOj via the chemllumlnescent reactions
SO* + 0 - S03* (U)
S03* - SOj + hv (X ^3500 8] (5)
The reaction of carbon monoxide with atomic oxygen is also
accompanied by chemiluminescence However, a relatively smaller
fraction of the evolved energy appears as emitted radiation in
the visible spectral range U ^'(350A) and the present detection
limit for carbon monoxide is 100 ppra. It is believed that the
cnemiluminescence (X 1.38902) quantum efficiency for the reaction
of CO with atomic nitrogen, that produces CN, is sufficiently
nigh to utilize that reaction for CO monitoring at low concen-
trations .
It Is visualized that a source monitoring system based on
chemiluminescent reactions with atomic oxygen ana atomic 1 itro-
gen (for CO) can readily be developed. This will entail lowering
the sensitivity of the chemllumlnescent ambient atmosphere NO
detector It appears possible to develop a multifunctional x
system for simultaneous continuous monitoring of '10 , SO? .mil
CO in emission gases. This development will also require tlic
7
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design of an optical detection system for simultaneous measure-
ment of luminescence In three spectral ranges, characteristic
for each of the three major pollutants present In emission
gases.
Nitric oxide reacts readily with ozone
NO + Oj -~ NOj* + O2 (6)
The .measurement of chemlluminescent radiation emission [see
reaction (2)] associated with this reaction trer. IB) fta? reen
applied for nitric oxide monitoring In ambient air and in auto-
mobile emissions (ref. 19-21). The reaction rates of ozone
with HO:, S0s , and CO are much lower than the corresponding,
rate for the reaction with HO. Therefore, the monitoring of
the ehemiluinlnescence Intensity arising from the introduction
of gaseous ozone at low concentrations into emission gases Is
directly applicable only for the monitoring of their NO content
(ref 19).
The total N0X content of emission gase3 could be determined
by using a converter In which KOt Is quantitatively reduced to
MO. Need has been Indicated for the development of a reliable
converter that will allow th« use of the N0X ¦+ Ds chemilunlries-
cence monitor on all types or emission sources.
Correlation spectrometry (ref. 22-27) entails the matching
of absorption maxima of the material being analyzed with the
reference spectrum (correlation mask) contained in the vonitor-
lng instrument The intensity of radiation transmitted tnrough
the mask Is detected photoelectrlcally, it is related to tne
concentration of the analyzed substance
Correlation spectrometry has been employed most extensively
and successfully for remote (frequently aerial) monitoring of
atmospheric pollutants (N02, S0Z). This method has also been
used for SO2 monitoring In stack emission gases The optical
system of the sensor has beer inserted into the stack for the
latter purpose. To detect nitrogen oxides In stack emission
gases at the required low concentrations, it may be essential
to utilize a multiple-reflection optical system Monitoring
emissions external to the stack, subsequent to removal of par-
ticulate matter, would considerably simplify problems associated
with the protection of the optical components of the spectrometer.
The usefulness of a correlation spectrometer would be signifi-
cantly enhanced in the present application through the incor-
poration of multi-component pollutant detection capability
New maes spectrometrtc instruments have evolved duri~£ tie
recent years which provide somewhat reduced resolution arc range
capabilities, when compared with conventional laboratory systems,
8
for very significant reduction of complexity, weight, and cost.
Omegatrons (reT. 28-30), quadrupole (ref. 31,32). ion storage
(ref. 33), and single-foeusing mass spectrometers have been
developed. These have been used primarily Tor residual gas
analysis, space cabin atmosphere monitoring, and process control
instrumentation. With the exception of NOi and CO1 0 , CO
and N2, these mass spectrometers offer sufficient resolution
to separate the peaks associated with the major pollutants In
emissions emanating front fossil fuel-burning installations.
The natural abundance of the 0" Isotope In oxygen is 0.2J.
The parent peaks of KOj• and C0l,018 coincide In low-resolution
mass spectronetrlc measurements. Should It be necessary to
determine HO!, in addition to NO in emission gases, it will be
essential to Incorporate a 2oglc sub-system In the Instrument
that will automatically subtract the C0160" contribution from
the m/e = 46 peak intensity
To enhance the sensitivity of mass spectrometers, the
signal-to-noise ratio can be Improved by modulated beam opera-
tion For this purpose, the sample ion beam Is periodically
interrupted, the background contribution is measured, and a
correction is applied to the monitored m/e peaks The low-
resolution mass spectrometers will certainly provide sufficient
sensitivity, when operateJ in the modulated bean mode, for
emissions monitoring even In the low ppm range
Gas phase ionization can be induced with vacuum ultra-
violet radiation Nitric oxide and nitrogen dioxide have lower
ionization potentials (9.26 eV and 9.33 eV, respectively,
ref. 3^-io) than other gases commonly encountered In emission
gases. The Ionization potentials of S02) CO and C02 are 12.3^,
It 01, and 13.79 eV, respectively Thus, it is feasible to
effect selective photoionizaticn of nitrogen oxides in emission
gases (at X ¦= 1050 to 1350 A) and to monitor their concentra-
tions by conductivity measurement. Instruments designed for
high-altitude atmospheric NO detection and monitoring have
demonstrated the applicability of this method even for lov.-
concentration measurements.
The ionization potentials of most emission gas componepts
(major pollutants other than N0X, combustion products, and
atmospheric gases) are In the range of 12-1 'l eV. Therefore,
the selective photolonization technique does not appear to oe
applicable for multifunctional monitoring of pollutants emitted
by the major tipes of emission sources.
The discovery and development of lasers has opened neiv
avenues to pollution monitoring. These novel energy sources
provide colllmated, monochromatic radiation In the Infrared,
9
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visible, and ultraviolet spectral ranges. The main advantages
of laser energy sources - the very narrow wavelength range,
the Intensity, and the colllmatlon of emitted radiation -
must be evaluated with reference to advantages that can be
gained through these characteristics, the reliability and
stability of operation, and the cost.
It is well to appreciate that laser technology is still
in Infancy, significant discoveries that affect technological
developments occur with a high frequency. The versatility,
operational capabilities and reliability of lasers are continu-
ously improving, their cost Is diminishing.
Because of the inherent or attainable frequency coincidence
of some laser emission lines with absorption bands or vibrational-
rotational lines of pollutants, laser radiation absorption has
been utilized for pollutant monitoring (ref. 11-50) . Long-path
detection systems have been used for atmospheric monitoring
(ref 111,42). A laser spectrophone has been demonstrated to
be applicable for the analysis of NO in collected gaseous samples
at concentrations below 1 ppb (ref. 47-19).
Nitric oxide can be monitored by absorption measurement with
iodine (ref. 41,12), carbon monoxide (ref. 44,50), and InSb spin
flip Raman (ref. 47-49) lasers. All three approaches entail
the use of gas lasers. Carbon monoxide lasers, when operated
at liquid nitrogen temperatures, emit by vibrational-rotational
transitions in the broad spectral range extending from 1216 to
2013 cm-1. The emitted spectrum consists of approximately 200
pressure-broadened lines of ^0.05 cm 1 half-width (ref. 51,52).
The frequencies of some CO laser emission lines correspond to
absorption frequencies of NO, N02 , and S02 (ref. 50). The CO
laser, when operated at the liquid nitrogen temperature, could
in principle be used for multifunctional pollutant monitoring.
When operated at room temperature, the spectral range or radia-
tion emitted by the CO laser is much narrower (ref. 53) and it
does not encompass the fundamental vibrational frequencies of
NO 2 and SOj.
Precise tuning to the desired frequency, in the range of
5 to 6 gm, has been attained by Inducing spin flip stimulated
Raman emission from InSb by CO laser excitation (ref 47,54,55)
Thus, collimated radiation of very narrow spectral range can
be generated at frequencies that correspond to the centers of
NO absorption lines. Furthermore, monitoring can be conducted
at frequencies at which the atmospheric water causes no inter-
ference. The spin flip Raman laser has very recently been used
to determine NO concentrations below 1 ppt (ref 49). Strong
magnetic fields and cryogenic temperatures are required for the
attainment of spin flip Raman laser emission. Those requirements
10
are presently a distinct handicap in continuous source monitor-
ing. However, this novel technique is of definite Interest as
a laboratory method applicable with very low NO concentrations.
The application of semiconductor lasers has been proposed
for pollution monitoring (ref. 46). Emission at the desired
frequencies is attained by controlling the composition and
operating temperature of these lasers. The emission line width
attained with Pbi_xSnxTe is very narrow (<3.3 x 10 6 cm-1).
The thermal tuning range, attained through the control of diode
current is "*<40 cm Emission ranging from 6.5 to 32 ym has
been attained with Pbi_xSnxTe lasers through compositional vari-
ation (ref. 46). It Is visualized that PbSni_xSex can be induced
to emit at shorter wavelengths, which would encompass NO
(5.50 um) and CO (4.86 urn) absorption ranges (ref. 46).
Semiconductor laserB are generally less complex In design,
and physically more compact and rugged than gas discharge and
spin flip Raman lasers. Detection capability in the fractional
ppm range Is visualized with the former. Semiconductor laser
radiation absorption measurement is a technique that merits
thorough feasibility and utility evaluation for the monitoring
of emissions from stationary sources.
Extensive laser Raman scattering measurements have been
conducted with solid and liquid materials. To date, the lasers
have found little use in Raman scattering measurements with
gaseous compositions. Some interesting demonstrations of the
latter type of applications have entailed the measurement of
Raman-scattered laser radiation by pollutants in the atmosphere
(ref. 56).
Conventional gas Raman spectrometer systems contain a cylin-
drical gas sample cell that is positioned concentrically with
respect to linear- or spiral-shaped continuous discharge-type
excitation sources (ref. 57,58). The Raman-scattered radiation
is transmitted through an optical window that is mounted onto
one end of the cylindrical sample cell. To enhance the scattered
radiation collection efficiency, and to focus the scattered
radiation onto the entrance slit of the spectrometer, the gas
sample cell is equipped with allgnable mirrors In both ends.
A laser Raman gas cell with internal reflecting optica has
been described very recently by Uelsh (ref 59). The exciting
radiation is directed into the cell through the same slit that
is used for the collection of scattered radiation Multiple
traverses of exciting radiation through the cell enhance the
scattered radiation intensity.
11
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It would be of Interest to explore the usefulness of
laser-excited Raman emission measurement for the monitoring
of gaseous polluntants. In contrast to conventional Raman
scattering measurements with gases, It appears advisable to
use pulsed exciting radiation In conjunction with time-gating
techniques for Raman-scattered radiation detection Since
the Raman-scattering efficiency is proportional to the fourth
power of exciting radiation frequency, It would be desirable
to utilize an Intense, ultraviolet laser source (e.g., the
nitrogen laser, emitting at 3371 a).
To prevent oxidation of the mirrors in the Raman cell,
protective coatings are essential. A conventional high-resolu-
tion optical and time-gating photosensor system could be
utilized, monitoring the radiation intensity at frequencies
characteristic for the specific pollutants. To attain further
enhancement of the signal-to-noise ratio, the correlation
mask detection method could be employed for the measurement
of scattered radiation intensity.
A preliminary evaluation of the practicality could be
attained by calculations based on: (1) incident radiation
intensity, (2) optical system configuration, (3) Raman-
scattering cross sections of NO, N02, ana other pollutants,
and (-U) detector sensitivity.
Sources of pulsed, coherent Infrared radiation have made
it feasible to investigate vibrational energy transfer and
relaxation processes. A part of the evolved energy Is emitted
through vibrational fluoreecence at frequencies that are charac-
teristic -for~The—molecules"! The measurement of the vibrational
riuorescence intensity is therefore potentially applicable for
gas analysis. Some preliminary results, apparently affected by
the Indirect thermal excitation of the samples, have been
reported.
It appears desirable to evaluate critically the applica-
bility of vibrational fluorescence for the analysis of nitric
oxide and other pollutant gases at low concentrations. This
will require the design of a gas sample holder, equipped with
optical windows, that will minimize the indirect thermal exci-
tation contribution to fluorescence emission. It will also be
essential to investigate intramolecular energy transfer processes
to and from the pollutant species, to establish the effects of
gas composition and pressure on the efficiency of fluorescent
radiation emission. Furthermore, optimized detector system
design, usefulness of the correlation mask technique, and the
application of signal-integrating techniques merit evaluation
12
The utilization of the vibrational fluorescence technique
presents several options in the types of excitation Bources
that could be used. These sources must be evaluated In conjunc-
tion with appropriate detection methods. Similarly to other
methods involving the use of lasers, which have been recommended
for further evaluation, vibrational fluorescence measurement
lends itself to simultaneous multiple pollutant monitoring.
13
-------
r»;.w4q&ao.
r^vjwTie* awLKo i"r: scr;-.xi, x-s.-' wnr >••<¦-. a ^-znrasc.'rs
ID
NOMDISPERSIVE INFRARED
Beckman Instruments. Inc.. Model 315 Infrared Analyzer
Principle of Operation
Optically, the Beckman Model 315 Infrared Analyzer and Intertech
Corporation's Instrument Uras-2 (see the description on p. 18)
are quite similar.
OtfUAStO ttUWCI
W
CCPlTCtt) i hit
lAWPie oui
O COKfomm o< 9»mv
o O'HlW KCatClAtS
The diaphragm of the Model 315
Infrared Analyzer is part of a
capacitance-type detection system
of the sensor. The capacitance
changes , caused by cyclic pressure
differences in the two halves of
the sensor when NO Is present in
the sample cell, modulate a radio-
frequency signal from an oscilla-
tor. This signal is subsequently
demodulated, amplified, and U3ed
as input to an indicating meter
or to a recorder.
The Beckman Infrared Analyzer
can be equipped tilth either
filter cells or optical filters
to attain complete absorption
in the spectral range in which
interfering substances would
cause partial absorption.
Performance Specifications Indicated by the Manufacturer
Response time
Measuring ranges
Zero drift
Span drift
Reproducibility
903 electronic response in 0-5 sec. Instrument
response depends on sample cell size and gas
flow rate.
from parts per million to 1C0K :.'C
SIS for 8-hour period
612 full scale for 2^-hour pericd
normally uithin ±13 full-scale
Re ference
Bulletin 4055D, l.< ckraan Instruments, Inc.
15
-------
NONDISPERSIVE INFRARED
The Bendlx Corporation. UNOR 2
Principle of Operation
The operation or the UNOR 2 Instrument Is based on the detection
of radiation absorption In the sample compartment by means of a
tandem two-component sensor unit that contains the gas which Is
being monitored. The sensor geometries find gas pressures are
established to
maintain equal
pressure In the
two compartments
when the beam
is not attenu-
ated upon
iKSStTSS, U traversing the
125K-iT*" J sample compart-
S toftrvu dll ^ merit.
uirvii
9 DrMctton cftMwr
t fw cnw
10 It*' Httvriig cUtirr
11 Comrctlnf diml
if Cmtdof (umi
I] Ounriys of CiM(it«r
II Mrourin^ Mpllfiir
IS Ctitewt mic
II ftrterdtr
17 Cri pm
l» fetoa«i> l»i»rf«rMKr "Mic«lor
1} 1|4|(| filtr' In inltl
W Safely filter In im nilltt
The spectral
distribution of
transmitted energy
(I.e. , the inten-
sity as a function
of frequency) Is
dependent on NO
concentration in
the sample com-
partment. Because
of the structure
of the vibratlonal-
rotatlonal bands,
the heating of
gases In the two-sensor compartments Is affected by the spectral
distribution of incident energy. Thus, when absorption occurs in
the sample compartment, the microphone-type capacitor (13) responds
to the pressure difference between the two sensor compartments
which contain the absorbing gas. The sensor capacitance change
is converted by an amplifier to a direct current 3lgnal which is
amplified and subsequently either indicated or recorded by the
Instrument.
16
Performance Specifications Indicated by the Manufacturer
Response time 3 sec for 90* response
Measuring range 300 ppm to 50J NO
Zero drift • «l)t of full-scale
Reproducibility . within ±1J of full-scale
References -
"UN0R-2 Sinetrakl-Ultfarot-Gaaanalyaator," Bull a tin So. 7557,
H. Maihak A3, Hamburg, Germany.
Bulletin SB34Z-10S9, Bendlx Process Instruments Division.
17
-------
HONDISPERSIVE INFRARED
Intertech Corporation. Uras-2
Principle of Operation
The operation of Uras-2 Is based on the absorption or Infrared
radiation from a broad-spectrum source by the sample and subse-
quent measurement of radiation Intensities of the monitoring
and reference beams. Hie sample gas
<§T
-------
NONPISPERSIVE INFRARED
Mine Safety Appliances Company. LIRA Model 200
Principle of Operation
The absorption of infrared radiation, emitted by Nlchrome fila-
ments, Is monitored In the NO absorption spectral range.
r—*—i
• I
LIRA OPTICAL SYSTEM ¦{ SSSK,
The two nondlspersed infrared beams, which are periodically
Interrupted by the semicircular chopper, traverse the sample
and the reference cell, respectively. The transmitted radia-
tion is directed Into the sensor compartment. This compartment
contains the gaseous compound*s) whose concentration Is being
monitored in the gas sample. Incorporated In the sensor is a
microphone that is equipped with a membrane which responds to
pressure fluctuations in the sensor gas compartment. The
capacitance change of the microphone during the pressure fluc-
tuations is converted to an analog electrical signal that is
amplified and subsequently fed to a meter or recorded.
The detector and recorder systems are electronically balanced
when no radiation Is absorbed from the beam that traverses the
sample compartment. Absorption of radiation In the sample cell
causes unequal gas expansion in the sensor during the two con-
secutive beam-chopper half-cycles This results in different
capacitance values, the difference is recorded in terms of NO
concentration.
To prevent interference by compounds whlcn absor>, m the spectral
range of NO absorption, a niter cell is incorporated in tandem
with the sample cell. The niter causes total absorption In the
range in which the interfering compound would cau^e partial and
variable absorption
20
Performance Specifications Indicated by the Manufacturer
Response time : 90J of final reading In 5 sec (or 0.4 sec
as an added option).
Measuring ranges. From 0 to 100 ppm up to 0 to 100X full-scale
by using cells of different lengthB.
Zero drift . <1J of full scale In 2U hr.
Reproducibility : within tin of full scale.
Ambient temperature range. -1° to +*(9°C
Selectivity . results affected <1* by background varia-
tions during most analyses.
Reference -
Bulletin So. 070S-1Z, Mine Safety Appliances Company, Instrument
Division.
21
-------
NONDISPERSIVE VISIBLE AMD ULTRAVIOLET
duPont Company. Model 461 Photometric Analyzer
Principle of Operation
The monitoring with the duPont photometric analyzer Is based on
the measurement of the difference In absorption by the sample
at two separate wavelengths. The radiation from a light source
traverses a sample. The transmitted radiation is subsequently
split Into two beams. An optical filter that transmits radiation
at the wavelength at which the sample absorbs strongly (measuring
wavelength), and that absorbs radiation essentially completely
In the remaining portion of the spectrum, is positioned between
the sample and the phototube M. A second filter, which transmits
only at a reference wavelength, at which the substance that is
being monitored does not absorb, Is placed i'. the second part of
the split beam
The current from the phototubes, which is related to the Incident
radiation intensity, Is amplified. The output from the amplifiers
Is corrected for the logarithmic relationship oetween the inten-
sity and concentration. The differential output from the two
amplifiers is proportional to the concentration of the monitored
compound.
The Model 461 Photometric Analyzer is equipped with a sampling
system for quantitative oxidation of N0X to J<02 . When the system
Is operated in this mode, the 4360 8 mercury line is used for the
sample analysis beam, and the 5460 8 mercury line serves as the
reference beam. When the nitrogen oxides &/¦ ->yldlzed and deter-
mined as NO2, an eight-minute sample resident' time is required
to complete the oxidation.
22
Performance Specifications Indicated by the Manufacturer
1 sec standard, down to 0.001 sec available
0.05 to.4.0 absorbance unlt3
<1$ full-scale in 24 hrs
¦\-l/4* (usually limited by recorder)
in { of analyzer reading = (% accuracy of
calibration standard) + (ill of analyzer
reading) + (±1/4J of full-scale range).
0° to 100°C.
Response time
Measuring ranges
Zero drift
Reproducibility
Accuracy
Allowable temperature range:
References
DuPont 400 Spltt Beam Photometric Analyser Instruction Manual,
Instrument Products Division, E. I. duPont de Nemours 8r
Co. (Inc.).
DuPont 461 Nitrogen Oxides Analyzer System, Instrument Products
Division, E. I. duPont de Nemours & Co. (Inc.).
400 Photometric Analyser, Instrument Products Division, E. I.
duPont de Nemours & Co. (Inc.).
23
-------
ELECTROCHEMICAL
Dynaselences Corporation.
Air Pollution Monitor Model NX 130
Principle of Operation
The sensor operates on the voltammetrlc principle. The electrodes
and the electrolyte are enclosed in the sensor that Is equipped
with a semipermeable membrane. The nitrogen oxides diffuse
through the membrane and they are oxidized at the anode. The
rate of diffusion is proportional to the concentration of nitrogen
oxides In the atmosphere
to which the membrane Is
exposed.
Since the diffused N0X Is
subsequently quantitatively
oxidized to the nitrate
ion, the current 1b directly
proportional to the NOx con-
centration in the gas to
which the membrane is exposed.
The current measured with
the potentlostat is ampli-
fied and displayed in terms
of N0X concentration on the
meter.
The selectivity of the volt-
ammetrlc sensor is attained
through the appropriate
combination of electrodes,
electrolyte and membrane.
Information regarding the
Identity of materials used
in commercial instruments
has not been disclosed.
The sensor operating life may vary from 3 to 9 months. It Is
apparently frequently limited by the vaporization of the solvent
through the membrane. The sensor part of this electrochemical
systen is readily replaceable.
21
Performance Specifications Indicated by the Manufacturer
901 of full-scale In 18 sec.
0 to 500, 0 to 1500, and 0 to 5000 ppm
<1% of full-scale in 2U hrs
within ±2J of full-scale by using the Integral
Response time
Measuring ranges
Span drift
Reproducibility
meter. Within tlj of full-scale by use of
external potentlometrie recorder.
Ambient temperature range: +5° to i)9°C
Specificity . no response to N», Oj, CO, COj, hydrocarbons,
and water vapor
Reference
Operating Instruotiona for Dynaaoienoea Air Pollution Monitor,
Instrument Systems Division, Dynaselences Corporation, Subsidiary
of Whlttaker Corporation.
25
-------
ELECTROCHEMICAL
EnvlroMetrlcs. Inc.. Series NS-200A
With Type Sfc'JHZ and Type N76H2 Sensors
Principle of Operation
The principle of operation of this Instrument, equipped with
Interchangeable Faristor™ sensors, has not been fully disclosed
by the manufacturer. The Faristor*H sensors are reported to
be liquid-state devices that contain surfaces onto which the
pollutants are adsorbed. The adsorption 1b followed by electron
transfer either to or from the adBOrbate. The current flow Is
determined mainly by the rate at which the gas molecules reach
the catalytlcally active surface and by the valence change which
the adsorbed species undergo at this surface.
It 1b believed that the EnvlroMetrlcs sensors operate on the
voltarmnetrlc principle. Presumably the catalytlcally active
electrodes and the electrolyte are enclosed in the sensor that
Is equipped on one surface with a semipermeable membrane. The
membrane is exposed to the atmosphere being monitored. The
current generation by the sensor Is permeation-controlled by
the transport of the pollutant gas through the membrane. The
rate of permeation Is proportional to the pollutant concentra-
tion In the atmosphere.
The selectivity of the sensor is attained through the appropriate
combination of catalytlcally active electrodes and electrolyte.
Faristor-type sensor N76H2 is generally used for total N0X moni-
toring in combustion source emissions. A type S61H2 sensor Is
available for SO2 monitoring.
26
Performance Specifications Indicated by the Manufacturer
Response time 90J of full scale in 5 to 10 seconds
Measuring ranges continuously variable from 0-50 ppm to
0-10,000 ppm full scale. Linear response
In the 0-5,000 ppm range as a minimum.
Reproducibility within ±21 of full scale.
Ambient temperature range: -1° to +65°C.
TM
Specificity : type N76H2 Farlstor sensor is specific to
the total NO . When the HOj content in NO
is less than 10J, the total error in NO is
less than 21. This error increases to 10J
when NO2 content is 50> of the total NO .
The manufacturer recommends Series N-122
analyzer for NO monitoring in gases that
contain a high percentage of N0i in NO
x
Reference The Parietor Ptug-in Module, by
EnvlroMetrlcs, Inc.
27
-------
N0X MONITORING METHODS
CURRENTLY APPLIED IN PROTOTYPE SYSTEMS
28
-------
CHEMILUMINESCENCE (USING ATOMIC OXYGEN)
Principle of Operation
Electronically excited nitrogen dioxide Is produced upon
the reaction of nitric oxide with atomic oxygen
NO + 0 - N02« (1)
The deactivation of N02* is accompanied by light emission
KOi* - N02 + hv <*max ~ 6300 X) (2)
Nitrogen dioxide reacts with atomic oxygen without light
emission and at a faster rate than nitric oxide
NO j + 0 -~ NO + 02 (3)
The NO formed in reaction (3) will subsequently undergo chemi-
luminescent oxidation via reaction (1). Thus, the total
nitrogen oxides (NO ) content can be determined by reaction
with atomic oxygen.*
Prototype Instruments have been developed for the moni-
toring of NO in ambient air, based on reactions 1-3- The
sample and oxidant gas streams are combined in a spherical or
in a tubular reactor The atomic oxygen in the oxidant gas Is
generated by microwave or by electrical discharge prior to the
entry Into the reactor. Light emission Is monitored with a
photonultlpller detector. The photocurrent has been found to
be a linear function of NO concentration over the concentration
range from <1 ppb to at least 100 ppm.
SO: and CO react also with atomic oxygen by chemilumi-
nescent mechanism. Maximum detection sensitivities of 1 ppb
and 150 ppm, respectively, have been attained with these gases
Individually. The emission wavelength maxima of the N0-0,
SO2-O, and CO-O reactions (6300 8, 3500 %, and 4350 8) are
sufficiently separated for simultaneous monitoring of these
three pollutants with one instrument.
Commercial Equipment
None marketed at the present. Prototype instruments have
been built by Monsanto Research Corporator .
29
References
a Principle of operation. 17, 60, 61
b. Applications 17
c. Data 17
d. Specific Instrument description- 17
Discussion
Advantages
a Based on gas phase chemical reactions. Does not
require wet chemicals.
b. Can be used for simultaneous monitoring of NO ,
CO, and SOi. *
c. Rapid response (<1 sec.).
d. High reliability expected.
e. Utilizes inexpensive optica.
Disadvantages
a. The flow reactor requires continuous evacuation to
maintain P t 1-5 torr.
Conclusions
A potentially Inexpensive multifunctional instrument that
could be used for the monitoring of the major gaseou3 pollutants
emanating from stationary fossil fuel-burning emission sources.
Further development required to utilize this instrument for
multifunctional monitoring at concentrations encountered In
source emissions.
30
-------
CHEMILUMINESCENCE (USING OZONE)
Principle of Operation
The reaction of nitric oxide with ozone produces electroni-
cally excited nitrogen dioxide molecules-
NO + Oj - N02* + 02
The transition of N02* to the electronic ground state
N02* ¦» N02 + hv
Is accompanied by light emission in the 6000 to 8750 X region
Nitric oxide detectors have been developed that are based
on the measurement of light emission emanating from its reaction
with ozone. Nitric oxide- and ozone-containing gas streams are
combined at subatmospheric pressure in a spherical flow reactor
that is equipped with an optical window. Light emission is
measured with a photomultiplier tube. A linear response is
obtained at NO concentrations ranging from 1 ppb to 1J.
N02 reacts only slowly with ozone, producing NOj (ref. 62),
this reaction is not accompanied by chemiluminescent emission
at 6000 to 8750 8 Therefore, the described method Is directly
applicable only for the monitoring of NO in gases which also
contain N02 . It Is very probably suitable for stationary source
monitoring if the Instrument is equipped with a converter in
which the N02 Is quantitatively reduced to NO.
The above-described technique Is being developed for
ambient air and mobile source emissions monitoring.
Commercial Equipment
Available from AeroChem Research Laboratories, Inc The
Instrument can be purchased with an optional accessory for con-
verting N02 to NO prior to admission of the gas into the reactor.
References
a Principle of operation 18, 19
b. Applications 6, 19, 20
c. Data 19-21
d Specific Instrument descriptions- 19, 21
31
Discussion
Advantages
a. CO and S02 cause no Interference.
b. Rapid response (<1 sec).
c. High reliability expected.
d. Utilizes Inexpensive optics.
e. System is also applicable for ozone monitoring
Disadvantages
a. The chemiluminescent reaction of NO + 0S can be used
directly to measure the NO content of the gas. The
determination of the total NO content requires the
reduction of NO2 to NO.
b. The flow reactor requires continuous evacuation to
maintain P < 1 torr.
Conclusions
A useful instrument for the monitoring of NO over a very
broad concentration range.
Recommendat ions
Disseminate the results of prototype instrument evaluation
conducted by APC0, on ambient air and mobile source monitoring.
Ascertain reliable performance or develop a converter for rapid
quantitative reduction of N02 to NO. Conduct verification of
the combined system (converter and analyzer) in actual source
monitoring.
32
-------
OAS CHROMATOGRAPHY
Principle of Operation
Gas sample components are separated In a chromatographic
column and their quantities are determined with various detec-
tors (e.g., thermoconductivlty or flame Ionization measurement
devices) upon elutlon.
The gas sample Is transported In the column by an Inert
carrier gas (e.g., helium, argon). The separation of gas com-
ponents Is tased on the molecular weight-dependent differences
of gas phaBe diffusion rates, the Interaction with solid ad-
sorbents contained In the column, and the rate of diffusion
and the solubility in the stationary phase. Since these are
temperature-dependent phenomena, the efficiency of separation
In a column is affected by the temperature.
Nitrogen oxides are reactive gases. The residence time
of a sample In a chromatographic column Is long (i<2 to 15
minutes). Therefore, columns are used which are constructed
of materials Inert to NO and packed with support and stationary
phase materials that do not react with NO . Reactive chromato-
graphic systems (ref. 63), In which the analyzed substances are
caused to undergo reactions prior to detection do not need to
be Inert toward MO ,
x
Numerous references (ref. 64-88 represent those considered
most relevant as background information) pertain to the analysis
of NO by chromatographic techniques. It appears that only the
following column materials have been applied successfully for
the separation and quantitative determination of NOj and NO
Fluoropak 80 packing with SF-96 (a methyl silicone oil) as the
stationary phase (ref. 70, 76), sllanlzed glass beads (ref. 71),
firebrick support with fluorinated paraffin oil (ref. 7M), and
Porapak R and Q (ref 82).
Commercial Equipment
Although a variety or chromatographic instruments are
produced by a number of manufacturers, none appears to have
been specifically designed for continuous monitoring of emission
sources.
References
a. Principle of Operation 61-88
b. Applications 71-73, 79, 80,
c. Data: 63, 65, 67-7i, 76-82, 86-8B
d. Specific Instrument Descriptions- 63, 67-69, 71,
76-78, 80-88.
Discussion
Advantages
a. High sensitivity at low concentrations with sensitive
detectors.
b. High selectivity attainable.
Disadvantages
a. Lack of real-time analysis capability. Minimum
analysis time approximately two minutes.
b. Column pretreatment required prior to NO analysis.
Some column packing materials require prSlonged
pretreatment.
c. The separation characteristics of chromatographic
columns in NO separation frequently change upon
exposure to reactive gases and even upon admission
of air.
Conclusions
The susceptibility of chromatographic columns used for NO
analysis to the alteration of operational characteristics *
necessitates frequent recalibratlon and militates against their
use In continuous monitoring systems for source emissions.
31
-------
CONDENSATION NUCLEATION
Principle of Operation
Liquid or solid particles, 0.001 to 0.1 micron In diameter,
function as nucleatlon sites for water droplets In an atmosphere
supersaturated with water vapor. If the material that is to be
analyzed is gaseous, it is usually converted Into finely divided
liquid or solid form (by various chemical means) prior to con-
tacting with saturated water vapor and subsequent expansion.
The scattering of light by the aerosol, that is rapidly (^10 msec)
formed under controlled, reproducible conditions, is related to
the concentration of the nucleating substance. The lower detec-
tion limits with various substances have been found to range
from 1 ppb (for Hg, SO2) to 5 ppm (COi, CiHjCH). The reported
detection limit for NOt is 0.5 ppm.
Commercial Equipment
None currently marketed for NO monitoring. Prototype
models have been developed by the Gineral Engineering Labora-
tory and the Advanced Technology Laboratories of the General
Electric Company. Monitors based on condensation nuclei forma-
tion are marketed for SOj, Hg, NHs and CO analysis by the
Environment/One Corporation of Schenectady, N. Y.
References
a. Principle of operation 89, 90
b. Applications. 89
c. Data: 89
d. Specific Instrument description- 89, 90
Discussion
Advantages
a. A simple and potentially low-cost Instrument.
b. Rapid response (1-2 sec).
c. Portable
d. Can be developed into an Instrument for the
monitoring of HO and/or NO2.
35
Disadvantages
a. This instrument requires very reproducible removal
of particulate matter, down to the sub-micron level,
from the sample gas stream.
b. Can be subject to interferences when applied to
multicomponent samples of varying composition.
c. In the simplest design form the condensation nuclea-
tlon Instrument could be calibrated In terms of the
NO2 content of tne emission gases. This concentration
is a function of the NO/NO2 ratio and of the partial
pressure of constituent gases. To determine the total
N0x content, NO in the gas sample must be oxidized.
Conclusions
Because of the above-cited disadvantages, the condensation
nucleatlon technique does not appear useful for NO^ monitoring
in source emissions.
-------
CORRELATION SPECTROMETRY
Principle of Operation
Correlation spectrometry Is based on the matching of the
absorption maxima of the material being analyzed with the
reference spectrum (correlation mask) contained within the
monitoring Instrument. Spectral regions of maximum absorbance
by the compound being analyzed appear as regions of maximum
transmittance on the correlation mask, which Is iicorporated
In the exit aperture of the instrument.
The correlation spectrometer used for source monitoring
utilizes a built-in light source that emits strongly In the
spectral region of Interest. The sample cell (or a flow-through
unit) is positioned between the light source and the dispersive
element. The latter Is either a grating or a dirfraction prism.
The dispersed spectrum is directed onto the correlation mask.
The wavelengths of the dispersed beam and the corresponding
positions of the correlation mask are brought Into coincidence
with a desired frequency. This Is achieved by either sweeping
the dispersed spectrum across the correlation taask, or by
traversing this mask through the dispersed light beam.
The light Intensity transmitted through the correlation
mask Is detected with a photomultipller tube. The intensity
of transmitted light or its time derivative is related to the
concentration of the substance being analyzed.
Commercial Equipment
Correlation spectrometers for the monitoring of NO2 in
ambient atmosphere have been developed by Barrlnger Research,
Ltd., Rexdale, Ontario, Canada. Source monitors for SO2 have
been developed by the same firm. Similar monitors for NO^ are
reportedly under development by Barrlnger Research, Ltd.
(ref. 91). The correlation spectrometer type source monitors
are available in the United States from Combustion Engineering
Associates.
References
a. Principle of operation 22-27
b. Applications 25, 27, 92-91
c. Data: 25, 27
d. Specific Instrument description 2*4, 2^,, 27
37
Discussion
Advantages
a. Multifunctional operational capability attainable
through the use of more than one correlation mask.
b. Could be used for the monitoring of NO2 and/or NO
in the multifunctional mode.
c. Interference by compounds known to be present can be
eliminated through the use of only the non-coinciding
absorption bands in the mask, or by monitoring with
more than one correlation mask.
d. A high slgnal-to-noise ratio Is attained through
simultaneous measurement of absorbance by the sample
in several spectral ranges In which the major absorp-
tion bands appear.
e. Rapid response (<1 sec).
f. The lower detection limit with a 1-meter optical
path length through the sample Is believed to be
less than 1 ppm (ref. 27).
Disadvantages
a. The optical system of the self-contained source
monitoring system Is subject to damage when the
supply of air for the optics-protective air curtain
becomes discontinued.
b. When the instrument is used directly In the stack,
the varying particulate matter content would affect
the accuracy of results.
Conclusions
A novel and potentially useful technique that can very prob-
ably be developed to meet the required performance criteria for
source monitoring. To protect the optical components of the
instrument in source monitoring, it would be advantageous to
filter the gas stream. Filtration could be effected either
in or external to the stack.
38
-------
DIELECTRIC CONSTANT MEASUREMENT
Principle of Operation
The design Is baaed on the effect of the dielectric proper-
ties of the gaseous sample, introduced into the capacitor of a
Clapp oscillator, on the resonance frequency of the oscillator.
The output of this oscillator is beat against that of a reference
oscillator that contains only carrier gas In the capacitor.
The difference frequency between the two oscillator circuits
Is a linear function of the polarlzable additive content in
the carrier gas that traverses the sample compartment. The
lower detection limit for NOi has been reported as 200 ppb.
Commercial Equipment
None available.
References
a. Principle of operation: 95
b. Applications: 95. 96
c. Data 96
d. Specific instrument description: 95, 96
Discussion
Advantages
a. An Inexpensive instrument
b. Portable
c Rapid response
Plsadvantages
a. Nonspecific for compositions that contain more than
one polarlzable component. Stack emissions contain
a number of polarlzable compounds (I.e., C02, HjO,
SC2, NO, MOj, CO, and other compounds In smaller
amount s) .
Conclusions
The dielectric constant measurement Instrument would be
applicable for emissions measurement only as one of a multi-
instrument complement. Although these instruments may not
respond selectively or specifically to the components being
analyzed, a mathematical analysis of the responses of these
Instruments would provide the compositional information
39
ELECTRON-EXCITATION
Principle of Operation
A gas sample is exposed to electron beam Irradiation. A
fraction of the kinetic energy of electrons is transferred to
the gas molecules. A portion of this transferred energy appears
as fluorescent radiation of wavelengths that are characteristic
for the energy acceptor. The fluorescent radiation intensity
Is related to the acceptor concentration (ref. 97).
The electron beam excitation technique has been used In
aerodynamic studies for gas density measurement, flow visuali-
zation, temperature measurement, and also for gas composition
analysis [ref. 98).
Commercial Equipment
Electron beam instruments have not been developed commer-
cially for gas analysis.
References
a. Principle of operation 97
b. Applications: 98
c. Data: 99, 100
d. Specific instrument description- 99, ICO
Discussion
Advantages
a. Sample removal from gas stream is not necessarily
required
Disadvantages
a. Intensity of radiation emitted by NO upon exposure
to electron beam Irradiation is low.
b. Nitric oxide emission bands overlap with the Intense
emission bands of nitrogen.
c. Emission by nitric oxide occurs from a resonance
level, the emitted radiation is subject to self-
absorption .
d. High equipment cost
to
-------
Conclusions
It does not appear probable that electron beam excitation
of light emission by nitric oxide can be developed to a method
of required sensitivity for NO^ stack emissions monitoring.
41
OAS PHASE IONIC CONDUCTANCE
(INVOLVING CHARGE TRANSFER AND RECOMBINATION)
Principle of Operation
Some approaches to the utilization of gas phase ionization
measurement for K0X detection entail the determination of the
effect of its addition on the electrical conductivity of a car-
rier gas. The recombination of Ionized species in argon (ref.
101} and in argor.-ethylene mixtures (ref. 10?) is enhanced In
the presence of NO and NOj. Detection of 10~® to 10-1° mole
is feasible (ref. 101). Since the efTect of NOx on the conduc-
tivity of ionized argon is not specifically limited to these
oxides, the method has been used only in chromatographic eluant
detector devices.
The conductivity of a gaseous medium Is reduced very
rapidly and significantly when finely divided particulate
matter is Injected Into an ionized gas. The solid particles
serve as catalytic recombination slte3 for the charged parti-
cles. Detectors have been developed in which nitrogen dioxide
is allowed to react with ammonia. The aerosolized ammonium
nitrate that is formed Is Introduced into a chamber in which
gas phase ionization Is caused by an incorporated radioactive
(alpha) source. The reduction of ionic conductance in the
chamber Is related to the NOj content of the Introduced gas
sample (ref. 102, 103).
Commercial Equipment
An ionization monitor, based on the last of the above-
described operational principles is marketed by the Mine Safety
Applicance Company, Pittsburgh, Pa., under the name M-S-A
Billionaire.
References
a. Principle of operation- 101-lCt
b. Application- 103, 10^
c. Data 101-103
d. Specific Instrument Description- 101-1GU
12
-------
Discussion
Advantages
a. High sensitivity
b. Rapid response
c. Portable Instrument
Disadvantages
a. Lack of specificity.
b. Conductance measurements that entail particulate
solid formation require oxidation of NO.
Conclusions
Because of the lack of specificity for N0X detection with
emissions from stationary sources, the above-described methods
would only be of interest in conjunction with a gas-separative
technique.
1*3
INVERSE RADIOACTIVE TRACING
Principle of Operation
The inverse radioactive tracing method entails the reaction
of nitrogen dioxide with the Kr' '-hydroquinone clathrate. The
reaction of hydroquinone in the clathrate with NOj Is accompanied
by the release of radioactive Kr's (B-emltter, having a half-
life of 10.J yrs) which is monitored by radioactive counting
techniques. The rate of release of Kr in a flow-through sensor
unit Is proportional to NOj concentration In the analyzed es-3-
The minimum detected NOi concentration Is 2 ppm- HO does
not react with hydroquinone, nor does It displace Kr In the
clathrate.
Commercial Equipment
One or two Instruments were built by Tracerlab, formerly
a division of the Laboratory for Electronics, Inc. (ref. 105,
106). More recently, a prototype Instrument has been built by
Panametrics, Inc. to investigate the feasibility of automobile
exhaust monitoring by the Inverse radioactive technique
(ref. 107).
References
a. Principle or operation: 106-108
b Applications 106-108
c Data 107
d. Specific instrument descriptions 106, 107
Discussion
Advantages
a. A very simple and potentially low-cost instrument .
b. Portable Instrument.
c. Rapid response (<5 sec).
Disadvantages
a Requires an auxiliary oxidation unit for NO.
b. Water destroys the clathrates at relative huirl i e s
above 90t
c. S0i and SOj Interfere.
d The Kr" clathrate is consumed and requires rc,r.! icement
at periodic intervals (^3-6 months).
lti|
-------
Conclusions
Sufficient information has not been reported to definitively
evaluate the applicability of the Inverse radioactive tracing
method for the present purpose. The effect or temperature on
the rate of release of Kr'*, the extent of interference by S02,
the effect of water at relative humidities below 90X on the
reaction rate, and the effect of air flow rate need to be known
to evaluate the performance of this Instrument.
15
INTERFERENCE SPECTROMETRY
Principle of Operation
An Interferometer contains the following major components:
a broad-spectrum radiation source, a beam splitter, and a
radiation detector. The energy beam is divided with a partially
transparent beam splitter. One of the beam components Is
reflected from a mirror, which is traversed at a constant
speed, and the two beams are subsequently recomblned before
entering the sample compartment. Subsequent to transmission
through the sample, the radiation is focused onto a detector.
Recombination of the two beam components causes con-
structive and destructive interference. The recomblned beam
intensity at a given wavelength is a function of sample ab-
Borbance at this wavelength, and of the optical path lengths
of the two beam components.
The interferometers utilize portions of the electromagnetic
spectrum in a nondlspersed form. The detector output is re-
corded as a function of time (i.e., optical path lengths of
the two beam components). Computerized data reduction is used
to analyze the interferometer signal output and to convert it
to a conventional Bpectrum.
Commercial Equipment
Commercial Instruments are available fram the Block
Engineering Company, Cambridge, Mass.; Digllab, Inc., Cambridge,
Mass.; Idealab, Inc., Franklin, Mass.; and Beckman Instruments,
Inc., Fullerton Calif. Instruments offered by Beckman
Instruments, Inc.,_cover mainly the far infrared spectral
range (500 to 3 cm-1).
References
a. Principle of operation: 109-113
b. Applications 112, ll1)
c. Data: 109, UK, 115
d. Specific instrument description 109, 110, 113
111, 116
16
-------
Discussion
Advantages
a. The utilization of nondlsperslve optical systems
enhances the slgnal-to-nolse ratio of Interference
spectrometers, In comparison with corresponding
Instruments that contain dispersive optical systems.
High ultimate sensitivity can be expected through
further development of the Infrared lnterferometrlc
technique.
b. Applicable for simultaneous monitoring of NO , CO,
and SOa. x
Disadvantages
a. Instrumental malfunctions affect greatly the
reliability of results because of the data analysis
technique employed with this method.
b. The systems are presently still In an early evolu-
tionary phase and they are therefore quite costly.
Conclusions
The development of Interference spectrometrlc techniques
has greatly accelerated during recent years because of the
availability of high-speed computer equipment required for
data reduction with this method. Interference spectrometry,
In conjunction with data storage (for multiple scan analysis)
represents a potentially very useful technique for the analy-
sis of gas components at low concentrations. The utilization
of this method for NO monitoring in plant environment would
require the development of ruggedlzed instruments whose
operation would not be affected by temperature fluctuation
and other changes of the environment.
LASER RADIATION ABSORPTION
Principle of Operation
Certain laser emission lines coincide with the absorption
ranges of common air pollutants (ref. fll—^7» 50). Thus, the
attenuation of emitted laser radiation Intensity upon trans-
mission through gaseous samples can serve as the basis for
monitoring the concentrations of pollutants. The measurement
of the absorption of laser radiation has been considered pri-
marily for ambient atmospheric monitoring. However, It is
equally applicable for source monitoring. Enhanced sensitivity
can be attained by using folded-path sample cell3. The sensi-
tivity of laser spectroscopic measurements can also be increased
with spectrophone-type (optoacoustlc) detectors (ref. ^5, 117).
The third-strongest emission line of the Iodine laser at
1818.71) cm-1 coincides with the strongest absorption band of
NO (ref. ill, 1)2). Some carbon monoxide laser emission lines
coincide with the absorption lines of NO at 1780 to lSJ'tO cm"',
the Vi band of NOj at ^1620 cm 1, and the \>i band of SOa at
^1360 cm-1 (ref. 54,50).
To attain CO laser emission at the N02 and SO2 fundamental
frequencies, and thus multifunctional monitoring capability,
the laser must be operated at liquid nitrogen temperature
(ref. 51).
Frequency-tuning over "v-400 cm-1 spectral range, at wave-
lengths longer than those of the laser emission, has been
attained by the spin flip Raman technique (ref. 54 ,55,118,119) .
Tuning is attained by applying strong magnetic fields to the
semiconductor stimulated Raman scattering emitter. Stimulated
Raman emission from InSb has been Induced by CO and CO2 laser
Irradiation. The emitter must be maintained at cryogenic tem-
peratures [presently up to 77°K, conceivably to 190°K at some
later date (ref. 120)], and a trade-off exists between the
refrigeration and magnetic field strength requirements. The
spin flip Raman technique offers presently the capability
for NO monitoring at very low concentrations (<1 ppb), which
is of primary Interest in ambient atmosphere monitoring
The feasibility of using solid-state Junction lasers for
pollutant monitoring has been demonstrated (ref. ^6). Control
of emission frequency is attained by varying the Pb-to-Sn ratio
and the operating temperature. Using Pbj.jSn Te, emission was
attained through the range of 6.5 to 32 um, which does not
encompass the absorption bands corresponding to the fundamental
stretching frequency of NO and CO. It is believed (ref 46)
48
-------
that semiconductor materials with the required emission charac-
teristics ear he developed for absorption measurement at 5-33
and 4.66 ym.
Commercial Equipment
None available.
References
a. Principle of operation. 11-13, ^5—^7
b. Applications: 11-17, 50
c. Data: 11-1)7, 50
d. Specific Instrument description: 13, 15, 17, 51,
55, 117
Discussion
Advantages
a. Spectroscopic technique of very high sensitivity
because of the narrow spectral range of the light
source.
b. The narrow spectral range of laser emission also
assists In minimizing interferences.
c. Multifunctional operating capability with certain
laser light sources.
d. Very low (<1 ppb) minimum detection limit.
Disadvantages
a. Developmental status and relatively high cost of the
required laser radiation sources at the present time.
b. Complexity of systems, that is a contributing factor
to the high cost.
Conclusions
Present gas laser systems applicable for HO monitoring are
costly and they are not very rugged. It would bi desirable to
develop solid-state Junction lasers which would. tl) emit at
the absorption maxima of NO, (2) be physically compact and
rugged, and (3) be amenable to low-cost production.
19
LASER RAMAN SCATTERING
Principle of Operation
The interaction of electromagnetic radiation with polariz-
able molecules leads to scattering of radiation by these
molecules. Host of the scattered radiation is of longer wave-
lengths (Stokes lines) than the incident radiation. A small
fraction of scattered light is of shorter wavelengths (anti-
Stokes lines) than the Incident radiation The energy differ-
ence between the incident and scattered radiation corresponds
to the molecular vibrational frequencies.
Only a small fraction of Incident energy 13 scattered as
Raman radiation. The scattering efficiency of molecules Is
related to their polarlzability.
The feasibility of utilizing lasers for remote monitoring
of pollutant gas emission from stationary sources haB been
investigated (ref. 121-123). A multiple-reflection cell for
Raman scattering measurements with gaBeous samples, designed
specifically for use with laser excitation sources, has recently
been described (ref. 59).
Commercial Equipment
Laboratory Raman spectrometers utilizing lasers as radia-
tion sources are commercially available.
References
a. Principle of operation- 121-123
b. Applications: 122, 123
c. Data: 122
d. Specific instrument description. 57-59, 122
Discussion
Advantages
a. Potentially applicable for simultaneous monitoring
of NO^, CO, and SOj.
b. Rapid response (<1 sec).
Disadvantages
a. Intense laser light sources are still at an evolu-
tionary stage and their cost is presently high.
50
-------
Conclusions
It would be desirable to establish the low-concentration
analyBis capabilities of the laser Raman scattering technique
with gaseous materials, such as MO , SOi and CO. The nitrogen
laser, emitting in the ultraviolet spectral range (3371 S) Is
a preferred light source. To enhance the 3ignal-to-noise ratio,
the detector should be time-gated to the pulsed emissions from
the laser.
51
MASS SPECTROMETRy
Principle of Operation
In analyzing gases by mass spectrometric methods, the
molecules are subjected to bombardment by electrons which are
released by a thermolonic emission source. The electron-
molecule Interaction results In Ionization and/or dissociation
in a pattern that is characteristic for the molecule. When
the Ionization is conducted in a low-pressure environment,
Immediate recombination of the charged particles can be pre-
vented. The ions are accelerated in an electric field and
the monoenergetlc ion beam is introduced Into the magnetic
field. Traveling in a circular path, the radius of curvature
is proportional to the square root of the mass for singly-charged
particles. Electrometer-type detectors, positioned to Intercept
the circular path of the charged ionic fragments, are used to
measure the relative concentrations of the species with dif-
ferent m/e ratios (ref 124).
The time-of-f light mass spectrometer utilizes a similar
ionization source. The ions are accelerated Into a linear
drift tube. The time required by the ions to travel from the
entrance port of the drift tube to the collector plate Increases
with the m/e ratio of the species. An outstanding feature of
the tlme-of-fllght mass spectrometer is the high speed at which
samples are analyzed (<0.1 msec) (ref. 121!}.
Omegatron is a mass spectrometer that Is based on the
cyclotron resonance principle (ref. 28-30), it has high sensi-
tivity but only moderate resolution. Because of these charac-
teristics, its small size, and bakeout capability, the omegatron
Is used for residual gas analysis In vacuum systems.
The quadrupole mass spectrometers utilize an electrical
quadrupole field to effect ion separation (ref. 31,32) This
field is created by applying rf and dc voltages to four
parallel-positioned metal rods. Opposite rods are connected
lr\ pairs ami adjacent rods are of different polarity. The ion
trajectories depend on their m/e ratios and on the quadrupole
field parameters. With selected field parameters, only ions
of specific m/e ratio Impinge upon the detector, other ions
are neutralized on the charged rods.
The quadrupole mass spectrometers have a high sensitivity
because of their large acceptance angle. Their design does not
entail the use of magnets. Complete mass ranges are scanned
In less than C.25 sec.
52
-------
An Ion storage mass spectrometer has been recently
reported (ref 33). The separation of Ions is effected In
a three-dimensional quadrupole electric field. The ions are
stored on wire mesh electrodes, from which they are freed upon
pulsing for detection by an electron multiplier The ion stor-
age mass spectrometer is a very small device, it has good
resolution at low (<100) m/e values.
Commercial Equipment
Numerous manufacturers market mass spectrometers (ref. 125).
Some companies listed in the cited reference market moderately-
priced instruments of sufficient resolution which have the
required sensitivity for N0X stack emissions measurement
References
a Principle of operation 28-33, 124
b Applications 126-131
c Data 126-131
d Specific instrument description- 28-31
Discussion
Advantages
a. Sapid response (<1 seel.
b. Multifunctional operational capability attainable
c. The lower detection limit Is ?1 ppm.
d Temperature fluctuations are not expected to affect
instrument response greatly
Disadvantages
a. Automated correction for the contribution of minor
gas components (HCHO, C2H6) to the major m/e peak
of NO may be required.
b. Continuous evacuation required.
c. Conditioning of mass spectrometers by exposure to
nitrogen oxides may be required
d Need for interface accessories for sampling transfer
that will not remove the oxides of nitrogen from the
sample.
53
Conclusions
Significant advances have oeen made during recent years
in the development of moderate-cost mass spectrometers. In
view of the advantages that these instruments (I.e., omegatrons,
quadrupole and ion storage mass spectrometers) offer, it appears
advisable to evaluate their utility in stationary source emis-
sions measurements.
5*
-------
PARAMAGNETISM
Principle of Operation
Magnetic susceptibility of nitric oxide (+1.461 x 10"' in
cgs units per mole at 293°K) and of nitrogen dioxide (+0.150 x
10"') has been used as the basis for their determination In gas
mixtures. The gas mixture Is passed through a magnetic field
that deflects paramagnetic molecules. Two electrically heated
wires are mounted in the sensor assembly. The latter is posi-
tioned in the magnetic field in such a manner that the paramag-
netic molecules are deflected from one of the heated wires In
the direction of the other. This causes one of the wires to
be cooled and the adjacent wire to be warmed. For given para-
magnetic molecules, the temperature differential between the
two wires is related to the concentration of these molecules.
Commercial Equipment
None known to be marketed.
Reference
Ref. 132
Discussion
Advantages
a. Rapid response.
b. A simple and potentially low-cost Instrument.
c. Portable.
d. CO and SO2 cause no interference.
Disadvantages
a. It would be desirable to reduce NO In the gas to
NO, that has high magnetic susceptibility, for the
attainment cf maximum signal strength. Conversion
of the mixed oxides of nitrogen to either NO or NOz
Is essential because both species are paramagnetic.
b. Oxygen has a magnetic susceptibility of 3 .*49 x 10"'
in cgs units per mole at 293°K. The concentration
of oxygen In emission gases is high (^3J) and It does
not remain constant. Oxygen would therefore strongly
Interfere kith NO determination.
55
Conclusions
Because of the above-cited serious disadvantages, the
paramagnetism measurement is not a useful method for NO
analysis In gas mixtures that contain oxygen.
56
-------
PHOTOIONIZATION
Principle of Operation
Nitric oxide has an Ionization potential of 9.26 eV (ref.
34-37) • The corresponding value for NO2 13 9.63 eV (ref. 37 — 39)
The ionization potentials of other stationary source emission
gas components have values above 12 eV (ref. 3^ and 133). Thu3,
it is feasiole to monitor the N0X content in emission gases by
selective photolonlzation with ultraviolet radiation having
wavelengths in the 1050 to 1350 A range.
Selective photolonlzatlon in an ion chamber, in conjunction
with ion collection current determination, has been applied for
the measurement of NO concentration in the earth'b atmosphere
(ref. 134 and 135). This method was found to provide a very
high detection sensitivity (<<1 ppb).
Commercial Equipment
A prototype commercial instrument for NO analysis, based
on photolonization measurement, is at an advanced development
stage, Commercialization is projected by Walden Research
Corporation for the spring or summer of 1972 (ref. 136).
References
a.. Principle of operation; 3U, 36,
b. Application: 135
c¦ Data: 134
d. Specific Instrument description:
Discussion
Advantages
a. With emission gases, response specific to NO .
b. High sensitivity.
c. Rapid response.
d. Portable instrument.
Disadvantages
a. Interference by aromatic hydrocarbons, which may be
encountered in some emission source,.
37, HO, 134, 137
*t0, 13J)
57
Conclusions
The selective photolonization method, involving the
measurement of gas phase ionization Induced by short-wavelength
ultraviolet radiation (1050-1350 8) merits further experimental
development and evaluation.
Limitation of applicability with sources that emit pollu-
tants of low ionization potential (e.g., benzene, IP = 9.25 ev),
should be established.
58
-------
VIBRATIONAL FLUORESCENCE
Principle of Operation
Subsequent to the absorption of Infrared radiation of a
given wavelength by molecules In the gas phase, thermal equi-
librium Is re-established by emission of radiation and by
colllslonal energy transfer.
A requirement for absorption of radiation Is the corres-
pondence of the quantum energy of Incident radiation to the
energy requirement for a quantum-mechanically allowed
vlbrational-rotational transition. The radiation emitted by
molecules is of equal or longer wavelength than the incident
radiation. Emission of radiation that occurs subsequent to
vibrational excitation Is known as vibrational fluorescence.
The wavelengths of vibrational fluorescence emission correspond
to those known from absorption spectra.
Commercial Equipment
None available.
References
a. Principle of operation: 138-140
b. Applications- 141-1'is
c. Data: 141-113, 115
d. Specific Instrument description 138, 141—143
Discussion
Advantages
a. Enhanced specificity of detection may be attained
by vibrational fluorescence measurement with com-
pounds that have coinciding absorption regions.
Compounds which absorb radiation In the 3ame spectral
region frequently exhibit fluorescence in non-
overlapping spectral ranges.
b. Rapid response.
Disadvantages
a. Compensation or correction for indirect (thermal)
excitation will be required for quantitative
analytical measurements
59
Coneluslons
Vibrational fluorescence measurement has become practi-
cally feasible through the development of intense monochromatic
emission sources, the lasers. Such measurements have recently
been applied in Interesting and significant studies pertaining
to Intra- and lntermolecular energy transfer processes (ref.
138, 139).
Attempts have been made to apply vibrational fluorescence
measurements for analytical purposes (ref. 143). The
reported data (relatively intense emission at quantum energies
higher than the quantum energy of exciting radiation) suggest
that a sizeable fraction of the measured effect arose from
indirect excitation.
Vibrational fluorescence measurement represents a new and
potentially very useful gas analysis technique. It appears
highly advisable to conduct studies with the objective of
defining the potential of this technique for analytical appli-
cations, specifically for NO monitoring In the present case.
Such studies must be conducted with full cognizance of energy
transfer processes that affect the emission characteristics of
specific components in gas mixtures of variable compositions.
Means for minimizing or compensating for the contributions to
emission which result from energy transfer between the gas and
its containing walls must also be Investigated.
6o
-------
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