United States
Environmental Protection
Agency
Environmental Monitoring Systems
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-85/063 Jan. 1986
SEPA Project Summary
A Cryogenic Preconcentration—
Direct FID (PDFID) Method for
Measurement of
NMOC in Ambient Air
Frank F. McElroy, Vinson L Thompson, and Harold G. Richter
Accurate measurements of atmos-
pheric concentrations of nonmethane
organic compounds (NMOC) are nec-
essary in the application of photochem-
ical models that are used by states in
developing the control strategies needed
to achieve compliance with ambient air
quality standards for ozone. NMOC
measurements obtained with available
continuous NMOC analyzers have often
been of inadequate quality. Speciated
gas chromatographic measurements,
though adequate, are excessively diffi-
cult and expensive where speciated
data are not needed.
A simplified cryogenic preconcentra-
tion, direct flame ionization (PDFID)
method that is sensitive and provides
accurate measurements of ambient
NMOC concentrations has been devel-
oped and standardized sufficiently to be
recommended for use by state and local
air pollution control agencies in the
development of their ozone control
plans. Recent refinements to the
method are discussed, an automatic
remote sampling system is described,
and the performance (precision and
accuracy) of the method is character-
ized, based on results from utilization of
the method for NMOC analysis of 1375
air samples collected from 22 sites
during the summer of 1984.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Accurate measurements of ambient
concentrations of nonmethane organic
compounds (NMOC) are important to the
control of photochemical smog because
these organic compounds are primary
precursors of atmospheric ozone and
other oxidants. Achieving and maintain-
ing compliance with the National Ambient
Air Quality Standards for ozone thus
depends largely on control of ambient
levels of NMOC.
A number of photochemical dispersion
models have been developed to describe
the quantitative relationships between
ambient concentrations of NMOC and
other compounds (e.g., NO,) and sub-
sequent downwind concentrations of
ozone. An important application of such
models is to determine the degree of
control of NMOC that is necessary, in a
particular area, to achieve compliance
with applicable ambient air quality stand-
ards for ozone. To achieve this purpose,
the models require input of accurate data
on ambient concentrations of NMOC.
The more elaborate theoretical models
generally require detailed organic species
data. Such data must be obtained by
analysis of air samples with a sophisti-
cated, multicomponent gas chromato-
graphic (GC) species analysis system.
Simpler empirical models such as the
Empirical Kinetic Modeling Approach
(EKMA) require only total NMOC concen-
tration data—specifically, the average
total NMOC concentrations from 6:00
AM to 9:00 AM.
Until recently, ambient NMOC meas-
urements for EKMA were often obtained
with commercial, continuous NMOC
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analyzers. However, measurements from
these instruments have been shown to be
unreliable, particularly at concentrations
below about 0.5 ppmC, due to a variety of
instrument-related problems. These prob-
lems included (1) the indirect, subtractive
nature of the measurement process
employed (total organic compounds
minus methane), (2) non-uniform per-
carbon response for different compounds
due to oxygen interference, (3) inadequate
sensitivity, and (4) interference from
water vapor. Thus, the usefulness of
NMOC measurements obtained with
these instruments is limited.
The GC speciation method provides
more accurate and reliable ambient
measurements. Utilizing cryogenic pre-
concentration followed by GC separation
and flame ionization detection (FID), the
technique provides quantitative, identi-
fied, species concentrations of the C2 to
Cio compounds typically observed in
ambient air. NMOC measurements may
be obtained by summing the individual
species concentrations. The cryogenic
sample preconcentration greatly en-
hances the sensitivity of the method while
effectively minimizing interference from
methane and oxygen, allowing direct
measurement of various organic species
with little variation in the per-carbon
response for most compounds of interest.
However, the GC speciation method
requires sophisticated analytical equip-
ment, a high level of operator skill and
experience, and considerable time per
analysis, making such measurements
expensive. This expense is often not
justified in EKMA applications where
speciated data are not required.
A number of researchers have contrib-
uted to the development of a simplified
NMOC method—derived from the specia-
tion method—that eliminates the GC
separation and much of the operator skill
required but retains the cryogenic pre-
concentration for good sensitivity, the
FID for selectivity, and the inert carrier
gas for uniform per-carbon response.
This method has been further refined,
tested, and sufficiently standardized to be
an available and recommended method
for the measurement of ambient concen-
trations of NMOC for EKMA or other
applications. The method is identified as
the preconcentration, direct flame ioniza-
tion detection (PDFID) method for NMOC.
Method Description
Figure 1 shows a schematic diagram of
the basic analytical apparatus. Major
components include a sample volume
metering system, a six-port gas valve, a
Vacuum
Valve
Absolute
Pressure Gauge
! Sample
Valve
Low
Pressure
Regulator
Vacuum
Pump
Glass
\*^\\ Beads
Cryogenic
Sample Trap
(Liquid Argon)
Hydrogen
Air
Sample
Canister
Figure 1. Schematic diagram of system for analysis of total non-methane hydrocarbons by
cryogenic preconcentration and flame ionization detection.
cryogenic preconcentration trap, and a
FID-integrator-recorder system.
The vacuum pump is used first to reduce
the pressure in the vacuum reservoir to a
known selected absolute pressure setting.
With the gas valve in the sample (trapping)
position, sample air is drawn through the
trap, which is immersed in liquid argon
(-186°C). The volume of air passing
through the trap is measured by the
increasing pressure (diminishing vacuum)
in the reservoir, and the sample flow is
stopped at a second selected pressure.
The trap condenses NMOC while permit-
ting air and methane to pass through.
During the trapping mode, helium carrier
gas passes through the gas valve directly
to the FID.
Following the trapping mode, the gas
valve is switched to the inject position,
directing the helium carrier gas through
the trap, in the direction opposite to the
previous sample flow, before passing to
the FID. The cryogen is then removed, and
the trap is heated to approximately 90°C.
Organic compounds collected in the trap
revolatilize and are swept into the FID by
the carrier gas. The resulting peaks are
integrated and converted to ppmC by an
NMOC calibration curve, prepared pre-
viously by using propane-in-air concen-
tration standards. Use of the same precise
reservoir pressure readings for each
trapping cycle results in a constant sample
volume and allows calibration of the
system with known concentration stand-
ards without quantitatively measuring the
actual sample volume.
A detailed description of the PDFID
method, incorporating changes and re-
finements developed to date, is provided
in an appendix to the full report.
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Refinements
Apparatus. Various types of compo-
nent flame ionization detectors, integra-
tors, and chart recorders could be used to
assemble the requisite analytical appa-
ratus. However, a recent-model laboratory
gas chromatograph provides the most
advantageous and expedient way to
obtain the necessary functions, even
though no GC column is used. Such an
instrument contains a high quality
temperature-controlled FID, precision gas
flow controls, and a properly interfaced
signal integrator/recorder in a conven-
iently integrated system. Further, a
chromatograph instrument also provides
a temperature-programmable oven for
warming the trap, and capability for
controlling the valve operation and other
procedural sequences. In addition, it
provides a convenient mechanical facility
for properly mounting the trap, valve, and
other components so as to minimize the
lengths of interconnecting lines and keep
them heated above ambient temperature.
These advantages help to minimize vari-
ability in the analytical measurements.
Trap. Use of a small, U-shaped trap
packed with 60/80 mesh glass beads (as
opposed to a multi-turn unpacked trap)
provided better uniformity of per-carbon
response to various paraffinic and olefinic
hydrocarbon compounds and also more
repeatably shaped peaks.
Water Interference. Some positive
interference from moisture in the sample
air was observed, manifested as a rela-
tively uniform and predictable shift in the
FID baseline during sample injection. This
effect can be minimized to a large extent
by (1) carefully observing the character of
the baseline shift using humidified zero
air and an expanded recorder scale, (2)
adjusting the trap heating rate for uniform
shift, and (3) programming the integrator
to correct or compensate for the shift.
Remote Sampling. Collection of air
samples at remote sites provides a
number of advantages, including (1) col-
lecting integrated (e.g., 3-hour) samples,
(2) shipping and storing samples for
convenient central laboratory analysis,
(3) analyzing samples from several sites
with a single analytical system, and (4)
capability for repeat analysis of samples
or collection of duplicate samples for
quality assurance. NMOC samples have
shown to be stable for several weeks
when collected and stored in stainless
steel canisters whose surface has been
specially treated by the SUMMA process
(Molectrics, Carson, CA). A sampling
system, consisting of a metal bellows
pump, hypodermic needle, filter, timer,
and special electric solenoid valve, allows
for automatic unattended collection at
remote sites of 3-hour integrated ambient
air samples in canisters. Such a system
has been successfully demonstrated in a
22-site network operated for 12 weeks
during 1984. Following analysis, the
treated stainless steel canisters can be
readily and easily cleaned and evacuated
for reuse.
Method Performance and
Test Results
A 22-site, 12-week NMOC monitoring
project during the summer of 1984
provided PDFID method performance data
under actual field-use conditions. Three-
hour air samples were collected in treated
stainless steel canisters and shipped to a
central laboratory for analysis using the
PDFID analytical system. In all, 1375 valid
samples were collected, ranging in con-
centration from 0.06 ppmC to 4.75 ppmC.
Overall completeness (number of valid
samples obtained divided by the number
expected) for the 22 sites, located in the
eastern and central areas of the country,
was 90.6 percent.
Calibration drift was observed with
daily calibrations at the beginning of each
day and calibration checks at the end of
the day. Daily zero and span drifts are
shown in Table 1.
Analytical precision for the method was
assessed from the differences observed
between the original analyses and later
repeat analyses for 28 of the samples.
The results are shown in Table 2. Since
the mean difference is considerably
Table 1. Daily Calibration Drift
Maximum
Minimum
Mean
Standard
deviation
Zero Drift
ppmC
+0.013
-0.016
+0.00016
0.0022
Span Drift
%
14.1
-10.1
+ 1.47
4.06
smaller than the standard deviation, it
can be concluded that the mean is not
significantly different than zero, suggest-
ing that the time delay between the first
and second analyses had no significant
effect on the concentration measurement.
Overall precision was assessed from
the differences between the analyses of
58 pairs of duplicate samples collected
simultaneously in duplicate, paired can-
isters. Accordingly, this overall precision
assessment includes both analytical var-
iability and variability contributed by
collection and storage of the air samples
in the canisters. The results are shown in
Table 2. The overall precision is only
slightly worse than the analytical preci-
sion, indicating that collecting and storing
the air samples in the canisters added
relatively little to the overall variability.
Accuracy is undefined because the
NMOC measurements encompass an
unspecified mixture of various organic
compounds. Accuracy relative to internal
propane standards was assessed with
audit samples, prepared by diluting NBS-
traceable propane standards with zero air
into clean canisters at a pressure similar
to the pressure used for ambient samples.
The overall regression slope and intercept
for 73 audit samples were 1.0296 ±
0.0154 and 0.0210 ± 0.0277 (95%
confidence interval), respectively, indica-
ting about a 3% positive bias and a small
fixed offset.
Accuracy relative to GC speciation
analysis was assessed by comparing the
results from 336 samples that were
independently reanalyzed by GC specia-
tion (sum of species) analysis. A linear,
orthogonal regression of these data is
illustrated in Figure 2. The resulting slope
of 1.081 and intercept of 0.015 indicated
a modest bias of approximately +8% for
the PDFID method compared to GC speci-
ation analysis.
Accuracy relative to an independently
operated PDFID analysis system was
assessed by comparing the results from
120 samples that were reanalyzed by the
independent system. An orthogonal linear
regression resulted in a slope of 1.032
Table 2.
Precision
Analytical Precision
Overall Precision
Number of differences
flange of differences
Mean difference
Std. deviation of differences
Range of percent differences
Mean percent difference
Std deviation of % differences
28
-0.14 to+0.27 ppmC
+0.025 ppmC
0.107 ppmC
-38.3% to +30.7%
0.2%
12.7%
58
-0.41 to+0.22ppmC
-0.026 ppmC
0.119 ppmC
-67.8% to 47.8%
-3.1%
17.4%
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United States
Environmental Protection
Agency
Official Business
Penalty for Private Use $300
EPA/600/S4-85/063
Center for Environmental Research
Information
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