United States
                   Environmental Protection
 Environmental Monitoring Systems
 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).
  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

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
Pressure Gauge
        ! Sample
                                                                   \*^\\ Beads
                                                            Sample Trap
                                                           (Liquid Argon)

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.

  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

Zero Drift
Span Drift
+ 1.47
                             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.
                               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
                    -0.14 to+0.27 ppmC
                       +0.025 ppmC
                        0.107 ppmC
                     -38.3% to +30.7%
-0.41 to+0.22ppmC
   -0.026 ppmC
    0.119 ppmC
 -67.8% to 47.8%

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