xvEPA
United States
Environmental Protection
Agency
Environmental Monitoring Syste
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-82-019 July 1982
Project Summary
Laboratory Evaluation of
Non-Methane Organic Carbon
Determination in Ambient Air
by Cryogenic Preconcentration
and Flame lonization Detection
#v
R. K. M. Jayanty and A. Blackard
\
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Results of this study demonstrate
the feasibility of a technique for ana-
lyzing samples of ambient air for gas-
eous nonmethane organic compounds
(NMOC) using a cryogenic trap to
both preconcentrate and separate
NMOC from methane (CH4). The
NMOC is subsequently measured by
warming the trap to release the
NMOC and channeling the concen-
trated sample of NMOC into a modi-
fied commercial flame ionization
detector. The system response per
carbon atom is linear and uniform for a
large group of hydrocarbons analyzed
singly or in mixtures. Analyses of aro-
matic hydrocarbons indicate a reduced
per-carbon response that varies with
each compound but is linear with con-
centration. Precision is within ±5 per-
cent for standard gas calibrations
and generally within ±10 percent for
ambient samples. Accuracy for ambi-
ent air samples has been determined
to be ±15 percent by comparison with
compound-specific GC analysis. Experi-
mental results also show no signifi-
cant effect from humidity over a wide
range of concentrations (75 to 5,000
ppbC). The analytical method is sim-
ple, rapid, and cost effective, and the
NMOC measurements can be used to
establish a basis for control of hydro-
cP
W^ C O ^L
te carfiarvemissterts irHttver to meet oxi-
"* danfjjrrtgria levels. \\.
,. This Pfoject Summny was devel-
oped by ^PA 's Envintnmenta^Moni-
toring Sy&ems Laboratory, Research
Triangle Park, NC, to announce key
findings of the research project that is
fundocumented in a separate report
ofThe same title (see Project Report
ordering information at back).
Introduction
Ambient nonmethane hydrocarbons
and nitrogen oxides (NO*) are primary
precursors of ozone (63) and other oxi-
dants, which are key constituents of
photochemical smog. Current strate-
gies for controlling photochemical oxi-
dants depend on abatement of non-
methane organic carbon (NMOC) as the
primary means of control. A variety of
photochemical models have been devel-
oped to describe the quantitative rela-
tionships between ambient concentra-
tions of precursor organic compounds
and subsequent downwind concentra-
tions of ozone.1 An important applica-
tion of such models is to determine the
degree of control of organic compounds
that is necessary in a particular area to
achieve compliance with applicable
ambient air quality standards for
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ozone. ' Simple empirical models such
as the Empirical Kinetic Modeling Ap-
proach (EKMA) require total NMOC con-
centration data, specifically the aver-
age total NMOC concentrations for 6a.m.
to 9 a.m. daily.2
For many EKMA applications, NMOC
measurements are required at urban,
center-city-type sites.2 The moderately
high NMOC concentrations typically
found at such urban sites can be mea-
sured adequately by commercially avail-
able continuous (or semicontinuous)
NMOC analyzers. However, if transport
of precursors into an area is to be con-
sidered, then NMOC measurements up-
wind of the area also are necessary.2
Upwind NMOC concentrations are like-
ly to be very low (less than a few tenths
of 1 ppm) and, therefore, may not be
measured adequately by conventional
NMOC analyzers. GC measurements of
individual NMOC species can be used
by summing the various components
to obtain a total NMOC. But for EKMA,
the species data are not needed, and
species analysis cost is high.
The method described herein can be
used instead to obtain the requisite,
upwind NMOC measurements by direct
measurements. Also, bag or canister
samples from urban sites can be
brought to the upwind site for subse-
quent analysis. The higher concentra-
tions at the urban sites tend to minimize
the effect of losses or contamination of
the bag or canister samples, while the
low, upwind concentrations are mea-
sured directly. Thus, all measurements
can be made with a single analytical
system.
Analytical System
The analytical system consists of a
trapping system to preconcentrate the
sample and a total organic carbon (TOC)
analyzer. The sample trapping system
serves two primary purposes: (1) it
separates methane and air from the
Office of Air Quality Planning and Standards
Uses, Limitations and Technical Basis of Proce-
dures for Quantifying Relationships Between Phot-
ochemical Oxidants and Precursors EPA-450/
2-77-021 a, U S Environmental Protection Agency,
Research Triangle Park, North Carolina, November
1977 120pp
2 Office of Air Quality Planning and Standards Guid-
dance for Collection of Ambient Air Nonmethane
Organic Compound (NMOC) Data for Use in 1982
Ozone SIP Development, and Network Design and
Siting Criteria for the NMOC and NO, Monitors.
EPA-450/4-80-011, U S. Environmental Protec-
tion Agency, Research Triangle Park, North Carolina,
June 1980. 27pp
NMOC sample, and (2) it concentrates
the NMOC, which enhances the method's
sensitivity. Figure 1 shows a schematic
of the system. The design of the cryogeni-
cally cooled, open tubular sample trap
and the use of either liquid argon or
liquid oxygen as a cryogen are critical
points for this technique.
A detailed description of the operating
procedures used for this system is pre-
sented in the project report. Briefly, a pre-
cisely measured volume of sample air is
drawn through the analysis system by
monitoring the pressure rise in an
evacuated reservoir. As this air sample
is drawn through the cryogenically
cooled, open tubular trap, the NMOC are
concentrated on the trap's inner surface
either by adsorption or condensation.
Absolute Pressure
Gauge
After the desired volume of ambient
sample air has passed through the trap,
a helium carrier gas is directed through
the trap and into the TOC analyzer.
When the trap is purged of residual
methane and oxygen and when the
baseline becomes steady (usually taking
2 to 5 minutes), the Dewar of cryogen is
removed fro/n the trap, and the trap is
heated to release the NMOC as a single
peak (or a few peaks) as seen by the TOC
analyzer.
A variety of sample collecting tech-
niques may be used, including clean
evacuated canisters, Tedlar® bags, or
Teflon® bags. Ambient air or calibra-
tion standards may also be sampled
directly from a sample manifold.
Two-Stage
Regulator
• Shut-off Valve
Needle
Valve
•\
V3
Single-stage
Regulator
Beckman 400
Pressurized Sample
Canister
Figure 1. Schematic of analysis system showing three sampling modes.
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Results and Discussion
Two prototype NMOC analysis sys-
tems were evaluated. The first prototype
was delivered to the Research Triangle
Institute (RTI) by the U.S. Environmental
Protection Agency (EPA) after construc-
tion and some preliminary evaluation by
J. M. McBride and Dr. W. A. McClenny
of EPA; the second was constructed by
RTI in an effort to produce a portable
unit and to incorporate some minor
changes to improve cryogenic trapping.
The system response to known mixtures
of hydrocarbons in air is summarized
in Figure 2. FID response peaks are inte-
grated and plotted for various loadings,
in nanograms carbon. Analysis of 9.56
ppmV methane in air gave no response.
All parafinic and olefinic hydrocarbons
show approximately equal and linear
response per carbon atom. The per-
carbon responses for toluene, benzene,
and ethylbenzene are approximately 19
percent lower and the responses for
xylenes are 50 percent lower than those
for the parafinic and olefinic hydrocar-
bons. To identify the cause of reduced
system response to aromatics, several
experiments were performed. In the
first, two traps (100 cm long) were
placed in series. When p-xylene was
sampled, no p-xylene was collected in
the second trap, implying a 100-percent
trapping efficiency in the first trap. To
determine the flame response for p-
xylene, an experiment was performed
by direct injection of p-xylene and pro-
pane (for reference) into the flame. The
results for two different concentrations
(704 and 1,030 ppbC) showed the
response of p-xylene to be 30 percent
lower than the propane response in
both cases. The 30-percent lower
response of p-xylene is apparently due
to nonuniform FID response. The
remaining 20 percent difference between
p-xylene and the propane responses is
unknown but could be due to losses in
sampling lines. The same is probably
true for low response of benzene, ethyl-
benzene, and toluene. Further work is
needed to substantiate these specula-
tions.
Several operational parameters were
varied to determine system sensitivity.
The humidity of the sample air varied
from ~15,000 to 30,000 ppm at approxi-
mately 23°C and caused negligible
response variation (±5 percent) in trial
runs using propane in air, provided that
the trap and lines were not contami-
nated. When contamination was pre-
sent, unusually broad peaks and long
tailing were observed. The effect of vari-
10.000
9000
8000
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Table 1. Comparison of NMOC Concentration in Ambient Samples Collected in Stainless Steel Canisters and Analyzed by
NMOC Method and Gas Chromatography
GC Analysis NMOC RSD*
(as NMOC) Analysis NMOC NMOC-GC
Sample No.
Date
(ppbCJ
IppbC)
(ppbC)
Percent Error]
1 1/9/81
2 1/12/81
3 1/13/81
4 1/14/81
5 1/14/81
6 1/16/81
7 1/16/81
8 1/20/81
9 1/20/81
10 1/20/81
11 1/20/81
12 1/27/81
846
380
945
120
129
120
390
769
533
328
223
549
443
929
500
984
175
188
—
437
352
469
365
252
469
422
11
12
2
6
10
3
6
1
2
9
12
11
+ S3
+ 120
+ 39
+ 55
+ 59
+ 68
+213
- 64
+ 37
+ 29
- 80
- 21
Mean + 45
+ 9.8
+31,6
+ 4.1
+45.8
+45.7
+56.7
+ 12.0
+27.7
-12.0
+ 11.2
+ 13.0
-14.6
- 4.7
Mean +17.4
* Relative Standard Deviation of NMOC Results (RSD) -
percent Error ^ MMOC result - GC result,
GC result
Deviation
'ean
•x WO.
R. K. M. Jayanty and A. Blackard are with the Research Triangle Institute.
Research Triangle Park. NC 27709.
Frank F. McElroy and William A. McClenny are the EPA Project Officers (see
below).
The complete report, entitled "Laboratory Evaluation of Non-Methane Organic
Carbon Determination in Ambient Air by Cryogenic Preconcentration and
Flame lonization Detection," (Order No. PB 82 -224 965; Cost: $ 10.50, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officers can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
0000329
t
b LldRAKY
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AGEMCV
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