Environ. Sci. Technol. 1982, 16, 57-61
Determirqtion of Low Levels of Total Nonmethane Hydrocarbon Content in Ambient Air Robert D. Cox," Maureen A. McDevitt, Kenneth W. Lee, and Gary K. Tannahlll Radian Corporation, Austin, Texas 78758
A simple, yet highly efficient method has been devel-
oped for the determination of low levels of total nonmethane hydrocarbons (NMHCs) in ambient air. The method consists of cryogenically concentrating nonmethane hydrocarbon species and thermally desorbing these directly into a flame ionization detector, without the use of a chromatographic column. This method provides an accurate, cost-effective means of acquiring NMHC data for at.mospheric modeling studies. Although the method requires manual operation, it provides data which cannot accurately be obtained with automated techniques. The method was used to study NMHC levels upwind of Nashville, TN. Critical procedures for the collection of ambient air samples are also discussed. Hydrocarbon levels in the atmosphere are of considerable interest since these are important precursors of atmospheric oxidants (primarily ozone). Regulatory policies are attempting to control atmospheric oxidants by decreasing emissions of oxidant precursors, particularly hydrocarbons and oxides of nitrogen. To comply with these policies, regulatory agencies and industries are faced with the task of monitoring hydrocarbons and other oxidant precursors. To aid in the control of atmospheric oxidants, a number of photochemical simulation models have been developed which attempt to quantify relationships between photochemical oxidants and their precursors. These models require a variety of data inputs for hydrocarbons ( I , 2 ) ,ranging from complete species identification to only total nonmethane hydrocarbon data. In addition, most models for urban areas allow corrections for upwind ozone precursors, usually in the form of a total nonmethane hydrocarbon (NMHC) value. Although a great deal of interest exists in atmospheric hydrocarbon levels, the absence of reliable analytical procedures for the measurement of atmospheric hydrocarbons has impeded investigations in this area by limiting the quality of data which are available for photochemical simulation models. Since methane does not participate in atmospheric oxidant formation, it is the nonmethane hydrocarbon fraction which is of particular interest. Atmospheric hydrocarbon levels are normally measured with continuous methane/total hydrocarabon analyzers. Although these instruments generally provide reliable data at high nonmethane hydrocarbon levels (many urban areas), they do not provide reliable data for the measurement of nonmethane hydrocarbon concentrations less than 1 ppmv C (ppm (by volume) C) (I,3-5). Nonmethane hydrocarbon data may also be acquired by collection of integrated (1-3-h) air samples which are analyzed chromatographically. In this case individual hydrocarbon species are determined over two or three chromatographic runs and summed if a NMHC value is required. Although this technique is much more reliable than continuous methane/total hydrocarabon analyzers and provides a large amount of information for photochemical dispersion models, it is still unsuitable in other cases. In instances where only NMHC data are required, hydrocarbon speciation and summation is an extremely 0013-936X/82/0916-0057$01.25/0
time-consuming and expensive procedure for obtaining this type of data. A modified chromatographic method has been developed which provides a means for rapid and accurate analyses of ambient air for total nonmethane hydrocarbon (NMHC) content. The method is based on cryogenically trapping NMHC species with liquid oxygen and thermally desorbing these directly into a flame ionization detector. Omission of a chromatographic column reduces the amount of data which can be obtained but still provieds accurate NMHC data, The primary advantage of this technique is a significant decrease in analysis time when compared to obtaining the same data from summation of multiple chromatographic runs. In addition, data obtained by this method are much more accurate than those obtained with continuous methane/nonmethane analyzers. Maintaining sample integrity while collecting integrated ambient air samples is a difficult task. Since the analytical method described in this paper requires procedures for sample collection, some discussion of sampling procedures will also be presented. Several parameters have been found to be critical for maintaining sample integrity when collecting integrated samples. Experimental Section A schematic diagram of the analytical system is presented in Figure 1. The system basically consists of a cryogenic trapping apparatus, a gas chromatograph equipped with an FID, and an electronic integrator. All tubing used was l/s- in. 0.d. stainless steel with the exception of the transfer line from the eight-port valve to the FID which was 1/16-in.0.d. stainless steel. Nonmethane hydrocarbon species were trapped on 4 in. of 80/100 mesh glass beads packed in l/s-in. 0.d. stainless-steel tubing and cooled with liquid oxygen. Gas flow through the trap was controlled with an eight-port stainless steel valve (Valco No. V-8-HTa). Several three-way valves were used to select input of samples, standards, or nitrogen purge gas to the system. All flow through the system was effected by means of positive pressure of the external gas cylinders or sample containers. The volume of gas passed through the trap was measured by collecting it in a fixed-volume reservoir and monitoring the pressure of this with a high-precision gauge (Airco No. 60010). Normally, a 2.8-L reservoir was used for collection of sample volumes in the range of 0.1-2.0 L. The uncertainty involved in measuring the sample volume in this manner is less than 0.01 L. The eight-port valve and the cryogenic trap were mounted inside a chromatograph oven to allow rapid thermal cleaning while purging the system. Also, this minimized the amount of tubing between the valve and the detector. The chromatograph used was a Tracor 560A equipped with a flame ionization detector. The signal from the chromatograph was processed with a Hewlett-Packard HP-3388 chromatographic data system. Liquid oxygen (bp -183 OC) was used as the cryogenic coolant since liquid nitrogen (bp -210 "C) will also condense oxygen in the air sample, which will cause disturbances when rapidly injected to the FID detector. Liquid
@ 1981 American Chemical Soclety
Environ. Sci. Technol., Vol. 16, No. 1, 1982 57
A
GC
ULTRAPURE? N2
1
1
B
C
SAMPLE VENT
OVEN
VALCO
”
1
COLLECTION RESERVOIR
I
1 1
-
U
Figure 2. Typical responses to hydrocarbon-containingsamples: (A) propane, 0.28 ppmv C; (8)C2-CI2 n-alkane mixture; (C)ambient sample collected upwind of Nashville, TN, 0.15 ppmv C; (D) amblent sample collected upwind of Louisville, KY, 0.42 ppmv C. (Numbers above peaks correspond to retention times in minutes). I /L ULTRAPURE N2
Table I. Calibration Slopes for Hydrocarbon Species compd methane ethane propane c2-C6
concn, ppmvC
slope, area/L
3.20 0.188 0.282 0.529
90 1370 2040 3670
ECNCa 0.004
1.006 1.000 0.960
Effective carbon number contribution using propane as reference. SAMPLELOAD
SAMPLE ANALYZE
Figure 1. Schematic diagram of the concentrationand analysls system.
argon (bp -186 “C) can be substituted for liquid oxygen but is considerably more expensive. However, if this method is to be used in field studies, safety factors may require the use of liquid argon. Before sample analysis the cryogenic trap was cooled for 2 min with liquid oxygen. During this time the sample inlet valve was opened and the sample lines were purged and vented through position 7 of the eight-port valve. Following the cooling period, the eight-port valve was switched to the “sample load” position and the desired volume of gas was passed through the cryogenic trap and into the sample reservoir. When the final pressure had been reached in the reservoir, the sample inlet valve was closed and the pressure recorded. The pressure was then released from the reservoir before switching the carrier flow through the trap (eight-port valve to “analyze” position). Releasing the reservoir (and trap) pressure in this manner alleviated problems due to excess pressure surges to the FID and minimized detector disturbances during valve switching. The flame was often extinguished when pressures greater than 3 psig were switched directly to the detector. Once the appropriate sample volume was passed through the trap and excess pressure relieved, the eightport valve was switched to the “analyze” position, routing the carrier gas through the cryogenictrap and into the FID. After a 1-min delay period the liquid oxygen was removed and replaced with boiling water. The purpose of the 1-min delay was to allow the FID time to stabilize following the valve switch and to remove any methane which remained on the trap. Results Typical responses to hydrocarbon standards and samples are presented in Figure 2. The FID signal was monitored from the time when the valve was switched to the “analyze” position to allow observation of any gross disturbances to the detector. However, integration was 58
Environ. Sci. Technol., Vol. 16, No. 1, 1982
not initiated until after the 1-min delay period. A small noise spike was observed when the liquid oxygen was removed from the trap. This appeared in all samples and blanks and was not included in the integration. The valving of the trapping system was set up in the “forward-flush” instead of a “back-flush” mode to allow sufficient separation between this spike and the hydrocarbon peaks. Single hydrocarbon species eluted from the trap as a sharp spike (Figure 2A) while a C2-CI2mixture (Figure 2B) eluted from the trap as a broader (30-s)peak. Ambient samples (Figure 2, C and D) usually produced several distinct peaks. Areas for these peaks were summed by the integration system. Overall analysis times ranged from 10 to 15 min. The short analysis times allowed preparation of multipoint calibration curves for quantitative analysis of samples. Calibrations were prepared by analyzing varying volumes of a 0.1 ppmv C propane standard, and quantitation was based on electronic integration of peak areas. Although peak height responses were reproducible for single hydrocarbon species, ambient samples generally produced multiple peaks which were not reproducible on the basis of peak height. Responses of the analytical system to different carbon compounds was evaluated with standards of methane, ethane, propane, and a mixture of C& n-alkanes. The system was calibrated with each of these by passing volumes from 50 to 1000 mL of respective gas standards through the trap. Calibrations for these compounds are shown in Figure 3. Linear calibrations were obtained for up to 1 L of propane or the Cz-C6 n-alkane mixture. A linear calibration was obtained for ethane at low analysis volumes, but slight losses of this compound were observed when volumes above 400 mL were used. This was probably not observed in the alkane mixture since ethane comprised only 10% of the carbon content in this case. Methane gave no response at volumes below 700 mL and only a slight response at 1000 mL. The slopes of the hydrocarbon calibrations shown in Figure 3 are presented in Table I. Also listed are the
3000
2500
; i 2000 C
a m 'y
4,
1500
Ly
c
g
=r
1000
500
0
200
.
400
CH4
A BO0
800
io00
Volume Sampled (mL)
Figure 3. Responses to various n-alkanes: ethane at 0.188 ppmv C; propane at 0.282 ppmv C; C& n-alkane midure at 0.529 ppmv C; methane (A)at 3.20 ppmv C.
effective carbon number contributions (ECNCs) for each gas standard. These were calculated by using the following equation: ECNC = R,C,/(R,CJ where R, is the response (slope) to the standard, which is propane in this case, R, is the response to the sample compound, and C, and C, are carbon concentrations (ppmv C) of standard and sample, respectively. An equivalent FID response to all carbon compounds will produce ECNCs of 1. With this technique ECNC values for the nonmethane hydrocarbons agreed to within 4% for the Cz-cS hydrocarbon mixture. The c1-cG normal hydrocarbons were chosen for this study to evaluate the efficiency of trapping and desorption and the FID response to these compounds. Although the hydrocarbon content of ambient air consists primarily of aliphatic hydrocarbons, aromatic hydrocarbons such as benzene, toluene, xylenes, and terpenes are also present. These would be expected to give a somewhat different ECNC. However, a single response factor was used for all organic species, as is done with continuous nonmethane analyzers. Ethane was used to evaluate trapping efficiencies for the Cz hydrocarbons, which are the most volatile species of interest. The boiling point of ethane (bp -89 "C) is between that of ethylene (bp -104 "C) and acetylene (bp -84 "C). Therefore, these species were expected to demonstrate retentions similar to that of ethane. The minimum detectable quantity (MDQ) of nonmethane hydrocarbon was 4 X 10-lomol of C, which corresponds to approximately 6 ng of propane. When a 500-mL sample was used, the detection limit was 0.02 ppmv C (pL of C/L) or 0.007 ppmv propane. The MDQ and the detection limit are higher than those generally attainable by GC/FID. This is primarily due to filtering and base-line rejection functions performed by the integrator. In this case the MDQ is not defined as a signalto-noise ratio but as a minimum integrated area (50 counts). No efforts were made to decrease the MDQ since all samples encountered thus far have fallen within acceptable working ranges on the method. Highly reproducible and linear results were obtained with this method. Multipoint calibrations generally yielded correlation coefficients (CCs) greater than 0.99. The average CC for
56 calibrations over an 8-week period was 0.997. Precision for the analysis of standard gas mixtures (c,-c6) was generally within 3% (coefficient of variation (CV)) in the 0.1-1.0 ppmv C range. Precision at the detection limit (0.02 ppmv C) was approximately 15%. Ambient samples in the concentration range of 0.05-1.0 ppmv C were mu& reproducible to within 10% (CV). Experiments were performed to determine the effect of sample flow rate through the cryogenic trap on hydrocarbon trapping efficiency. The effect of sample flow rate was evaluated by passing constant volumes of ethane and propane standards through the trap at flow rates from 25 to 200 mL/min. In this range, different flow rates appeared to have no effect on trapping efficiencies for these hydrocarbons. It was also found that the moisture content of the sample did not have any adverse effects on hydrocarbon trapping efficiency. Excessive moisture in the sample could possibly cause clogging of the cryogenic trap, increased trapping of methane or decreased trapping of other hydrocarbons, and suppression of FID response. Analysis of standard gas mixtures of propane with artificially prepared humidities ranging from 20% to 90% demonstrated no discernible effect due to moisture. Methane was included in a gas mixture which was prepared at 60% humidity, and the response obtained was equivalent to that for only propane, indicating that water did not cause increased retention of methane. No significant re uction in flow rate through the cryogenic trap was observe when samples containing high moisture content were analyzed, demonstrating that clogging was not a problem.
%
Sampling Procedures The analytical method presented here requires that air samples be collected in the field and transported to a laboratory for analysis. For ambient air sampling, 1-3-h integrated samples are normally collected in Tedlar bags. Recent studies have shown that this material, poly(viny1 fluoride) (PVF), can contribute significant amounts of hydrocarbon contamination to collected air samples, presumably due to residues of solvents used in the extrusion process (6,7). Teflon, a fluorinated ethylenepropylene copolymer (FEP), was also evaluated as a material for use in collection of air samples, and similar problems were shown with this material (8). The sampling procedure used in this work involves modifications of a previously recommended procedure (9). Integrated air samples were collected in Tedlar bags and immediately transferred to precleaned stainless-steel canisters. This procedure minimized the amount of contact time between air samples and Tedlar surfaces and provided the pressurized samples required for analysis. Since the stainless-steel canisters could be heated, they were much easier to clean than the Tedlar bags. Also, samples can be transported in the canisters; whereas this is not possible with Tedlar bags. Even for a 3-h contact time between the samples and Tedlar bags, significant amounts of hydrocarbon contamination by the bags were observed. Bags were tested by filling them with UHP air (
