Technique for Monitoring Toxic VOCs in Air: Sorbent Preconcentration

Environ. Sci. Technol. 1996, 30, 1939-1945

Technique for Monitoring Toxic VOCs in Air: Sorbent Preconcentration, Closed-Cycle Cooler Cryofocusing, and GC/MS Analysis KAREN D. OLIVER,* JEFFREY R. ADAMS, AND E. HUNTER DAUGHTREY, JR. ManTech Environmental Technology, Inc., P.O. Box 12313, Research Triangle Park, North Carolina 27709

WILLIAM A. MCCLENNY U.S. Environmental Protection Agency, National Exposure Research Laboratory, Air Measurements Research Division, Methods Branch, Research Triangle Park, North Carolina 27711

MATTHIAS J. YOONG AND MICHAEL A. PARDEE XonTech, Inc., 7027 Hayvenhurst Avenue, Van Nuys, California 91406

ELIZABETH B. ALMASI AND NORMAN A. KIRSHEN Varian Chromatography Systems, 2700 Mitchell Drive, Walnut Creek, California 94598

An automated gas chromatographic (autoGC) system was used to collect and analyze both nonpolar and polar volatile organic compounds (VOCs) in ambient air. This system combines the use of dual multiadsorbent traps to provide continuous air sampling for 57 min of each hour; a dry helium purge to remove extraneous gases, including some residual water vapor retained in the sorbent packing; thermal desorption of analytes onto a VOC-focusing trap cooled by a small Stirling-cycle refrigerator; and GC/mass spectrometric detection using ion trap technology. Cleanliness, linearity, method detection limits (MDLs), precision, and accuracy of the autoGC were determined for 41 VOCs. For most of the compounds tested, MDLs were less than 0.10 ppbv, response was linear over the 1-40 ppbv range, accuracy was (20%, and trap-to-trap precision was (20%. Linear response for a set of polar VOCs was also determined over the 5-50 ppbv range. The autoGC was successfully operated in a mobile laboratory at a field site in Axis, AL, for 10 days, during which time the system was in operation 24 h/day with minimal interruptions. This autoGC is designed for monitoring subsets of the 97

S0013-936X(95)00631-6 CCC: $12.00

 1996 American Chemical Society

VOCs among the 189 hazardous organic compounds that are listed in Title III of the Clean Air Act Amendments of 1990.

Introduction The U.S. Environmental Protection Agency (EPA) is developing techniques to monitor hazardous pollutants to determine their prevalence and trends in concentration. A subset of the 189 species in Title III of the Clean Air Act Amendments (CAAA) of 1990 (1) includes 97 volatile organic compounds (VOCs) (2). In the past, the monitoring of toxic VOCs in air has been performed by various methods, a number of which have been written as guidance by the U.S. EPA in a compendium of methods (3). For VOCs, the guidance is to use either solid adsorbents, canisters, or specially treated filters for sampling. Concentration of gases followed by separation on a high-resolution capillary column is the most generally applicable analysis approach. Some of the more recent research on methods and techniques for monitoring has been with automated gas chromatographs (GCs) using hourly sampling and analysis on a continuous basis with almost complete (up to 57 min in 1 h) time coverage (4). The analytical approach is similar to that proposed in Compendium Method TO-14 entitled The Determination of Volatile Organic Compounds (VOCs) in Ambient Air Using SUMMATM Passivated Canister Sampling and Gas Chromatographic Analysis, but it uses direct sampling from air instead of from a canister. Such a monitoring approach may eventually find use in monitoring stations as part of a national network to address risk assessments for hazardous VOCs. This network might be similar to the automated GC monitoring network developed for ozone precursor hydrocarbons under Title I of the CAAA (5). One deficiency with the TO-14 method is that it does not work well when monitoring water-soluble VOCs because of the type of water management system used (permeable membrane dryer). This paper documents the successful application of an automated GC approach that is similar to the TO-14 approach but alters the water management system so that polar compounds can also be detected. The approach combines the use of a small sample volume and a trap filled with hydrophobic multisorbents for concentration during sample collection. Water vapor breaks through the multisorbent trap during sampling to a degree depending on the specific multisorbent combination; after sampling, a dry helium purge of the multisorbent trap is used to remove extraneous gases, including some additional water vapor. A cryotrap cooled by a small Stirling-cycle refrigerator is used to focus the sample prior to GC/mass spectrometric (MS) analysis using above-ambient oven temperature programming of the GC column. The sensitivity of the GC/MS ion trap system (MDLs of 0.01-0.03 ppbv for 60-cm3 samples of VOCs in air) (6) compensates for the small sample volumes. Commercially available autoGC systems for monitoring VOCs incorporate different but competitive water management systems. [Commercial systems include Entech * To whom all correspondence should be addressed; fax: 919541-3566; telephone: 919-541-2337.

VOL. 30, NO. 6, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Schematic diagram of XonTech 930/940 preconcentrator and Varian Saturn II GC/MS. MFC, mass flow controller; SV, solenoid valves that facilitate the flow of gases through the sorbent traps. CV-1 and CV-2 are GC valves that are used (1) to route the flow of helium and sample to the appropriate traps and (2) to transfer the concentrated VOCs in a helium stream from the sorbent trap to the cryotrap during desorption. CV-3 is a GC valve that is used to route the concentrated VOCs in helium (1) to the cryotrap for focusing at -165 °C and (2) to the GC after desorption at 100 °C. (The use of a six-port valve allows the Model 940 to be adapted to other applications in which a different GC carrier gas flow rate is required.)

Instruments, Inc., Simi Valley, CA (Microscale Purge and Trap); Tekmar Company, Cincinnati, OH (Moisture Control Module); Graseby/NuTech, Smyrna, GA (Controlled Desorption Trap); Dynatherm Analytical Instruments, Inc., Kelton, PA (Thermal Desorption Unit); Varian Chromatography Systems, Walnut Creek, CA (Variable Temp. Adsorb. Trap); and others.] One type of design allows an initial co-collection of VOCs with water vapor on a cooled surface. This is followed by temperature programming of the surface to increase the vapor pressure of VOCs above the water. The VOCs are entrained in a purging stream of neutral carrier gas while most of the water remains on the trap. The VOCs are then trapped again for subsequent thermal desorption and analysis. Pankow has explained the basis for this approach in a paper on the analysis of VOCs in water, soil, and sediment samples (7). Designs based on the separation of VOCs from water condensed from the air include those of several commercial companies (e.g., the first three listed above). Other commercially available autoGC systems have been designed to use a serial combination of two uncooled multisorbent traps (8) or have simply concentrated a small sample volume and used a sensitive detector (e.g., the last two listed above). The water management approach taken here is a combination of multisorbent breakthrough and dry purge on a primary trap, followed by reduced temperature focusing on a secondary trap, and the use of a small sample volume. Our objective in the paper is to demonstrate a system design that uses this water management approach, operates without the use of consumable liquid cryogen, and provides a high level of specificity in compound identification by using MS analysis. An example of the effectiveness of the system in the field is also provided.

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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 6, 1996

Experimental Method As shown in the schematic diagram in Figure 1, a commercially available XonTech Model 930 organic vapor concentrator (XonTech, Inc., Van Nuys, CA) equipped with two multisorbent traps was interfaced to a XonTech Model 940 cryofocusing trap, which is also commercially available. All tubing and transfer lines were 1/16-in. fused-silica-lined stainless steel. The Model 930 two-trap system allowed the collection and dry purging of sample on one trap while the other trap was simultaneously cycled through the desorb, clean, and cool modes. The multisorbent traps (10 × 1/8 in., stainless steel) contained 0.05 g of Tenax-GR, 0.04 g of Carbotrap, and 0.51 g of Carbosieve S-III (9). The Model 940 cryotrap focused the VOCs on its trap after they were desorbed from the Model 930 sorbent trap. The VOCs were then desorbed onto the GC column by heating the Model 940 trap. This trap consists of a length of smalldiameter, thin-walled, stainless steel tubing (10-in. length, 0.042-in. outside diameter, 0.035-in. wall thickness). This tubing was wrapped around a copper coil form that was connected to the expander of a Stirling cooler (Hughes, Electron Dynamics Division, Torrance, CA). An electric heater, wrapped around the tubing/copper coil form assembly, was used to heat the trap and desorb the collected VOCs onto the GC column. During the evaluation, the Model 940 cryotrap was interfaced to a Saturn II GC/MS (Varian Chromatography Systems, Walnut Creek, CA), which utilizes ion trap mass analyzer technology. A description of the operation of the concentrator, cryotrap, and GC/MS system follows. The Model 930 concentrator was programmed to collect whole air samples for 57 min of each hour at a flow rate of 5 cm3/min through the sorbent trap for a total sample volume of 285 cm3. The Model 930 was designed for direct

FIGURE 2. Cycle timing for XonTech/Saturn automated GC system.

analysis of ambient air. In our evaluation, air samples in canisters were introduced to the Model 930 by connecting a 0-10 standard cm3 mass flow controller (Tylan, Torrance, CA) to the canister and setting the flow at 8 cm3/min. A tee was placed between the canister/mass flow controller and Model 930 inlet, and this allowed 5 cm3/min of sample to be pulled into the Model 930 by the sample pump while 3 cm3/min excess sample flow was vented. This mass flow controller and vent tee configuration provided for ambient pressure sample collection from pressurized canisters. After the 57-min sample collection, the trap was purged with 20 cm3/min of dry helium for 3 min to reduce residual water vapor. The trap was then heated from ambient temperature to 230 °C , and the analytes were desorbed from the trap for 5 min with a reverse helium flow. The trap was then cleaned by heating it to 235 °C for 15 min while purging it with helium. During the remaining 40 min, the trap was allowed to cool to ambient temperature. This cycle was repeated on the two traps, as shown in the timing diagram in Figure 2, to provide continual air sampling for 57 min of each hour. The Model 940 Stirling cooler was operated without actuation of its six-port GC valve. In this mode of operation, as shown in Figure 1, a 5 cm3/min flow of carrier gas from the GC was directed to the Model 930 six-port GC valve and subsequently to the Model 940 and GC column. The 5 cm3/min flow of helium continually passed through the cryofocusing trap in this configuration. The Model 940 system was programmed to begin cooling the cryofocusing trap at 40 min into the sampling cycle of the Model 930. The trap was cooled to -165 °C for collection of VOCs. As the VOCs were desorbed from the Model 930 sorbent trap, they were collected for 5 min on the Model 940 cryofocusing trap. The analytes were then desorbed onto the GC column by heating the Model 940 trap to 100 °C at an average rate of 10 °C/s. The GC/MS system used a DB-1 capillary column (60 m × 0.32 mm × 1.0 µm, J&W Scientific, Folsom, CA) for separation of analytes, and the carrier flow was split 4:1 at the head of the column such that 1 cm3 of helium entered

TABLE 1

Saturn II MS Operating Conditionsa scan range (amu) scan rate (s/scan) RF storage level background mass (amu) segment breaks tune factors AGC target emission current (µA)

47-300 0.8 (4 µscan per analytical scan) 210 DAC converter steps 45 70/78/150 120/70/100/70 between 10000 and 20000 15

a Abbreviations used: RF, radio frequency; DAC, digital-to-analog converter; AGC, automatic gain control.

the column and 4 cm3 was vented. This 4:1 split reduced the sample volume that was routed to the detector from 285 to 57 cm3. The GC oven temperature was programmed as follows: 35 °C for 5 min, a 6 °C/min ramp to 210 °C, and a 0.84-min hold at 210 °C, for a total analysis time of 35 min. The MS operating parameters are listed in Table 1, and the quantitation ions for the target compounds are included in Table 2. A representative chromatogram of the VOC mixture at 2.5 ppbv is shown in Figure 3. Test gas mixtures for the evaluation were prepared by dynamic dilution of low-concentration (ppmv level) gaseous standards in high-pressure cylinders in humidified scientificgrade air (National Specialty Gases, Research Triangle Park, NC) at ∼70% relative humidity (10). SUMMA-polished stainless steel canisters (SIS, Inc., Moscow, ID) were then pressurized to 30 psig with the dilute gas mixtures for use in the experiments discussed below. The relative humidity of the gaseous standards in the canisters varied from 35 to 65%, depending on the canister pressure at the time the sample was extracted. The gas standards included a mixture of the 41 VOCs on the EPA Method TO-14 (3) target list at nominal concentrations of 1-2 ppmv each in nitrogen (Alphagaz, Walnut Creek, CA) diluted to 0.5, 5, 10, 20, and 40 ppbv and a mixture of 10 polar compounds in nitrogen from two cylinders at nominal concentrations of 10 ppmv each (Scott Specialty Gases, Plumsteadville, PA) diluted to 5, 10, 25, and 50 ppbv. The polar compounds were


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TABLE 2

XonTech/Saturn AutoGC System Method Detection Limits (ppbv) and Precision for VOCs no.

compound

quantitation ion (amu)

detection limita (ppbv)

precisionb (% RSD)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

dichlorodifluoromethane chloromethane 1,2-dichloro-1,1,2,2-tetrafluoroethane chloroethene bromomethane chloroethane trichlorofluoromethane 1,1-dichloroethene dichloromethane 3-chloropropene 1,1,2-trichloro-1,2,2-trifluoroethane 1,1-dichloroethane cis-1,2-dichloroethene trichloromethane 1,2-dichloroethane 1,1,1-trichloroethane benzene carbon tetrachloride 1,2-dichloropropane trichloroethene cis-1,3-dichloropropene trans-1,3-dichloropropene 1,1,2-trichloroethane toluene 1,2-dibromoethane tetrachloroethene chlorobenzene ethylbenzene m,p-xylene styrene 1,1,2,2-tetrachloroethane o-xylene 4-ethyltoluene 1,3,5-trimethylbenzene 1,2,4-trimethylbenzene benzyl chloride m-dichlorobenzene p-dichlorobenzene o-dichlorobenzene 1,2,4-trichlorobenzene hexachlorobutadiene

85 49 85 62 94 49 101 61 49 76 101 63 61 83 62 97 78 117 63 130 75 75 97 91 107 166 112 91 91 104 83 91 105 105 105 91 146 146 146 180 225

0.044 0.53 0.072 0.18 0.093 0.81 0.038 0.18 0.27 0.023 0.058 0.71a 1.40 0.034 0.14 0.11 0.096 0.039 0.17 0.076 0.051 0.30a 0.15 0.038 0.062 0.059 0.058 0.036 0.049 0.10 0.050 0.035 0.029 0.053 0.054 0.27a 0.070 0.019 0.050 0.96 0.069

4.41 4.87 3.86 2.75 5.71 5.21 2.49 2.26 4.00 3.05 2.92 3.39 4.39 2.71 7.40 2.92 4.81 2.50 3.53 4.36 2.71 3.33 3.88 2.58 2.48 3.72 2.68 3.70 2.50 3.27 3.24 2.74 2.46 2.76 2.99 4.29 2.70 2.65 2.79 2.96 2.92

a Detection limit listed is the higher of the two values for trap 1 and trap 2. For compounds 12, 22, and 36, MDLs on the opposite trap were 0.30, 0.11, and 0.10, respectively. b Precision was determined by averaging % RSD values from both traps.

methanol, ethanol, isopropanol, butanol, acetone, methyl ethyl ketone, acetonitrile, acrylonitrile, methyl methacrylate, and ethyl acrylate.

Results and Discussion The evaluation of the system included (1) checks on cleanliness, (2) investigation of the linearity of response of the 41 VOCs and 10 polar VOCs, (3) determination of precision, (4) determination of the method detection limits (MDLs) for the 41 VOCs, and (5) investigation of the accuracy of the method by analysis of an independently prepared audit standard. Additionally, the XonTech/GC/MS system was set up in a mobile laboratory and used to monitor VOCs in ambient air for 10 days in Axis, AL. Cleanliness. Cleanliness of the system was evaluated by analyzing samples of humidified scientific-grade air. With the exception of benzene, which was consistently detected at 0.2 ppbv, background concentrations of the 41 VOCs ranged from undetectable to 0.07 ppbv. Run-to-run carryover of target compounds was investigated by analyzing samples of humidified scientific-grade air immediately following the analysis of a 10-ppbv calibration standard of

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the 41 VOCs. For the humidified scientific grade air analyses, carryover of VOCs was minimal, with trap 1 concentrations ranging from undetected to less than 0.4 ppbv for all compounds except chloromethane, 1,2,4trichlorobenzene, and hexachlorobutadiene. These compounds were detected at concentrations between 0.9 and 1.4 ppbv. Trap 2 concentrations ranged from undetected to less than 0.1 ppbv for all compounds except 1,1dichloroethane, 1,2,4-trichlorobenzene, and hexachlorobutadiene, which were detected at concentrations between 0.2 and 0.4 ppbv. Linearity. Linearity of response of the 41 VOCs was investigated by using the XonTech/Saturn autoGC system to analyze samples at nominal concentrations of 0.5, 5, 10, 20, and 40 ppbv for the 41 VOCs. Also, linearity of response of the polar VOCs was investigated by using a HewlettPackard 5890 GC-flame ionization detection (FID) system (11), instead of the GC/MS, to analyze samples at nominal concentrations of 5, 10, 25, and 50 ppbv. The FID was used for detection of the polar VOCs because the operating parameters for the MS were not optimized for analysis of those compounds. A linear regression analysis of the

FIGURE 3. Reconstructed total ion current chromatogram of 41component VOC mixture, 2.5 ppbv, 57 cm3 after 4:1 split. (Peak numbers correspond to compound numbers in Table 2.)

resulting data was performed for each compound on each trap, and good linearity was observed for all compounds with the exception of methanol, which did not chromatograph well with this sampling and analytical system. For all of the 98 linear regression analyses, R2 was g0.99. Representative linearity plots for o-xylene and acrylonitrile are shown in Figure 4, and the linear regression analysis results are included in the Supporting Information. Several additional three- and four-point calibrations were performed during this evaluation, and the XonTech/Saturn autoGC always responded linearly when tested with nominal concentrations of 1-40 ppbv of the 41 VOCs. As an additional investigation of linearity of the VOCs, the sample collection time was decreased from 57 to 27 min so that a 135-cm3 sample of a 5 ppbv standard was collected and compared to a 285-cm3 sample of the same 5 ppbv standard. The resulting concentrations from the two sets of analyses were compared for each compound on each trap in order to generate ratios to compare to the expected value of 0.47. The average percent difference was determined to be -0.70% for trap 1 and 3.42% for trap 2. Percent differences for individual compounds were within (10% with the exception of dichloromethane on both traps (25% difference) and benzene on trap 2 (12% difference). Method Detection Limits. MDLs for the 41 VOCs were calculated for each of the two traps by using seven replicate analyses of the standard at a nominal concentration of 0.5 ppbv per compound. The MDLs were calculated by using the following formula:

MDL ) t(n-1, 1-R )

0.99)S

where S is the standard deviation of replicate analysis and

FIGURE 4. Linearity of response of (a) o-xylene, 1-40 ppbv, and (b) acrylonitrile, 5-50 ppbv.

t is the Student’s t-value appropriate to a 99% confidence level and a standard deviation estimate with n - 1 degrees of freedom (t ) 3.143). The higher of the MDLs (trap 1 or trap 2) for the 41 VOCs is reported in Table 2. In general, the detection limits ranged from 0.019 to 1.40 ppbv (cis1,2-dichloroethene) and were less than or equal to 0.10 ppbv for many compounds. Precision. Precision was determined by analyzing 24 consecutive samples (12 per trap) of a nominal 5 ppbv standard of the 41 VOCs. The mean, standard deviation, and percent relative standard deviation (% RSD) were calculated for each compound on each trap. The trap 1 and trap 2 % RSD values were averaged and are reported in Table 2. The mean RSD values ranged from 2.26 to 7.40%. Plots of concentrations of individual compounds versus time for the replicate analyses showed excellent stability over the 24-h period. Another test of system precision was performed by comparing the analytical results from trap 1 and trap 2 for a 5 ppbv standard of the 41 VOCs. The percent difference

(trap 1 - trap 2)/(trap 1 + trap 2) × 200 was calculated for each compound for nine sets of analyses. The difference between traps was found to be within (20% for all compounds, and the difference for most compounds was within (10%. For the nine data sets that were compared, the average percent difference for the 41 VOCs ranged from -1.67 to 1.41%. Trap-to-trap precision for


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TABLE 3

Audit Accuracy Results XonTech/Saturn compound chloroethene bromomethane trichlorofluoromethane 1,1-dichloroethene dichloromethane 1,1-dichloroethane trichloromethane 1,1,1-trichloroethane benzene carbon tetrachloride 1,2-dichloropropane trichloroethene toluene 1,2-dibromoethane tetrachloroethene chlorobenzene ethylbenzene styrene 1,1,2,2-tetrachloroethane o-xylene a

certified cylinder ppbv (95% CI 4.60 4.45 4.71 4.45 8.83 5.29 4.83 5.47 10.10 4.50 10.20 4.96 4.88 4.87 5.09 10.30 10.20 3.69 a 13.10

1.50 1.00 0.20 0.90 2.10 0.40 0.90 2.00 0.80 0.60 0.90 0.70 0.40 1.10 0.50 1.90 1.60 1.30 2.80

concn (ppbv)

difference between traps (%)

difference from “true” (%)

4.4 4.9 4.7 4.3 10.4 4.8 5.2 5.0 10.1 5.0 10.4 5.3 5.3 5.0 5.2 10.6 10.4 6.6 9.7 10.7

0.0 -0.81 -2.5 4.9 -7.1 -1.9 -2.7 -3.4 -2.3 0.61 0.38 3.4 -1.1 2.2 -0.97 3.6 -0.48 0.45 -2.1 2.3

-4.3 11.2 0.0 -4.5 18.1 -9.5 8.3 -9.1 0.0 11.1 2.0 6.0 8.2 2.1 2.0 2.9 2.0 78.6 a -18.3

Compound present but not certified.

the polar compounds on the XonTech/GC-FID system was determined by comparing trap 1 and trap 2 values and using the equation above for five sets of analyses at concentrations from 5 to 50 ppbv. The difference between traps was within (8% for all compounds. Accuracy. As a test of system accuracy, an analysis was performed on a canister prepared from an independent audit cylinder traceable to a National Institute of Standards and Technology standard, which was obtained from the EPA Atmospheric Research and Exposure Assessment Laboratory, Quality Assurance and Technical Support Division, and originally made by Research Triangle Institute (Research Triangle Park, NC). The analytical results were submitted to the Quality Assurance and Technical Support Division and compared with the audit “known” values. The results of this comparison are given in Table 3. Except for styrene, which has historically been difficult to determine quantitatively in our laboratory, all concentrations either were within the 95% confidence interval for the certified value or met the (20% criterion for acceptance. The difference between the traps for the audit compounds ranged from -7.1 to 4.9%. Field Operation. The XonTech/Saturn autoGC system was operated in a mobile laboratory for 10 days in March 1994 at a field site in Axis, AL. A three-point calibration (5, 10, and 20 ppbv) for the 41 VOCs was carried out at the beginning of the field study. After the initial multi-level calibration, periodic calibrations were performed throughout the duration of the study by using a 10 ppbv standard of the VOCs. The autoGC system sampled ambient air continually 24 h/day from March 16 to March 24 with relatively few interruptions. The XonTech 930/940 operated flawlessly during this time, and the Varian Saturn GC/MS worked well except for a burned-out filament (which was easily corrected because the Saturn is equipped with two filaments) and some computer problems, which slowed data processing and eventually required rebooting the system. Installation of both an upgraded computer and a more recent version of the Saturn software allowed us to

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FIGURE 5. CS2 (0) and 1,1,2-trichloro-1,2,2-trifluoroethane (Freon 113, ×10) (9) concentrations on March 22, 1994, at Axis, AL.

operate without such problems on a later field study in Nashville, TN, in June 1995. During the study in Alabama, the detected concentrations of the 41 VOCs were usually less than 1 ppbv with the exception of several samples in which 1,1,2-trichloro-1,2,2trifluoroethane was as high as 10 ppbv. Sub-parts-perbillion by volume concentrations of R- and β-pinene were also detected in several of the samples. The 1-h continuous sampling technique allowed resolution of an exposure event for carbon disulfide (CS2), which was detected in several samples at 200-400 ppbv, as shown in Figure 5. This event was not resolved by the State of Alabama’s air monitoring instrumentation because that system collected 6- and 24-h integrated air samples. Varying concentrations of 1,1,2trichloro-1,2,2-trifluoroethane (up to 10 ppbv) were also detected in several samples during the same time period as the CS2 event. Although CS2 coeluted with 1,1,2-trichloro1,2,2-trifluoroethane as shown in Figure 6, discrimination between the two compounds was never a problem because

or commercial products does not constitute endorsement or recommendation for use.

Supporting Information Available

FIGURE 6. Quantitation masses for CS2 (76) and 1,1,2-trichloro-1,2,2trifluoroethane (101).

full-scan mass spectra provided by the Saturn allowed identification of compounds, and the characteristic ions of 76 for CS2 and 101 for 1,1,2-trichloro-1,2,2-trifluoroethane were used for quantitation.

Conclusions The XonTech/Saturn II autoGC provided reliable sampling and detection of VOCs as determined by our tests of cleanliness, linearity of response, accuracy, and precision. This autoGC is suitable for monitoring volatile hazardous air pollutants such as those listed in Title III of the CAAA by using a multiadsorbent concentrator tube. This allows the reduction of residual water vapor by breakthrough during both sampling and subsequent dry purging with helium. With the multisorbent-dry purge approach, polar VOCs are retained on the multisorbent tube rather than being removed from the sample stream, which occurs when water management techniques include the use of a permeable membrane dryer. After drying, a cold trap allows refocusing of the sample at cryogenic temperatures without the use of liquid cryogen. The initial temperature of the GC oven can be programmed from above-ambient temperatures, a feature that is often useful in field and laboratory settings. The ion trap detector provides detection at partsper-trillion sensitivity levels for a 285-cm3 sample volume, which was split 4:1 at the head of the GC column. The split allowed delivery of a small (57-cm3) sample volume to the detector, further reducing the interferences from residual water vapor. Future research will include optimizing the GC/MS parameters for detection of polar VOCs and expanding the list of polar VOCs that can be determined with this autoGC.

Acknowledgments The research described in this paper has been funded wholly or in part by the U.S. Environmental Protection Agency under Contracts 68-D0-0106 and 68-D5-0049 with ManTech Environmental Technology, Inc. Mention of trade names

Two tables giving linear regression results for multi-level calibrations for 41 compounds using XonTech Model 930/ 940 Saturn II system from December 1995 and one table giving linear regression results for polar compounds using XonTech/ GC-FID from January 1994 (26 pages) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supporting information from this paper or microfiche (105 × 148 mm, 24× reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St. NW, Washington, DC 20036. Full bibliographic citation (journal, title of article, names of authors, inclusive pagination, volume number, and issue number) and prepayment, check or money order for $46.50 for photocopy ($48.50 foreign) or $12.00 for microfiche ($13.00 foreign), are required. Canadian residents should add 7% GST. Supporting information is available to subscribers electronically via the Internet at http://pubs.acs.org (WWW) and pubs.acs.org (Gopher).

Literature Cited (1) Fed. Regist. 1990, June. (2) McClenny, W. A.; Evans, G. F.; Oliver, K. D.; Daughtrey, E. H., Jr.; Winberry, W. T., Jr. In Proceedings of the 1991 U.S. EPA/ A&WMA International Symposium on Measurement of Toxic and Related Air Pollutants; VIP-21; Air & Waste Management Association: Pittsburgh, PA, 1991; pp 367-374. (3) Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air; Office of Research and Development, U.S. Environmental Protection Agency: Research Triangle Park, NC, 1988; EPA-600-4-84-041. (4) McClenny, W. A. In Proceedings of the International Conference Volatile Organic Compounds in the Environment; London, England; Leslie, G., Perry, R., Eds.; Indoor Air International: Rothenfluh, Switzerland, Oct 27-28, 1993. (5) McClenny W. A.; Gerald, N. O. Environ. Lab. 1994, 60, 37. (6) Almasi, E.; Kirshen, N. The Determination of Volatile Organic Compounds (VOCs) in Air by the TO-14 Method Using the Saturn II GC/MS; GCMS18: 0292; Varian Chromatography Systems: Walnut Creek, CA, 1992. (7) Pankow, J. F. Environ. Sci. Technol. 1991, 25, 123. (8) McClenny, W. A.; Oliver, K. D.; Daughtrey, E. H., Jr. J. Air Waste Manage. Assoc. 1995, 45, 792. (9) Levaggi, D. A.; Oyung, W.; Zerrudo, R. V. In Proceedings of the 1992 U.S. EPA/A&WMA International Symposium on Measurement of Toxic and Related Air Pollutants; VIP-25; Air & Waste Management Association: Pittsburgh, PA, 1992; pp 857-863. (10) Oliver, K. D.; Pleil, J. D. Automated Cryogenic Sampling and Gas Chromatographic Analysis of Ambient Vapor Phase Organic Compounds: Procedures and Comparison Tests; TN-4120-8502; Northrop Services, Inc.sEnvironmental Sciences: Research Triangle Park, NC, 1985. (11) Oliver, K. D.; Adams, J. R.; Daughtrey, E. H., Jr.; McClenny, W. A.; Yoong, M. J.; Pardee, M. A. Technique for Monitoring Ozone Precursor Hydrocarbons in Air at Photochemical Assessment Monitoring Stations: Sorbent Preconcentration, Closed-Cycle Cooler Cryofocusing, and GC-FID Analysis. Urban Atmos./Atmos. Environ., in press.

Received for review August 25, 1995. Accepted February 11, 1996. X ES9506317 X

Abstract published in Advance ACS Abstracts, April 15, 1996.


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Technique for Monitoring Toxic VOCs in Air: Sorbent Preconcentration

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