Anal. Chem. 1993, 65,1932-1935
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Determination of Cq-Ca Hydrocarbons in Air You-Zhi Tang,’ Quang Tran, and Philip Fellin Concord Environmental, 2 Tippett Road, Toronto, Ontario, Canada, M3H 2V2
W. K . Cheng Canadian Petroleum Products Institute, 275 Slater Street, Suite 1000, Ottawa, Ontario, Canada, K I P 5H9
Ian Drummond Canadian Association of Petroleum Producers, 350-7 Avenue, SW, No. 2100 Calgary, Alberta, Canada, T2P 3N9
INTRODUCTION Health implications of occupational exposure to hydrocarbons has been an area of controversy. In the past, minimal concern was raised over exposures to light hydrocarbons since the lighter fraction of hydrocarbons, i.e., the C1-C4 fraction, was believed to have little health impact. These hydrocarbons were only of concern as simple asphyxiants’ or because of their combustible properties. This view of the lighter hydrocarbons’ potential effects has changed as the result of new studies.* The accurate measurement of the lighter hydrocarbons has become an important goal for occupational hygienists, especially those working for the petroleum industry due to the potential for exposure. However, light hydrocarbons are normally excluded from health effect assessments of volatile organic compounds (VOCs), due to difficulties in the determination of specific light hydrocarbon compounds. Adsorbent tubes are widely used by occupational hygienists to collect VOCs for personal exposure monitoring in the workplace. Application of these adsorbent tubes for quantitative collection of light hydrocarbons, however, has not been as successful as for the heavier VOCs, due to their high volatility, which leads to breakthrough during sampling. The “total” hydrocarbons normally sampled by charcoal tubes thus do not include the lighter fraction. Therefore, cold traps (cryogenic methods) have been used for collection of light hydrocarbons in ambient air with adsorbent tubes.S5 Also, individual quantitation of these compounds requires chromatographic resolution by means of long GC columns combined with temperature programs using a subambient initial t e m p e r a t ~ r e . Cryogenic ~,~ procedures complicate both sampling and analytical protocols and result in significant extra costs and inconvenience. In addition, the results may still not be satisfactory for many applications. Advances in both sampling and analytical techniques will make the health effect assessment of light hydrocarbons more accessible. We have recently developed a practical, reliable, and costeffective sampling and analytical method for measurement of light hydrocarbons in the occupational environment for the petroleum industry. The study comprised three major tasks. The first one, concerned with the development of appropriate analytical methods, is described in this note. Results of the other two tasks, concerned with the evaluation of sampling methods under controlled laboratory conditions and field testing, are published e l ~ e w h e r e . ~ , ~ (1) des Tombe, K.; Verma, D. K.; Stewart, L.; Reczek, E. B. Am. Ind. Hyg. ASSOC. J. 1991, 52, 136-144. (2) Drummond, I. Appl. Occup. Enuiron. Hyg., in press. (3) McClenny, W. A.; Pleil, J. D.; Holdren, M. W.; Smith, R. N. Anal. Chem. 1984, 56, 2947-2951. (4) Netravalkar, A. J.; Mohan Rao, A. M. Chromatographia 1986,22, 183-186. ( 5 ) Cudrey, R. A,; Walther, E. G.; Malm, W. C. J.Air Pollut. Control Assoc. 1977,27, 468-470. (6) Supelco. Supelco Rep. 1990, 9 (6), 2-4.
EXPERIMENTAL SECTION There are 33 hydrocarbons having four carbons or less; some of them are thermally unstable and some of them cannot be obtained from normal commercial s0urces.l Although it is unlikely that all these light hydrocarbons occur in an air sample simultaneously, it is always beneficial to develop methods for analysis of as many of these compounds as possible. The 17 light hydrocarbons (C1-C4) identified in Figure 1 were selected for testing. Hexane, benzene, and toluene were added to the list of test compounds to explore the possibilityof simultaneous analysis of both light and heavier compounds. Pentane was also used for evaluation of GC columns. Scotty I analyzed gases (Scott Specialty Gases, Plumsteadville, PA) were used for calibration and selection of GC conditions. Gas standards were prepared by injection, with a gas-tight syringe, of known concentrations and volumes of gases from the Scotty cans into a Tedlar gas sample bag (SKC Inc., Eight-Four, PA), and the bag was filled with a known volume of nitrogen (Linde extra dry, Union Carbide Canada Limited, Toronto, ON.). The volumetric measurements were made by using a calibrated rotameter and accurate measurement of the filling time. Seven different types of samplers were tested for air sampling. The Carbotrap 200 and 300 thermal desorption tubes, both 4and 2-mm id., were supplied by Supelco Canada, Oakville, ON. Originally, the CONCAWE (the oil companies’ European Organization for Environmental and Health Protection) tube was made from l / 4 in. 0.d. X 3.6 in. long stainless steel tubing, packed with 200 mg of Chromosorb 106 (80/100 mesh) and 300 mg of activated coconut charcoal (60180 mesh).9 In order to fit in the Supelco thermal desorber, it was custom-made by Supelco with the same types and amounts of adsorbents but with a 4 mm i.d. x 6 mm 0.d. X 11.5 cm glass tube. The 4-mm-i.d. charcoal tube for thermal desorption was also custom-made by Supelco, with 600 mg of activated charcoal, and is referred to as the thermal desorption charcoal (TDC) tube. Both CONCAWE and TDC tubes were also available in 2-mm-i.d. sizes. Normally the 4-mm tube is used for sampling and the 2-mm tube is used, if necessary, as a “focusing”tube for minimizing peak broadening during the initial transfer of collected compounds to the GC. The contenta of these four types of adsorbent tubes are listed in Table I. They were extracted using thermal desorption and were thermally cleaned before use. The SKC charcoal tubes (400-mg front and 200-mg backup sections) were extracted using carbon disulfide. The SUMMA 0.85-Lcanisters were from Biospherics Research Corp., Hillsboro, OR. The critical orifice personal samplers (COPS) were made by MDA Scientific, Lincolnshire, IL. They are not commercially available and were provided by the Canadian Association of Petroleum Producers (CAPP). The COPS, 100 mL in volume, was made of stainless steel and was equipped with a built-in critical orifice. Before use, the canister-type samplers were heated in a hot water (100“C)bath and flushed continuously with humidified air for 30 min. They were then filled with zero air and evacuated by a vacuum pump (PN. 7411-70, Canadawide (7) Tang, Y. Z.; Cheng, W. K.; Fellin, P.; Tran, Q.; Drummond, I. To be submitted to Am. Ind. Hyg. Assoc. J . (8) Tang, Y. Z.; Fellin, P.; Tran, 4.;Cheng, W. K.; Drummond I. To be submitted to Am. Ind. Hyg. Assoc. J . (9) Coker, D. T.; van den Hoed, N.; Saunders, K. J.;Tindle, P. E. Ann. OCCUP. Hyg. 1989, 33, 15-26.
0003-2700~93/0365-1932$04.00/00 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 85, NO. 14, JULY 15, 1993 1938
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Retention Time (min) Figure 1. Separation of test compounds: (1) methane; (2) ethylene; (3)acetylene; (4) ethane; (5) propylene; (8) propane; (7) cyclopropane; (8)
propyne; (9)lsobutane; (10)isobutene; (11) 1-butene; (12) 1,3-butadiene;(13)+butane; (14)trans-2-butene; (15) 1-butyne; (16)c/s-Bbutene; (17) 2-butyne; (18)hexane; (19)benzene; (20)toluene. (A) Direct gas injectlon (0.5 mL); (B) desorbed from 2-mm tube; (C) desorbed from 4-mm tube. Column: J&W OS-Q, 30 m X 0.53 mm; injection port temperature, 250 'C; detector temperature, 250 O C ; column flow rate, hellum 5 mL/min; makeup gas, hellum 30 mL/min; air, 300 mL/min; hydrogen, 30 mL/mln; temperature program, 50 O C for 1 min, 3 OC/min to 80 O C , 8 OC/mln to 200 O C . Table I. Thermal Desorption Adsorbent Tube. tube Carbotrap 200 Carbotrap 300 CONCAWE TDC
adsorbents glass beads Carbotrap B Carbosieve S-I11 Carbotrap C Carbotrap B Carbosieve S-I11 Chromosorb 106 activated charcoal activated charcoal
mesh 70180 20140 60180 20140 20140 60180 801100 60/80 60180
amount (mg) 4mm 2mm 80
200
20 48
350
84
300 200 125 200 300 600
72
48 30 50 75 150
a Adsorbents are listed in the order of their occurrence in the tubes, starting with the front adsorbent; sampling was conducted with air flowing from the front to the rear of the adsorbent bed, whereas thermal desorption was conducted with flush gas flowing from the rear to the front of the adsorbent bed.
Scientific Ltd., Toronto, ON). Air was drawn into the samplers by vacuum, and sampling flow rates were controlled by calibrated critical orifices. Samples from the SUMMA canisters and COPS were either withdrawn and injected onto the GC directly using a syringe or, if necessary, preconcentrated on a 2-mm Carbotrap 200 tube by drawing the air through the tube by a pump and then thermally desorbed onto the GC. All samplers were examined for blank levels and for recovery efficienciesof target compounds. The effects of sampling conditions on the performance of these samplers are reported elsewhere.'J An HP5890 gas chromatograph (GC) equipped with a flame ionization detector (FID) and cryogenic capability was used for sample analysis. GC signal integration was performed by a HP3393A integrator. Sample injection was made by using a Hamilton No. 901 1O-rLsyringe for liquid samplesanda Hamilton 1002SN 2.5-mL gas-tight syringe for whole gas samples through the injection port of a thermal desorption unit (Dynatherm Analytical Instruments, Inc. Model 850 thermal tube desorber
and Model 851 temperature controller) supplied by Supelco Canada. After sample collection, the 4-mm thermal desorption tube was inserted in the desorber's desorption chamber and the analytes were desorbed to the 2-mm-focusing tube in the desorber's secondary trapping chamber. The focusing tube was then transferred to the desorption chamber to thermally transfer the analytes to the GC column. This approach eliminated the costly cryofocusingprocedure while it minimized peak broadening of the CZand Cs hydrocarbons. For some field samples, this approach was not necessary because only a few Cz and Cs hydrocarbons existed and the GC resolution was adequate even without refocusing. For these circumstances, samples collected were directly desorbed into the GC column using the desorber. Three different types of GC columns were tested for analysis of C1-Ch hydrocarbons based on results of a literature review,lVMJ&lSincluding a SPB-1megabore thick-film column (60 m X 0.53 mm X 5 rm) from Supelco Canada; an OPN/Porasil C packed column (16 ft X l/g in. glass, 80/100 mesh) from Waters Associates, Inc., Milford, MA; and a GS-Q megabore column (30 m X 0.53 mm) from J&W Scientific, Folsom, CA.
RESULTS AND DISCUSSION Separation of Light Hydrocarbons. The first step in this study was the selection of a suitable GC column and GC conditions. Normally, subambient temperatures are needed for separation of low molecular weight hydrocarbons if liquid(10)Supelco. Supelco Rep. 1989,8 (3),5-7. (11)Supelco. Supelco GC Bull. 1983, 760F. (12)Supelco, Supelco GC Bull. 1989, 722G. (13)Mindrup, R. J. Chrornatogr. Sci., 1978,16, 380-389. (14) de Nijs, R. C. M.; de Zeeuw, J. J. Chromutogr. 1983,279,41-48. (15)Schmidbauer,N.; Oehme, M. HRCICC,J. High Resolut. Chrornatogr. Chrornatogr. Cornrnun. 1985,8, 404-406. (16) Pelz, N.; Dempster,N. M.; Shore, P. R. J . Chromatogr. Sci. 1990, 28,230-235. (17) de Zeeuw, J.; de Nijs, R. C. M.; Henrich, L. T. J . Chrornatogr. Sci. 1987,25, 71-83. (18)Peters, A.; Sacks, R. J. Chrornatogr. Sci. 1991,29, 403-409.
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phase columns are used.5JO Cryogenic procedures result in increased costs and inconvenience, and therefore, the 60-m Supelco SPB-1 megabore thick-film column was tested without using subambient initial temperatures. A maximum of 1 2 resolved peaks were observed for t h e 17 light hydrocarbons injected onto the SPB-1 column using temperature programs with an initial column temperature of 30 "C. Four pairs/groups of compounds coeluted: acetylene and ethylene; propylene, propane, and cyclopropane; isobutene and 1,3-butadiene; and 1-butyne and trans-2-butene. The last eluting C4 hydrocarbon (2-butyne) was well separated from the heavier hydrocarbons, facilitating the analysis of light hydrocarbons in the presence of heavier compounds. Although subambient initial temperatures are required when an OPN Porasil column is used for separating light hydrocarbons, this packed column was still selected for comparison purposes since it has a higher chromatographic efficiency than other packed columns commonly used for separation of light hydrocarbons.11J3 A total of 14 peaks were resolved at best from the mixture of 17 C1-C4 hydrocarbons using temperature programs with an initial column temperature of -30 "C. All C1 and Cz compounds were resolved. The three pairs of compounds that could not be resolved were propylene and cyclopropane, propyne and 1-butene, and isobutene and trans-2-butene. Pentane eluted before l-butyne, and hexane eluted close to the last eluting Cq hydrocarbons (2-butyne),causing potential difficulties in analyses in the presence of heavier compounds. The GS-Q column is a porous-layer open tubular (PLOT) column. PLOT columns are rapidly gaining popularity because of their significant retention of permanent gases and light organic compounds, which leads to their application in the separation of these compounds without the inconvenience of subambient temperature operation.lP17 However, some precautions are required when the GS-Q column is used. Spikes on the baseline were observed when high carrier flow rates were used (>8 mL/min He). The spikes were likely caused by polymer particles blown into the FID from the PLOT column by high carrier flows. This phenomenon has also been observed by another PLOT column user.18 Reconditioning and aging of the column did not correct this problem. A carrier flow rate of 5 mL/min was selected as the suitable flow for use in this work, and under this condition no spikes were observed. The flow rate for thermal desorption was normally 30 mL/min, and therefore, it was necessary to split the flow from the desorber before directing it to the GS-Q column. The GS-Q column was superior to the OPN Porasil column for this application, not only because of the elimination of the requirement for cryogenic operation of the GC but also due to its better resolution for the light hydrocarbons and its suitability for analysis of heavier hydrocarbons (Figure 1A). With an initial column temperature of 50 "C, light hydrocarbons were separated in groups according to their carbon numbers. Altogether, 10 C1-C3 hydrocarbons exist, and complete resolution of the eight target compounds was achieved with this column. Cyclopropylene and 1,2-propadiene were considered too unstable and were not tested in our study. The two pairs of compounds not resolved by this column were isobutene/l-butene and trans-2-butenell-b~tyne. The last eluting Cq hydrocarbon (2-butyne, no. 17 at 19.23min) was well separated from the heavier hydrocarbons. The GS-Q column was therefore selected for analysis of light hydrocarbons. When analytes are thermally desorbed onto the GC from an adsorbent tube, losses in resolution may occur due to peak broadening caused by prolonged sample desorption/injection. If a subambient initial column temperature is used, peak
broadening is eliminated because of cryogenic focusing of the analytes a t the column head. Another approach is retrapping of the analytes using an adsorbent tube with a smaller inner diameter than that of the sampling tube and then desorption to the GC from the focusing tube. Figure 1also demonstrates how the diameter of the adsorbent tube affected the GC resolution, especially for early-eluting compounds (Cz and C3). Direct gas injection produced sharp peaks of CZand C3 hydrocarbons and therefore baseline resolutions. When the analytes were desorbed from the 4-mm tube to the column, broad and poorly resolved peaks were observed for C2 and C3 hydrocarbons. Desorption from the 2-mm tube provided a better resolution than from the 4-mm tube. Therefore, the 4-mm tube, which allowed greater sampling flow rates and sample volumes, was usually used for sampling. The analytes collected by the 4-mm tube were desorbed, transferred to the 2-mm tube for focusing, and then desorbed onto the GC. This process eliminated the need of cryogenic operation, but losses of CZhydrocarbons occurred during the preconcentration if the desorption volume exceeded 300 mL. Use of the focusing tube could be avoided if the resolution was sufficient for a particular analysis where less analytes existed. Sampling for Light Hydrocarbons. Recoveries of test hydrocarbons from samplers were examined. The recovery efficiencies from the canister and COPS, and the thermal desorption or extraction efficiencies (with CSz) from the adsorbent tubes, were found to be 100 f 10%. However, the collection efficiencies of adsorbent tubes varied with compound volatility and sampling conditions. The collection efficiencies were examined using a test chamber with known concentrations of test compounds in the laboratory7 and field evaluated by comparison with canister results.8 Methane was not collected by any of the adsorbent tubes but was successfully collected by canister-type samplers. The Carbotrap 200 tube was found to be the preferred adsorbent tube, among all those tested, for sampling light hydrocarbons. Hydrocarbons with three or more carbons were collected by all adsorbent tubes, but only the Carbotrap 200 tube was capable of collecting Cz hydrocarbons. The sample volume allowed before breakthrough would occur was very low however. Collection of Cz hydrocarbons occurred mainly on the Carbosieve $111 in the multibed adsorbent tube. A Supelco bulletinlg indicated that the breakthrough volume for ethane on Carbosieve S-I11was 2.95 L/g, but no test conditions were specified. Based on this figure, the breakthrough volume of the Carbotrap 200 tube for ethane was estimated as 1.0 L since 350 mg of the Carbosieve S-I11 was contained in each Carbotrap 200 adsorbent tube (Table I). The specific retention volume of the Carbosieve S-I11for ethane was later reported (also by Supelco personnel) as 0.98 L/g at 20 OC.zO This value, based on which the retention volume of the Carbotrap 200 tube for ethane was estimated as 0.34 L, disagreed with that reported in ref 19. The breakthrough volume (which is normally defined as the volume at which the analyte concentration in air after passing through the adsorbent tube reaches 5 % of that before the adsorbent tube) is dependent on the chromatographic efficiency of the adsorbent tube and is always smaller than the retention volume.21,22 Also, the other two C2 hydrocarbons, ethylene and acetylene, were retained less efficiently than ethane by the Carbotrap 200 tube.7 This suggested that breakthrough was anticipated for all Cz hydrocarbons when the sample (19)Supelco. Supelco Sample Handling Bull. 1987,850. (20)Betz, W. R.;Maroldo,S. G.; Wachob, G. D.;Firth, M. C. Am. Ind. Hyg. ASSOC. J. 1989,50,181-187. (21)Gallant, R.F.; King, J. W.; Levins, P. L. EPA Report, EPA-600/ 7-78-054, 1978. (22)Grisp, S. Ann. Occup. Hyg. 1980,23, 47-76.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993
volume was 0.34 L a t 20 OC, resulting in underestimates of CZhydrocarbons. The Carbotrap 300 tube contained 125mg of the C a r h i e v e S-I11 (Table I) and the breakthrough volume for CZhydrocarbons was expected to be approximately one-third that of the Carbotrap 200 tube. It appeared to be impractical to use the Carbotrap 300 tube for sampling CZhydrocarbons. The collection efficiencies of the TDC, CONCAWE, and SKC tubes for light hydrocarbons were found to be similar to that of the Carbotrap 300 tube. The SKC charcoal tube was extracted with 1-2 mL of CSz and an aliquot of 1 pL was injected onto the GC for analysis,leading to sample dilutions of 1000-2000 times and thus much lower method sensitivities. Larger sample volumes, however, resulted in severe losses of C3 hydrocarbons. The SKC charcoal tube proved to be suitable for sampling hydrocarbons with four and more carbons, with a sampling flow rate of ca. 100 mL/min and sample volumes of up to 40 L. Analysis of Field Samples. Air samples were taken inside the compressor building of a petroleum refinery using canisters and adsorbent tubes. The major C144 hydrocarbons found were methane, ethane, propane, butane, and isobutane, and their average concentrations in an 8-h sampling period are listed in Table 11. Methane was not collected and ethane was poorly collected by adsorbent tubes, and they were determined by using canister sampling and syringe injection. The canister samples were then preconcentrated on the focusing tube followed by thermal desorption and analysis for CBand C4 hydrocarbons a t lower concentrations. In order to obtain information on potential time-weighted-average (TWA) exposures and yet not exceed the low breakthrough volume of the adsorbent tube, sampling flow rates below 5 mL/min were used, which were much lower than sampling rates of 30-300 mL/min normally used to obtain air samples of approximately 10 L or more in occupational hygiene sampling with activated charcoal tubes (100-mg front and 50-mg backup section^).^^^^ For hydrocarbons with three and four carbons, the Carbotrap (23)NZOSH Manual of Analytical Methods; U.S. Department of Health, Education, and Welfare, NIOSH,GPO Washingon, DC, 1984. (24) OSHA ManualofAnalyticalMethode;U.S.Department oflabor, Occupational Safety t Health Administration; GPO Washington,DC, 1985.
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Table 11. Analysis Results of Field Samples 8-h-averaee concna (mdrn3) Carbotrar, 200 compd canister 0.5-L sample 1.2-L sampleb methane 11.7 ethane 2.16 0.618 0.598 propane 0.962 1.07 0.982 isobutane 0.088 0.078 0.071 n-butane 0.131 0.133 0.116 a Based on duplicate measurementa. * A 1.2-L air sample for each of the two consecutive 4-h sampling periods.
200 tube results agreed with the canister results with i 2 0 % . However, the Carbotrap 200 result for ethane was very poor, likely due to breakthrough even with sample volumes of only 0.5 L. Sampling was conducted a t temperatures above 30 OC, and breakthrough could be less severe a t lower temperatures. Compared with sampling by canister and sampling by adsorbent tube followed by solvent extraction, sampling with thermal desorption adsorbent tubes followed by GC-FID analysis provided a faster and more sensitive way for determination of Cz-C4 hydrocarbons in air. The method provided an accuracy of *20 % ! and a precision of f 1 5 % RSD for hydrocarbons with three or more carbons. Sampling for Cz hydrocarbons exhibited some limitations. The detection limit was ca. 1pg/m3 of each hydrocarbon, if 1L of air was sampled with the Carbotrap 200 tube. Thermal desorption adsorbent tubes, though more expensive than disposable adsorbent tubes used for solvent extraction, were reusable and much less expensive than the canister. They were also, compared with canisters, easier to deploy for personal exposure monitoring and large-scale air quality surveys. For C1 and Cz hydrocarbons, the canister sampling was still the method of choice unless direct-sampling GCs are available for monitoring on site.
RECEIVED for review November 18, 1992. Accepted April 5, 1993.
