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Anal. Chem. 1988, 6 0 , 509-512
material samples used in this study is appreciated. fiberglass uick
Registry No. Sulfur, 7704-34-9; carbon disulfide, 75-15-0. LITERATURE CITED (1) Markuszewski, Richard; Mroch, Davkl R.; Norton, Glenn A.; Straszheim, Warren E. I n Fossil Fuels Utilization: Envkonmental Concerns; ACS Symposium Series 319; Markuszewski, R., Blaustein, B. D.. Eds.; American Chemical Society: Washington, DC, 1988; pp 63-74. (2) Norton, Glenn A.; Mroch, David R.; Chrisweii, Colin D.; Markuszewski, Richard I n Processing and Utilization of High Sulfur Coals-II; Chugh, Y. P., Caudie, R. D., Eds.; Eisevier: New York, 1987; pp 213-
glass vial
33.1 ---.
so1 u t i on
Figure 1. Wick evaporation apparatus.
This wick evaporation method should be useful in testing this hypothesis. In this application example, the quantities of analyte recovered were not determined by weighing, but it is certainly feasible to weigh wicks before and after evaporation to determine the amounts of residue deposited on the wick tip. ACKNOWLEDGMENT The assistance of James L. Hofer and Gary L. Bryant in performing sulfur determinations on the crude humic-like
(3) Chrisweii, Colin D.; Shah, Navin D.; Kaushik, Surender M.; Markuszewski, Richard I n Proc. 4th Annu. Pittsburgh Coal Conf .; Plttsburg PA, Sept. 28-Oct. 2, 1987; pp 937-943. (4) Chrisweii, Colin, D.; Mroch, Davld R.; Markuszewski, Richard Anal. Cbem. 1986, 58 319-321. (5) Markuszewski, Richard; Chiotti, Premo; Chrisweii, Colin D.; Kaushik, Surender M.; Natarajan, G.; Norton, Glenn A,; Shah, Navin D. "Chemical Cleaning of Coal Using Molten Caustic Leaching and Regeneration of Reagents"; Fossil Energy Annual Report (Oct. 1, 1985Sept. 30, 1986); Ames Laboratory, Iowa State University: Ames, IA. (6) Uddin, Zamir. Ph.D. Dissertation, Iowa State University, Ames, IA, 1985.
RECEIVED for review August 26,1987. Accepted October 23, 1987. Ames Laboratory is operated for the US. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. This work was supported by the Assistant Secretary for Fossil Energy through the Pittsburgh Energy Technology Center.
Determination of Methyl Bromide in Air Samples by Headspace Gas Chromatography J a m e s E. Woodrow, Michael M. McChesney, a n d J a m e s N. Seiber*
Department of Environmental Toxicology, University of California,Davis, California 95616 Methyl bromide is extensively used in agriculture (4 X lo6 kg for 1985 in California alone (1))as a fumigant to control nematodes, weeds, and fungi in soil and insect pests in harvested grains and nuts. Given its low boiling point (3.8 "C) and high vapor pressure (-1400 Torr at 20 "C), methyl bromide will readily diffuse if not rigorously contained. When used as a soil fumigant, where the material is injected into the soil and immediately covered with a plastic tarp, significant amounts will escape (2);subsequent tarp removal will result in further releases to the atmosphere. Venting of fumigation sheds will also result in short-term releases of relatively high concentrations (>80 ppm) to the atmosphere (3). Primary human and animal exposure will be by inhalation, especially for those individuals in the general vicinity of the treatment sites. The time-weighted average (8 h/day, 40 h/week) threshold limit value for methyl bromide in air is 5 ppm ( ~ 2 0 mg/m3) (4). This level is recommened to prevent serious neurotoxic effects and pulmonary edema. In general, halocarbons at parts per million levels in air will affect the central nervous system and cause liver and kidney dysfunction. Thus, it becomes imperative that a simple and fast, yet accurate, method be available to determine exposure levels to methyl bromide, and other halocarbons as well. Furthermore, the method should be able to handle high sample throughput to provide an extensive data base for risk assessment studies applied to population exposure to these chemicals. Methods for determining methyl bromide and other halocarbons in air vary widely (5-9). A common practice is to trap the material from air on an adsorbent, such as polymeric resins, followed by thermal desorption either directly into the analytical instrumentation or after intermediary cryofocusing 0003-2700/88/0360-0509$01.50/0
(5,8). While in some cases analytical detection limits were reasonable (parts per million range), many of the published methods were labor intensive and required special handling techniques that precluded high sample throughput. We describe here a method for the sampling and analysis of airborne methyl bromide that was designed to handle large numbers of samples through automating some critical steps of the analysis. The result was a method that allowed around-theclock operation with a minimum of operator attention. Furthermore, the method was not specific to methyl bromide and could be used to determine other halocarbons in air. EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer Model Sigma 2000 gas chromatograph coupled to a Model HS-100 autosampler was used to analyse the methyl bromide samples. The instrument was modified so that the carrier gas entered the system at the head of the column, with a fraction flowing through the transfer line, in order that flow could be maintained during vial pressurization and headspace sampling (Figure 1). The pressurization and carrier gases had separate sources and pressure controls. In the usual configurationsupplied by the manufacturer, carrier gas flow through the transfer line and then through the gas chromatographic column. With this configuration, the carrier gas flow is interrupted during vial pressurization and headspace sampling. Sample Preparation. Glass tubes filled with about 3 mL of charcoal each (Lot 120; SKC-West,Fullerton, CA) (Figure 2) were either spiked directly with 0.05-100 pg of methyl bromide (Matheson, East Rutherford, NJ) or were used to adsorb the compound from an air stream. In the latter case, two tubes were connected in series to form a sampling train and the intake glass wool was spiked with 3 and 10 pg of methyl bromide in separate tests to determine trapping efficiency. The sampling train was 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. BO, NO. 5. MARCH 1, 1988 computing integrator programmed to integrate peaks valley-tovalley.
GCCOLUMN
TRANSFER
SAMPLING
\
EXHAUST
Figure 1. Configuration of gas chromatograph for methyl bromide analysis by headspace sampling. INTAKE
f
TOYACIRIM PUMP
Figure 2. Charcoal air sampling lube. connected via a manifold to a modified ac-powered high-volume air sampler (10)which pulled air at 0.1 L/min through the ehareoal tubes for 4 h. In a separate study using the same air sampler Configuration [ l l ) ,three tubes connected in series were used to sample air in an agricultural region of California where methyl bromide is commonlyused as a soil fumigant in strawberryculture. Methyl bromide standards were prepared by spiking the condensed material to preweighed, Teflon septum-sealed vials containing hexane (Resi-Analyzed;J. T. Baker Chemical Co., Phillipsburg,NJ).The pure methyl hromide was contained in a lecture bottle equipped with a needle valve. The material was recovered hy inverting the bottle and opening the valve just enough to allow 1-2 mL of the liquid to empty into a glass vial chilled to dry-ice temperature (-78.5 "C). A gas-tight syringe (Hamilton Co., Reno, NV) chilled to the same temperature was used to transfer -23 BL of methyl bromide to the preweighed, sealed vial containing 3.5 mL of hexane and from which about 0.7 mL of headspace had been evacuated with a gas-tight syringe. The vial was weighed again and additional hexane was injected into the vial to adjust the methyl bromide concentration to 10 pg/&. After preparation, the standard was stored in a freezer (-10 "C) and replaced with a fresh standard every 1-2 weeks. Aliquots for spiking were removed through the septum by using a gas-tight syringe. Analysis. The contents up to, but not including, the last polyurethane foam (PUF)spacer F i e 2) of each charcoal-filled air sampling tube were emptied into separate 22-mL glass headspace vials (Perkin-Elmer, Nonvalk, CT), which were immediately sealed with Teflon-coated septa. Approximately 2.8 mL of air was evacuatd from each vial by using a gas-tight syringe and replaced with 2.8 mL of benzyl alcohol (AR grade; Mallinckrodt Chemical Works, St. Louis, MO, purified hy distillation before use). The samples were then thermostated at 110 OC for 15 min in the headspace apparatus and pressurized with nitrogen to 35-37 psig for 0.5 rnin and the equilibrated headspace was sampled for 0.3 min. Gas chromatographywas accomplished hy using a 1.8m x 0.32 em (o.d.1 stainless steel column packed with 100/120 mesh Porapak Q (Supelco, Bellefonte, PA) at 140 "C and a B3Nielectron-capture detector at 300 "C. Carrier and make-up gas (both nitrogen) flows were 20 and 40 mL/min, respectively. The nitrogen source was set at 27-30 psig. Quantitation was done hy comparing peak heights with those of standard injections. Peak heights were measured by usi-g a Perkin-Elmer Model LCI-100
RESULTS AND DISCUSSION Sampling of equilibrated headspace is well suited to the determination of volatile analytes in a low volatility matrix. The vapor above the liquid extract should be enriched only in those compounds whose high vapor pressure and low solubility in the extracting solvent would favor the vapor phase. Because of this, a gas chromatogram of the vapor should he relatively "clean" compared to the chromatogram of the liquid extract. Furthermore, careful choice of a high-boiling extracting solvent would virtually eliminate the large solvent response common with techniques that include direct liquid solvent injection. We chose benzyl alcohol as the desorbing solvent because of its boiling point (-205 "C)and its ability to quantitatively remove methyl bromide from charcoal; methyl bromide vapor density was the same for equivalent spikes to charcoal, subsequently desorbed with benzyl alcohol, and benzyl alcohol without charcoal. The headspace sampler used in this study was a programmable mnltisampling system composed of a pneumatically operated injection system, a thennostated sample vial carousel, and an electronically controlled sample magazine. Sampling is based on a pneumatic halanced pressure principle (IZ), which avoids the disadvantages associated with gas syringes, such as change of partial pressures of the volatiles due to reduced pressure in the syringe. Briefly, the septum of the thennostated sample is pierced by the hollow sampling needle, the vial is pressurized, and then, through automatic valve switching, an aliquot of the headspace is injected onto the column by using the vial pressure as the driving force. After sampling, the vial pressure is vented and the vial is returned to the sample magazine. The instrument is programmed to allow for the various steps from loading the sample in the carousel to sampling the headspace so that there is no delay time from vial to vial. Instrument configuration shown in Figure 1was crucial for the success of the method. The instrument was designed originauy to be used primarily with a capillary column, through which carrier gas flow represents only -2% of the total gas flow through the detector compared to over 30% through the packed column used in this study. A brief disruption (
