Anal. Chem. 1988, 60,508-509
508
TECHNICAL NOTES Wlck Evaporation: A Technique for the Isolation of Soluble Analytes from Volatile Solvents Colin D. Chriswell* and Richard Markuszewski
Ames Laboratory, Iowa State University, Ames, Iowa 50011 Wick evaporation is a simple and effective specialized technique useful for recovering small amounts of soluble, nonvolatile analytes from volatile solvents. In the simplest form of this technique, a fiberglass wick is immersed in a solution contained in a narrow-mouth vial. The solution is drawn up by the wick by capillary action and is evaporated from the wick above the top of the vial, leaving a deposit of the soluble analyte on the tip of the wick. The advantage of this technique is that a small amount of analyte is concentrated on a very small area, i.e., the wick tip. In contrast, the direct evaporation of the solvent without the wick present can lead to dispersal of the small amount of analyte over a relatively large surface area. Despite its obvious utility, this wick evaporation technique, which has been used in our laboratory for over 10 years, has not been reported to our knowledge in the literature and appears to be unknown to most analysts. In past work at the Ames Laboratory, wick evaporation has been applied to the isolation of polyaromatic hydrocarbons from diethyl ether extracts of drinking water. The technique is likely to be applicable to similar applications in which small amounts of low-volatile analytes are isolated into volatile solvents. In the present work, the isolation of elemental sulfur from small samples of coal-derived humic-like materials is described as another example of the utility of this technique. In this application, wick evaporation serves both as a means for separating the soluble elemental sulfur from insoluble humic-like materials and as a means for concentrating the separated elemental sulfur in a small, accessible area on the wick tip. EXPERIMENTAL SECTION Preparation of Coal-Derived Humic-like Materials. The 10-g samples of Illinois No. 6 and Pittsburgh No. 8 coal were chemically cleaned by the molten caustic leaching (MCL) process in our laboratory (1-3). Subsequent water washing of the coal caustic cakes formed during MCL yielded caustic solutions containing soluble organic materials, referred to as humic-like materials, as well as other components. Acidification of the spent caustic solutions to pH 1 f 0.5 with hydrochloric acid resulted in the precipitation of crude humic-like materials contaminated with acid-insolubleand occluded inorganic substances. Typically, 100 to 200 mg of the crude humic-like materials were produced from each coal sample. Routine determination of parameters such as ash content, total sulfur, and heating value consumed most of the humic acids produced, leaving only small amounts (less than 50 mg) of material for determination of the elemental sulfur content. Isolation of Elemental Sulfur from Crude Humic-like Materials. A sample of humic-like materials resulting from the molten caustic leaching of Pittsburgh No. 8 coal was found to contain 56% total sulfur, both by the use of Fisher total sulfur analyzer and by an ion chromatography procedure developed in our laboratory ( 4 ) . A 25-mg portion of this sample was placed in a 1-dram glass vial, 1 mL of redistilled carbon disulfide was added, and the sample was dispersed by placing the vial in an ultrasonic cleaning bath for 15 s. A fiberglass wick, formed by
twirling glass wool between the fingers, was placed in the vial, and the top was trimmed in such a way that about 5 mm of the wick extended above the top of the vial (see Figure 1). The vial was allowed to sit in a hood until all the carbon disulfide had evaporated (approximately 30 min). CAUTION: Carbon disulfide is poisonous, malodorous, and very flammable. Because of its low flash point, it can be ignited by hot steam pipes. The wick was then removed from the vial, and the analyte removed from the tip by scraping. Total sulfur was determined, using the Fisher analyzer, in the analyte from the wick tip and in the insoluble humic acids that remained in the bottom of the vial. RESULTS AND DISCUSSION Elemental sulfur is soluble in carbon disulfide, whereas humic-like materials and most inorganic salts are insoluble. Thus, elemental sulfur in solution was carried with the solvent up the wick and was deposited on the tip of the wick when the carbon disulfide evaporated. The humic acids and other insoluble materials remained in the vial. The material deposited on the wick tip visually appeared identical with sublimed sulfur, and analysis for total sulfur showed it to have a purity of 96%. Thus, essentially only elemental sulfur was separated from the crude humic-like material by wick evaporation. Both the starting crude humiclike material and the residue remaining in the vial after wick evaporation were black powders. However, the starting crude humic-like materials contained 56% total sulfur, whereas the residue after wick evaporation was found to contain only 6% total sulfur. The remaining 6% sulfur is consistent with the organic sulfur content of coal-derived humic-like materials observed previously in this laboratory (5). Thus, essentially all elemental sulfur was removed from the crude humic-like material by the wick evaporation technique. The technique was applied to one other sample of humiclike materials derived from Pittsburgh No. 8 coal and to two additional samples of humic-like materials derived from an Illinois No. 6 coal. In each case, it was confirmed that essentially all the sulfur present in the crude humic-like materials was in the form of elemental sulfur. The finding that crude humic-like materials contained high concentrations of elemental sulfur was not totally unexpected because it is difficult to conceive of any sulfur-containing species that could be present in the crude humic-like materials and give rise to the high levels of sulfur determined-typically 20-6070. However, the presence of elemental sulfur in these samples had to be confirmed because previous work ( 4 ) has identified the sulfur species in the spent caustic solutions from which the humic-like materials were isolated, and elemental sulfur was not found. The wick evaporation technique proved to be a rapid, convenient, and effective method of confirming that elemental sulfur was present as the major source of sulfur in very small samples of coal-derived material. It is now hypothesized that polysulfides (e.g., Na2S5),which are known to be present as major sulfur-containing components of spent caustic solutions (6),react or decompose during the course of the isolation process to form elemental sulfur.
0003-2700/S8/0360-050S$01.50/0 @ 1988 American Chemical Society
509
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 James E. Woodrow, Michael M. McChesney, a n d James 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 configuration supplied 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
