Anal. Chem. 1987, 59, 2670-2673
2870
Determination of Carbon Disulfide in Natural Waters by Adsorbent Preconcentration and Gas Chromatography with Flame Photometric Detection Ki-Hyun Kim and M. 0. Andreae*' Department of Oceanography, Florida State University, Tallahassee, Florida 32306
Carbon disulfide (CS,) in natural waters is determined at the picomoiar level by a comblnation of preconcentration on Carbosieve G and gas chromatography with a flame photometric detector. The volatile carbon disulfide is stripped out of up to 1.8 L of sample by a nitrogen gas stream and preconcentrated on a Carbosieve adsorption tube. Thls tube is then attached to the gas chromatograph/flame photometric detector system and heated to desorb CS2, whkh Is collected in a liquidnltrogen-cooled trap, released by heating the cold trap, separated on a Chromosll 330 column, and detected by a flame photometric detector. The CS, peak Is recorded and integrated eiectronicaiiy. The detection iimlt is ca. 1 pmoi L-' CSp The analytical precision is 9 % Results of analyses of natural water samples are presented.
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Carbon disulfide (CS,) was first observed in seawater by Lovelock ( I ) . Since then several attempts have been made to evaluate the geochemical cycle of CS2 and to identify its role in the global sulfur budget by assessing its atmospheric concentration (2-7) and oxidation rates (7-10). Although there have been some analytical methods developed for the determination of atmospheric CS2, they are not directly applicable t o the determination of CS2 in an aqueous matrix due to lack of analytical sensitivity or problems with chromatographic separation, especially between CS2 and dimethyl sulfide (DMS) (11). Using gas chromatography with an electron capture detector, Lovelock ( I ) reported that the natural background levels of CS2 in seawater are on the order of 10 pmol L-' CS2. On the basis of his report, we first investigated the gas chromatography/electron capture detection (GC/ECD) technique. However, with this method, we experienced serious interference problems with organ0 halogen species depending on the types of columns utilized for separation. Therefore, we developed a new procedure combining gas chromatography/flame photometric detection (GC/FPD) with the Carbosieve preconcentration method which had been ktroduced for atmospheric measurements of CS2 by Tucker e t al. ( 5 ) . Here we present the first report of an analytical method for the determination of CS2 in an aqueous matrix a t environmental levels. EXPERIMENTAL SECTION Preconcentration Apparatus. A schematic diagram of the preconcentration apparatus is shown in Figure 1A. The two stripping vessels with a capacity of up to 2 L are constructed of Pyrex glass with a 40/35 ground glass fitting. A nitrogen gas stream is introduced into the water samples through fritted glass bubblers. This parallel configuration is efficient for testing the precision and accuracy of the analytical system and also allows the simultaneous purging of two samples to reduce analysis time. Present address: Max-Planck-Institut fur Chemie, Postfach
3060, D-6500 Mainz, Federal Republic of Germany.
The outlet of the vessel is connected by Teflon tubing to a potassium carbonate filled (K2C03)drying tube (to prevent water vapor from wetting the Carbosieve adsorbent) followed by the Carbosieve adsorption tube. This adsorption tube serves to preconcentrate the volatile CS2 from the water samples. The adsorption tube consists of a 6 mm o.d., 4 mm id., 20 cm long glass tube in which a 1-cm section of 80-100 mesh CarbosieL-: G adsorbent (Supelco,Inc., Bellefonte, PA) is held by two small glass wool plugs. The breakthrough volume of this adsorption tube is ca. 50 L. Analytical Apparatus. A Hewlett-Packard Model 5890 gas chromatograph with a flame photometric detector was interfaced with the CS2 desorption system. A schematic diagram of the instrument is depicted in Figure 1B. Two independently regulated helium carrier gas streams are introduced into the system. The first He carrier gas stream, which supplies the desorption system, passes a 12.7 mm o.d., 80 cm long stainless steel tube scrubber packed with activated charcoal (50-200 mesh, Fisher Scientific, Pittsburgh, PA) and molecular sieve (Union Carbide type 4A, Fluka Chemical Co., Hauppauge, NY). The desorption section is composed of an adsorption tube and a heating device made of about 1 m of 5 m-l Chrome1 wire wrapped around the tube and connected to a variable transformer. The end of the desorption section is connected to a six-way rotary valve (Altex series 202, Rainin Instrument Co., Woburn, MA). When this six-way valve is in the load position, the carrier gas stream from the desorption section (flow rate 100 mL m i d ) passes through a 6 mm o.d., 20 cm long glass U-tube filled with 45-60 mesh Chromosorb W-AW-DMCS, which serves to trap cryogenically the CS2 desorbed from the adsorption tube. At liquid nitrogen temperatures, no breakthrough has been observed with this trap over trapping times of up to 2 h. A second He stream serves as chromatographic carrier gas and passes through a 2 m long, 3 mm 0.d. FEP Teflon column packed with Chromosil330, a specially treated silica gel (Supelco, Inc., Bellefonte, PA), which is operated isothermally at 50 "C and separates CS2from other sulfur compounds, in particular DMS. After cryogenic trapping, the six-way valve is switched to the inject position and the second carrier gas stream transports the eluted CS2 into the Chromosil330 column. The end of the column is connected to a flame photometric detector. This detector consists of a Pyrex burner and a photomultiplier system which are similar to those described by Andreae and Barnard (12). The optimal gas flow rates for the burner were found to be as follows: H2, 110 mL min-'; He, 80 mL min-I; and air, 100 mL min-'. Standards. Liquid standards were prepared by a stepwise procedure. Primary standards were prepared by injecting a known amount of liquid CS2 into a known amount of ethylene glycol. The flask was sealed and shaken for about 1h to ensure complete mixing. The resulting solution was further diluted stepwise until working standards with final concentrations of ca. 0.15 pmol of CS2/pL of glycol were obtained. Microliter amounts of this standard solution were then added to degassed water to obtain the working standards. A CS2 permeation device with a permeation rate of 6.9 ng min-' S(CSJ at 24 "C (Vici Metronics, Santa Clara, CA) was used as an alternative standard and cross-checked against liquid CS2 standards. A similar method was used by Andreae and Barnard (12)for the preparation of DMS standards. Operational Procedures. Water samples (up t o 1.8 L) are loaded into the stripping vessels, and the drying and adsorption tubes are attached to the outlets. The nitrogen purging gas stream is then passed through the vessels at a flow rate of 600 mL min-I
0003-2700/87/0359-2670$01.50/0 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 59. NO. 22, NOVEMBER 15. 1987 * 2671
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Table I. Results of Stripping Efficiency Test
stripping time, min
cumulative stripping time, min
amt of C S , found, pmol of
cs*
mean
recovery,
cumulative
std dev
%
(a) 98.6 pmol of CSo added to 1 L of degassed water; flow rate, 500 mL m i d ' bine replicate samples) 5 5 5 5 10 30 60
5 10 15 20 30 60 120
72.7 18.5 6.2 2.3 2.1 1.9
