2100
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
Flame Photometric Determination of Carbon Disulfide in Air after Specific Preconcentration Jean Godin,
Jean-Louis Cluet, and Claude B o u d h e
INSERM U 722, Laboratoire de Toxicologie, UER des Sciences Pharmaceutiques, 92290 Chatenay-Malabry, France
total sulfur estimation and the second (poly(pheny1ether), H3P04) for separation of sulfur compounds (6). For H2S and COS separation, the second column was replaced by a special silica gel column (Tracor) ( 7 ) . To assess the efficiency of CS2trapping by different absorbent mixtures, an automatically controlled injection device (Intersmat, France) was connected to the gas valve of the analyzer, allowing injection of a fresh gas sample every 3 min. For headspace determinations, a septum injector was placed in front of the column; the temperature was maintained at 60 "C. Gas syringes of 0.1- to 1-mL volume (type A2, Precision Sampling Corp., USA) were used throughout the study. Atmospheres contaminated with known amounts of CS2were prepared with a permeation tube maintained in a thermostated chamber (Tracor 412) as described previously (8). In this experiment, the mean CS2 permeation flow was 7.75 ,ug/h. Procedure. Sampling. Air contaminated with CS2was drawn through the absorbent cartridge at a flow rate of 150 mL/min. Flow rate, sampling time, and absorbent weight can all be modified. Analysis. The content of one cartridge was poured into a headspace flask containing 2 mL of 58% AR hydriodic acid (Prolabo). The flask was promptly capped and placed in a water bath at 40 "C. Two hours later an aliquot of the gas (usually 0.1 to 1 mL) was injected into the analyzer. Calibration. Calibration curves were constructed by running cartridges containing known amounts of CS2in exactly the same way as the samples. Daily standardization of the analyzer was simple and consisted of treated standard solutions of sodium diethyldithiocarbamateAFt (Carlo Erba, Italy) which, under these conditions, produce known quantities of CS2.
A new technique for determining carbon disulfide in alr has been developed. It Is based on concentrating this contaminant on a chromatographic support treated with sodium azide and hexamethylphosphorotrlamlde (HMPT) and then applying a headspace technique followed by flame photometry. This method is simple, accurate and allows analysls of a large number of samples. Among the different factors affecting quantitative determination of CS2 in air which were studied, temperaturedependent partial hydrolysis of CS2 leading to the formation of H,S and COS is the most important.
The use of large quantities of carbon disulfide by the chemical industry necessitates monitoring of this compound both in industrial and ambient atmospheres. It is therefore of interest to develop simple and rapid methods for sampling and determination of this pollutant. Colorimetry based on copper diethyldithiocarbamate formation has been extensively used for many years but is not very suitable for field measurement ( I , 2). Methods involving the trapping of CS2 on solid phases, like active charcoal sampling (3)and subsequent analysis with the flame photometric detector (FPD) are increasingly used because of their greater sensitivity and feasibility ( 4 ) . The ambient air concentration of CS2 has also been determined by cryogenic trapping a t -196 "C ( 5 ) . The aim of this work is to describe a new method for CS2 sampling and determination, both in industrial areas and in ambient air. This method is sufficiently easy and quick and requires only commercially available material, Le., a Tracor H M 270 analyzer fitted with a septum injector, and reagents. Carbon disulfide is first specifically retained on a treated support of high efficiency. The support is then placed in a headspace flask and the gas phase thus released is used for CS2 measurement by a flame photometry detector.
RESULTS AND DISCUSSION In the first part of this work, we tested the CS2 trapping efficiency of several mixtures. Air samples containing small amounts of CS2were drawn through cartridges and injected into the analyzer every 3 min by means of an automatic gas valve. The time interval was measured between CS2 entry a t the inlet of the cartridge and its exit a t the outlet. The best results were obtained with a mixture containing either sodium azide or a secondary amine (e.g., piperazine) and HMPT on Chromosorb W. HMPT greatly improved the capacity of the absorbent. Sodium azide was chosen because of its instability and because of the lability of the azidecarbon disulfide addition product. The efficiency of this trapping mixture was very high. As an example, CS2a t a concentration of 174 ,ugf L air, was drawn through 100 mg of treated Chromosorb a t a flow rate of 4.5 L / h for 6 h, 45 min before any trace of CS2 could be detected a t the outlet of the cartridge. In the second part of this work, we attempted to determine CS2 in the form of N3CS2-. Spectrophotometric determinations was not used because of insufficient sensitivity (9). Further, extraction of the addition product by organic solvents prior to chromatographic analysis is not satisfactory because of the presence of sodium azide and HMPT and also because these solvents usually prolong chromatography and require a back-flush system to avoid detector flame extinction. For all these reasons, we chose a technique involving the destruction of the azideCS2 addition product in acid medium and the volatilization of CS2. Among the different compounds
EXPERIMENTAL Reagents. The absorbing reagent was prepared as follows: 10 mL of HMPT (Prolabo, France), 10 mL of distilled water, 2 g of NaN3 (Prolabo),and 50 mL of ethanol (96% v/v) were successively placed in a 200-mL flask. The flask was gently stirred until the sodium azide had completely dissolved. Ten grams of acid-washed Chromosorb W (60-80 mesh) were then added. Ethanol and a large quantity of water were eliminated under vacuum in a rotary evaporator. The mixture can be stored without deterioration for long periods in tightly-capped vessels. Apparatus. Absorption cartridges were prepared by placing 80 t o 100 mg (about 1 cm) of treated support in glass tubes 4 cm long (i.d., 0.4 cm). The support was maintained by two glass wool plugs. For storage, cartridges must be capped at both ends by glass stoppers and small pieces of PTFE tubing. For release of CS2,40-mL headspace flasks were used (Pierce Chemical Co., Rockford, Ill.). They can be replaced by inexpensive 60-mL flasks fitted with Bakelite screw caps (Sovirel, France). The cap was lined with a PTFE disk 0.2 mm thick. A silicone rubber spacer was placed between the PTFE disk and the Bakelite cap. Both cap and spacer had a syringe bore of 5-mm diameter. CS2 determinations were performed with a Tracor HM 270 analyzer fitted with two chromatography columns, the first for 0003-2700/79/0351-2100$01.00/0
C
1979
American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
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decreased, forming a second peak. Judging by the retention times on the poly(pheny1 ether)-H3P04 column, the second peak might correspond to carbonyl sulfide and/or hydrogen sulfide. These compounds were readily identified on a silica gel column. COS and H2S formation may result from partial hydrolysis of the azide-carbon disulfide addition product in accordance with following scheme:
I
~i i
N,-C
130 -1VE
-r
n
Figure 1. CS2 generation from N3CS2- in a headspace flask
Table I. Precision Studya within a set of samples drawn from same
cartridge
COS and H,S
determination of CS, after complete conversion into COS and H,S
//
\ 6 0
determination of CS, without conversion into
2101
5.342 * 0.131 (20) 0.645 i 0.016
(22) -
within a set of samples drawn from different cartridges on different days 3.833 t 0.125 (16)
1.291 * 0.065 (14) 1.563 t 0.193
(18)
a Mean and standard deviation (number of determinations), results expressed in p g .
tested hydriodic acid completely released CS, within 2 h at 40 "C (Figure 1) whereas acetic acid, iodine + acetic acid, o-phosphoric acid and hydrochloric acid released only small amounts of CS2 during the same period. At higher temperature (80 "C), regeneration of CS2 was completed in less than 30 min, but under these conditions, condensations in syringes increased the variability of the results. This is why all incubations were performed at 40 "C. The volume of hydriodic acid had little effect on the results in the 2- to 6-mL range. Table I shows the results of reproducibility experiments with fresh samples. The detection limit may be improved by using smaller headspace flasks or increasing injected sample volume to several milliliters. Using 40-mL headspace flasks and 1-mL gas phase injections, the CS2 detection limit per cartridge was 1.2 ng or twice the background noise. However, since the reagents in the flasks contained CS2, it was not possible to attain this limit (mean of eight experimental determinations: 1.97 ng, s = 0.434). Great care was taken to avoid contamination of flasks or syringes. A standard curve can be made by injecting 0.1 mL of gas phase coming from the headspace flasks. So, the CS, quantity in cartridges is determined by the relation: log ng CS2 = 0.495 log R 1.339
+
where R = height of the peak (mm) x FPD attenuation (range: 10-7). Large variations in experimental conditions such as sampling time, air flow rate, flask volume, size of the sample injected, and the detector attenuation enabled determination of a wide range of CS2 concentrations in air. Another advantage of the method used is the possibility of making repeated determinations in a short time (less than 2 min per chromatographic measurement). A single peak appeared on chromatograms when cartridges were treated immediately after sampling; however, when they were kept at room temperature for several hours, this peak
S
HOH
-+
[
N3
