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infrared spectrum of approximately 75 ng of a polyethyl-
Figure 2. Infrared spectrum of a carbon-filled co-polymer generated through a 3 0 0 - ~ maperture
Figure 3.
from the fillers. Figure 2 shows the spectrum of a 2+m, thin section of a carbon-filled polystyrene-butadiene copolymer blend. The record was generated using a 0.3-mm jewel aperture. Figure 3 shows the spectrum of a polyethylene film approximately 75 ng generated through a 0.25-mm aperture. Both spectra were recorded between 0 to 100% transmittance. This microtomy technique has been of great value when studying the homogenity of mixed polymeric systems. Small areas showing abnormal physical features by microscopy can be isolated and analyzed by FTIR to obtain chemical information. Sample preparation also includes pressing materials into jewel apertures. This is accomplished by manually compressing the sample into the aperture using two thingauged, flattened needle tips while viewing under the microscope. It is important to note this sample may or may not require KBr as a matrix. Owing to the micro amounts of sample used, contamination is always a problem to consider. Preventive measures
included the following steps: 1) Use a low-powered microscope to check all equipment and apparatus coming in contact with the sample. 2) Clean all needles, stainless steel apertures, and other major metal parts by rinsing in various solvents, e.g., methylene chloride and acetone, and heating over a low-flame micro burner. Check under a microscope and use a tungsten needle to remove any remaining material before storing in a clean glass container. The watch jewel apertures can be cleaned by rinsing in hot water and air drying. 3) View the sample with a microscope while using a small mortar and pestle to grind the KBr matrix. Also make sure both the pestle and mortar surface are smooth to prevent loss of material in digs and crevices. 4) Most important, do not touch prepared samples; use clean tongs or tweezers.
ene film
LITERATURE CITED (1)D. H. Anderson and 0. E. Miller, J. Opt. Soc. Am., 43,777-779 (1953). (2) P. R. Griffiths and F. Block, Appl. Spectrosc., 27, 431-434 (1973).
RECEIVEDfor review April 24,1975. Accepted July 9,1975.
Evacuated Gas Sampling Valve for Quantitative Head Space Analysis of Volatile Organic Compounds in Water by Gas Chromatography William F. Cowen, W. J. Cooper, and Jerry W. Highfill
U.S. Army Medical BioengineeringResearch and Development Laboratory, Fort Detrick, Md. 2 170 1 The quantitative analysis of volatile, soluble organic compounds in water has been reported by Weurman ( I ) , Ozeris and Bassette (2); and Kepner et al. ( 3 ) . These authors sealed the water sample in a bottle, removed a sample of the head space gas with a syringe, then injected the sample through the septum of a gas chromatograph for separation and quantitation of the volatile compounds. Our preliminary tests with this method of head space sampling have resulted in septum failures, with consequent loss of sample and unsatisfactory analytical reliability. In the work reported here, the construction of a device for septumless injection of head space gas is presented and its performance compared with that for syringe injection.
EXPERIMENTAL Apparatus. Gas Chromatography. A 6-ft long, 2-mm i d . glass column was packed with G P 0.4% Carbowax 1500 on Carbopack A (Supelco, Inc., Bellefonte, Pa.). All studies were performed with a Hewlett-Packard 5750B gas chromatograph equipped with flame ionization detectors and interfaced with an Autolab System IV integrator (Spectra-Physics, Santa Clara, Calif.) for acquisition of
retention time and peak area data. After injection of sample, the column temperature was held at 80 "C for 2 min, then increased a t 2 OC/min to 100 OC. Gas flows were: helium carrier, 10 ml/min; hydrogen, 30 ml/min; and air, 250 ml/min. The injector and detector blocks were maintained a t 170 and 150 "C, respectively. Gas Sampling Deuice. Figure 1 shows a diagram of the gas sampling device for septumless injection of head space gases. A Valco 8-port gas sampling valve with %-in. zero dead volume fittings (purchased from Aadco, Rockville, Md.) was fitted with two l-cm3 stainless steel IA-in. 0.d. sample loops and was connected into the helium carrier gas line of the gas chromatograph between the instrument's flow controller and the Y l 6 - i ~0.d. capillary coil in the injector block. T h e connection of the valve to the capillary coil was made with Ih-in. 0.d. stainless steel tubing, using the %-%6-in. Swagelok connector supplied with the instrument. The valve and sample loops were mounted outside the chromatographic column oven and were warmed to 70 OC with heating tape, as measured a t the body of the valve. A stainless steel Luer-Lok syringe needle (Yale 24 gauge, l-in. length, SGA Scientific Inc., Bloomfield, N.J.) was soldered to the ferrule end of a Swagelok %-in. port connector for connection to a Whitey toggle valve (VI) (both available from Crawford Fitting Co., Cleveland, Ohio). This valve was connected to the Valco valve by a short length of l/~-in.0.d. stainless steel tubing to provide a leak-tight sample input line. The remaining port
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
2483
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Figure 1. Schematic of the gas sampling device for head space
analysis of the Valco valve was connected to the helium and vacuum carrier gas lines via Whitey toggle valves V2 and V3, respectively, by means of a Swagelok union tee. A 0.05-Torr vacuum was supplied by a Welch Duo-Seal pump. The helium cylinder used to supply carrier gas to the gas chromatograph also supplied helium for back flushing the Valco valve sample loops and syringe needle, via a union tee split, at a cylinder gauge output pressure of 40 1b/im2. When vented through the Valco valve sample loop and syringe needle, the backflush flow was about 4.7 l./min. The helium supply and vacuum lines shown in Figure 1 were constructed with ?&in. 0.d. copper tubing. The rubber septum of the gas chromatograph inlet port was sealed from the carrier gas stream with two to three layers of aluminum foil placed in back of the septum, preventing interferences from volatile organics which bled off the heated septum. Standard Solutions. Combined standard solutions were prepared by adding 10 p1 of ACS reagent grade chemicals to 11. of distilled water. This stock solution was then diluted as desired with distilled water. Head Space Bottles. The addition of sodium sulfate to "salt out" volatile organic compounds has been shown by Bassette et al. ( 4 ) to increase the gas chromatographic peak heights by 4 to 7 times in the headspace analysis of these compounds in water. Accordingly, sodium sulfate (1.2 g per bottle) was weighed into 15-ml serum bottles, which were then heated a t 340 "C for 8 hr. The bottles were cooled to room temperature in a vacuum desiccator, then capped under a nitrogen atmosphere in a glove bag. Rubber flangewith-slotted-plug-type stoppers were sealed onto the bottles with aluminum crimped sealers. Procedures. Internal Standardization, Two procedures were used for adding the isobutyl alcohol internal standard to the test solutions. In procedure A, 2 ml of test solution were injected with a syringe into the head space bottle, followed by 0.1 ml of a solution of 42 fil/l. isobutyl alcohol in water. In procedure B, 1 ml of 50 pl/l. isobutyl alcohol solution was added to a 25-ml volumetric flask and diluted to volume with test solution. Two milliliters of this mixture were then injected into the head space bottle. In all injections into head space bottles, a 25-gauge needle was used to bleed excess pressure during sample addition. The final concentration of internal standard in both procedures was 2 pl/l. Analysis of Head Space Gas with t h e Gas Sampling Device. The head space bottles were immersed to their necks in a closed water bath at 70 "C for at least 70 min. They were then removed from the bath and immediately sampled with the gas sampling de2484
vice. As shown in Figure l , position l, loop l was chromatographed while loop 2 was being prepared for sampling through the following sequence: 1) during the chromatography of loop 1,valves VI and V2 were closed while V3 was open to evacuate loop 2 (assisting in removal of volatile residues from that loop); 2) a t the conclusion of the loop 1 chromatographic run, VS was closed and VI and V2 were opened to provide a 1-min helium backflush through loop 2 and the syringe needle; 3) VI and V2 were closed and V3 was opened for 1 min to evacuate loop 2 for sample loading; 4) V3 was closed, the stopper of a head space bottle was pierced with the syringe needle, and VI was opened for 30 sec; 5 ) VI was closed and the Valco valve switched to position 2, placing loop 2 into the carrier gas stream. These steps were then repeated for preparation of loop 1 for sampling. The peak areas of the resulting chromatograms were reported as the ratios of peak area to the internal standard peak area. Peaks in the chromatograms were identified by comparing peak retention times to the retention times found with solutions of individual compounds in water. Analysis of Head Space with a Syringe. After the head space bottles had been heated to 70 "C for a t least 70 min, a Hamilton No. 1001 gas-tight syringe was filled and emptied three times into the head space bottle before removing a 1-ml sample for injection through the rubber septum of the chromatograph. The bottles were removed from the water bath just before sampling with the syringe. The syringe was cleaned between runs by flushing with nitrogen several times. In preparation for each injection, the syringe was supported above the surface of the water in a closed bath at 70 "C for 10 min. For comparison of the gas sampling device and the syringe, a random sequence of 10 combined standard (2 plA.1 solutions were analyzed by each method. The standard solutions were prepared by procedure A. A fresh rubber septum was placed into the gas chromatograph at the start of the tests and was sealed from the carrier gas stream by aluminum foil before each gas sampling valve injection. The foil was removed prior to each syringe injection to simulate the commonly used procedure for syringe injection of head space gases. Carry-Ouer Tests. T o evaluate the carry-over from the gas sampling valve, nine sets of three combined standards in the sequence 0.5-10.0-0.5 pl/L were run. The relative peak areas of the first and second 0.5 pl/l. standard were compared in each set to find the carry-over from the 10.0 pl/l. standard. All standards were prepared by procedure B. Standard Curves. Combined standards (0.1,0.5, 1.0, 2.0, 5.0, 8.0, and 10.0 pl/l,) were prepared according to procedure B. Using the gas sampling device, the standards were run in order of ascending concentration. Blanks were prepared with distilled water and internal standard. The data from four consecutive standard series run on the same day were fitted by least-squares for each chemical in the combined standard solutions.
RESULTS AND DISCUSSION Preliminary tests with the gas sampling device were performed in a factorial design experiment with three factors, each a t two levels as follows: A) heating period of head space bottles in the 70 "C water bath, 10 min and 70 min; B) valve-sample loop temperature, 30 and 70 "C; and C) time period for loading sample gas into evacuated sample loop, 2 and 30 sec. Acetone standards of 2 p1/1. concentration were used in these tests. Only A produced significant differences (
