Document zzE3kgmrnwVYdv7xb4LnnDezm

R&S 004144 / L C4 i A/ / ` / /it] 'C-c. W-2,; / THE DETERMINATION OF VINYL CHLORIDE MONOMER IN 7-, I. -- i` i- M. - l- CHARCOAL-LOADED PERSONNEL MONITOR TUBES USING THE^Z- // AUTOMATED GAS CHROMATOGRAPHIC HEAD SPACE TECHNIQUE A' A. Scbcstyen and II1. B. Thompson The Perkin-Ehner Corp. Norwalk. Connecticut 06S56 r I ^Z. ABSTRACT The head space method described herein is a simple and accurate, totally auhmi.m d. way to determine the amount of vinyl chloride monomer tYCMi which has been ad sorbed in a personnel monitor collection tube containing activated charcoal. The rela tively short analysis cycle time (6 minutes! and relative accuracy labour 2';) wi!i allow for a high throughput (over 200 samples per day) of accurate results. INTRODUCTION Gas chromatography (GC) has been shown (1-3) to be a preferred method for measuring residual vinyl chloride monomer (VCM) in many matrices. The preferred GC method for the analysis of absorption tubes used in per sonnel monitoring devices employs carbon disulfide as a desorbant. Alternate GC methods which avoid the use of CSi have been published (4, 5). This paper describes an other alternative GC technique wliich avoids Die use of CS2. In this method, the "static head space" sampling technique is used in conjunction with relatively safe desorbants. The head space technique is widely used for determining volatile compounds in complex and/or non volatile matrices (6, 7). Basically the head space sampling technique is accomplished by establishing an equilibtium between the sample and the gas phase above the sample, and subsequently sampling and analyzing the gas phase. Three relatively safe alternative desorbing agents were ex amined: ethyl alcohol (ETOH), benzyl alcohol (BA). N,N.dimethy] acetamide (DMAC). A VCM loaded sample charcoal tube consisting of a 100 mg front portion and a 50 mg second portion, was opened and each section was placed into separate standard 24 ml sample vials. A measured amount of desorbing agent was added to each vial. The vial was then sealed and equili brated in the water bath at a specific tempeiature. After equilibration had taken place, the F-42 automatically sam pled the head space in the vial, which then consisted of a mixture of the desorbing agent and the VCM. After the VCM peak eluted, the desorbing agent was backfiushed out of Die column. When the chromatographic analysis was completed, the instrument automatically sampled the sub sequent vials in sequence. In selecting desorbing agents for headspace work, certain criteria must be met: -- There may be no peaks that interfere with the VCM. -- Must be able to be backfiushed out of the stripper column. -- Must reach headspace equilibrium in a relatively short time. -- Must be a good desorber and have a good recovery rate. -- Must be relatively non-toxic. The three desorbers that were tried met most of the above criteria. EXPERIMENTAL The Perkin-Elmer Model F-42 automated gas chromato graphic head space analyzer was used with the automatic backfiush system and the Model 1 compiling integmior. The following procedure was used in ecjluatine each o: the desorbing agents. Charcoal sample tubes were loaded with identical volumes of 5 ppm VCM gas. Tire volume pulled through'was 571 mL at a sampling rate of 100 ml. min. After loading the sample tubes wiih a known aim,mo of VCM gas, the tubes were opened and the charcoal u.:\ separated into two vials: the front section :r> one vial. :iie rear (backup) section in another. One ml. of Die desorbing agent was added to the from section 1100 mg of charcoaii. To the backup section (50 mg of charcoal). 0.5 mL was added. The vials were immediately sealed and were placed in the thennostatted sample turntable. Ten minutes was allowed before sampling was started. Using a ten mmuie cycle lime, a time study was made showing tire equilibra tion period vs. time. When equilibration was reached On* large increase in area counts shown). Die standard dc-ia* lion was calculated using the concentration printout.' or the Model 1. At tire end of the run time' for all from 'cction charcoals, the `'backup'' section v.a> run to nuke 'lire no breakthrough occurred: and none had occurred. TaKe 1 shows time vs. cone, for each of the three desorbing agents. All three of the desorbing agents met tire first two cri teria previously listed. The differences me in the last three criteria. As can be seen in Table 1. the I'TOH desorbed VCM samples reached equilibrium first, followed b> the DMAC samples and last by the BA samples. Due to the close retention of the ETOH peak to the VCM peak, fire set up of the backfiushing is not easy. For that reason ETOH is ruled out. The benzyl alcohol takes a long time to reach equilibrium. Therefore the authors recommend the use of DMAC as the desorbing agent, even though :t has a lower recovery percentage (about '-0U). DMAC Iras a short equilibrium time, about 30 minutes, and the tuckflush timing is also made easier. Instrumental - The following condirions were set <<n instrument. The temperature settings were oven. 65 C . injector, 150 'C: detector, 150 C: needle. 150 'C: ware* bath, 60 *C for ETOH, 90 *C for BA and DMAC. The time settings were inject = 3 s, analysis = 30 s. F = BF set at 4 min, S = as needed, usually 0. Tire flows, with the inurn ment in the standby position, were set as follows il> Disconnect column from FID. (2) Measure and adjust fiow to obtain 24 mL/min using column pressure reguLtor. (3) Start the instrument cycle. While the instrument :s in the backfiush cycle, measure the flov. again. Flow should be adjusted to 24 mL/min, using the backfiush pre-ure regulator. Connect the column to the FID. ignite the fume. Run through an instrument cycle with range and atten uation at XI. Zero Die baseline during the analysis time ,CHROMATOGRAPHY NEWSLETTER Vol 7 No. 2 August, 1979 'Q  using the course and fine suppression control. When the huckfiushing cycle occurs, adjust the backflush pressure regulator to correct for small offsets in the baseline. Check laseline this way several times. Set the FID pressure litors at optimum settings which are approximately: air - 3.7 atm. and ll; - 2.4 atm. Obtain the retention time for the VCM peak, about 60 s. Then obtain the retention time for the solvent peak. DM AC and BA have greater than 45 min retention time, so the retention time is not critical. The ETOH retention time is approximately 140 s. First set the analysis time at about 1/3 of the retention time of the VCM peak. Run a sample of VCM + ETOH. Increase the analysis time by six seconds until the VCM peak appears on the recorder. If the analysis time is too long, the ETOH peak will elute. The optimum setting just allows the VCM peak and not the ETOH solvent peak to elute. Quantitation - Calibrate the charcoal tube with a known amount of VCM. Take a Tedlar bag, fill it with a known concentration, C (ppm) of VCM/air or VCM/N2 gas mixture. Pull through a calibration charcoal tube, with a personnel monitoring pump, n counts of this gas mixture. If the personal sampling pump has a sampling volume of L mL/count and has a rate of Y counts per minute, then the volume (mL) pulled through the charcoal tube per min ute is Vm = (L mL/ct) x (Y ct/min) The total volume (mL) pulled through the charcoal tube is (1) VT = VVCM = VM x t. where t is the time in minutes. The total weight of the ad sorbed VCM is expressed by the formula (-) WVCM = Vx x C x 2.5S ng (2.5S ng = weight of VCM in 1 mL of 1 ppm gas), value is proportional to the head space concentraof the VCM which is desorbed by a desorbing agent in a sealed vial that has been equilibrated at a specific tem perature. The loaded and desorbed charcoal is then used as the externa] calibration standard. (The external standard can be used because of the excellent repeatability of the sam ple size). Knowing that the desorption efficiency of the VCM on the calibration charcoal is the same as on the sample charcoal, the weight of the desorbed VCM in the unknown sample is (3) wuVC.M = Auvcm/AV'CM * WyCM where Avc.M is the area counts of the calibration VCM peak of weight Wy^yj (as calculated earlier) in the de sorbed calibration charcoal, and AuVCM is the area counts of the VCM peak of weight WUVCM in the desorbed un known charcoal. The average air concentration of the unknown VCM (in ppm) during the time the personnel monitor charcoal sample tube was worn is (4) Cu = Wuvcm/(Vu x 2.5S x 10'3) or combining equation (2), (3) and (4) (5) Cu = AuVCM/AVCM * vTA u x C x 1 O'6 The concentration for each unknown can be calculated automatically using the Model 1 automatic integrator or similar data system. The Vj and Vu are entered for each sample. DISCUSSION Having selected DM AC as the best overall desorbing nt, runs were made with varying concentrations and umes of VCM gas loaded on sample charcoal tubes. These runs were made to assure that the desorption rate of the VCM from the charcoal remains constant for varv- TABLEI Test of Desorption Reagents* ETOH 60C Thermostatted Samples BA 90C Thermostatted Samples DMAC 90C Thermostatted Samples Min Cone. Cone. Cone. 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 Mean = Sigma RSD = Recovery = 100.6 129.3** 112.5 115.6 115.4 125.4 123.8 134.7 118.5 127.6 127.0 129.2 121.5 126.1 125.9 129.4 134.0 116.1 122.3 124.2 6.5 5.2% 95% 85.0 95.4 105.5 122.0 129.8 140.6 138.2 141.7 147.0** 146.9 154.9 152.3 149.1 146.1 151.3 152.4 150.7 155.0 151.5 150.6 150.7 2.9 1.9% 94% 5.0 102.0 114.0** 112.5 113.2 115.1 110.8 116.7 114.3 111.3 115.9 116.4 117.7 115.9 112.1 109.3 113.2 113.9 2.4 2.1% 80% *The experimental conditions were: Column: 0.2% Carbowax 1500 on Carbopack C. Col. temp.: 65 C. VCM reten tion time: 60 s. FID x 80 Attn. " Equilibrium reached, ing amount loaded. Although this study was written using the approved NIOSH charcoal tubes for sampling, the desorption method from charcoal can be used on different amounts of char coal loaded in different ways. Adjustments may be needed in the volume of desorbing agents used or desorption times. But overall results should be as good as shown in this paper. REFERENCES 1. Method N'o. P&CAM-178 in NIOSH Manual ofAnalytical Meth ods, Dept. HEW, USPHS, NIOSH, 1014 Broadway, Cincinnati, Ohio, 45202. 2. L. D. White, D. G. Tavlor, P. A. Mauer, and R. E. Kupel, Am. Ind. Hyg. Assoc. J. 31, 225 (1970). 3. A. R. Berens, L. B. Crider, C. J. Tomanck and J. M. Whitney, J. Appl, Poly. Sci. 19, 3169 (1975). 4. J. E. Purcell, Amerb. Lab., May, p. 99 (1975). 5. D. H. Ahlstrom, R, J. Kilgour and S. A. Liebman, Anal, Chem. 47. 1411 (1975). 6. H. Hachenberg and A. P. Schmidt, "Gas Chromatopaphic Headspace Analysis", Heyden & Son, New York, 1977. 7. B, Kolb, J, Chromatop. 122, 553 (1976). CHROMATOGRAPHY NEWSLETTER ,30 VoI. 7 No. 2 August, 1979