Simultaneous determination of molecular hydrogen, sulfide, and


Chem. , 1986, 58 (6), pp 1255–1258. DOI: 10.1021/ac00297a064. Publication Date: May 1986. ACS Legacy Archive. Cite this:Anal. Chem. 58, 6, 1255-1258...
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Anal. Chem. 1086, 5 8 , 1255-1258

Simultaneous Determination of Molecular Hydrogen, Sulfide, and Sulfate in Solution Samples from Experimental Hydrothermal Systems Sir: In the experimental studies of redox reactions under hydrothermal conditions, knowledge of the fo, (or the equivalent fH,) in the reaction system is a requisite for the thermodynamic interpretation of observed results. Several methods of fixing, regulating, and measuring fo, have been devised and used (1-6) but each of them has its own serious limitations. Another method of determining fo,, which is advantageous over existing methods in that it requires no extraneous substance to be brought into the reaction vessel and that it is applicable to relatively low temperatures, is possible simply by measuring the concentration of the Hz commonly present in reducing hydrothermal solutions. The measured Hz concentration, cHn,is translated to fo, according to the following equation, which is an expression for the equilibrium, Hz + ' / 2 0 2 = HzO: Here, fHzodenotes the water fugacity in the system, K, is the constant of the equilibrium, and Y is the fH,/CH, quotient. Numerical values of K , and of fH,O in pure water are Wellknown (7-9) and Y also has been evaluated by us recently for dilute Hzin s u b and supercritical water (10). Water fugacities in solutions could be estimated with fair certainties from existing data, but little has been known about the Y in solutions. However, from some preliminary data on the Y values in NaCl solutions (II), solubility data of gases in salt solutions at high temperatures (12),and physicochemical considerations to the possible interrelationship between the salt effect on fHzO and that on Y , it can be deduced that Y is not very sensitive to dissolved electrolytes as [email protected] not and that, accordingly, the Y values in water can be used for dilute solutions in a limited range. In the case of solutions having an ionic strength of 0.05 m, for example, this approximation would not introduce errors greater than 3% (positive) in the estimation of fo,. Thus, the new method is practicable. We have presented a pertinent gas chromatographic technique for solution samples withdrawn from the Dickson-type hydrothermal apparatus (13); Ziegenbein and Johannes (14) also have described techniques to puncture the sealed metal capsules used with the popular cold-seal hydrothermal apparatus and to analyze the liberated solutions by gas chromatography. Hydrothermal redox and related reactions of sulfur compounds are of great geochemical importance. A large volume of data has been accumulated concerning them but some important reactions need to be studied more precisely. The above method therefore is applied first to sulfur systems. A simple procedure of solution analysis developed for and being used in these studies is described below; i t is an extension of our previous technique (13).

CONSTRAINTS ON ANALYSIS Only sulfide and sulfate can become dominant sulfur species in hydrothermal solutions in our experimental temperature and pressure ranges below 500 OC and 1kbar (15),and hence the present analysis is assumed to determine only those species and H2 simultaneously. Precisions better than 2% were desired for all the three components, but obtaining this level of precision was difficult for sulfate in many samples because, in equilibrated systems, the sulfate/sulfide ratio and cH, are interdependent such that the ratio becomes very small as cHZ becomes large enough for measurement. Thus, it was most important in the present analysis to minimize spurious sulfate 0003-2700/S6/0358-1255$0 1.50/0

(below approximately 1% of initial sulfide) resulting from the oxidation of sulfide, incomplete separation of sulfate from sulfide, and sulfate contaminations. The maximum capacity of the hydrothermal reaction vessel (collapsible gold bag placed in the Dickson-type apparatus (16)) was 140 cm3, while the total number of samplings in a run was presumed to reach 60. These figures and the experimental 7'-P conditions limited an appropriate mean size of sample to be below 1.5 mL. In our procedure of cHZmeasurement outlined previously (13),the solution sample was taken into a plastic syringe and shaken with an aliquot of N2 and the resulting gas mixture was analyzed by using a gas chromatograph (GC). We decided to use the same technique here and combine it with the sulfate determination by means of the standard ion chromatograph (IC). Then, since the sulfide in the sample was not amenable to both GC and IC, a choice had to be made for a sulfide-fixing reagent to be put in the sample receiver (plastic syringe) prior to sampling, on the assumption that the fixed sulfide could subsequently be oxidized to sulfate. An ideal reagent should have a simple or stable composition containing nothing to interfere with the Hz and sulfate determinations and should provide efficient fixation of sulfide into a fast-settling, stable, colored precipitate; the volume of the fixing reagent also was limited below 1.5 mL. On the IC used in this study, OH-, CH3CO2-,C1-, Br-, and NO< ions higher than 150, 50, 8, 2, and 2 mM, respectively, interferred seriously with 1 pM sulfate, carbonate affected the peak height of sulfate, perchlorate was deleterious to the anion separator column (17), and metal ions in concentrations to form precipitates in the eluent flow also had to be avoided. It was also highly desirable that the oxidation of the precipitated sulfide be carried out within the sample receiver in order to avoid the error accompanying the transfer of analyte.

EXPERIMENTAL SECTION Apparatus. The sampling setup attached to the hydrothermal apparatus is shown in Figure 1; the sampling valve (SV) is a three-way, one-on pressure valve; i.e., the upper and the lower port are connected through. The GC (Shimazu GC-GAM) was equipped with a TCD and a dual separating column (3 mm i.d. X 5 m) packed with Porapak-Q; N2was used as a carrier gas. The IC (Dionex Model 10) was equipped with a 4 X 250 mm anion separator column, a 9 X 100 mm suppressor column, and a 0.1-mL sample loop; 1.5 mM Na2C03(pH 10.6) was used as an eluent with a flow rate about 150 mL/h. The sulfate concentration was determined from the peak height; the precisions were approximately f5, 2, 1, and 0.5% for 0.5, 1, 2, and higher than 5 ppm of SO?-, respectively. Reagents. Solutions of cadmium acetate (0.4 M) and NaOH (0.4 M) were prepared by dissolving the reagent grade chemicals in high-purity water. Stock MnOz powder was prepared by heating the reagent in water to remove soluble sulfate impurities and drying. Reagent grade concentrated (30%) H20zwas used as supplied; its sulfate content was determined previously. Sample Receiver (SR).Ordinary plastic syringes for medical use (Terumo Corp.; y-ray sterilized),having a maximum capacity of 12.5 mL and attached with a plastic three-way stopcock, were used as SRs. This brand and size of syringe was chosen because it best satisfied the requirements of SR; namely, it should be oxidant-free, chemically and thermally stable (not attacked by alkaline, concentrated Hz02heated to 65 "C), and transparent or translucent and provide smooth, light plunger action with good air-tightness. A gold ball having a diameter slightly larger than the diameter of the syringe orifice was put in the syringe to facilitate the agitation of content. Before use, all syringes were 0 1986 American Chemical Society

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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986

Sciences, Inc.; 3 mm in diameter and 0.45 Mm in pore size) and injected into the IC immediately. Then the SR was reweighed, 1 mL of the NaOH solution was added through the same filter (the precipitate caught by the filter was carried back), and the SR was warmed in an oven kept at 60-65 "C; 1 mL of hot (ca. 80 "C)H202was also added and the SR was allowed to stand in the same oven for 30 min with the stopcock open. After the SR was cooled to room temperature, a minute amount of Mn02was added through the orifice and the remaining H202 was decomposed. (SR was cooled with running tap water when the decomposition appeared to become too vigorous.) A portion of the resulting solution was filtered off and injected into the IC.

RESULTS AND DISCUSSION

"I

IP

Figure 1. Sampling setup: EF, electric furnace; SV, sampling valve: HN, hypodermic needle: SFR, sulfide-fixing reagent; SR, sample re-

ceiver. filled with water, heated to 65 "C to remove possible sulfate contaminations, and dried. Syringes to be reused were first immersed in 1 M acetic acid. Procedures. Typically, 0.25 g of the Cd solution and 0.70 g of the NaOH solution were added to an SR and shaken (by using a simple reciprocating mechanism) with pure N2 four times to purge dissolved air and the SR was closed with a minimum volume of N2 inside and weighed. Another plastic syringe containing a known volume (approximately 9 mL) of pure N2 was attached to the upper port of the SV as seen in Figure 1,and a flow of N2 was directed from the stopcock of this syringe through SV to the needle pipe attached to the lower port of SV. (SR was not attached to the needle hub yet.) While the Nz flow was continued, a small volume of hydrothermal solution was taken aside and the dew staying in the needle hub was wiped off; then SR was attached to the hub and the N2 flow was led into water in a cup. After a while, the N2flow path was cut off at the terminating stopcocks, and a solution sample (about 1.5 mL) was taken into the SR. Then, the sample was joined with the N2 transferred from the top syringe and shaken up for 2 min, and the gas phase wm finally returned to the top syringe. After the stopcocks of both syringes were turned off, the top syringe was taken over to the GC. The SR and the needle pipe were detached from SV and weighed for the weights of two solutions, one in these parts together and the other only in the SR (in order to give the concentrations of volatile and nonvolatile components, respectively). SR was then wrapped with aluminum foil in order to prevent light and allowed to stand with the stopcock upward until the precipitate settled. A portion (larger than 0.8 mL) of the supernatant fluid was filtered off through a small syringe filter (Gelman

The preliminary tests noted below were performed with water and H2S gas instead of the actual hydrothermal solution samples. Normally, the sulfide-fixing reagent mixture and about 1.5 mL of water were taken into an SR and shaken with N2, the SR was shut off enclosing about 9 mL of Nz, and then 0.983 (f0.005) mL at STP of H2S prepared in a small plastic syringe was introduced with care to prevent air contamination; subsequently the mixture was treated as described above (i.e., the SR was wrapped with aluminum foil, and so forth). In order for the true Hz concentration of the sample to be obtained, any formation or consumption of H2within the SR must be avoided. In this respect, it should be noted that CdS is a semiconductor photocatalyst having the potential to split water to Hz and O2 under the action of visible light. The evolution of H2, however, does not take place in the simple mixture of CdS and water (18) but requires an effective electron donor and becomes efficient when an appropriate redox catalyst is added. Within our concern, such combinations as follows are known to be able to evolve H2: CdS-EDTA (19),platinized CdS-a variety of organic compounds such as alcohols, saccharides, and amino acids (20), and CdS-RuOZ-HzS (21). In our SR, acetate ions, a small amount of silicone oil (used as lubricant for the plunger), a gold ball, and a trace of gold originating from the hydrothermal reaction vessel are invariably present with CdS suspension and trace of free sulfide. What is known about the effects of those subsidiary materials is only that acetic acid is ineffective even in the presence of platinized CdS (20). We performed therefore a few test analyses to see if H2 is formed within the SR. In one of them, neutralized chloroaurate solution was added to the initial mixture in the SR to give a gold concentration of 1mM (an unrealistically high value for the actual samples), and in addition, the separation of gas phase was done 30 min after the introduction of H2S. No Hz was detected throughout these tests, however. In this context, zinc and copper(I1) acetate solutions were considered and tested to some extent as alternatives to the Cd solution; ZnS and CuS are not photosensitizers capable of evolving H2 under visible light and they have been recommended by some analytical chemists (22,23)as fixation forms of sulfide. We confirmed that those solutions, made weakly alkaline (pH 9-11) with reference to the pH of the eluent of IC, had no problem with regard to H2but concluded that both of them were inadequate for the present purpose mainly because significant sulfite and sulfate signals resulted from them on the anion chromatogram for the first filtrates. (Sulfite ions are partially oxidized to sulfate in the eluent flow of IC (24).) The odor of H2S was noticeable with the filtrates from the Zn solution but not with those from the Cu solution. Thus, the cause of the sulfite and sulfate signals is judged to be incomplete fixation of sulfide for the former and to be oxidation of sulfide by Cu(I1) ions in the liquid phase or on the surface of CuS particles for the latter. It is also known that CdS suspension is comparatively liable to oxidation in aerated water in the presence of light (22,25,

ANALYTICAL CHEMISTRY, VOL. 58,NO. 6, MAY 1986 Table I. Hz, Sulfide, and Sulfate Concentrations in an BeSa-Fel$3-Pe~0~-Hz0System Equilibrated under Varied Pressures at 600 K

amt of sample no.

pressure, bar

4 5 6

1000 1000 1000 550 550 1000 1000 800 800 600 600 400 400 200 200 200 160 160 140 140 140 140

7

\

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

cm3

Hz,

amtof sulfide,

amtof sulfate,

date

(STP)/g

mmol

kmol

11/18 11/28 2/09 2/27 4/03 4/10 4/16 4/23 5/01 5/07 5/09 5/17 5/20 5 / 29 5/30 5/31 6/10 6/12 6/19 6/21 7/02 7/29

6.26 6.26 6.39 7.97 8.00 6.23 6.30 6.84 6.84 7.64 7.81 8.77 8.93 11.28 11.09 11.14 12.13 11.78 12.23 12.21 12.10 12.35

3.82 3.92 3.86 4.92 4.96 3.82 3.93 4.18 4.18 4.72 4.74 5.41 5.40 6.66 6.50 6.55 1.02 6.93 7.04 7.10 6.99 7.04

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8

nd nd nd 7 8

I 7 5 6 5 5 8 5 4 3 3 5 4 8 3

26). In fact, we observed that sulfate concentration increases slowly under the condition but stops increasing when light is cutoff. Aluminum foil, therefore, was used as mentioned in the procedure section because in our case, small volumes (less than 4 pL) of air originating from the sulfide-fixing reagent and accompanying the motion of the plungers of the top and bottom syringes were commonly present in the SR. Koch and Stolz (27) observed that CdS samples prepared by precipitation from solution can be decomposed at 80-90 "C in weakly alkaline HzOz,and after decomposing solid CdS samples to determine C1- dopant by IC, they destroyed the excess peroxide by boiling. Encouraged by their work, we tried to oxidize the CdS precipitate by use of HzOzat lower temperatures and decompose excess peroxide within the SR. At room temperature, addition of only HzOzresulted in irregular, incomplete oxidation (67-96%) although the color of CdS disappeared immediately. Satisfactory results could be obtained constantly under the conditions indicated above within the tolerance of the plastic syringe. The level of spurious sulfate and the recovery of sulfide (as sulfate) were critically examined by using six simulated samples (three a day) as mentioned above. The results could be summarized as follows: (1) Sulfate concentrations in the first filtrates were found between 0.8 and 1.8 pM (mean, 1.1); subtracting the 0.5 pM of sulfate originating from the sulfide-fixing reagents, the extent of the undesirable oxidation of sulfide was calculated to be between 0.002% and 0.010% (mean, 0.005%) of sulfide. (2) The sulfate concentrationswere apparently independent of either the ratio of Cd solution to NaOH solution in the range 1:2.5 to 1:3.3 or the time intervals between the air purge, introduction of HzS, and filtering off of the first supernate (even if both were extended to 50 min without moving the syringe plungers). (3) All the recoveries of sulfide were 100 0.5%. The 1.1PM of sulfate is regarded as the background level of the following analyses. In order to see if original sulfate ions coprecipitate with the CdS particles, test analyses similar to those described above were performed by adding small amounts of sulfate to the sulfide-fiiing mixtures. The sulfate concentrations in the first filtrates were observed between -0.3 and +0.1 pM relative to the expected values (about 20 pM). Thus, the coprecipitation

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was concluded to be negligible.

With this procedure, the determination of sulfide becomes more inaccurate as the sulfate/sulfide molar ratio in the original sample increases. This occurs because a fairly large portion of the first sulfatebearing solution cannot be separated from the rather voluminous CdS-Cd(OH)2 precipitate. For sulfide to be determined within a precision of 270,the molar ratio needs to be below 8 or so even if the sulfide is present in a large amount. However, if it is, the CdS precipitate could be rinsed within the SR and thus the limitation on the sulfate/sulfide ratio would be relaxed substantially. Finally, we will show an example of analysis on actual hydrothermal solution samples. Table I lists H2, sulfide, and sulfate concentrations measured in a system intended to represent the pyrite-pyrrhotite-magnetite-water system equilibrated at 600 K. With a few points being still unclear about the nature of this experimental system, we do not regard the listed data to be definitive for the intended system, but they will be enough to show the performance of the present procedure. Satisfactory reproducibilities and consistencies have been obtained for Hzand sulfide, but the results for sulfate are not good enough in comparison to the reproducibility of f l pM expected from the results of test analyses. Better results should be obtained by more careful elimination of sulfate contaminations from all the parts, including the open void in the sampling valve, with which the sample comes into contact and by preparation of the IC in its best condition (e.g., to give a smooth base line) at every time of sampling. The pyrite-pyrrhotite-magnetite assemblage, which defines both fs, and fo, as a function of temperature and pressure, is of great importance for hydrothermal experiments (28,29) as well as for the discussion of fs,-fo, conditions in natural systems. The simple system composed of the mineral assemblage and water deserves detailed study, where measuring cH2,along with concentrations of sulfide and sulfate, is almost indispensable. This point, however, will be clarified in a subsequent paper. The fugacity/concentration quotient of H2S in water at elevated temperatures and pressures will be evaluated in the near future. Then, the simultaneous determination of H2 and sulfide will acquire a new importance as a means of measuring fs, in experimental hydrothermal systems. Registry No. H2S, 7783-06-4; Hz, 1333-74-0; 02, 7782-44-7; sulfide, 18496-25-8; sulfate, 14808-79-8. LITERATURE CITED Eugster, H. P. J. Chem. Phys. 1957, 26, 1760-1761. Shaw, H. R. Science 1963, 739, 1220-1222. Chou, I-M.; Eugster, H. P. €OS, Trans. A m . Geophys. Union 1976, 57, 340. Gunter, W. 0.; Myers, J.; Wood, J. R. Contrib. Mineral. Petrol. 1979, 70,23-27. Potter, J. M. Abstracts with Programs, Annual Meeting of the Geological Societv of America. 1980. .. D 502. --Macdonald, D.;-Scott, A. C.; Wentrcek, P. J . Electrochem. SOC. 1981. 728. 250-257. Robie. R. A.;Hemingway, B. S.; Fisher, J. R. U . S . Geol. Surv. Bull. 1978, No. 7452. Burnham, C. W.; Holloway, J. R.; Davis, N. F. Spec. Pap.-Geol. SOC.A m . 1969, No. 132. Haas, J. L., Jr. Geochim. Cosmochim. Acta 1970, 3 4 , 929-932. Kishima, N.; Sakai, H. Earth Planet. Sci. Lett. 1984, 67,79-86, Kishima, N. unpublished work, ISEI,Okayama University, 1985. Drummond, S. E. Ph.D. Thesis, Pennsylvania State University, 1981. Kishima, N.; Sakai, H. Geochem. J . 1984, 78, 19-29. Ziegenbein, D.; Johannes, W. Neues Jahrb. Mineral., Abh. 1977, 130, 145-149. Barnes, H. L.; Czamanske, G. K. "Geochemistry of Hydrothermal Ore Deposits"; Barnes, H. L., Ed.;Holt, Rinehart and Winston: New York, 1967; Chapter 8. Rytuba, J. J.; Dickson, F. W. Proc, Int. Assoc . Genesis Ore Deposits Symp. 4th 1974, 2 , 320-326. Dionex Application Note 18: Dionex Corp.: Sunnyvale CA, 1979. Darwent, J. R.; Porter, G. J . Chem. Soc., Chem. Commun. 1981, 145- 146. Harbour, J. R.; Wolkow, R.; Hair, M. L. J . Phys. Chem. 1981, 8 5 , 4026-4029.

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(26) Sakata. T.; Kawal, T. “Energy Sources through Photochernlstry and Catalysls”; Gratzel. M., Ed.: Academic Press: New York, 1983; Chapter 10. (21) Borgarello, E.; Kalyanasundararn, K.; Gratzel, M.; Pelizzetti, E. Helv. Chim. Acta 1982, 85. 243-248. (22) Pomeroy, R. Anal. Chem. 1954, 2 6 , 571-572. (23) Kivalo, P. Anal. Chem. 1955, 2 7 , ’1809. (24) Hansen, L. D.; Richter, E. E.; Rolllns, D. K.; Lamb, J. D.; Eatough, D. J. Anal. Chem. 1979, 4 7 , 633-837. (25) Hlllebrand, W. F.: Lundell, G. E. F.; Bright, H. A,; Hoffman, J. I. “Applied Inorganic Analysis”, 2nd ed.; Wiley: New York, 1953; p 723. (26) Krasnovskil, A. A,; Brln, G. P.; Luganskaya, A. N.; Nikandrov, V. V. Dokl. Akad. Nauk SSSR 1979, 2 4 9 , 896-899. (27) Koch, W. F.; Stolz, J. W. Anal. Chem. 1982, 54, 340-342.

(28) Raymahashay, B, C.:Holland, H. D. Science lQ68, 162, 895-896. (29) Crerar, D. A.; Susak, N. J.; Borcsik, M.; Schwartz, S. Geochim. Cosmochim. Acta 1978, 42, 1427-1437.

Noriaki Kishima Institute for Study of the Earth’s Interior Okayama University Misasa, Tottori-ken, 682-02, Japan

RECEIVED for review October 15,1985. Accepted January 28, 1986.

Sorption of Benzene and Toluene by Poly(tetrafluoroethy1ene) during Headspace Analysis Sir: We wish to convey a word of caution about the use of poly(tetrafluoroethy1ene) (PTFE)-coated septa or PTFEcovered stirring bars in connection with the sample vials used in headspace analysis attachments for gas chromatographs (GC). We have two such headspace systems: one is a commercially available Perkin-Elmer Model HS-100 headspace option coupled to a Perkin-Elmer Sigma 2000 gas chromatograph and the other is a headspace attachment we designed and constructed for a Varian Model 3700 gas chromatograph (1).

EXPERIMENTAL SECTION Part of our interest in headspace analysis centers around its use to determine physical properties such as solubilities ( 2 , 3 ) , partition coefficients (3),activity coefficients (3),etc. of compounds with appreciable vapor pressures. In these determinations we cannot make use of conventional calibration curves but relate the GC response of the vapor above a stirred solution to the response of a vapor of known partial pressure. Such an approach does not allow for a cancellation of errors due to effects such as solute adsorption or absorption. Therefore, it is necessary that we screen our system for potential sources of these types of errors. As part of this screening process a series of duplicate headspace experiments designed to determine the solubilities of benzene and toluene in aqueous sodium chloride solutions of varying ionic strengths were conducted. The experiments were conducted using 125-mL septum bottles (Wheaton “400” clear glass) as equilibration vessels and involved determining the solubility of the hydrocarbon in 50 mL of solvent at 25.00 “C. The bottles were thermostated and shaken for a minimum of 72 h prior to analysis. The experiments varied only in the type of stirring bars used (glass or PTFE covered). A detailed discussion of the experimental procedure can be found elsewhere (3). In two series of samples the vapor was not analyzed, but the PTFE-covered bars were removed and subjected to multiple headspace extraction (MHE) analysis. One of these series contained 87.9 mg of benzene and 50 mL of water (-73% of the benzene needed for system saturation), the other contained 34.7 mg of toluene and 50 mL of water (-80% of the toluene needed for system saturation).

RESULTS AND DISCUSSION The results of the solubility determinations are listed in Table I. A plot of the logarithm of these solubilities as a function of ionic strength yields a series of straight lines, as expected (4). However, the difference in solubility of a given

hydrocarbon a t a particular ionic strength in the presence of PTFE- or glass-covered bars is only a qualitative indication that the PTFE bars are sorbing hydrocarbon. The amount being sorbed cannot reliably be determined from the solubility data since the heterogeneous equilibrium between the aromatic compound and the P T F E depends on the particular stirring bar used. MHE analysis of the bars yielded amounts of benzene ranging from 1.34 to 3.09 mg with the mean value being 2.07 mg with a standard deviation of the mean of 0.38 mg. The corresponding values for the bars from the toluene samples were 1.87-2.86 mg with a mean value of 2.44 mg and a standard deviation of the mean of 0.21 mg. For both cases a mean value of 26.5 pmol of benzene or toluene was sorbed. In both cases the slopes of the respective standard MHE curves were a little more than 3 times the slopes of the MHE plots for the PTFE bars. This latter observation can be taken to indicate the benzene and toluene were substantially absorbed into the PTFE and is similar to the behavior observed by Kolb for MHE analysis of residual vinyl chloride monomer in poly(viny1 chloride) (5). It should be noted that the mean amount of benzene and toluene recovered in the MHE analyses would be sufficient to produce over 500 layers of benzene or toluene if only adsorption occurred. We believe that these results demonstrate that PTFEcovered magnetic stirring bars (0.8 cm by 2.5 cm bars manufactured by Bel-Art Products, Pequannock, NJ) absorb up to several milligrams of benzene or toluene under the experimental conditions we used. Since at least one manufacturer (Perkin-Elmer) offers a magnetic stirrer and PTFE-covered stirring bars as options for their headspace analysis attachment, we felt that we should convey to the scientific community what our experiments have shown. If PTFE septa and/or PTFE-covered magnetic stirring bars must be used in an analytical procedure, one would be well advised to determine the extent and the rate at which they may sorb organic molecules. Bars should treated in a consistent fashion between uses so as to minimize any error. Much of our work precludes the use of PTFE, so we employ aluminum-coated septa and glass-covered bars. Fortunately we have been able to have small 0.5 mm by 1.27 mm glasscovered magnetic bars made by Bel-Art Products for the 25-mL vials used in our Perkin-Elmer Model-100 headspace unit.

0003-2700/86/0358-1258$01.50/00 1986 Amerlcan Chemical Society