Anal. Chem. 1983, 55, (7) Lores, E. M.; Bristol, D. W.; Moseman, R. F. J . Chromafogr. Sci. 1978, 16, 358-362. ( 8 ) Lores, E. M.; Meekins, F. C.; Moseman, R. F. J . Chromafogr. 1980, 188, 412-416. (9) El-Dib, M. A. J . Assoc. Off. Anal. Chem. 1971, 5 4 , 1383-1387. (IO) Bhatia, I. S.; Bhatia, M. !%;Singh, S.; Bajaj, IC L. Ann. Chim. (Paris) 1976, 7 , 7-8. (11) Ai-Ghabsha, T. S.; Rahim, S. A,; Townsend, A. Anal. Chim. Acfa 1976, 8 5 , 189-194. (12) US. Environmental Protection Agency Method 607, Nitrosamines Fed. Regist, 1979, 44 (Dec 3), 79496-69500. (13) Interagency Testing Cornmlttee; Receipt of Fourth Report and Request for Comments Fed. Regist. 1979, 44 (June I), 31866-31889. (14) Giaser, J. A.; Foerst, D. I..; McKee, G. D.; Quave, S. A.; Budde, W. L. Environ. Sci. Technoi. 1981, 15, 1426-1435.
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(15) Riggin, R. M.; Luca!s, S. V.; Cole, T. F.; Birts, M. A. Final Report on Contract No. 88-03-2952; US. Environmental Protection Agency, Environmental Monitoriing and Support Laboratory, Cincinnati, OH, 1983.
RECEIVED for review March 2,1983. Accepted June 29,1983. This research was supported by U.S. Environmental Protection Agency, Contract Number 68-03-2952. The contents of this paper do not necessarily reflect the views and policies of U.S. Environmental Protection Agency and the mention of trade names and commercial products does not constitute their endorsement.
Determination of Trace Levels of Water in Gaseous Samples by Gas Chromatography with Helium Ionization Detection Fikry F. Andrawes American Cyanamid Company, Chemical Research Division, Stamford Research Laboratory, 1937 W e s t M a i n Street, Stamford Connecficut 06904
A method for the determlnatlon of traces of water In gaseous samples by gas chromatography with a helium lonlzatlon detector was developed. A deactlvated glass-llned stainless steel column packed with deactlvated Porapak N provlded adequate separation of water from both actlve and Inert gases. Gaseous sample!, are Introduced with a syrlnge by on-column Injection, Into a modlfled Injection port. A water permeation tube generated various concbntratlons of water In a gaseous stream for standardlzatlon purposes. The method Is h e a r In the range between 2 ppm and 100 ppm; however the lower detectable limit was dependent upon the amount of water in the system blank. Thls blank originates from the sample introductlon system and was not completely ellmlnated. Water in butadlene and sulfur hexafluorlde was determlned to demonstrate the appllcabillty of the method.
The determination of traces of water is one of the most difficult and most important analytical problems. Water is universally present, is highly polar, and adheres to almost all surfaces. The Karl Fisher titration method (1)has been used for water determination for half a century and is the most widely used method today. This method however has some limitations, such as interference from compounds that readily undergo oxidation/reduction side reactions with the reagents used (2, 3), it does not lend itself readily to the analysis of either solids or gases, andl it has a modest detection limit of only a few micrograms. There are several other methods for water determination such as the azeotropic distillation method, the dew-point method, and electrolytic, gas chromatographic, near-infrared spectrometric, and infrared spectrometric procedures. With the exception of gas chromatography, most of these methods have limited applications and are not commonly used. Gas chromatography on the other hand has become more popular for water analysis and has completely replaced the Karl Fisher method in some laboratories. GC has a number of advantages: it is a direct and fast method, it detects other compounds present, and it lends itself to a variety of applications. The major limitation of the GC method for water analysis is the relatively modest detection limit. The thermal conductivity detector, which is the most com-
monly used GC detector for water analysis, has a lower dotection limit of only i3 few micrograms. To improve the dotectability of the gas chromatographic method, concentration and chemical derivatization have been used. Precolumn concentration with a cold trap has been reported ( 4 ) ,as well as chemical derivatization of water to organic compounds followed by flame ionization detection. Converting water to CzHz over calcium carbide is the most common procedure (5-7) while other derivatizing agents such as lead tetraacetate (8) and dimethoxypropane (9) are less commonly used. Concentration and chemical derivatization techniques however have their own limitations and are only used due to the lack of a direct and more sensitive method. The helium detector provides response to all compounds with an ionization potential below 19.8 eV. Water has an ionization potential of 12.8 eV and therefore could be detecteld readily and with good sensitivity. This sensitive detector was suggested for direct trace determination of water (lo), but it was not successfully used due to some difficulties in its opleration. Recently some of these problems were resolved (11-14). In this work, traces of water in gaseous samples were analyzed and a specially designed sample introduction system was evaluated. Potential applications as well as advantages and limitations are described.
EXPERIMENTAL SECTION A Hewlett-Packard 6750 gas chromatograph was used in this work. The chromatograph was used only for its oven capability and was fitted with a Varian 1700 electrometer, a Varian helium ionization detector, and an Ortek high-voltage power supply. Tbe bucking current circuit of the electrometer was modified to measure directly the actual value of the current as previously reported (12). The detector was operated at an applied potential of 200 V. To this system a high-temperature gas sampling valve (Valco Instruments) or a modified injection port was fitted as a means of sample introduction. The gas sampling valve was operated at 150 " C and the modified injection port was operated at 110 "C. A schematic diagram of the system including the modified injection port is shown in Figure 1. The modified injector is similar to one that has been described previously (11), and it was made in-house by press fitting the sampling chamber onto the front of the injector port to form a leak-free seal. The sample to be analyzed is continuously flowing through the Sam. pling chamber in which the needle is stationed at all times. To
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SAMPLE CYLINDER
4-WAY
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Figure 1. Schematic diagram of the system.
HELIUM PUR I FI ER
HELIUM CYLINDER
analyze a sample, 50 r L of the gas is withdrawn from the sample chamber, the sample is then injected through the on-column injector and the needle withdrawn back to the sampling chamber. The column was manufactured from l/s in 0.d. glass-lined stainless steel tubing and packed with 80-100 mesh Porapak N. Both column and packing were deactivated with dimethyldichlorosilane. Teflon wool was used to plug the column ends. The column was conditioned at 150 "C for 48 h with helium flow at a rate of 34 mL/min, measured at the detector exit at room temperature. Standards of HzO in a helium gas stream were generated by a permeation tube filled with water. An AID calibration system (Analytical Instrument Development, Inc.) was used to house the permeation tube which was kept at 30 "C. The glass housing of the AID calibration system was also deactivated with dimethyldichlorosilane. All tubing used in this work was carefully cleaned and dried. The helium carrier gas was ultrahigh purity grade and was further purified on a Supelco purifier (Supelco Inc.). The carrier gas for the analytical column was the same as that for the calibration system as shown in Figure 1. Sulfur hexafluoride and 1,3-butadiene were from lecture bottles purchased from Matheson. Chromatographic signals were recorded on a 1mV full scale Omega strip chart recorder and were integrated on an Hewlett-Packard 5390.4 integrator.
RESULTS AND DISCUSSIONS While several columns (15)have been used for the analysis of water by use of a thermal conductivity detector, the type of column that is suitable for use with a HID is limited by the magnitude of the background current at the operating temperature. Preliminary evaluation of Porapak, carbon molecular sieve, Carbopack coated with Carbowax 20 M, Chromosorb 101, and a wide-bore open tubular column coated with Emulphor showed that the Porapak materials are more suitable for HID applications. They provide lower background current at HzO elution temperature and provide adequate peak shape. Silanizing the column and the packing, as well as plugging the column ends with Teflon wool, all minimize adsorption and reduced peak tailing. In our preliminary work we used a gas sampling valve for sample injection. The valve was conditioned as recommended by the manufacturer, by briefly heating it to 350 "C and actuating it several times prior to cooling to the operating temperature. After this conditioning procedure was completed, we still observed a high detector background current relative to that obtained at room temperature. Also when the ultrahigh purity helium (blank) was sampled, a considerable water peak was observed. The conditioning procedure was repeated several times over a period of a week, after which the detector background current was reduced to a usable level of about 30% above that measured a t a valve temperature of 25 "C. We also found that the valve blank still contained
p
I
TIME (MIN.)
AHzO 150°C
Figure 2. Effect of gas sampling temperature on the water blank.
a significant water peak and that the magnitude of this blank was valve temperature dependent. Figure 2 shows the system blank at gas sampling valve temperatures of 150 "C and 275 "C. It appears that the source of water in the blank is the valve itself. I t is also unlikely that the amount of water in the valve can be completely purged because the valve seal is constructed of fluorocarbon filled with polyimide material, which probably contains some water on the surface as well as trapped inside. It is also apparent that water persists at trace levels even after extensive conditioning and that the blank water peak could possibly be caused by the heat of friction releasing some water from the valve body when the valve is actuated. The blank was reproducible (relative standard deviation was 4% for six blank samples) and the system was linear between about 5 and 100 ppm. The lower detecton limit is dependent upon the blank value. When the system was used to analyze for water in butadiene and sulfur hexafluoride, water was detected in both compounds. But we also found that the valve was ghosting a butadiene peak even 48 h after analyzing butadiene. The ghosting, high blank value, and the tailing of the water peak associated with the gas sampling valve prompted us to investigate a different approach to sample introduction. An on-column syringe injection system was developed to provide a sample reservoir and to protect the injection site from atmospheric contamination. When the injection system was tested, an improved peak shape was observed and the system blank was much smaller than that seen with the gas sampling valve. However, a new problem arose with the syringe injection. A syringe exposed to the atmosphere will have water adsorbed on the surface. Water is the most abundant adsorbed atmospheric gas on glass surfaces and is slowly removed in a dry atmosphere. A newly installed syringe which had been flushed several times with the sampling gas still required many injections before constant results were obtained as shown in Figure 3. The initial high values shown in Figure 3 are a result of slow
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
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iD 0 0.
0 0.
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Figure 3. Consecutive syringe injectlons with newly installed syringe for a standard sample of waiter in helium (2.4 pprn).
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I
Figure 4. Chromatograms of consecutive syringe injections of an 8.75 ppm standard of water heliurn. Upper chromatograms are for blank injections.
removal of some of the hydrolated layer on the glass surface as well as in the needle. The pattern shown in Figure 3 was also noticed when the gas sampling valve was in use and opened to the atmosphere. To minimize this we installed the four-way valve in line witlh either the modified injection port or the gas sampling valve to keep the valve or the sampling chamber dry. The septum or syringe was changed only a t the end of the working day to allow overnight equilibration. The water blank peak shown in Figure 4 is much smaller than that introduced by the gas sampling valve. This blank is probably caused by removal of water from the glass surface by the heat of friction generated from the injection. This was confirmed when a syringe needle was introduced into the column and a sample from the column carrier gas was withdrawn from and then injected back into the column producing a water peak. However, merely puncturing the septum with the syringe needle did not release an:y significant amount of water. An atmospheric leakage through the syringe can be ruled out; because the sample blank did not show any amount of air comparable to that of water. When the modified injector wm used to analyze a standard sample of 8.75 ppm of water, a8 shown in Figure 4, the peak shape was greatly improved over the gas sampling valve. The relative standard deviation for six samples was 7.4% and the system is linear between 2 ppm and 100 ppm.
Flgure 5. Chromatograms of water in 1,3-butadiene and In sulfur hexafluoride sample introduced by syringe.
Many applications demonstrate the potential usefulness of this method, but in this work the determination of water in gaseous butadiene arid in sulfur hexafluoride was selected. 1,3-Butadiene is uaied in the production of CI6-poly(buta1,3-diene) rubber. The presence of moisture in the monomer will deactivate the catalyst and can cause blockage in thle reaction. To avoid this the monomer's water content should be less than 20 ppm (16,17).Analysis of water in butadiene is done by a number of methods. The two most important methods are converting water to acetylene on a calcium carbide reactor followed by flame ionization detection (6, 7) and analyzing liquified gas by thermal conductivity detection ( 2 7 ) . Both methods are known to have limitations. Analysiis of water in butadiene lby using HID is shown in Figure 5. The concentration of water was found to be 8.25 ppm with a relative standard deviation of 7.4% for six injections. A second example demonstrating the use of the HID in measuring water is in the analysis of sulfur hexafluoride. The major use for this gas is in large gas-insulated circuit breakers by utility companies. Moisture present in SF6 could react with decomposition products to start corrosion. The amount of water in SF6 should not exceed 11 ppm. When water was determined in the lecture bottle of SF6, it was found to contain 12.75 ppm. The reproducibility of this determination is 6.3% RSD (six injections). A chromatogram of water in SF, i s shown in Figure 5. There are several difficulties in accurately determining trace levels of water in either liquid, solid, or gaseous sampleai. Because it is ubiquitous, contamination is largely unavoidable. The chromatographic methods which rely on chemical conor concentrations by liquefication are version of H 2 0 to C2H[, relatively insensitive and also have the attendant problems associated with indirect determinations (e.g., determining trapping efficiency). 'The helium detector on the other hand offers a direct, rapid means of detecting low level concentrations. In this investigation we have also defined some of the problems likely to be encountered in the determination.
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Although background values due to contamination will limit the lower detection level, these levels can be easily monitored on the HID because of its sensitivity. Careful sample introduction procedures will also aid in limiting background problems, and Once a constant level is attained, determinations can readily be made on a routine basis with a high degree of precision. Registry No. Water, 7732-18-5; 1,3-butadiene,106-99-0;sulfur hexafluoride, 2551-62-4.
LITERATURE CITED Fisher, K. Agnew. Chem. 1935, 48, 394. Hachenberg, H. “Ifidustrial Gas Chromatographic Trace Analysis”; Heyden: London, 1973; pp 1933-1941. Egovlile, M. J.; DellaMonica, E. S. J . Chromatogr. 1981, 212, 121-125. AuBeau, R.; Champeix, L. et Mme.; Reiss, J. J. Chromatogr. 1984, 16, 7-21.
(5) Kaiser, R. Chromatographla 1989, 2, 453-461. (6) Knight, H. S.; Weis, F. T. Anal. Chem. 1982, 34, 749-751. (7) Latif, S.; Haken, J. K.; Wainwright, M. S. J. Chromatogr. 1983, 258, 233-237. (8) Musha, S.; Nishimura, T. Bunsskl Kagaku 1985, 14, 803. (9) Erley, D. S.Anal. Chem. 1957, 8 9 , 1564. (10) Hachenberg, H. “Industrial Gas Chromatographic Trace Analysis”; Heyden: London, 1973; pp 133-141. (11) Andrawes, F. F.; Brazell, R.; Gibson, E. K. Anal. Chem. 1980, 52. 891-896. (12) Andrawes, F . F.; Bayer, T.; Gibson, E. K. J . Chromatogr. 1981, 205, 4 19-424. (13) Andrawes, F. F.; Bayer, T.; Gibson, E. K. Anal. Chem. 1981, 53, 1544-1545. (14) Andrawes, F. F.; Gibson, E. K.; Bafus, D. A. Anal. Chem. 1980, 5 2 , 1377-1379. (15) Mindrup, R. J . Chromatogr. Scl. 1978, f 6 , 380-389. (16) Wilhite, W. F.; Hollis, 0. L. J . Chromatogr. Sci. 1988, 6 , 84-88. (17) Wang, W.; Ding, X.; Wu, X. J. Chromatogr. 1980, 199, 149-159.
RECEIVED for review May 2, 1983. Accepted July
1, 1983.
Coadsorption of Mixed Anionic and Cationic Surfactants in Reversed-Phase Liquid Chromatography Way-Yu Lin, Muoi Tang, John J. Stranahan, and Stanley N. Deming* Department of Chemistry, University of Houston, Houston, Texas 77004
The combined effects of mixed sodium octanesulfonate and octylamine surfactants on the liquid chromatographic retention times of anillne, phenylethylamlne, benzenesulfonic acid, and chromotroplc acld are described by a thermodynamic model that assumes electrostatic interaction of charged solutes with anionic and catlonlc surfactants in the adsorbed phase.
The intentional addition of charged surfactants to aqueous mobile phases has proven to be a valuable factor in achieving improved separation of ionic mixtures in reversed-phase liquid chromatography (1-7‘). The separation of charged solutes can be enhanced either by increasing their retention times with surfactants of charge opposite to that of the solutes or by decreasing their retention times with surfactants of charge similar to that of the solutes (8, 9). To date, however, the use of mixed anionic and cationic surfactants does not appear to have been exploited in reversed-phase liquid chromatography. A mixture of anionic and cationic surfactants will exhibit marked deviations from ideal mixing of individual components in various physicochemical properties such as surface tension, conductivity, viscosity, solubilization, and solubility (10-12). Although anionic and cationic surfactants are not generally used together for applied purposes (13), studies of their coadsorption are of fundamental interest because of the enormous synergistic effects they have on surface and interfacial properties ( I O ) . In this paper, we present the results of an experimental study for measuring coadsorption and interaction effects between anionic and cationic surfactants using reversed-phase high-performance liquid chromatography. A retention model is derived that describes the retention behavior of different solutes in this mixed system.
THEORY Stranahan and Deming (14) have recently proposed a quantitative, thermodynamic model based on the assumption
that simple Langmuir adsorption of an added, charged surfactant at the stationary phase/mobile phase interface is one of the major factors governing the retention behavior of solutes in ion-interaction chromatography. Six basic assumptions were adopted from the work of Locke (15): either a porous microparticle or superficially porous silica support is used; surface hydroxyl groups on the silica support are all chemically bonded to an alkyl group; the organic layer is not cross-linked or polymerized; simple adsorption occurs which produces a monolayer; the eluent may contain one or more solvents, small samples are used to approximate infinite dilution. Locke’s model (15) can be stated
where the subscripts i, 1, and a correspond to the solute component I, the bulk liquid (mobile) phase, and the adsorbed (stationary) phase, respectively, Ki is the distribution coefficient of component I, Vo is the average molar volume, yi is the activity coefficient of component I, u is the interfacial tension of the system, uio is the interfacial tension of pure I in equilibrium with the solid adsorbent; si represents the area on the surface occupied by 1mol of I, R is the gas constant, and T is the temperature. In eq 1,the terms R, T , u p ,si, yil, Vlo, and Vao will be constant or approximately constant for a given component I and a given chromatographic system where only the concentration of surfactant modifiers is varied. We now consider the effects on Y~~of adding surfactants to the system. Adsorption of One Surfactant. We first consider the adsorption of a nonreacting surfactant on the surface of the stationary phase. The surfactant molecules may be thought of as adsorbing at so-called “vacant sites” (S) on the stationary phase surface. The adsorption process for an anionic surfactant A can be viewed as A+S*AS (2) The equilibrium constant KA is given by
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