198
Anal. Chem. 1991, 63,198-202
(8) Evans, J. F.; Kuwana, T. Anal. Chem. 1979, 57, 358. (9) Bowling, R. T.; Packard. R. T.; McCreew, R. L. J . Am. Chem. SOC. 1989, - 7 7 7 , 1217. (10) Gerwirth, A. A.: Bard, A. J. J . Phys. Chem. 1988, 92, 5563. (11) Bowling, R. J.: McCreery, R . L.; Pharr, C. M.; Engstrom. R . C. Anal. Chem. 1989. 67. 2763. (12) Gonon, F. G.f Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53, 2763. (13) Feng, J.-X.; Brazell, M.; Renner, K.; Kasser, R.; Adams, R. N. Anal. Chem. 1987, 59, 1863. (14) Wang, J.; Tuzhi, P.; Villa, V. J. Nectroanal. Chem. 1987, 234, 119. (15) Chien, J. B.; Saraceno, R. A.; Ewing, A. 0. Redox Chemistry and Interfacial Behavior of 8 l o l o g l ~Mokcules; l Plenum Press: New York. 1988; pp 417-424. (16) Bowling, R.; Packard, R.; McCreery, L. J . Nectrochem. SOC. 1988, 1605. (17) McCreery, R . L.;Packard, R. T. Anal. Chem. 1989, 67. 775A. (18) Poon, M.: McCreery, R. L. Anal. Chem. 1986, 5 8 , 2745. (19) Poon, M.; McCreery, R. L. Anal. Chem. 1988, 6 0 , 1725. (20) Poon. M.: McCreery, R. L. Anal. Chem. 1987, 59, 1615.
(21) Dayton, M. A.; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1980, 52, 946. 1842. A. G.; Dayton, M. A.; Wightman, R. M. Anal. Chem. 1981, 53, (22) Ewing, (23) Kovach, P. M.: Ewing, A. G.; Wilson, R . L.; Wightman, R. M. Anal. Chem. 1984.. IO. - _ - . 215. -(24) Wightman,-R. M.; Paik, E. C.; Borman, S.; Dayton, M. A. Anal. C b m . 1978, 50, 1410.
RECEIVED for review June 8, 1990. Accepted November 2, 1990. This work was supported, in part, by grants from the Office of Naval Research and the National Science Foundation. A.G.E. is a recipient of a Presidential Young Investigator Award from the National Science Foundation (CHE8657193), an Alfred P. Sloan Research Fellow, and a Camille and Henry Dreyfus Teacher-Scholar.
Low-Level Moisture Generation Francois Mermoud,' Michael D. Brandt, and James McAndrew* American Air Liquide, Inc., 5230 S. East Avenue, Countryside, Illinois 60525
Two complementary methods have been developed and successfully tested for the generation of moisture concentrations In the 0.02-50 ppm-v range in various gaseous media. The first method uses catalytic recombination of H, and 0, over a hot Pt/Rh catalyst, whlie the second is based on the principle of permeation of water through a membrane. The balance gas is air in the first method, but any gas may be used with the second method, provided that it Is compatible with the membrane material. Tests were done over the whde range of concentrations with two different hygrometers to verify the reproduclbiHty of the methods. A 5-10% (relative) reproduciblity of the measured concentration (method dependent) was observed. The accuracy of the methods was determined by a frost-pdnt hygrometer, which was previously calibrated at the NIST and is certified to f0.7 O C for dew points of -75 O C and up (this corresponds to an uncertainty of approximately *lo% in the moisture concentration). For the lower ranges, a volumetric dilution of higher molsture concentrations by means of mass flow controllers adds a further 1-3% (relative) to the overall uncertainty depending on the number of dliutlon steps. The molsture concentrations thus generated can be used to calibrate hygrometers and to study the lnteractlons of low moisture concentrations with tubing, valves, etc.
INTRODUCTION The demand for gases of high or ultrahigh purity is continuously increasing especially for applications in the electronics or semiconductor industries. Among the impurities that are critical for the applications in which these gases are used (I1, moisture requires particular attention. Moisture interacts strongly with all surfaces with which it comes into 'Present address: F i r m e n i c h SA, Case Postale 239,1211 Geneve
8, Switzerland.
0003-2700/91/0363-0198$02.5010
contact and is relatively nonvolatile, which makes it very difficult to remove from gas-handling systems. Whether it is for gases contained in a cylinder or for piping distribution networks, a low water concentration in the gas phase must be obtained. For inert gases (N, included), these levels are well below 1 ppm (by volume). Low moisture measurements can also be used for the verification of the proper preparation of the gas-handling system (2). Concurrently, the accurate determination of the moisture concentration in a gas system is a delicate task and may be influenced by the contributions from the sampling lines (2) or from the analyzers themselves. Incorrect moisture measurements can also be obtained from inadequately calibrated analyzers or from deteriorated cells. Moreover, all the different analyzers that may be suitable for monitoring parts per million (ppm) or sub-ppm levels of moisture in a gas (3),such as frost-point hygrometers ( 3 , 4 ) , electrolytic cells (3-5) (i.e., coulometry), aluminum oxide capacitive sensors (3,4,6),vibrating quartz crystals (3,7),mass spectrometry (3,B-IO),infrared spectroscopy (3, 1 1 , 12), or gas chromatography (3,13),require calibration of the sensor in some fashion, and until recently, the calibration could only be verified down to the ppm level because reliable moisture standards were not readily available at lower concentrations. As a consequence, the validity of moisture measurements taken for concentrations below 1ppm is g e n e r a l l y questioned because of the lack of standards below this level. Although frost-point hygrometers are well accepted as an absolute method, they are only used with confidence down to approximately 1ppm. Below that value, corresponding to a dew point of -76 "C, there are questions about the validity of their measurements: (1) coimpurities in the gas could form, or initiate too soon, a deposit on the mirror or nucleate moisture condensation too soon; (2) extremely long periods for the buildup of a frost layer create the possibility that the moisture source may vary during this time. Unlike other atmospheric contaminants, for which ppm or sub-ppm standards can accurately by prepared in cylinders, the preparation of low level moisture standards is not easy 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 3, FEBRUARY 1, 1991
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(3). Adsorption-desorption phenomena occurring in the cylinder itself and in the pressure regulation system make it extremely difficult to manufacture reliable moisture standards in cylinders. On the basis of these considerations, we have been working on the problem of reliably generating low concentrations of moisture in a gas. Two complementary methods which generated from 0.02 t o 50 ppm-v moisture concentration were successively developed and are reported here. EXPERIMENTAL SECTION A schematic diagram of the hygrometric calibration bench manifold and moisture generation methods is presented in Figure 1. The transfer standard used in this experiment is a chilled mirror hygrometer (1331 XR/ 1500 hygrometer, General Eastern, Watertown, MA) that has been previously calibrated a t NIST (National Institute of Standards and Technology), formerly NBS, in the range -75 to -50 "C against their two-pressure humidity generator (14). The manifold allows the generated moisture standards to be distributed to hygrometers for calibration (15) and evaluation, or through various components of high-purity gas distribution system, and then to a hygrometer in order to evaluate the behavior of the distribution system toward moisture. A pressure-regulating system is used to maintain a constant adjustable pressure in the manifold. (a)Catalytic Recombination. The operating principle of this generator, using a direct catalytic recombination of O2 and H2, has been presented elsewhere (16). Briefly, a misture of H2 (typicallyin N2)in one cylinder is blended volumetrically by means of a series of mass flow controllers (UNIT Instruments, Orange, CA) with a mixture of 02 (stored in the same balance gas in a second cylinder). The blended mixture is dried over highly activated molecular sieves before passing through a hot Pt/Rh catalyst. The residual H2 concentration after recombination was undetectable with a trace analytical reduction gas detector, verifying that the recombination yield is 99%. The contribution
of the delivery system to the uncertainty is i2%, while that of the H2 concentration in the standard mixture is i l %(analysis by GC/HID), so that the total uncertainty is approximately i3% (relative). Concentrations of moisture below the range of certification of the chilled mirror hygrometer were generated by nitrogen gas downvolumetric dilution with dry (50 ppb H20) stream of the reaction. (b) Permeation. Commercially available permeation systems (G-Cal,GC-Industries, Chatsworth, CA) were used. These devices use a silicone permeation membrane in between liquid water and a vapor-phase moisture chamber, with a second membrane between this vapor-phase chamber and the diluent gas. This arrangement increases their stability and reproducibility for low-level moisture generation (17). Further improvement is achieved by our configuration (Figure 1) particularly by the use of backpressure regulation (18) a t 20 psig. The temperature of the permeation device is maintained a t 50 f 1 "C by a heater and temperature controller (Omega (N9000) with a Type T thermocouple. The flow of the diluent gas is accurately controlled by means of a mass flow controller (UNIT Instruments) and is usually varied in the range 300-3000 cm3/min. The diluent gas is dried before passing over the permeation device. The residual moisture concentration of the system was experimentally determined to be 20 ppb (see discussion below). The permeation rates of the tubes were individually determined by the manufacturer using gravimetry. However, when we used these data to calculate expected moisture concentrations in given flows, the results differed by up to 30% from the measurements determined by using the chilled mirror hygrometer. This is shown in Table I. However, once we used the measured moisture concentrations to derive a corrected permeation rate, reproducibilities within f3-10% were routinely achieved (see Figure 2 and later discussion). In order to verify the measurements made by an electrolytic hygrometer (Aquamatic+, Meeco, Warrington, PA), data were taken in parallel with the frost-point hygrometer over the range of operation of the latter (see Figure 1). All the measurements
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Table I. Comparison between the Moisture Permeation Rates Given by the Manufacturer and the Rates Calculated from the Observed Moisture Measurements tubeo 1
2 3 4
5 6b 7b
gravimetric 3050 3010 425 1680 79 130 550
permeation rate, ng/min hygrometer % ' diffC 3205 3496 485 2043 113 103 503
u, n
5 14 12 18 30 26 9
= 3-10d 115 63 21 96 18 4 29
"Corresponds to the tubes identified in Figures 2 and
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1
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.
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not indicate a chronological order. bTube obtained after data displayed in Figure 3 were collected. c (Hygrometer - gravimetric)/ (hygrometer X 100). d n varies with the tube as this is a compilation of data collected over a period of time. u: standard deviation of hygrometer determination.
for moisture concentrations below 0.2 ppm were obtained with the electrolytic hygrometer only.
RESULTS AND DISCUSSION The core of this study was the availability of precise reliable moisture measurements, a t least for the early part of the investigation. For this, it was necessary to guarantee the measurements of a hygrometer, preferably of the absolute type, that could be used as a transfer standard. Although the accuracy of the frost-point hygrometer was guaranteed by the manufacturer with NIST traceability, it seemed best to extend this certification below 50 ppm (below -48 "C). Thus, the hygrometer was sent to NIST before experiments began for a precise calibration at five different dew points selected in the range -50 to -75 "C. This certification procedure was repeated at the end of the experiments (3 years later) to verify that no aging had occurred in our instrument. The results of these calibration tests are reported in Table 11. The agreement can be seen to be well within the specifications of the frost-point hygrometer, which is f0.7 "C (this corresponds to approximately *lo% in moisture concentration). This hygrometer was then used to measure the moisture concentrations in the 5-50 ppm range generated by the catalytic recombination of H2 and 02. This range was selected because it fell entirely within the domain of certification of the frost-point hygrometer, and all the data points were obtained without a seconary dilution step. By use of a mixture containing 100 ppm H2 in balance N2 and assuming a 99% conversion efficiency, a theoretical concentration of moisture was determined. All the moisture concentration measurements matched well with the theoretical values, within the range of accuracy of the analyzer (see * in Figure 3). At each moisture concentration, the average value was measured by the hygrometer using the identical setup over a period of several months. Each determination took at least a day and had a spread of f 5 % . This first series of moisture values was then volumetrically diluted, by a factor adjustable between 1 and 20 using a dry gas (ca. 50 ppb residual H,O), by means of MFC No. 4 (Figure
0.0 1 0
1
2 3 4 5 6 7 8 9 1011 1 2 1 3 1 4 1 5 1 6 number of
analysis
Flgure 2. Study of the long-term reproducibility of the generation of moisture concentrations by the permeation method. The permeation rates in Table I were used to calculate the generated concentrations: (W) 13.11 ppm (tube l), (+) 4.53 ppm (tube 2),(*) 1.93 ppm (tube 3), (0)0.38 ppm (tube 3), ( X ) 0.059 ppm (tube 5).
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Figure 3. Generatiin of moisture using the catalytic recombination and the permeation methods. The moisture sources were ( 0 )tube 1, (+) tube 2,(X)tube 3, (0)tube 4, (A)tube 5, (V)"dry" N, (*) catalytic recombination.
1). Again, we observe an almost perfect match between calculated and measured concentrations (Figure 3). This correlation verified the performance of the frost-point hygrometer beyond the range of calibration by NIST. Thus, the domain of performance was extended down approximately 0.2 PPm.
Table 11. Calibration Tests Run at NIST for the Frost-Point Hygrometer dates
-50 "C
NIST moisture generator -49.97 frost-point hygrometer' -50.16 Aug 1988 NIST moisture generator frost-point hygrometer" a Conversion from the thermometer resistance measurements. July 1985
I
-55 "C
-60
dew-point range, "C "C -65 "C
-70 "C
-75 "C
-55.13 -55.16 -55.07 -54.94
-60.09 -60.23 -59.89 -59.69
-70.05 -69.81 -70.06 -69.60
-74.14 -73.19
-64.28 -64.49 -65.02 -65.05
ANALYTICAL CHEMISTRY, VOL. 63, NO. 3, FEBRUARY 1, 1991
Although the catalytic recombination gave satisfactory results, the method was limited by a series of drawbacks: (1) a limitation to 0.2 ppm for the lowest value that could be generated, due to the combination of limitations in the available hydrogen concentration in mixtures and the limitations of the two-stage dilution scheme; (2) the presence of a bulky manifold, difficult to move for calibrating field analyzers; (3) an oxygen-containing balance gas; (4) long equilibration times due to a complex mixing manifold; (5) complications in working at various pressures while simultaneously maintaining a constant mixing ratio; (6) high residual moisture content (ca. 50 ppb), due to the large manifold. Consequently, another method was searched for that would accommodate most of these limitations. Permeation through an organic membrane is a well-known method for generating low-level standards in a balance gas. However, in many applications, a very precise temperature control is necessary to maintain a constant permeation rate of the permeate through the membrane. This constraint is a serious impediment to the application of the technique to inexpensive, portable, and reliable systems. Recent developments, however, in the membrane material (18)and in the system configuration (17)allowed us to build a simple moisture generator based on commercially available permeation tubes. A series of tubes with different permeation rates were acquired from the same manufacturer. By maintaining the tubes a t a constant nominal temperature (usually 50 "C) and by varying the flow of a dry gas passing over the permeation tube (MFC No. 5, Figure l),it is possible to generate a known moisture concentration in the dilution gas, according to the formula
c = -K p + B F
where C is the concentration of moisture in the gas (ppm-v), K is the molar gas constant (KH20(00~) = 1.234), P is the permeation rate of the membrane (ng/min) at a given temperature (a manufacturer's datum, see Table I), F is the gas flow rate (cm3/min), and B is the background moisture concentration (ppm-v) measured without permeation tube. In the course of the present study, the flow over the permeation device was controlled by a single mass flow controller (MFC No. 5, Figure l),which limited the range of moisture concentration that was attainable for each tube to 1order of magnitude (the range of flows attainable with reasonable accuracy with a single mass flow controller). We chose to limit ourselves to a single dilution step in order to decrease the number of variables in the study. For more general applications, it is possible to further dilute the doped gas with a dry gas to extend the dynamic range of concentrations available from a single permeation tube (setup not displayed in Figure 1). In contrast to the catalytic recombination, no simple calculation is possible with permeation tubes to determine theoretical moisture concentration values. One must rely on a determination by gravimetry, as done by the manufacturer. Surprisingly, we have observed discrepancies between permeation rates calculated by using measured moisture concentrations and eq 1and those determined by gravimetry. The difference between the permeation rate determined by gravimetry and that determined by direct measurement of moisture concentration is somewhat disappointing. In principle, the gravimetric method is capable of greater precision, and it would also free the hygrometry bench from its use of the frost-point hygrometer (certified by NIST) as a transfer standard. When most of the data collected here were collected, the permeation tube manufacturer carried out the gravimetric
201
calibration in an oven at 50 O C (the recommended operating temperature of the tubes). Because tubes were exposed to ambient humidity and static conditions in the oven, it is perhaps not surprising that the permeation rates were different from those observed under flowing, dry nitrogen a t 25 psig. Following our suggestion, the tube manufacturer now mounts the tubes on a gas manifold mounted in the oven, so that the tubes may be kept under conditions similar to those of their use, during calibration. Preliminary indications are that this method (which was used for tube 7 in Table I) gives more satisfactory results. The above effect would be expected to give rise to determinations of permeation rate by grayimetry which are too low relative to those determined by hygrometry. This agrees with observation for all cases in Table I except tube 6. This tube, however, has a low permeation rate, which presents problems for both methods. Small weight losses due to rapid permeation through the membrane by trace impurities in the water or evaporation of trace impurities from the tubes, for example, could account for this difference. Even though the present limitations preclude the use of the gravimetrically determined permeation rates, permeation rates determined by hygrometry may be used to give data that are reproducible within f3-10%, as illustrated in Figure 2. These may then be used to certify other hygrometers using the frost-point hygrometer as a basis, both within and outside its range. Once recalibrated by hygrometry, the stability of the tubes proved to be very good. In Figure 2, a few points from some of the tubes used during our studies are presented. Each of these data points were separated by periods as long as months and usually include complete removal and reinstallation of the tubes in the test bench. Initially we tested the tubes with the higher permeation rates (tubes 1,2,and 4, Table I) in order to verify the linearity of the dilution system within the range of confidence of the transfer standard, as previously determined. All the moisture concentrations generated between 15 and 0.85 ppm were reproducible within 3-570, which was within the range of reproducibility of the transfer standard. The dilution system applied to the permeation device showed a very good linearity, and all the data points matched very well with the calculated values (Figure 3). While collecting these data points, we verified the performance of a Meeco Aquamatic+ electrolytic hygrometer, specially designed for low-level moisture measurements (19), by connecting it in parallel with the frost-point hygrometer as illustrated in Figure 1. The agreement between the two hygrometers was excellent. Once the accuracy of the method and the performance of the second hygrometer were verified for higher moisture concentrations, permeation tubes with lower permeation rates (tubes 3 and 5, Table I) were installed. The highest moisture concentrations that could be generated with these tubes, approximately 1.35 and 0.35 ppm, respectively, were still within the range of the previously observed performance for the transfer standard. The measurements were checked in parallel with the electrolytic hygrometer. The dilution scheme used for tubes with a higher permeation rate was applied, and the measured concentrations again agreed well with the theoretical values (see Figure 3). The background moisture level ( B in eq 1)was 50 ppb for the moisture concentrations generated by catalytic recombination and 20 ppb for those generated by permeation tubes (due to the greater complexity of the catalytic recombination manifold). The approach that was taken in this study was designed to counter many of the questions generally raised when dealing
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 3, FEBRUARY 1, 1991
with sub-ppm moisture measurements. The good correlation between calculated and measured moisture levels, both within and beyond the range of calibration of the transfer standard, gives a good assurance about the ability of both methods to generate reliable moisture concentrations. There are clearly advantages and disadvantages in using either method, and thus, both methods remain complementary. The major advantage of the catalytic recombination method lies in the fact that it allows an easy determination of theoretical values. Once all the parameters are set for assuring a 99% conversion (as previously described 15),the theoretical concentrations can be easily calculated from the known H2 concentration in the standard mixture. It has, however, many disadvantages, as discussed earlier. Interestingly, the major advantage of the catalytic recombination becomes the major drawback of the permeation system, while all the concerns with the catalytic recombination are readily solved by using the permeation method. Indeed, with the second method, the major shortcoming is the precise determination of gravimetric permeation rates. As previously mentioned, by carrying out this determination under conditions as close as possible to those of operation, it appears possible to eliminate this drawback. At present, we will still require a transfer standard. However, once precisely calibrated, the permeation-based moisture generator is highly useful. I t is compact, allowing a short stabilization time and better control of the residual moisture level (on account of its simplicity), typically 20 ppb or below. Since no standard mixture in a cylinder is necessary, continuous operation is possible. The short stabilization time (typically less than 1 h, including contributions from the hygrometer) and the small size of the equipment also allow an easy transportation for the periodic recalibration of field hygrometers. It can also be used over a wide range of pressures and with a variety of balance gases (providing that they are compatible with the membrane material). Conversely,the catalytic recombination method requires a manifold to purify and mix the gases before catalysis, is limited to atmospheric pressure, requires an excess of hydrogen or oxygen in one of the mixtures and in the final product, and has a higher residual moisture level (ca. 50 ppb). I t also requires cylinders of gas mixtures and is in general unsuitable for field operation. In the present study, the lowest moisture concentration generated was 20 ppb, and the limitation for this came for the level of residual moisture in the system. In more recent data, this residual level was slightly reduced, allowing the generation
of moisture concentration in the 10 ppb range. Indeed, only the availability of very dry gas and of noncontaminating parts can set the limitation to the lowest level that can be achieved for generating moisture.
CONCLUSIONS The generation of moisture in the low ppm to ppb range has been realized by the use of two complementary methods. The accuracy of both methods and their reproducibility over a long period of time have been determined for the range 0.02-50 ppm-v. The hygrometry test bench implemented in our laboratory that was used to verify the performance of the moisture generators is now used for the testing or periodic recalibration of hygrometers, as well as for the calibration of subsequent moisture generators. The latter are now routinely used in our laboratories for the control of field hygrometers. Other applications of the moisture generation system include the study of the behavior of components used in high-purity gas intallations toward moisture concentration changes. LITERATURE CITED Osburn, Cariton, M.; Berger, Henry; Donovan, Robert P.; Jones, Gary W. J. Environ. Sci. 1988, March/April, 45-57. Mitzuguchi, Y.; Ohmi, T.; Sugiyama, K.; Nakamwa, M. 10th SymposC um on ULSI Ultra Clean Technology, Tokyo, 1989. Carr-Brion, K. Moisture Sensors in Process Control, Eisevier Applled Science Publishers Ltd: Barking U.K., 1986. Mehrhoff, T. K. Rev. Sci. Instrum. 1985, 56, 1930-1933. KeMel, F. A. Anal. Chem. 1959, 3 1 , 2043-2048. Hasegawa, Saburo. Proc. 30th Nectr. Compn. Conf. (IEEE, New York) 1980, 386-391. Collier, Peter L.; Blakemore, Colin B. Microelectron. Manuf. Test. 1987, April, 16-17. Kambara, Hideki; Kanomata, Ichiro. Anal. Chem. 1977, 4 9 , 270-275. Mltsui, Yasuhiro; Kambara, Hideki; Kojima, Masuo; Tomita, Hiroshi; Katoh, Kenji; Satoh, Kunitaka. Anal. Chem. 1983. 55, 477-481. Sugiyama, K.; Kakahara, F.; Abe, M.; Okumura, T.; Ohmi. T.; Murota, J. 9th ICCCS Proc. 1988, 322-340. Mucha, John A. Appl. Spectrosc. 1982, 3 6 , 393-400. Mucha, J. A. Barbalas, L. C. ISA Trans. 1986, 25, 25-30. Latif. S.; Haken, J. K.; Wainwright, M. S. J. Chromatogr. 1983, 258, 233-237. Hasegawa, Saburo; Little, John W. J. Res. Nat. Bur. Strand. 1977, B l A , 81-88, Mermoud, Francois; Brandt, Michael D.; Weinstein, Barry L. SolM State Techno/. 1989, May, 59-61. Jehanno, P.; Turck, G. Bull BNM 1985. 62. 17-22. Chand, R. Gas Emitting Device. US. Patent 4,399,942, Aug 23, 1983. Mermoud, F.; Brandt, M. Gas Generating Device. U S . Patent 4,849,174, July 18, 1989. Zatko, David; Ragsdale, Daniel J. 39th Pittsburgh Conference and Expos. Anal. Chem. Appl. Spectrosc., (New Orleans) 1988, reprint from paper 1249 (courtesy of D. Zatko).
RECEIVED for review April 26,1990. Accepted November 8, 1990.
