Gas-Solid Chromatography of Organic Acids and Amines Using Steam Containing Formic Acid or Hydrazine Hydrate as Carrier Gases Akira Nonaka Institute for Optical Research, Kyoiku Unicersity, 22-1 7, Hyakunincho-3, Shinjuku-ku, Tokyo, Japan Gas-solid chromatography can be carried out for samples of free organic acids and amines using carrier steam containing 10% formic acid or 10 to 20% hydrazine hydrate. The mixed carrier vapors are introduced into the column by pumping aqueous solutions of formic acid or hydrazine hydrate into a vaporizing port set in the GC system. The adsorbents, such as diatomaceous firebrick and porous glass beads, can be used as stationary solids, without any coating. The FID can be employed as a detector with the mixed carriers. The effect of steam and the added polar vapors is so significant that acid and amine samples are eluted very rapidly without any marked tailing. By changing the carrier vapor from the acidic to a basic one, acid and amine samples can be analyzed on the same column. Chromatography of lower and higher fatty acids and their alkaline salts, lower and higher fatty amines, etc. is reported.
RECENTLY, STEAM CARRIER GAS-SOLID CHROMATOGRAPHY, SSC, has been reported to be useful for the analyses of polar organic samples, where inorganic porous adsorbents and pure water vapor were employed ( I ) . In SSC, carrier steam was considered to have the ability not only to remove the tailing effect from elution peaks but also t o promote more rapid effluence of the samples. The function of the steam carrier gas in SSC was explained in terms of the multilayer formation of adsorbed water which could make significantly smaller the edsorption energy of the adsorbent surface for the sample molecules. The effect of using steam in SSC was so significant that most polar sampleqand high boiling samples could be eluted from ordinary porous stationary solids very rapidly without any marked tailing. Even in SSC, however, adsorbents for the stationary solid must be modified by a nonvolatile acid or base, if samples having strong acidity or basicity are to be analyzed. Therefore, the use of carrier vapors more polar than steam was desired and examined by the present author for analyzing acidic or basic samples by GSC with ordinary inorganic adsorbents. Vapors of formic acid and hydrazine hydrate were useful for the elution of samples of acids and amines, respectively, when they were used as carrier gases in GSC, although the effectiveness of formic acid vapor as a component of a mixed carrier gas in G L C of acidic samples had been reported (2). For practical use by the present author, however, these vapors had been diluted with steam before they were introduced into a column head, because they were too corrosive to be brought into G C systems in the form of pure vapor as carrier gases. Formic acid and hydrazine hydrate were suitable for use as components of a mixed vapor carrier gas which contained steam as a principal component, because they have boiling points similar to that of water. If they had boiling points much lower than water, they would not be adsorbed so strongly on the stationary solid and their deactivation effects would be rather weak a t temperature conditions where steam (1) A. Nonaka. ANAL.CHEM., 44,271 (1972). (2) R. G. Ackman and R. D. Burgher, ibid.,35, 647 (1963).
carrier gas could be used; on the other hand, if the polar component to be mixed with steam has a boiling point much higher than that of water, the mixed vapor carrier gas must be used at column temperatures much higher than 100 “C, that is, a t temperature conditions where the separation of light molecules would become rather imperfect. Another reason why formic acid and hydrazine hydrate were picked up from many organic or inorganic polar materials is that they were not found to have any remarkable influence o n the level of the base current and the sensitivity of the FID used in the present GC experiments. The mixed carrier vapors were introduced into columns by the method of “column head vaporization” of the liquid mixtures, which were being pumped at a constant flow rate into the column head or a vaporization port maintained a t a temperature above the boiling point of the liquid mixture. This method of preparing carrier vapors makes it very easy not only to obtain mixed vapors having exactly constant ratios of mixing for the carrier gas, but also to measure and control the flow rates of the carrier vapors accurately. When a liquid pump, which must be one of a small capacity and controlled a t a constant flow rate, is employed for introducing vapor carrier gas into a vapor carrier GC system, the system becomes much simpler and easier to operate, even if the carrier vapor used is one of a single component, since it can remove a boiler which must be controlled very carefully at a constant pressure to generate a stable flow rate of carrier vapor. In the present stage, where vapor carriers, as well as permanent-gas carriers, have been found to be very useful for GC in general use, the application of a liquid pump for supplying carrier gas to G C systems may not only eliminate the high pressure gas cylinders for carrier gases from the G C systems but also may make them portable and convenient to use. Organic acids and amines could be eluted from the same stationary solid by using mixed carrier vapors composed of steam and formic acid and steam and hydrazine hydrate, respectively. This fact is particularly noteworthy, because it means that in gas-solid chromatography various kinds of samples which are classed differently, polar and nonpolar, low boiling and high boiling, or acidic and basic, could be eluted effectively from one kind of stationary solid by using the carrier vapors or mixed carrier vapors corresponding to the nature of the samples to be applied; this is contrary t o ordinary gas chromatography using permanent gases. Since it may be rather easy to change the kind of carrier vapors or the mixing ratios of mixed carrier vapors when a liquid pump is employed for introducing carrier vapors, this sort of vaporsolid chromatograph will be very useful even for general use. EXPERIMENTAL
Apparatus. PUMPINGSYSTEM FOR CARRIER VAPOR. A constant-flow-rate liquid pump or a pumping system constructed of a water container, a needle valve, and a highpressure gas cylinder was utilized to pump the liquid mixture ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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Figure 1. Schematic diagram of the system for supplying carrier vapor (a) Column head and carrier-vapor generating portion, (b) constantflow-rate liquid pump, (c) another type of pumping system. 1, carrier-vapor generating port; 2, sample injection port; 3, analytical column; 4, electric heater for vaporizing carrier and sample; 5, electric heater for rubber septum; 6, capillary tube for leading carrier material; 7, glass syringe; 8, driving mechanism; 9, water container; 10, high pressure gas cylinder; 11, needle valve
for a carrier vapor into the GC system (Figure 1). When the former was utilized, flow rates of the carrier vapor could be measured more easily and controlled more precisely. O n the other hand, the latter pumping system could be instantly assembled by chromatographers so as to supply a vapor carrier gas into a GC system for laboratory use. The constantflow-rate liquid pump (JP-20 G C , Furue Saiensu Co., Tokyo) was constructed of a glass syringe (10 or 20 ml) and a screw rod which could push the plunger of the syringe a t a certain rate from 0.01 to 0.3 mm/min, while the pumping flow rate ranged from 2 to 100 pl/min. A mechanism driving the screw rod had been set so as to push the plunger at a given rate by arranging the reducing gears in a proper ratio. This pump must be able to press the liquid at a rather high pressure, that is, up to about 5 atmospheres, in order to obtain a steadily flowing carrier vapor, because the carrier vapor generated from the liquid at the vaporizing portion and introduced into the column head cannot flow through the column to the end unless it has a n excess pressure against the loss of pressure occurring through the column. In the case where a flask and a needle valve were used as a pumping system, a rather high pressure would also be needed to send the liquid to the GC system for introducing carrier vapor, although it was provided from a gas cylinder maintained above the pressure required. Since the gas cylinder was necessary only for providing the pressure, almost all kinds of inert gases in a cylinder could be used unless they were quite soluble in the liquid. Teflon (Du Pont) tubing having 0.5-mm i.d. was used for leading the liquid from the pumping system to the vaporizing port for generating carrier vapor. VAPORIZING PORTFOR CARRIER VAPOR. To obtain carrier vapor continuously from the liquid mixture pumped into the GC system, a proper vaporizing port, which had a small space (5-10 cm3) packed with silica granules (about 30 mesh) and which was maintained a t 150 to 180 "C by a electric furnace, was equipped. The head portion of the analytical column or the sample injection port, maintained usually at a rather high temperature, also served as a vaporizing port for obtaining the carrier vapor from the flow of liquid mixture, if it was pumped directly into the head portion of the analytical column, as seen in Figure l a . In this case, the specially prepared vaporizing port for carrier vapor could be eliminated. 484
It was not difficult to supply the heat necessary for converting the liquid mixture into a vapor carrier gas, and to maintain the head portion at a constant temperature for obtaining a steady flow rate of carrier vapor, since the flow rates of the liquid mixture brought into the vaporizing portion were usually very small and the heat of vaporization of the liquid mixture may also be small, that is, at most several watts. However, it seemed that a slight uneveness in the carrier vaporization caused noise in the detector, and the noise level was about 3 X A on the output of the F I D used in the present apparatus. The value of the noise level will be reduced to less than 5 x A, as shown in the case where an ordinary inert carrier gas is used in the FID, by improving somewhat the structure of the vaporizing port. ANALYTICAL COLUMN. Chromosorb G AW 30/60 mesh (Johns-Manville, New York, N.Y.), as an adsorbent of diatomaceous earth and Corning GLC-100 60B0 mesh (Corning Glass Works, Corning, N.Y.) as an adsorbent of porous glass were examined for efficiency and stability as stationary solids in this experiment, where rather active vapors were used as a carrier gas, and these adsorbents showed satisfactory results for the purpose of these investigations. Both of these adsorbents were used without any modification or coating process, packed into glass column tubing of 2- to 3-mm i.d. and 2 to 4 m in length. When Corning GLC-100 was used as a stationary solid, a comparatively long column was necessary for separating mixed samples, since the specific area of the adsorbent was rather small. SAMPLEINJECTION.In order to construct an injection port, the head of the analytical column tube was stretched up to the rubber septum set for sample injecting, although about 20 cm of this column tube in this head portion was left empty. Samples were usually injected through the rubber septum into the empty portion of the column head with a microsyringe. AIR BATH. An air bath having dimensions of 28 cm X 28 cm X 10 cm made in our laboratory was thermostated by an Ohkura Electronic EC52/2 and an electric heater (80 to 800 watts) maintained between 100 to 400 "C i 0.1 "C. The portion of the silicone rubber septum through which samples were injected and the sample evaporating portion were heated by their own, independently placed electric heaters. The thermostat for the detector was not prepared, because the distance from the column end to the hydrogen flame was only a few millimeters in the F I D used in this experiment. A laboratory-made hydrogen flame ionization DETECTOR. detector was employed. A cone-type nozzle (4 mm high and 7 mm 0.d.) made of fused quartz was attached almost directly on the end of the analytical column so that the fuel gas (a mixture of hydrogen and nitrogen) could pass through the fine gap between the nozzle bottom and the column end and come to the exit of the nozzle together with the carrier gas accompanying the sample materials eluted. By using this type of FID, sample materials eluted from the end of the column would not be lost by adsorption in the detector. The electrometer which had been attached to an Ohkura Gas Chromatograph 2000 was employed, and it was able to amplify the ion current to record on chromatographic charts A full scale. The recorder used was an Ohkura up to 25G. Reagents. All chemicals used were of reagent grade quality. Distilled water was used for the carrier vapor. Procedure. Separation columns were maintained a t a constant temperature between 120 and 250 "C in most cases, and the inlet portion of the silicone rubber septum and the sample vaporizing port were kept at 150 to 170 "C and at 150 t o 400 "C,respectively, according to the nature of the samples to be analyzed. The carrier flow rates were 5 to 55 mgjmin (about 9 to 100 m1,'min in vapor volume). Water-soluble samples were made into an aqueous solution of 0.1 or less, and water-insoluble samples were made into
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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Figure 2. Chromatograms of fatty acids mm X (b) Chromosorb G AW 30/60, non-coated, 2 mm X ( c ) Corning GLC-100 60/80, non-coated, 2 m m X (d) Corning GLC-100 60/80, non-coated, 2 m m X 1, acetic; 2, propionic; rz-caproic; 4, enanthic; (a) Chromosorb G AW 30/60, non-coated, 2
2 1 4 4 5,
m; column temp, 125 “C; carrier vapor, H2O-lOZ HCOOH, 96 ml/min m; column temp, 150 “C; carrier vapor, HzO-lOZ HCOOH, 9.7 ml/min m; column temp, 130 “C; carrier vapor, HzO-lOZ HCOOH, 18 ml/min m; column temp, 200 “C; carrier vapor, H20-10 Z HCOOH, 22 ml/min lauric; 6, myristic; 7, palmitic; 8, stearic acid
an aqueous suspension for injecting. The samples were injected with a microsyringe 1 kl or less in size. RESULTS AND DISCUSSION
Fatty Acids. Figure 2 shows typical chromatograms of free fatty acids under various chromatographic conditions in this study. With the column of Chromosorb G A W and carrier steam containing 10 (wiw) formic acid, both lower fatty acids and higher fatty acids were eluted in a n order related strictly to their boiling points, as seen in Figure 2a and Figure 2b, respectively. However, in the GC system where Corning GLC-100 was used as a stationary solid, the lower fatty acids (acetic acid to enanthic acid) were eluted in reverse order in relation to their boiling points, as shown in Figure 2c, while higher fatty acids (lauric acid to stearic acid) were eluted in the regular order (Figure 24. This is probably due to rather strong interactions of lower fatty acid molecules to the surface of the glass bead adsorbent, which may be more hydrophilic than the diatomaceous one, although the fact that lower alcohols are eluted from the column of water layer stationary phase in reverse order in relation to their carbon numbers has been reported (3). The profiles of elution peaks of higher fatty acids in these GC systems, in general, showed a more or less leading effect. This effect was particularly significant in the case of the use of Corning GLC-100 a t a rather low column temperature. This leading effect may be due to the nonlinear adsorption isotherms of the stationary solids ( 4 , 5), that is, due to the type I11 adsorption isotherms which are convex to the pressure axis, according to the classification by Brunauer et ai. ( 6 ) . This type of adsorption isotherm corresponds to the fact that the differential heat of (3) L. H. Phifer and H. K. Plummer, Jr., ANAL.CHEM.,38, 1652 (1966). (4) D. de Vault, J. Amer. Cliem. SOC.,65, 532 (1943). ( 5 ) L. D. Belyakova, A. V. Kiselev, and N. V. Kovaleva, Bull. SOC.Chin?.Fr., 1967, 285.
(6) S. Brunauer. “The Adsorption of Gases and Vapours, Vol. 1,” Princeton University Press, Princeton, N.J.. 1945.
adsorption increases with the amount of adsorption (7). This tendency of the differential heat may be explained by the assumption that the adsorbed sample molecules could interact with each other o n the surface of the adsorbent by their migration, and the attractive forces between adsorbed molecules might be larger than the attractive forces between adsorbed molecules and surface molecules. Therefore, the adsorbent surface cannot be considered to have significant chemisorptive heats to any higher fatty acid molecules under the conditions where steam and formic acid vapors are used as the carrier gas. This type of surface adsorption, which consists mainly of physisorption, may make the elution of the sample easier. It is also reasonable that the fatty acid samples are eluted with comparatively large retention volumes and with less skewed elution peaks on the diatomaceous adsorbent, because the surface of the diatomaceous adsorbent is considered to be less hydrophilic and to interact with the fatty acid molecules more strongly. Owing to these nonlinear adsorption isotherms, however, the profile of the elution peak becomes wider and the position of the top of the peak drifts backward when the amount of the sample injected is too much (8). Therefore, exact measurements of the retention volumes cannot be carried out unless the sample size is limited to any value; the maximum size permissible depends on the kind of the sample, the kind of adsorbent used in the column, the temperature of the column, and the content of formic acid in the steam carrier gas. For example, a stearic acid sample is limited to a size of about gram in the case of a 2-mm i.d. column of Corning GLC-100 and 10% formic acid-steam carrier gas at a column temperature of 105 “C. In the case of Chromosorb G AW, on the other hand, it is possible to introduce a larger sample size, that is, about 10-7 gram at the same temperature and carrier flow. In general, the higher the molecular weight (7) D. M. Young and A. D. Crowell, “Physical Adsorption of
Gases,” Butterworths, London, 1962. (8) A. V. Kiselev and Y . I. Yashin, “Gas-Adsorption Chroma-
tography,” Plenum Press, New York-London, 1969. ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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Figure 3. Chromatograms of lower monoalkylamines (a) and higher monoalkylamines (b) ( a ) Chromosorb G AW 30/60, non-coated, 2 m m X 4 m; column temp, 130 "C; carrier vapor, H 2 0 - 2 0 z H2NNH2-H20, 9 ml/min (b) Chromosorb G AW 30/60, non-coated, 2 mm X 4 m; column temp, 200 "C; carrier vapor, H20-10 % H z N N H ~ - H ~ O22, mlpin. 1, rr-propylamine; 2, /?-butylamine; 3, amylamine; 4, /I-hexylamine; 5, pi-octylamine; 6, laurylamine; 7, stearglamine
1
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2
1
0
Figure 4. Chromatogram of ethylenediamine and ethanolamine Corning GLC-100 60/80, noncoated, 2 m m X 4 m; column temp, 150 "C; carrier vapor, H20-10% H2NNH2-H20, 19 ml/ min. 1, n-butylamine; 2, ethylenediamine; 3, ethanolamine
of the sample and the higher the column temperature, the larger the sample size may be. The preferred content of formic acid in the mixture carrier vapor was about 10% by weight. When the content of formic acid in the carrier vapor was less than 5 %, the elution peaks of higher fatty acids on the adsorbent Corning GLC100 showed profiles widened by a significant leading effect. O n the other hand, with the adsorbent of Chromosorb G AW, the use of less formic acid in the carrier vapor brought about a rather extended retention time and a marked tailing in the elution profiles for higher fatty acids. The use of steam carrier vapor containing more than 10% formic acid gave n o increase in the chromatographic efficiency for either the adsorbent Corning GLC-100 or Chromosorb G AW, but it had some disadvantages owing to the corrosive action of 486
formic acid on the chromatographic apparatus a t high temperatures. Using the same chromatographic systems as used in the separation of free fatty acids, sodium salts of fatty acids could be eluted with the same retention volumes as those of the corresponding free acids, as was the case where acidic columns were used with pure steam carrier gas (I) or with ordinary carrier gases (9, IO). It seems that the sodium salts of fatty acids are readily hydrolyzed into the free acids by formic acid contained in the carrier vapor at the sample injection port, because formic acid has the strongest acidity of the monobasic fatty acids. Dibasic aliphatic carboxylic acids, such as malonic, succinic, and adipic acid, could not be eluted using the present chromatographic system, while these samples could be eluted with pure steam carrier gas on acidic columns modified by phosphoric acid (I). This may be due to the fact that acidity of formic acid is not so much stronger than those of these dibasic acids that formic acid cannot prevent the dibasic acid sample molecules from being adsorbed on the basic sites of adsorbent by its own adsorption to the sites. Amines. Figure 3 shows typical chromatograms of lower monoalkylamines and higher monoalkylamines obtained by using a column of Chromosorb G AW and carrier steam containing 20% (w/w) or 10% hydrazine hydrate (H2NNH2. H 2 0 ) . The profiles of the elution peaks of lower monoalkylamines up to n-octylamine were almost symmetrical, and the resolution about these elution peaks also was satisfactory (Figure 3u). The mixed samples of monoalkylamines lower than propylamine were not separated completely from each other with the Chromosorb G column and the mixed carrier vapor. Di- and trialkylamines, such as di- and trimethylamine and di- and tributylamines, were also eluted after being separated from each other or from the corresponding monoalkylamine in this G C system. Higher monoalkylamines, such as laurylamine, cetylamine, and stearylamine, could be introduced as samples to the GC system having a Chromosorb G A W column of 4 m or less a t a column temperature of 130°C or more, with a retention time less than fifteen minutes. For example, stearylamine was eluted in 14 minutes from a Chromosorb G AW column of 4 m a t 200 "C with a mixed carrier vapor of steam and 10% hydrazine hydrate at a flow rate of 22 ml/min (Figure 36). The elution profiles of these higher fatty amines were slightly distorted by a tailing effect which appeared to increase with an increase in the molecular weight of the amines and to decrease with a rise in the column temperature. Aromatic amines such as pyridine, aniline, toluidine, quinoline, etc. were satisfactorily eluted under chromatographic conditions similar to those described above. The hydrazine-containing steam carrier gas caused the hydrochloric salts of these amines to elute from columns of Chromosorb G AW. The preferred content of hydrazine hydrate in the steam carrier gas was 10 t o 2073 (w/w). Hydrazine hydrate is so corrosive that it may attack all apparatus in the chromatograph, and no more effective elution could be expected with a hydrazine hydrate content higher than 2 0 z . When a steam carrier gas containing less than 10% hydrazine hydrate was used, however, the elution of amines samples becomes rather slow and the profiles of the elution peaks were skewed forward by a tailing effect. (9) G. F. Thomson and K. Smith, ANAL.CHEM., 37, 1591 (1965). (10) I. J. Krchrna, J. Gas Clzromatogr., 6, 457 (1968).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
On the other hand, when Corning GLC-100 was used in a column for analyzing monoamines involving higher fatty amines with steam carrier gas containing hydrazine hydrate, the elution of these samples was so rapid that the elution peaks could not be separated from each other. However, this column was suitable for the analyses of more polar or more basic materials, such as amino-alcohols and diamines, if it was used with hydrazine-containing steam carrier gas, while with the column of Chromosorb G AW it seemed to be difficult to cause these materials to elute even by using such a basic carrier vapor. Figure 4 shows a typical chromatogram of ethylenediamine and ethanolamine obtained with a 4-m Corning GLC-100 column and a mixed carrier vapor of steam and 10% hydrazine-hydrate.
Because it is well known that a vapor in a condition near liquefaction has a high second virial coefficient, which is significant in the case of a vapor having functional radicals or polar groups in its molecules (1 1-13), the interactions between carrier vapor and sample molecules are expected to exist and function to help the sample molecules to be eluted easily from the solid phase or liquid phase of the column, although straightforward evidence for the interactions in gas-chromatography is yet to be provided. On the other hand, that the peculiar effects of vapor carrier gases in gas-solid chromatography are mainly due to the multilayer adsorption of carrier vapors has been proved to some extent by the author’s experiments of vapor adsorption, which will be reported in the near future.
CONCLUSIONS
Gas chromatography will be improved to a great extent by making use of various kinds of vapor or mixed vapor other than pure steam as a carrier gas, and this improvement will be made very easily by using a liquid pump for supplying the vapor carrier gas to the G C system. However, in cases where a carrier vapor favorable to the samples to be applied can be chosen, it will be necessary to find a sensitive detector which is capable of detecting the sample elution from the carrier vapor, whatever kind of carrier vapor it may be.
RECEIVED for review August 23, 1972. Accepted November 17.1972.
(11) J. D. Lambert, G. A. H. Roberts, J. S. Rowlinson, and V. J. Wilkinson, Proc. Roy. Soc., A196, 113 (1949). (12) J. H. P. Fox and J. D. Lambert, ibid., A210, 557 (1952). (13) G. S. Kell, G. E. Mclaurin, and E. Whalley, J. Chem. Phys., 48,3805 (1968).
Design of Molecular Effusion Separators Using Porous Silver Membranes M. A. Grayson and J. J. Bellina, Jr. Me Donnell Douglas Research Laboratories, McDonnell Douglas Corp., St. Louis, Mo. 63166 An apparatus and technique for the determination of the conductance of frits for use in molecular effusion separators is described. The conductances of porous silver membranes of various pore sizes are determined and the effects of pore volume on the conductance is discussed. These data along with other pertinent information for the design of molecular effusion separators are tabulated. The requirements for the design of molecular effusion separators are logically elucidated and the solution of the design parameters is presented in nomographic form. The theoretical performance of molecular effusion separators and means of optimizing their yield are discussed.
MOLECULAR EFFUSION SEPARATORS for combining a gas chromatograph with a mass spectrometer were first described by Watson and Biemann ( I , 2 ) . In devices of this type, the carrier gas (helium) is preferentially removed from the gas stream entering the mass spectrometer by pumping it through a frit of small pore size (-1 pm). Proper operation of the device depends upon establishing a condition of molecular flow through the frit--i.e., the mean free path of the gases in the separator must be approximately one order of magnitude greater than the diameter of the average pore (1-4). If this condition is not maintained, the separator ceases to enrich the (1) J. T. Watson and K. Biemann, ANAL.CHEM., 36,1135 (1964). (2) Ibid., 37,845 (1965). (3) J. T. Watson in Chapter 5 of “Ancillary Techniques of Gas Chromatography,” L. S. Ettre and W. H. McFadden, Ed., WileyInterscience, New York. N.Y., 1965, p 145. (4) P. D. Zemany, J . Appl. Pliys., 23,924 (1952).
eluate going to the mass spectrometer and functions as a splitter. In practice, the condition for molecular flow through the frit is established by controlling the pressure in the separator with restrictions between the G C and the separator (GC-SEP restriction) and the separator and the MS (SEP-MS restriction). Recent papers (5-9) have described the use of porous silver membranes (PSM) (Selas Flotronics, Box 300, Spring House, Pa. 19477) as frits for molecular effusion separators. Despite the well-known physical phenomena occurring in the molecular effusion separator, minimal data are available to aid in the design of these devices. If the conductance of a given area of frit of certain pore size were known, design of molecular effusion separators would be straightforward.Also, the operating flow ranges of a given device could be easily determined. EXPERIMENTAL
The conductance of an element through which gas flows is governed by the relationship (10): (5) R. F. Cree, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1967. (6) M. Blumer, ANAL.CHEW,40, 1590 (1968). (7) M. A. Grayson and C. J. Wolf, ibid., 42.426 (1970). (8) A. J. Luchte and D. C. Damoth, Amer. Lab., p 33. Sept. 1970. (9) M. A. Grayson and R. L. Levy, J . Chromatogr. Sci., 9, 687 (1971). (10) S. Dushman, in Chapter 2 of “Scientific Foundations of Vacuum Technique,” revised by D. G. Worden, J. M . Lafferty, Ed., John Wiley & Sons, Inc., New York, N.Y., 1962, p 80. ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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