US3455092A

US3455092A - Gas analyzer inlet system for gaseous state materials

Info

Publication number: US3455092A
Application number: US511756A
Authority: US
Inventors: Peter M Llewellyn
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1965-12-06
Filing date: 1965-12-06
Publication date: 1969-07-15
Anticipated expiration: 1986-07-15
Also published as: JPS5310476B1; SE333265B; DE1673239A1; GB1109160A; US3421292A; US3429105A; DE1673239B2

Description

July 15, 1969 p, M. LEwE LYN 3,455,092

GAS ANALYZER INLET SYSTEM FOR GASEOUS STATE MATERIALS Filed Dec. 1965 I 64 65 In T H Ii :'f: \\w CHROMATOGRAPH x 2 7| MASS sPEcTPoMETER 42 43M 69 T Tl 67 FIG 4 VACUUM FROM c'As PUMP Fl 6 3 (HR0MAT0GRAPH3937 5| 46 2 28- if??? 47 W I 26 FFJL: 3=4| 3| 49 32 l I 33 To VACUUM PUMP 4 43 22 24 FIG. I INVENTOR I" W PETER M. LLEWELLYN A4 T0 n t 7 VACUUM 25 .BY a? PUMP 22 v 2| ATTORNEY U.S. Cl. 55158 12 Claims ABSTRACT OF THE DISCLOSURE An inlet system for a gas analyzer is disclosed for separating a gaseous constituent from a mixture of gases while at the same time allowing the fluid pressure conditions on the downstream side of the separator to remain substantially independent of the fluid pressure conditions on the upstream side. A permeable membrane which is free of holes and of which the ratio of the permeabilities of the permanent gases to the nonpermanent gases is less than unity comprises the separating element. The inlet system has particular utility as a means for joining to gether a gas chromatograph and a mass spectrometer so as to combine the analytical characteristics of each in the analysis of a single quantity of test material.

The present invention relates generally to inlet systems for gas analyzers and more particularly to a gas inlet system which separates the permanent gases from other gaseous state materials which are to be analyzed. Permanent gases are those gases commonly found in air which have a boiling point substantially below zero degrees centigrade.

Often times in analyzing gaseous state materials, such as naturally occurring gases and vapors originating from solids and liquids, extraneous gases are present which renders the analysis diflicult to accomplish and in some cases virtually impossible. In many cases, the analysis is complicated because of difliculties encountered in separating the extraneous gases from those of interest. This is especially characteristic of the permanent gases and particularly when such gases are present in concentrations which are orders of magnitude greater than those of the gaseous state materials of interest. For example, conventional mass spectrometers are capable of handling gases at a maximum flow rate of approximately 10 torr liters/ second, although under optimum operating conditions this figure generally is maintained at 10- torr liters/second. In those cases where such gas analyzers are employed to investigate various gases separated by a gas chromatograph, only a fraction of the gases issuing from the chrmatograph can be analyzed. This is because the flow rate of the issuing gases is generally about 1 torr liter/ second. It is not unusual to find the concentration ratio of the carrier gas (generally a permanent gas) and gases of interest to be of the order of 10 to 1. Under such conditions, reducing the flow rate of the mixture to the spectrometer while maintaining the concentration ratio of the constituents constant would render mass analysis insensi tive. This is clearly seen when it is considered that the actual concentration of the gas of interest would be re duced to such a low level that the mass spectrometer would be relatively insensitive to its presence.

In addition, when a gas chromatograph is coupled to a gas analyzer, it is necessary to provide a pressure drop between the chromatographic column, which operates at atmospheric pressure, and the analyzer, which operates at pressures ranging from to 10- atmospheres depending upon the particular gas analyzer employed.

To overcome these problems, it has been the practates l tice to enhance the concentration ratio in favor of the gas of interest by extracting the carrier gas from the mixture while simultaneously lowering the flow rate of the gases issuing from the chromatograph and reducing the pressure. Typical apparatus for accomplishing the foregoing are described in the articles Breakthrough in Identifying GC Effluent Fractions by J. Throck Watson, Laboratory Management, July 1965, and Use of a Mass Spectrometer as a Detector and Analyzer for Eflluents Emerging From High Temperature Gas Liquid Chromatography Columns, by Raynar Ryhage, Analytical Chemistry, April 1964. Although such apparatus are arranged to preferentially remove a particular permanent carrier gas present, they also remove some of the gas of interest. Hence, it is seen that where the concentration of the gas to be removed is considerably greater than the gas of interest, care must be taken to insure against the removal of too much of the gas of interest and concomitant insuflicient enhancement of the concentration of the gas of interest. Furthermore, the degree of concentration enhancement of such devices is effected by fluctuations in pressure and gas flow.

Other techniques are also employed to accomplish the separation of permanent gases from other gases of interest. These techniques generally involve collecting samples of the mixture in a cold trap and subsequently liberating the collected gases for introduction into appropriate gas analyzers. Because such techniques require discontinuous steps of collecting, handling and introducing, they generally are less desirable than the aforementioned.

Considerable advantage is therefore to be gained by the provision of an inlet system for introducing gaseous state materials into gas analyzers which discriminates against permanent gases without detrimentally impeding the introduction into the analyzer of other gases to be analyzed. Other advantages will be realized where the inlet system is adapted to receive material in the liquid phase and allow gaseous diffusion of the material into the analyzer.

The present invention provides an apparatus which accomplishes the foregoing and thereby overcomes those limitations and disadvantages characteristic of the prior art devices. More particularly, the gas inlet system of the present invention comprises a membrane mounted to hermetically seal one end of an apertured support member which is in gas flow communication with a vacuum pump. Preferably, the membrane is constructed from materials selected from the group consisting of polymers and stationary liquid phases. Stationary liquid phases are those liquid materials employed in chromatographic columns to partition materials to be separated. Comprehensive lists of such materials can be found in numerous publications, one being Gas Chromatography by Ernst Bayer, published by Elsevier Publishing Company, New York, 1961, Tables 2, l3 and 14.

Polymers and stationary liquid phases are characterized by generally being free of holes. Hence, gaseous state material can pass through such material only by diffusion. However, in order to diffuse through the membrane, the gaseous state material must first be captured by the membrane either by entering into solution therewith or adhering thereto. Although most gases can be captured by such membrane materials, the permanent gases generally will not be captured, especially at elevated temperatures, i.e., substantially above zero degrees centigrade.

This property of the membrane materials is employed to facilitate analyzing selected gaseous state materials. In operation, the vacuum pump establishes a pressure difference between opposing surfaces of the membrane. The material which is to be analyzed is communicated to the surface of the membrane at the higher pressure. Gaseous state materials, except the permanent gases, will readily enter into solution with the membrane material and diffuse therethrough. The gaseous state materials which diffuse through the membrane may then be directed to any suitable gas analyzer for analysis. It is important to note that the ratio of the gases of a mixture passed by the membrane is independent of pressure and gas flow fluctuations.

Contrary to the principle of operation of prior art devices, the inlet system of the present invention accomplished the enrichment of selected gases of a mixture by extracting for use the selected gases from the mixture while rejecting the extraneous permanent gases. Such an inlet system is characterized by being free from those limitations imposed on the prior art devices by their very nature.

Accordingly, it is an object of this invention to provide apparatus for enhancing the concentration of selected gaseous state materials at the expense of other gaseous state materials of a mixture being introduced into a gas analyzer.

More particularly, it is an object of this invention to provide apparatus for conveying selected gaseous state materials into a gas analyzer while rejecting permanent gases.

It is a further object of this invention to provide apparatus for extracting selected gaseous state materials exclusive of permanent gases from a mixture including permanent gases for introduction into a gas analyzer.

It is a further object of this invention to provide a gas inlet system for gas analyzers to facilitate communicating gas chromatographs directly thereto.

It is still a further object of this invention to provide a gas discriminating inlet system for gas analyzers whose gas enhancement characteristic is substantially independent of pressure and gas flow fluctuations.

Additional objects and advantages of the invention will become apparent from the following description and claims considered together with the accompanying drawing, of which:

FIGURE 1 is a cross sectional view of one embodiment of the gas inlet system of the present invention.

FIGURE 2 is an enlarged cross sectional view of the area delineated by line 2-2 in FIGURE 1.

FIGURE 3 is a cross sectional view of a stationary liquid phase embodiment of the membrane employed in the gas inlet system of the present invention.

FIGURE 4 is a cross sectional View of a two stage embodiment of the gas inlet system of the present invention.

FIGURE 5 illustrates one use of the gas inlet system of the present invention.

Referring to FIGURE 1, the gas inlet system of the present invention is seen to include a support member 11 defining an aperture 12. A membrane 13 is mounted to one end 14 of member 11 to hermetically seal that end of the member. As shown, the hermetic seal is accomplished by utilizing a metal vacuum joint of the type disclosed in US. Patent 3,208,758, entitled Metal Vacuum Joint, inventor Maurice A. Carlson et al. In such an arrangement, the membrane 13 is mounted between two soft metal gaskets 17 of the joint. The hermeteic seal is formed by clamping the gaskets 17 between the member 11 and top plate 18 which defines an aperture 19 for communication with a source of gaseous state material.

An end 21 of member 11 is hermetically connected by, for example, a conduit 22 brazed thereto, to a suitable vacuum pump (not shown). The vacuum pump is operated to establish a pressure differential across membrane 13, with the lower pressure in the region defined by membrane 13, member 11 and conduit 22. This pressure differential facilitates diffusion of gaseous state materials through the membrane.

The effectiveness of membrane 13 in rejecting permanent gases while allowing other gaseous state materials to pass therethrough is influenced by the material of the 4 membrane, the thickness of the membrane and the temperature of the membrane. As noted hereinbefore, materials selected from the polymers and stationary liquid phases have been found to work satisfactorily. However, other materials will work if relative to gaseous state materials they are free of holes, if the permanent gases will not enter into solution with the material. As utilized herein, entering into solution is defined as a process of condensation and then mixing of the gaseous state material in the surface layers of membrane 13. (See Physics and Chemistry of the Organic Solid State edited by David Fox, Mortimer M. Labes and Arnold Weissberger, published by Interscience Publishers, New York, 1965, vol. 2, p. 517.).

In those instances where stationary liquid phases form the membrane 13, a suitable reservoir supporting structure must be provided. For example, a reservoir constructed from a polymer or a fine screen mesh capable of supporting the liquid by surface tension would be suitable.

The effectiveness of the membrane 13 in allowing gases to diffuse therethrough is inversely related to the thickness of the membrane. It is a particularly important consideration where the gas of interest is a minute part of a mixture, e.g., one part in 10 Under such circumstances, a membrane thickness of less than 20 mils is recommended.

All materials defined hereinabove are characterized by an optimum operating temperature range within which a gaseous state material will enter into solution therewith. Generally, this range falls within the extremes of to 400 centigrade.

An inlet system of the type described hereinabove may be employed to introduce gaseous state materials into a gas analyzer from various gas sources. For example as illustrated in FIGURES 4 and 5, such an inlet system is employed to introduce gases originating from a gas chromatograph. However, gaseous state material originating from a liquid or even volatile solids can be introduced by the inlet system. In such cases, a liquid, for example, would be placed in a reservoir defined by the top surface 22 of membrane 13 and the apertured top plate 18. If the vapor pressure relative to the material of membrane 13 is too low to render a quantity of gaseous state material which can be detected by a gas analyzer, the vapor pressure may be increased by heating the liquid.

- In one embodiment constructed in accordance with FIGURE 1, a five mil thick polysiloxane polymer membrane 13 having a cross sectional area of about one square inch was employed. In order to furnish additional support for membrane 13, a perforated support 23 was secured beneath membrane 13. A gas mixture including for example one part hexane gas per (10 parts helium was directed over the top surface 22 of membrane 13. The gas mixture diffusing through membrane 13 was monitored by a gas analyzer and found to be enriched in hexane gas by a factor of 500.

Additional gaseous enrichment or discrimination may be achieved by covering at least surface 22 of membrane 13 with a coating 24 of stationary liquid phase. This is best depicted in FIGURE 2. The thickness of the stationary liquid phase coating 24 is a matter of choice, generally selected in accordance with those considerations mentioned supra with respect to membrane 13'.

An alternative construction of a membrane is illustrated in FIGURE 3. As depicted therein, a stationary liquid phase 26 is confined within a closed liquid tight relatively thin walled vessel 27 constructed from a gas permeable material. Preferably, the vessel material is selected from the polymers. By providing vessel 27 with supporting tabs 28 mounted circumferentially therearound, the vessel 27 can be mounted in the inlet system in the same way as membrane 13. (See FIG- URE 1.)

It has been found that by providing a plurality of spaced apart membranes arranged in staged fashion along the gas flow path, the enrichment of selected gases at the expense of the permanent gases can be enhanced many orders of magnitude. In FIGURE 4, a two stage embodiment is illustrated which includes a first polymeric membrane 31 supported in hermetically sealed relation by a first soft metal annular gasket 32 and a first perforated disc 33 between first and second

annular members

34 and 36 respectively of a metal vacuum joint. A second polymeric membrane 37 is mounted spaced above membrane 31 to define a chamber 38 therebetween. Membrane 37 is hermetically mounted by a second soft metal annular gasket 39 and a second perforated disc 41 between an aperture recessed cap 42 and second member 36. Cap 42, member 36 and gasket 39 form a metal vacuum joint.

A first vacuum pump (not shown) is mounted in gas tight relation to first member 34 by a conduit 43 brazed thereto. The vacuum pump is operated to facilitate the difiusion of gaseous state material through first membrane 31. A second vacuum pump (not shown) is her metically communicated to chamber 38 by a passageway 44 of a selected gas conductance defined by second member 36. The second vacuum pump is operated to establish a predetermined pressure in chamber 38 higher than that established by the first pump and to extract out a portion of the gaseous state material entering chamber 38 through second membrane 37. The amount of gas extracted by the second pump will depend upon the relative gas conductances of first membrane 31 and passageway 44. The amount of gas passing through second membrane 37 is determined by the pressure differential between chamber 38 and chamber 46 defined by recessed cap 42 and membrane 37. For a given gas and membrane material, the conductance of membrane 31 is determined by its thickness and cross sectional area. The type of gas of interest and membrane material fixes its solubility constant and its dilfusion rate through the membrane, hence the permeability of the membrane to a given gas.

In one application of the two stage inlet system, a gas mixture of 0.1 microliter of heptane, octane, nonane and decane contained in helium issuing from a gas chromatograph at a gas flow rate of 60 milliliters per minute was introduced at atmospheric pressure into chamber 46 through an inlet 47. The thickness and cross sectional area of membrane 37 were one mil and one square inch respectively. The helium gas flow rate through membrane 37 was found to be above 0.002 cubic centimeter per second while the gas flow rate of decane was 2 cubic centimeters per second. Hence, the enrichment of decane in the gas mixture entering chamber 38 was a factor of about 1000.

To preferentially remove helium from the gas mixture contained in chamber 38, the length and cross sectional area of passageway 44 was adjusted to be 0.5 inch and 0.003 square inch respectively. A mechanical type vacuum pump was employed to remove the gases emerging from passageway 44 and to establish a chamber pressure of about 100-200 microns, a helium gas flow rate through passageway 44 of about cubic centimeters per second and a decane gas flow rate of about 2 cubic centimeters per second. Hence, an additional enrichment by a factor of 2000 was obtained; the overall enrichment of decane being about 2x10 This is exceedingly better than has been accomplished in the prior art when it is considered that enrichments of only 50-100 are commonly obtained.

If it is desired to improve the discrimination or enhancement of certain gases directed through membranes 31 and 37, a selected stationary liquid phase coating 49 and 51 may be applied to one surface of the membranes 31 and 37 respectively. By proper selection of the coating material, the gas enrichment may be enhanced orders of magnitude over that achieved without the coatings.

Also, by proper selection of the coating material, discrimination between polar and non-polar gaseous state materials may be accomplished.

Considering now FIGURE 5, a single stage inlet system 61, such as illustrated in FIGURE 1, is shown as employed with a conventional mass spectrometer 62 gas analyzer to inspect efiluent as it issues from a gas chromatograph 63. A gas flow across a stationary liquid phase coated polymeric membrane 64 is caused by the flow from the gas chromatograph 63. A pressure differential is established across membrane 64 by connecting a vacuum pump 67 to one end of the bar member 68 of a T-type gas conduit 69, the other end of the bar 69 mounted to membrane 64. The stem member 71 of conduit 69, conveys a portion of the gas mixture which passes through membrane 64 to the gas ionizer 72 of spectrometer 62. The ionized gases are accelerated and directed to a magnetic field established by sectored magnets 73 whereat they undergo mass separation. The mass separated gases are detected for analysis by collector 64. Instead of the parallel gas flow connection of vacuum pump 67 and gas analyzer 62, the pump could be connected in series with the analyzer 62 and the membrane mount to pump through the analyzer.

The gaseous state material inlet system of the present invention can be adapted to other gas analyzers as well. For example, infrared or microwave mass analyzers could be connected to receive the gases from stem 71 of conduit 69. Furthermore, gas measuring devices such as ion gauges, sputter-ion vacuum devices, thermal conductivity devices or other equivalent means can be coupled to stem 71 of conduit 69 to monitor the effluent emerging from gas chromatographs.

Supplementary benefits evolve from the use of the inlet system of the present invention. For example, in those instances where the inlet system is employed to remove helium from a gas mixture, sputter ion pumps may be employed to evacuate the relatively helium free portion of the system. Furthermore, since for a given membrane various gases will have diiferent solubility constants and diffusion rates, the various gases will require longer times to pass through the membrane. Hence, the inlet system of the present invention could be employed in some cases to time-dependent separation of gases of nonpermanent gas mixtures.

The inlet system has been described as operating with a pressure differential across the membrane where one of the pressures is maintained below atmospheric. However, the inlet system can be employed in environments requiring a differential between pressures in the range above atmospheric. In suchcases, suitable pumps would be employed in place of the vacuum pumps. However, even in those cases, a pressure reduction means must be employed to couple the inlet system to the gas analyzer.

What is claimed is:

1. An inlet system for introducing gaseous state materials from a source into a gas analyzer comprising a membrane constructed of a material free of holes and in which the permeability of permanent gases and the permeability of other gases is in a ratio of less than unity, mounting means for hermetically mounting said membrane in a gas flow path between said gas analyzer and said source of gaseous state material, and means for connecting a vacuum pumping means to establish a pressure difierential across said membrane with the lower pressure region being between said membrane and said analyzer.

2. The inlet system according to claim 1 wherein said membrane comprises a sheet of polymeric material.

3. The inlet system according to claim 2 wherein said membrane has a thickness less than twenty mils.

4. The inlet system according to claim 1 wherein said membrane comprises a coating of stationary liquid phase covering a layer of polymer.

5. The inlet system according to claim 1 wherein said membrane comprises a gas permeable liquid impervious reservoir means supporting a stationary liquid phase.

6. The inlet system according to claim 5 wherein said reservoir means is a polymeric vessel.

7. The inlet system according to claim 1 wherein said mounting means is a vacuum seal means including an apertured base member and a recessed cap member having an inlet and outlet communicating with said recessed portion adapted for mounting to said base member with the recess in facing relation thereto, said inlet adapted for connection to a gas chromatograph, said membrane is constructed of materials selected from the group consisting of polymers and stationary liquid phase and is hermetically mounted between said cap and base memher in covering relation with the aperture of said base member, said vacuum pumping means is connected to said base member, and means are provided for connecting a gas analyzer to receive gases which pass through said membrane.

8. The inlet system according to claim 7 wherein said vacuum pumping means and gas analyzer are connected to said base member by a T-shaped conduit having a bar member and stem member, said bar member connected between said vacuum pumping means and base member, said stem member connected to said gas analyzer.

9. The inlet system according to claim 8 further defined as comprising a perforated support member mounted below said membrane to render support thereto, said membrane including a layer of polymer of a thickness less than twenty mils having an upper surface coated with a stationary liquid phase.

10. A gas monitoring system comprising a gas chromatograph from which an effiuent issues, a gas analyzer means disposed in a gas flow relation to receive at least a portion of said effiuents issuing from said chromatograph, at least one membrane constructed of material free of holes and in which the permeability of permanent gases and the permeability of other gases is in a ratio of less than unity hermetically mounted in said gas flow path between said gas analyzer means and chromatograph, and means for connecting a vacuum pump to establish a pressure differential across said membrane with the lower pressure region being between said membrane and said gas analyzer means.

11. An inlet system for a gas analyzer means for separating a nonpermanent gaseous state material from a mixture of the gaseous state material and a permanent gas and for introducing said separated gaseous state material into the input of said gas analyzer means, said inlet system including means forming an input chamber having an inlet and an outlet such that said mixture may be passed through said input chamber, means forming an output chamber in communicating relationship with a vacuum pump means and with an input to said gas analyzing means, said input and output chambers being adjacent and separated by at least one permeable membrane constructed of a material free of holes and of which the permeability of permanent gases and the permeability of nonpermanent gases is in a ratio of less than unity such that only said gaseous state material is communicated through said membrane to the input of said gas analyzer means.

12. In a gas analyzing system including a gas chromatography means, a mass spectrometer means and an inlet system for operatively connecting the gaseous effluent flow path of said gas chromatography means and the input flow path of said mass spectrometer means, said inlet system comprising means forming an inlet chamber and an outlet chamber separated by a permeable membrane, said inlet chamber including means for directing said efliuent flow into communication with said membrane such that a gaseous constituent thereof is captured by said membrane and caused to permeate therethrough into said outlet chamber, pressure reducing means communicating with said outlet chamber and said input flow path such that said gaseous constituent may be introduced into the input of said mass spectrometer means at a substantially reduced pressure.

References Cited UNITED STATES PATENTS 2,045,379 6/1936 Bennett 158 X 2,892,508 6/1959 Kohman et a1 55--l6 3,246,450 4/1966 Stern et a1. 5516 3,285,701 11/1966 Robertson 55-197 X REUBEN FRIEDMAN, Primary Examiner J. L. De CESARE, Assistant Examiner US. Cl. X.R.