GB1582869A

GB1582869A - Gas curtain device and method for transfering matter between a gas and a vacuum

Info

Publication number: GB1582869A
Application number: GB1979877A
Authority: GB
Original assignee: University of Toronto
Current assignee: University of Toronto
Priority date: 1977-05-11
Filing date: 1977-05-11
Publication date: 1981-01-14

Description

(54) GAS CURTAIN DEVICE AND METHOD FOR TRANSFERRING MATTER BETWEEN A GAS AND A VACUUM -U1) We, THE GOVERNING COUNCIL OF THE UNIVERSITY OF TORONTO of Simcoe Hall, Toronto, Ontario, Canada, a corporation incorporated under the laws of the Province of Ontario, Canada, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to the transfer of matter between a vacuum chamber and-a gaseous medium. The invention will be described primarily with reference to the detection and analysis of trace gases by an analyzer such as a mass spectrometer, and the consequent transfer of ions from a gaseous medium into a vacuum chamber.In the -following description the term "sample gas" will be used as meaning the trace gas to be detected together with the carrier gas in which it is contained.

In prior art apparatus for detecting and analyzing trace gases contained in large quantities of air or other carrier gases, problems in achieving extreme sensitivity exist for a number of reasons, including the following.

Normally the mass spectrometer or other detector used must be located in a vacuum chamber, while the'sample gas must be supplied from outside the vacuum chamber. In order to preserve the vacuum, the orifice through which' the gaseous components are admitted into the vacuum chamber must be very small. In some cases pressure staging has been used to permit a larger orifice, but due to practical limitations on pump sizes the gain in orifice size has been relatively minor. The small orifice limits signal transmission into the vacuum chamber and hence limits the sensitivity of the device.

According to a first aspect of the present invention there is provided apparatus for transferring matter between a vacuum chamber and a gaseous medium, comprising: (1) a vacuum chamber having a first inlet orifice therein, said vacuum chamber having an interior surface, (2) a gas curtain chamber outside said vacuum chamber and having a second inlet orifice therein, said gas curtain chamber also having an outlet orifice connected to said first inlet orifice, (3) conduit means for supplying said gaseous medium, at a first flow rate towards said second inlet orifice (4) supply means for supplying -a curtain gas which has a low reactivity with said matter -and which when deposited in solid phase at a predetermined temperature has a vapour pressure substantially less than atmbspheric pressure, at a second flow rate, said supply means being connected to said gas curtain chamber for supplying said curtain gas to said gas curtain chamber at said second flow rate and said second flow rate being such rélative to said first flow rate that said curtain gas will flow out of said gas curtain chamber into said conduit means through said second inlet orifice and will also flow into said vacuum chamber through said outlet orifice and said first inlet orifice, (5) -means connected to said vacuum chamber for cooling at least a portion of said interior surface thereof to said predetermined temperature whereby to condense said curtain gas on said portion of said surface, thereby evacuating said vacuum chamber, (6) and means associated with said conduit means, said gas curtain chamber and said vacuum chamber for moving said matter along a path extending through said gaseous medium in said conduit means, through said first inlet orifice and said curtain gas, and through said outlet orifice and said second inlet orifice into said vacuum chamber, whereby said curtain gas serves to block ingress of said gaseous medium into said vacuum chamber while permitting passage of said matter between said vacuum chamber and said conduit means, and by its condensation substantially enables maintenance of a vacuum in said chamber.

According to a second aspect of the present invention there is provided a method of transferring matter between a vacuum chamber and a gaseous medium, comprising: (a) supplying said gaseous medium at a selected flow rate to a first region, (b) selecting a curtain gas which, when deposited in solid phase at a predetermined temperature, has a vapour pressure substantially less than atmospheric pressure, and which has a low reactivity with said matter (c) directing said curtain gas into a second region adjacent said first region at a flow rate sufficient relative to said selected flow rate to prevent said.gaseous medium from entering said second region, (d) directing at least some of said curtain gas from said second region into said vacuum chamber, (e) cooling at least a portion of the'interior surface of said vacuum chamber of below said predetermined temperature, whereby to condense said curtain gas on said portion of said interior surface, thereby evacuating said vacuum chamber, (f) and moving said matter along a path extending from said first region through said second region and through the curtain gas in said second region, and into said vacuum chamber, whereby said curtain gas functions to prevent said gaseous medium from entering said vacuum chamber while permitting movement of said matter between said first region and said vacuum chamber, and also by its condensation functions to maintain a vacuum in said vacuum chamber.

The gas curtain of the invention, using cryogenically pumpable gas, together with cryogenic pumping in a connected vacuum chamber, may also be used for transferring other matter, e.g. in the form of solid materials, between a first gaseous medium and a second lower pressure gaseous medium (which for convenience we include in the term "vacuum" herein without significant transfer of one medium into the other, as will be explained. This aspect of the invention may be used, for example for transferring into a vacuum a wire or tape on which trace gases or liquids have been absorbed, or it rnay be used (as will be explained) for transferring objects such as razor blades, or other articles arranged in a continuous strip, into or out of a vacuum for further processing.It may be used, for example, to transfer electrons from a vacuum into air, e.g. for electron beam welding, at atmospheric pressure, or it may be used as a windowless aperture for photons.

In a third aspect of the invention there is provided a method of analyzing trace components in a vacuum chamber, comprisng: (a) selecting a curtain gas of low reactivity with said trace components, (b) directing said curtain gas into a first region adjacent said vacuum chamber, (c) admitting at least some of said curtain gas into said vacuum chamber through an orifice between said first region and said vacuum chamber, (d) maintaining a vacuum in said vacuum chamber and thereby expanding said curtain gas into said vacuum chamber about the axis of said orifice, (e) supplying a sample gas at a selected flow to a second region adjacent said first' region, said sample gas containing said trace components to be analyzed, (f) controlling the flow of said curtain gas to said first region to limit entry of said sample gas into said first region, (gD ionizing at least some of said trace components in said second region, thereby forming trace ions in said second region, (h) creating an electric field in said first and second regions to draw said trace ions from said first region through said second region and through said orifice into said vacuum chamber, so that said curtain gas functions to limit the amount of said sample gas entering said vacuum chamber and also functions as an ion window to permit ions to pass therethrough under the influence of said electric field, (i) directing said ions in said vacuum chamber along a path directed away from said expanding curtain gas therein and into an analyzer located in vacuum in said vacuum chamber, (j) and analyzing said ions in said analyzer.

In one embodiment of this aspect of the invention, the gas curtain prevents the sample gas from entering the vacuum chamber and thereby prevents particulates and water vapour from entering the vacuum chamber. The curtain, however, is transparent to ions which are drifted through it by the electric field, and the curtain therefore acts as an ion window. If for example the sample gas used is the effluent from a gas chromatograph, the gas curtain eliminates the need to match the flow into the vacuum chamber to the gas chromatograph flow. This permits use of a single size orifice, the size of which is limited only by the available vacuum pumping speed. Preferably but not necessarily, the gas of the gas curtain -is cryopumpable and cryopumping is used, increasing the pumping speed and hence the permissible orifice -size, and thus increasing the sensitivity of the system.

In another embodiment of this aspect of the invention, the flow rate of the gas curtain is controlled so that some of the sample.gas flow, as well as gas from the gas curtain, is permitted to enter the vacuum chamber. The gas curtain is in effect then pierced to a controlled extent by the sample gas. This also has various advantages, as will be explained.

According to a fourth aspect of the present invention there is provided a method of analyzing trace components in a vacuum chamber, -comprising: (a) selecting a curtain gas of low reactivity with said trace components, (b) directing said curtain gas into a first region adjacent said vacuum chamber, (c) admitting at least some of said curtain gas into said vacuum chamber through an orifice between said first-region and said vacuum chamber, (d) maintaining a vacUum in said vacuum chamber and thereby expanding said curtain gas into said vacuum chamber about the - axis of said orifice, (e) supplying a sample gas at a selected flow to a second region adjacent said first region, said sample gas containing said trace components to be analyzed, (f) controlling the flow of said curtain gas to said first region to admit a selected limited flow of said sample gas in a stream into said first region and through said orifice into said vacuum chamber, (g) controlling the direction of said curtain gas flow in said first region so that said curtain gas encircles and constrains the diameter of said stream of sample gas flowing through said first' region and through said orifice, (h) ionizing at least some of said trace components in said sample gas, (i) directing said ions in said vacuum chamber along a path directed away from said expanding curtain gas therein and into an analyzer located in vacuum in said vacuum chamber (j) and analyzing said ions in said analyzer.

Embodiments of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a diagrammatic view showing a first embodiment of the invention; Figure 1A is a graph of the ion creation region of the Figure 1 apparatus; Figure 2 is a sectional view along lines 2-2 of'Figure 1; Figure 3 is a sectional view along lines 3-3 of Figure 1; Figure 4 is a sectional view of the cryogenic pump of Figure. 1; Figure 5 is a graph showing fluid flow, electric field and ion paths; Figure 6 is a sectional view of modified apparatus of the invention; Figure 7 is a sectional view showing a modified vacuum chamber; Figure 8 is a sectional view on lines 8-8 of Figure 7; Figure 9 is a diagrammatic view showing another embodiment of the invention;; Figure 10 is a sectional view of a further embodiment of the invention; Figure 11 is a block diagram of a control system for the Figure 10 embodiment; and Figure 12 is a view similar to Figure 10 and showing another mode of operation of the Figure 10 system.

Reeerence is first made to Figures 1 and 2, which show one form of apparatus according to the invention. Such apparatus includes an ion reaction section generally indicated at 2, a gas curtain section 4, and a vacuum chamber (and analyzing) section 6. The reaction section 2 and the gas curtain section 4 are connected by an interface plate 8, while the gas curtain section 4 and the vacuum section 6 are:in turn connected by an orifice plate 10. Plates 8, 10 are separated by an insulator 11.

The reaction section 2 includes a bellmouth inlet 12 and a cylindrical duct 14 connected at 16 to a plenum 18. The plenum 18 is connected by duct 20 to a synchronous fan (not shown) which operates to draw the air or other gas to be analyzed through the bellmouth inlet 12 and'into the duct 14. Settling screens (not shown) may be provided in advance of the inlet 12 to eliminate vortices and to help provide laminar flow.

Located in the duct 14 is a central axially elongated electrode 22. The electrode 22 is supported in a position aligned with the axis of duct 14 by a triangular spider of insulating material such as nylon thread 24, as shdwn in Figure 2. A separate insulated wire, not -shown, is used to apply a desired voltage to the electrode 22. The portion of the duct 14 which surrounds the electrode 22 is insulated from the bellmouth 12 by an insulating joint diagrammatically indicated at 26, so that the wall portion of the duct 14 downstream of the joint'26 forms a second or outer electrode 28. Located between the two electrodes 22, 28 is ring-shaped ionizing device 30 such as a tritium foil.

In operating, air or other carrier gas containing the trace gas to-be analyzed is drawn into the cylindrical duct 14 by the synchronous fan at a rate such as to provide a flow which is laminar but which is of sufficiently high velocity to minimize the effects of matter diffusion.

As the mixture passes the ionizing device foil 30, ion creation occurs in the region 32, forming a mixture of positive and-negative ions in the gas. The ion creation region 32 is annular in form and its profile is shown in-Figure 1A, where distance from the foil is plotted along the Y axis (the foil being located at the origin) and the relative number of ions formed is plotted along the X axis.

During the ion creation, the beta rays from the tritium foil ionize components of the air or other carrier gas, resulting (after a series of reactions in the air or carrier gas) in the production of primary reactant ions. Some of the primary reactant ions then react,with molecules of the trace gas to form product ions from the trace gas. This results in a mixture of product ions and reactant ions. From this mixture, the product ions are to be preferentially selected and analyzed.

An electric field, caused by appropriate potentials' applied to the electrodes 22 and 28, the interface plate 8, and the orifice plate 10, is superimposed on the fluid flow. Ions are thus caused to drift with a local velocity = f + K1t, whereto is the local fluid velocity, Eb is the local electric field vector and K is the mobility of the species in question. The potentials applied to the device, and the geometry of the device, are arranged (as will be explained in more detail presently) such that the desired ions of selected polarity and any mobility are caused to converge in an approximately conical fashion from the reaction region 32 to a central region downstream of the front of the central electrode 22 and generally aligned with the axis of the electrode 22.The desired ions are thence carried forward in a concentrated flux toward a central aperture 34 in the interface plate 8. A - portion of the ions originating from the sample flowing through the reaction region 32 passes through the central aperture 34 in the interface plate; typically a flux concentration factor of between sixty and one hundred may be achieved.

The transfer of the concentrated ions into the vacuum and analyzer section 6 will now be described. The transfer occurs through the gas curtain section 4. The gas curtain section 4 is supplied with an appropriate curtain gas (such as CO2) which is selected to minimize reactions with the ions to be sampled and which preferably can be cryopumped in the vacuum section 6. The curtain gas, which acts as a curtain or gas membrane between the reaction section 2 and the vacuum section 6, is directed into the gas curtain section 4 by inlet ducts 40 (Figure 3) arranged to create a generally circular flow pattern having a circumferential component but directed generally radially inwardly in the gas curtain .section 4.The curtain gas, is supplied at sufficient flow to match the ingestion into the vacuum section 6 and to provide a small excess which effuses gently out through the central hole 34 in the interface plate 8, at sufficient flow to prevent passage of the carrier gas into the space between the interface plate 8 and the orifice plate 10. However, the concentrated ion flux is drawn forward, counter to this gentle outflow of curtain gas, by an appropriate attractive potential on the orifice plate 10, until these ions are caught up by the portion of the curtain gas flowing through the orifice 42 of the orifice plate and hence are carried into the vacuum section.

The vacuum section 6 includes (see also Figure 4) a cooling fluid reservoir 44 having fins 46. The reservoir 44 conveniently contains liquid nitrogen 48. As the curtain gas, containing the ions of interest, expands outwardly from the orifice 42, the molecules of CO2 impinge on the fins 46 and deposit there, reducing the pressure in the vacuum chamber. The fins 46 are formed with appropriate trapping surface geometry, as is well known in the art, of cryopumping, to maximize the trapping and depositing of the CO2 molecules. By this means, a high equivalent pumping speed can be achieved, so that an operating vacuum in the 10--' to .10-6 Torr range (suitable for mass spectrometry) can be obtained with an entry orifice 42 diameter of about .033 cm.This size is substantially larger than the .002 cm maximum or smaller size orifices conventionally used, so that the ion flux into the vacuum section can be increased by this invention at least by the ratio of hole areas typically by several hundred or more.

Once' a -vacuum has been established (a mild vacuum may initially be created, by convenient conventional means,' - e.g.- a small mechanical roughing pump, and then increased by cryopumping), the ions of interest entering the vacuum section 6 expand in a free jet 50 and are focussed by appropriate electrostatic ion lens elements (diagrammatical ly indicated-at 51) into 'a mass analyzer 52 such as a quadrupole mass spectrometer. The mass analyzer 52 analyzes the ions according to their charge-to-mass ratio and allows quantitative determination by ion counting or other appropriate conventional. techniques.

After the vacuum section has run for an appreciable period of time, CO2 frost will build up on the fins 46 and defrosting will be required. Sufficient fin'surface should therefore be provided to permit the vacuum section to operate for a sufficient interval before defrosting.

In 'one prototype df the invention which has been operated successfully, operation was carried on for a full week before defrosting was required, but operation for a few hours will frequently be sufficient. In addition, a small additional vacuum pump 54 can be provided to remove non-condensable impurities (such as nitrogen) from the vacuum chamber. The pump S4 conveniently may be a getter ion pump.

The interaction of the crossed electric and fluid fields in the Figures 1 to 4 embodiment will now be described in more detail, with. reference to Figure 5. In Figure 5, lines 56 represent the flow of the carrier fluid (which can be air), lines 58 represent the flow of the curtain gas, lines 60 are equipotential lines representing the electric field, and lines 62 represent the paths of the ions. Voltages V1, V2, V3 and V4 are applied to the electrodes 22, 28, and plates 8, 10 respectively. For analysis of positive ions, the voltages shown may all be positive, with V1 > V2 > V3 > V4.

It will be seen from Figure 5 that in the region between the needle 22 and the encircling electrode 28, the electric field is essentially radial' and the fluid flow lines are essentially axial. Thus assuming use of the voltages shown in Table I, as sample ions are created, the positive ions move at an angle inwardly towards the needle 22 and axially towards the interface plate 8. The negative ions are immediately separated and are drawn towards the duct wall, reducing the, amount of recombination. Ideally the positive ions just miss the tip of the needle 22 and are then drawn towards the aperture 34. Preferably the trajectories of the ions in the region between the needle and the duct wall converge conically towards a point just beyond the tip of the needle at an angle in the range between 30 and 60' degrees, preferably about 45 degrees.In the region downstream of the tip of the needle 22, the radial component of the electric field is small, so that radially inward movement of the ions is slight once they move downstream of the needle tip. However, the axial electric field between the needle'tip and the plate 8 is quite substantial, so that the ions are drawn at high speed relative to the local fluid velocities, towards the aperture 34.

As' the carrier fluid (e.g. air) approaches the interface plate 8, it is diverted radially outwardly, as indicated by the flow lines 56. In addition the curtain gas flowing from the aperture 34 in the interface plate meets the carrier fluid, creating a stagnation point ST1. At the stagnation point ST1 (which is on the central stagnation streamline) there is no moyement of fluid. However, in the region of the stagnation point ST1, the axial electric field is sufficiently high to cause the ions to move forward through the stagnation point ST1, through the counterflowing curtain gas, and through the aperture 34.

When the ions enter the space between the interface plate 8 and the orifice plate 10, they encounter a second stagnation point ST2, created by the division of the curtain gas as it divides to pass through aperture 34 and orifice 42. The axial electric field carries the ions through stagnation point ST2 and they then pass through the orifice 42. It will be noted that because the carrier fluid does not pass through either aperture 34 or orifice 42, any materials in the carrier fluid that might normally tend to clog an orifice will not have that result'in the embodiment here described.

It will be appreciated from the above discussion that the downstream tip of the needle 22 should be located upstream of the stagnation point ST1. If the tip of the needle 22 were located at or beyond the stagnation point ST1, then the fluid velocity at the top of the needle would have no axial component or would be reversed, and the desired sample ions would all be drawn onto the needle 22 (rather than just missing the tip of the needle) and would be lost. Locating the downstream tip of the needle 22 upstream of stagnation point ST1 ensures that a number of ions will be carried forward through the stagnation point.

Typically the spacing between the needle tip and the interface plate 8 is between about 3 and 8 cm, while the spacing between point ST1 and the interface plate 8 is about one or two cm.

In addition, it will be seen that when the ions enter the space between plates 8, 10, they then curve towards the orifice 42. The curved ion paths are caused by the flow of curtain gas into the orifice 42 and the superimposed electric field. To achieve this curved path effect, the voltages should be adjusted so that the speed of the ions due to the electric field as they pass through the space between the plates 8, 10 is not too high; otherwise the radius of the stream tube of ions drawn through the orifice 42 is unnecessarily reduced.

Instead of-providing a gaseous ion input to the gas curtain section 4 and vacuum section 6, the input may be in the form of a tape or wire 92, as shown in Figure 6. Ions of the desired sign may be focussed onto the tape or wire 92, where they are neutralized. Provided that the wire or tape 92 is surfaced with an appropriate material, such as graphite for organic ions, or with appropriate metals, zeolites, metal oxides, the former ions will be adsorbed on the surface of the tape or wire and will remain there until the tape or wire is heated. With appropriate selection of materials, simple adsorption of the trace molecules without prior ionization is a selective process and may also be employed. If desired, components of liquids, e.g. from a liquid chromatograph, may be adsorbed onto the tape or wire by conventional techniques, for analysis in the vacuum chamber.The tape or wire 92 is then fed through the aperture 34 in the interface plate 8, through the gas curtain and through the orifice 42 in the orifice plate 10, into the vacuum chamber 6. In the vacuum chamber the tape or wire 92 passes around capstans 94 which are electrified to pass currant through the section of tape or wire between them, heating the tape or wire to drive off the adsorbed matter bound thereto. The tape or wire 92 may then be returned via rollers 96 through an orifice 97 into the gas curtain and may be wound on a storage reel (not'shown).

Alternatively the tape or wire 92 may form an endless loop. A small hood 98 may be placed around the orifice 42 to generate some pressure within the hood, typically 0.2 to 5'torr, which is a pressure suitable for chemical ionization reactions'. Chemical ionization reactions may then be employed within the hood 98 to reconvert the desorbed molecules -into ions.

The hood 98 is typically insulated from the plate 10 by an insulating support section' 98a.

It will be seen that by using the tape or wire 92, traces of gases in air or other gases', or liquids may in effect be tape recorded, stored, and then played back by placing them in the apparatus of the invention which then serves as a readout station. The reaction section 2 of Figure 1 may if desired be used to create and concentrate ions from a trace gas into a central region where they may be captured by a tape or wire.

Although the use of CO2 as a curtain gas has been described, other gases may also be used, provided that they have a low-reactivity for use in positive or negative ion studies and provided that they can be cryopumped at a conveniently obtainable temperature: In some cases, gases of high reactivity may be acceptable, for example, ordinary H2O (steam).

Alternatively, various fluorinated hydrocarbon gases known under the trade mark freon and which are of low reactivity may be used. However, since a small amount of curtain gas diffuses out the aperture 34'in the interface plate 8, the gas used should normally be acceptable in a room or means provided to remove it. The flow of the curtain gas need not contain a circumferential component but may be purely radial. Preferably the curtain gas used should be a gas which, when deposited in solid phase, has a vapour pressure of substantially less than atmospheric, e.g. 10-4 torr, at a temperature attainable by the cryopumping surfaces.

If desired, mechanical refrigeration may be used instead of using-liquid nitrogen. In this event, a wider range of curtain gases may be used. By way of example, very low temperature refrigeration units are currently produced by Cryogenics Technology Inc.

(CTI), of Waltham, Mass., U.S.A. Some of these coolers can remove several watts of heat at temperatures as low as 20"K. This makes it possible to use gases such as argon, krypton (if available), or nitrogen as the curtain gases. Figures 7 and 8 show a typical vacuum chamber 6' having corrugated fins 46' extending around its periphery and suitable for use with mechanical refrigeration. The fins are of highly conductive metal, e.g. nickel plated copper, and are connected directly to the cold bar (not shown) of a suitable heat pump such as one made by CTI, above. A second cooling surface (not shown) may be provided encircling chamber 6' and may be connected to a second cooling point on the pump. The mass analyzer 52 may be located in a portion of the chamber 6' behind the fins 46'. Not shown is the insulation in which the vacuum chamber is packed.

It will be appreciated that instead of cryopumping the curtain gas, other means for evacuating the chamber may be used (e.g. large mechanical pumps). The advantages listed above for using curtain gas still apply. Other relatively low reactivity curtain gases may then be used, such as neon.

Although the invention has been described for use in connection with trace gas analysis, the technique of the invention, by which matter may be transferred into a vacuum using a curtain gas and cryogenic pumping, may be used in other applications. For example, razor blades during their manufacture may need to be transferred to a vacuum to be metal coated. In accordance with the invention, uncut razor blades in the form of a continuous strip may be transferred through the aperture 34 in the interface plate 8 and through the orifice 42 in the orifice plate 10 into the vacuum chamber. In such event the aperture 34 and the orifice 42 will take the form of narrow slits. The same procedure may be used for transferring the strip material from the vacuum chamber, using another aperture in the gas curtain or by using another gas curtain.Any strip material may be so transferred, for example strip material requiring vacuum deposition. Other bulkier items may also be fed through the aperture 34 and orifice 42 into the vacuum to be treated, so long as they are fed in the form of a continuous strip (if necessary with spacers between them), and so long as the gap between the edges of such strip and the border of the orifice 42 is kept to a minimum. The gas curtain and cryogenic pumping technique may also be used to provide a windowless aperture for electrons, photons, or any other matter directed towards the aperture from either direction. An example of such application is illustrated in Figure 9, which shows the gas curtain section 4 and the vacuum chamber 6. In the vacuum chamber 6 the mass analyzer 52 and associated lenses have been replaced by an electron gun 100 which directs a beam of electrons (indicated by-line 102) through orifice 42 and aperture 34 into the atmosphere. The electron beam 102 may be used for example for electron beam welding. The orifice plate 10 may be connected to an appropriate potential to attract the electronskthrough the orifice 42, while the gas curtain section 4 may use the same cryo pumpable gas (e.g. CO2)- as described in connection with the mass analyzer application of the invention.

The technique of the invention may also be used to transfer matter between two gaseous media, one at lower pressure than the other, without significant transfer of one medium into the other. For example, it may be used to transfer a strip of material from air into low pressure pure nitrogen or into a - low pressure inert gas, using water vapour or an appropriate freon gas as the curtain gas and using dry ice or other appropriate material (or even ordinary refrigeration coils), as the cooling means for cryo pumping the curtain gas.

Reference is next made to Figure 10, which shows a further embodiment of the invention.

Figure 10 shows a curtain gas chamber 198 defined by a metal interface plate 200 and a metal orifice plate 202 which are spaced apart by an annular insulator 204. Plates 200, 202 contain an-aligned aperture 206 and orifice 208 respectively. Plate 202 forms one end of a vacuum chamber 210 containing mass analyzer 212, while plate 200 forms one end of an ionization or reaction chamber 214. The vacuum chamber 210 is pumped by an appropriate means.

The sample gas containing the trace components of interest (either atoms or molecules) is supplied from a source 216 through a conduit 218 into the chamber 214. An electric discharge needle 220 is located in the wall of chamber 124 opposite the interface plate 200, in the path of flow-from conduit 218,.and is axially aligned with the apertures 206, 208.

Electric potentials of appropriate strength and sign are applied to the needle 220 and to the plates 200 202 to induce ionization between the tip of the needle 220 and the interface plate 200. The electric field lines'between these elements are indicated by lines 222 (the same line coding is used as in Figure 5); In operation the trace components may be ionized directly by the discharge from the needle 220 or by other discharge process (e.g. heat, an RF field, etc.). Alternatively the ionization process for the trace components may be indirect, through the sequence of steps in the chemical ionization process. In the latter case one or more chemical reagent gases may if desired be added to the sample flow via conduit 224 which leads into conduit 218.

Desired ion-molecule reactions then proceed in the chamber 224, with the ion products which'are produced dependent upon the gas mixture which is supplied.

A further method of producing a sample gas is as follows. The trace material to be analyzed may be dissolved in a solvent. For example, air containing the trace material may be bubbled through the solvent. If the'trace material is in water, than the water itself may be the solvent. Alternatively, the trace be collected on a solid substrate, e.g. charcoal, and then transferred to a solvent (e.g. benzene) by shaking the substrate in the solvent. The solvent may be benzene, methylene chloride, hexane, isooctane, or any other appropriate solvent.

The solvent, with the trace material therein, is then vaporized. This may be performed by injecting it into a stream of warm carrier gas. If a chemical ionization process is desired, then the-solvent may be used as a chemical ionization reagent, or an additional reagent can be added as part of or can be the entire carrier gas.

The trace components may also be emitted from a liquid chromatograph. In this case the liquid carrier will be vaporized before ionization.

Curtain gas is provided in the Figure 10 embodiment by inlet ducts 230 arranged to create a generally circular flow pattern having a circumferential component but directed radially inwardly. Any other means may be used to create a similar circular flow pattern. The curtain gas is provided via a feed conduit 232 from a curtain gas source 234. A valve 236 in the conduit 232 permits - control of the pressure in the curtain gas chamber 198.

The Figure 10 embodiment may be operated with the flow of curtain gas less than, equal to, or more than sufficient to match the flow through the orifice 208 into the vacuum chamber 210. Each-one of these conditions has specific advantages'. However, regardless of the flow; the electric field' in the chamber 214, caused by the voltages applied to needle 220 and plate 202, and continued in the same direction by a third voltage on plate 204, is arranged so that the ions are drifted, at velocities higher than the gas flows, out of the chamber'214, across the curtain gas in curtain gas chamber 198, and on into the vacuum chamber 210. Typical voltages and dimensions which may be used in the Figure 10 embodiment are given in Table I at the end of this description.Excess gas in the chamber 214 escapes or is pumped out via vents 240 in the outer wall of chamber 214 adjacent the interface plate 200.

When the curtain gas flow through the curtain gas feed conduit 232 matches or slightly exceeds the flow from curtain gas chamber 198 out through the orifice 208, then the flow of sample gas from chamber-214 into vacuum ,is blocked but ions are transmitted. This is the mode in which the Figure 1 apparatus is operated. The gas curtain then has the following advantages.

Firstly, the rate of flow of sample gas is independent of the flow rate into vacuum.

D'ifferent sources of sample flows, for example capillary column gas chomatographs, packed column gas chromatographs, direct air samples, samples desorbed- from the collector trap into a sample carrier gas, all have their own appropriate best gas flows which can now be accommodated without changing or matching the orifice 208 for various sample inlet systems.

Secondly, any sample gas can be used, cryopumpable or non-cryopumpable (such as helium), moisture laden or particle laden (such as normal 'air), and at almost any temperature. The curtain gas ensures that the orifice 208 will not become plugged with particulates, and because the curtain gas can be ultra-pure and dry and at an independently controlled temperature, problems of unwanted non-equilibrium clustering can be reduced.

Non-equilibrium clustering is a reaction which occurs in the adiabatic free jet expansion in the vacuum chamber, due to the cooling effect during the expansion. Since the molecules which cluster most severely around the trace gas ions are usually water molecules and other impurities, the non-equilibrium clustering can be drastically reduced by the use of a pure curtain gas.

Thirdly, the independence of the flow of the sample gas from the flow rate into vacuum can permit increased sensitivity when only a minute amount of sample gas is available (e.g.

from a small sample gas container or trap) and when the desired chemical reaction rate constants related to the ionization of the trace components are low. For example, the well known techniques of stop-column chromatography may be used, in which the flow of gas through the gas chromatrograph is stopped for a desired -interval while analysis is carried out on a given sample. Similarly, in the present invention, the gas flows may be adjusted to hold the sample gas in the chamber 214 for a longer period of time when entry of the desired trace components into the chamber 214 is detected. The entry of the desired trace components into the chamber 214 may be detected by the mass analyzer 212, since the output signal from the analyzer 212 will then begin to increase.A valve 242 may be provided in the conduit 218 to stop or reduce the flow of sample gas at this time, to permit the sample to be more fully ionized. If the pressure in the reaction chamber 214 falls due to the reduced sample gas flow, the flow of curtain gas may also be reduced at this time by valve 236, to maintain just sufficient curtain gas to prevent the sample gas from entering the curtain'gas chamber 198, but to avoid rapid flushing of the reaction chamber 214 with curtain gas.

If desired, and as shown in Figure 11, the increased output signal from the mass analyzer 212 when the desired trace components arrive in reaction chamber 214, may be amplified by an amplifier 250, the output of which is used to operate a threshold detector 252. When the signal at detector 252 rises above its preset trigger level for a period of time which is set sufficiently long to reduce the effect of transients, the detector 252 operates and adjusts valves 236, 242 (which may be solenoid valves) to preset conditions in which they control the gas flows to retain the current sample in the reaction chamber 214 for as long as possible. The valves 236, 242 may be reset to their normal conditions either manually or automatically, when the output of amplifier 240 falls below the threshold level of detector 252.

If desired, after the sample of interest has been stopped and analyzed, the reaction chamber 214 may be flushed out by increasing the curtain gas flow substantially for a short period of time.

A further advantage of the gas curtain is that the curtain gas may be an appropriate cryopumpable gas, so that the advantages of cryopumping may be used even though the sample gas itself may not itself be readily cryopumpable. This feature is'referred to in more detail previously in this application.

A second mode of operation of Figure 10 apparatus may be termed a "pierced curtain".

In this mode the curtain gas inflow is insufficient to match the required flow into the vacuum chamber 210 through the' orifice 208, the difference being made up by the sample gas flow. The curtain gas flow may then be controlled to yield, in effect, an infinitely variable orifice for the sample gas while retaining the maximum fixed open orifice as a window for ions. This situation is illustrated in Figure 12, in which the electric field lines are shown at 222, the curtain gas flow is indicated at 260, the sample gas flow at 262, and.the ion flux is shown at 264.

It will be seen that the curtain gas flow, because it occurs radially inwardly towards the axis of the aligned orifice 208 and aperture 206, encircles or sheaths the sample flow into-the vacuum chamber. As the curtain gas flow, indicated by lines 260, increases, it increasingly constricts the sample gas flow 262 towards the centre line or axis of the orifice 208 and aperture 206. This reduces the sample flow to such an extent as may be desired. Excess sample gas if any is discharged from vents 240.

In operation, the curtain gas flow may be controlled by monitoring the pressure in the sample gas conduit 218, by means of pressure transducer and indicator 266, and then adjusting the curtain gas flow with valve 236 two admit the desired amount of sample flow into the vacuum chamber 210. The sample flow admitted can be varied from the full sample flow (if the vacuum pumping capability is high enough) down to zero, in an infinitely variable manner. Again, the orifice 208 may be as large as the available vacuum pumping equipment permits, and again cryopumping is advantageous.

One of the advantages of the variable orifice mode of operation is that the effective variable orifice permits various desired sample flows to be matched conveniently to the flow into the vacuum. For example, a large orifice 208 may pass a larger flow than desirable for optimum operation of a capillary column gas chromatograph, thus pulling the columri pressure sub-atmospheric and degrading its operation. Sufficient supplementary curtain gas flow can be provided to eliminate this problem. If desired, the signal from pressure transducer and indicator 266 can be used to control the valve 236 to maintain the pressure in conduit 218 at the desired pressure.

The flow matching capability referred to above was also present in the previously described ion window mode of operation, where the excess curtain gas flow plus the sample gas were discharged through vents 240, but in the variable orifice mode of operation the flow matching capability includes the following further advantages.

Firstly, it will be seen that the ion flux lines 264 are arranged orthogonal to the electric field lines 242 in Figure 12. Thus, only that portion of the ion flux 264 which passes through the aperture 206 can contribute to the desired ion signal. Ions outside 'aperture 206 are incident on the metal plate 200 producing the field, where they are neutralized, and are re-emitted promptly as molecules if the source temperature is high enough. In the first mode, i.e. the ion window mode, these molecules will be swept out the vents 240. In the variable orifice mode, they are carried forward by the sample gas stream lines 262 and must pass through the central region where they have an opportunity to participate in the ion-molecule reactions again and to become re-ionized by contact with reagent ions.The re-ionized.trace molecules again thus contribute to the ion signal, increasing the total ion signal count.

A further advantage of the pierced curtain mode is that the surrounding curtain gas constrains the sample during the gas dynamic expansion immediately downstream of the orifice 208. This keeps the trace ions closer to the centre line or axis of the orifice 208, facilitating their focusing into the mass analyzer 212 while the surrounding gases are stripped away and removed.

Íf substantially all of the sample gas flow is admitted into the vacuum chamber 210, then the ionizing means may be located in the vacuum chamber 210 rather than in the chamber 214, and the electric fields caused by the potentials on needle 220 and plates 200, 202 may be removed. Instead, an ionizing flux may be provided in the vacuum chamber 210 by convenient means such as a high perveance magnetically collimated electron gun 270, which directs a stream of electrons indicated at 272 towards the stream emerging from the orifice 208 to create trace ions in the vacuum chamber by electron impact.Because of the -constraining effect of the curtain gas, which limits the expansion of the sample gas in the region immediately downstream of the orifice 208, the resultant sample ions will be created in a region in space more restricted towards the center line of the apparatus, resulting in more efficient focusing of the ions into the mass analyzer 212.

Although the curtain gas need not necessarily be cryopumpable in the ion transfer aspect of the invention, it is preferred that it be cryopumpable. Cryopumping the curtain gas permits increased pumping speeds with apparatus which is much smaller, lighter, and less costly than apparatus having conventional vacuum pumping systems having equivalent pumping speeds.

Typical cryopumpable curtain gases which may be used, depending on the temperature to which the vacuum chamber fins are cooled, are nitrogen, argon, carbon dioxide, oxygen (the latter for use with positive ions only), and appropriate freon gases. All will have a vapour pressure substantially less then atmospheric (typically 10-J torr or less) at a temperature to which the walls of the vacuum chamber can be conveniently cooled.

In addition, the sample gas itself may be cryopumpable. Such gas may be a cryopumpable chemical reagent gas such as water vapour, isobutane, propane, benzene, methylene chloride, hexane, isooctane, or other appropriate reagent. The sample gas may also be a gas such as nitrogen or argon. In that event, even when the sample gas is admitted to the vacuum chamber, cryopumping may still be used.

TABLE I -V10: + 1000 to + 3000 volts tut11: + 50 to + 300 volts V12: + 2 to + 20 volts The above voltages are for positive ion production and transfer. The sign of the voltages will be reversed for negative ions.

WHAT WE CLAIM IS: 1. Apparatus for transferring matter between a vacuum chamber and a gaseous medium, comprising: (1) a vacuum chamber having a first inlet orifice therein, said vacuum chamber having an interior surface, (2) a gas curtain chamber outside said vacuum chamber and having a second inlet orifice therein, said gas curtain chamber also having an outlet orifice connected to said first inlet orifice.

(3) conduit means for supplying said gaseous medium, at a first flow rate said second inlet orifice, (4) supply means for supplying a curtain gas which has a low reactivity with said matter and which when deposited in solid phase at a predetermined temperature has a vapour pressure substantially less than atmospheric pressure, at a second flow rate, said supply means being connected to said gas curtain chamber for supplying said curtain gas to said gas curtain chamber at said-second flow rate and said second flow rate being such relative to said first flow rate that said curtain gas will flow out of said gas curtain chamber into said conduit means through said second inlet orifice and will also flow into said vacuum chamber through said outlet orifice and said first inlet orifice, (5) means connected to said vacuum chamber for cooling at least a portion of said interior surface thereof to said predetermined temperature whereby to condense said curtain gas on said portion of said surface, thereby evacuating said vacuum chamber, (6) and means associated with said conduit means, said gas curtain chamber and said vacuum chamber for moving said matter along a path extending through said gaseous medium in said conduit means, through said first inlet orifice and said curtain gas, and through said outlet orifice and said second inlet orifice into said vacuum chamber, whereby said curtain gas serves to block ingress of said gaseous medium into said vacuum chamber while permitting passage of said matter between said vacuum chamber and said conduit means, and by its condensation substantially enables maintenance of a vacuum in said chamber.

2. Apparatus according to claim 1 wherein said vapour pressure at said predetermined temperature is less than 10-J torr.

3. Apparatus according to claim 2 and including means located in said conduit means for forming ions in said gaseous medium, said ions constituting said matter said means for moving including means for generating an electric field for moving said ions from said conduit means into said vacuum chamber.

4. Apparatus according to claim 1, 2 or 3 wherein said means for moving includes means for moving said matter from said gaseous medium into said vacuum chamber.

5. Apparatus according to claim 1, 2 or 3 wherein said curtain gas is selected from the group consisting of argon, nitrogen, carbon dioxide and a fluorinated hydrocarbon gas of the kind known under the trade mark freon.

6. Apparatus according to claim 2 wherein said means for moving comprises means for guiding said ions along a pre-determined path in said chamber, said portion of said surface encircling said pre-determined path.

7. Apparatus according to claim 6 and including mass analyzer means within said vacuum chamber for receiving and analyzing said ions.

8. A method of transferring matter between a vacuum chamber and a gaseous medium, com rising: (a supplying said gaseous medium at a - selected flow rate to a first region, (b) selecting a curtain gas which, when deposited in solid phase at a predetermined temperature, has a vapour pressure substantially less than atmospheric pressure, and which has a low reactivity with said matter (c) directing said curtain gas into a second region adjacent said first region at a flow rate sufficient relative to said selected flow rate to prevent said gaseous medium from entering said second region, (d) directing at least some of said curtain gas from said second region into said vacuum

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. TABLE I -V10: + 1000 to + 3000 volts tut11: + 50 to + 300 volts V12: + 2 to + 20 volts The above voltages are for positive ion production and transfer. The sign of the voltages will be reversed for negative ions. WHAT WE CLAIM IS:

1. Apparatus for transferring matter between a vacuum chamber and a gaseous medium, comprising: (1) a vacuum chamber having a first inlet orifice therein, said vacuum chamber having an interior surface, (2) a gas curtain chamber outside said vacuum chamber and having a second inlet orifice therein, said gas curtain chamber also having an outlet orifice connected to said first inlet orifice.

chamber (e) cooling at least a portion of the interior surface of said vacuum chamber of below said predetermined temperature, whereby to condense said curtain gas on said portion of said interior surface, thereby evacuating said vacuum chamber, (f) and moving said matter along a path extending from said first region through said second region and through the curtain gas in said second region, and into said vacuum chamber, whereby said curtain gas functions to prevent said gaseous medium from entering said vacuum chamber while permitting movement of said matter between said first region and said vacuum chamber, and also by its consdensation functions to maintain a vacuum in said vacuum chamber.

9. A method according to claim 8 wherein said vapour pressure at said predetermined temperature is less than 10-4 torr.

10. A method according to claim 9 wherein said gas is selected from the group consisting of argon, nitrogen, carbon dioxide and a fluorinated hydrocarbon gas of the kind known - under the trade mark freon.

11. A method according to claim 9 wherein said gaseous medium contains a trace material, said method including the steps of forming ions from said trace material in said first region, said ions constituting said matter; and generating an electric field for moving said ions from said first region into said vacuum chamber.

12. A method according to claim 11 and including the step of guiding said ions along a predetermined path in said- vacuum chamber away from said portion of said surface.

13. A method according to claim 12 including the step of analyzing said ions.

14. A method of analyzing trace components in a vacuum chamber, comprising: (a) selecting a curtain gas of low reactivity with said trace components, (b) directing said curtain gas into a first region adjacent said vacuum chamber, (c) admitting at least some of said curtain gas into said vacuum chamber through an orifice between said first region and said vacuum chamber, (d) maintaining a vacuum in said vacuum chamber and thereby expanding said curtain gas into said vacuum chamber about the axis of said orifice, (e) supplying a sample gas at a selected flow to a second region adjacent said first region, said sample gas containing said trace components to be analyzed, (f) controlling the flow of said curtain gas to said first region to limit entry of said example gas into said first region, (g) ionizing at least some of said trace components in said second region, thereby forming trace ions in said second region, (h) creating an electric field in said first and second regions to draw said trace ions from, said first region through said second region and through said orifice into said vacuum chamber, so that said curtain gas functions to limit the amount of said sample gas entering said vacuum chamber and also functions as an ion window to permit ions to pass therethrough under the influence of said electric field, (i) directing said ions in said vacuum chamber along a path directed away from said expanding curtain gas therein and into an analyzer located in vacuum in said vacuum chamber.

(j) and analyzing said ions in said analyzer.

15. A method according to claim 14 and including the step of maintaining the flow of said curtain gas to said first region at a rate sufficient to prevent said sample gas from entering said first'region.

16. A method according to claim 14 and including the step of controlling the flow of said curtain gas to said first region to permit a selected limited flow of said sample gas in a stream through said first region and through said orifice into said vacuum chamber.

17. A method according to claim 14 and including the step of controlling the flow of said curtain gas to maintain the pressure of said sample gas in said first region at a predetermined pressure.

18. A method according to claim 17 wherein said second region communicates with the outlet of'a sample gas producing device and including the step of controlling the flow of said curtain gas to prevent the pressure in said device from being drawn below atmospheric pressure.

19. A method according to claim 16 and including the step of controlling the direction and rate of flow of said curtain gas so that said curtain gas encircles and constrains the diameter of said stream of said sample gas flowing through said first region.

20. A metho'd according to claim 16 and including the step of controlling the direction and rate of flow of said curtain gas so that said curtain gas encircles said stream sample gas flowing through said orifice and constrains the expansion of said sample gas immediately downstream of said orifice in said vacuum chamber.

21. A method according to claim 14 and including the step of supplying said sample gas from a source of limited volume, so that said sample gas is limited in quantity, and controlling the flow of at least said sample gas for a selected interval to hold said trace gas molecules in said second region for an increased period of time, thereby to increase the number of trace gas molecules which are ionized.

22. A method according to claim 14 and including the step of substantially increasing the flow of said curtain gas for a selected period of time, to flush out said second region with curtain gas.

23. A method according to claim 16 and including the step of selecting a curtain gas which, when deposited in solid phase at a predetermined temperature, has a vapour.

pressure of substantially less than atmospheric, and further including the step of cooling at least a portion of the interior surface of said vacuum chamber to below said predetermined temperature, whereby to condense said curtain gas on said portion of said interior surface.

24. A method of analyzing trace components in a vacuum chamber, comprising: (a) selecting a curtain gas of low reactivity with said trace components, (b) directing said curtain gas into a first region adjacent said vacuum chamber, (c) admitting at least some of said curtain gas into said vacuum chamber through an orifice between said first region and said vacuum chamber (d) maintaining a vacuum in said vacuum chamber and thereby expanding said curtain gas into said vacuum chamber about the axis of said orifice, (e) supplying a sample gas at a selected flow to a second region adjacent said first region, said sample gas containing said trace components to be analyzed, (f) controlling the flow of said curtain gas to said first region to admit a selected limited flow of said sample gas in a stream into said first region and through said orifice into said vacuum chamber, (g) controlling the direction of said curtain gas flow in said first region so that said curtain gas encircles and constrains the diameter of said stream of sample gas flowing through said first region and through said orifice, (h) ionizing at least some of said trace components in said sample gas, (i) directing said ions in said vacuum chamber along a path directed away from said expanding curtain gas therein and into an analyzer located in vacuum in said vacuum chamber, (j) and analyzing said ions in said analyzer.

25. Apparatus for transferring matter substantially as any herein described with reference to and as shown in the accompanying drawings.

26. Method of transferring matter substantially as any herein described with reference to the accompanying drawings.

27. Method of analyzing trace components substantially as any herein described with reference to the accompanying drawings.