Miniature devices useful for gas chromatography

I claim:

1. A unitary gas chromatographic device comprising a body formed of a multiplicity of wafer members laminated together, the wafer members having mating surfaces with channels formed therein such as to define a plurality of gas chromatographic columns in the body, and further comprising injector means disposed in the body for injecting sample gas into the columns, and one or more gas valves disposed in the body for selecting one or more of the columns at a time to be receptive of the sample gas from the injector means.

2. A device according to claim 1 wherein each valve comprises a portion of the body having therein a first cavity and an adjacent second cavity with an inlet portion and an outlet portion, a membrane disposed in the body so as to separate the first and second bodies, a thermally expandable medium filling the first cavity, heating means for heating the medium to expand the medium such that the membrane is caused to distend into the second cavity, and a protrusion extending into the second cavity to a location proximate the membrane such that the outlet portion is closed off from the inlet portion by the distended membrane and open to the inlet portion when the membrane is non-distended, the whereby control of the heating means operates the membrane as a gas valve.

3. A device according to claim 2 wherein the membrane is formed of a nitride of boron or silicon.

4. A gas chromatographic device comprising a body having therein a gas chromatographic column with a gas inlet and a gas outlet, the column having inside column walls with an adsorbent phase being coated thereon, the body further having a detector cavity therein juxtaposed with the gas outlet to be receptive of sample gas from the column, the detector cavity having at least one component associated therewith in the detector cavity to provide detector means for detecting a characteristic of the sample gas, and the detector cavity and the at least one associated component having collective surfaces thereof in the detector cavity with an adsorbent phase being further coated on the collective surfaces, whereby the detector means constitutes an integral portion of the gas chromatographic column.

5. A device according to claim 4 wherein the body is formed of at least two wafer members having mating surfaces laminated together with at least one such surface having a serpentine groove therein to define the gas chromatographic column in the body.

6. A device according to claim 4 wherein at least one of the wafer members is formed of single crystal silicon.

7. A device according to claim 4 wherein the absorbent phase is a liquid phase.

8. A device according to claim 4 wherein the detector cavity has a cavity inlet receptive of a sample gas in a carrier gas and a cavity outlet for passage of the gas through the detector cavity and the detector means comprises an electrical resistive element extending laterally through the detector cavity and being receptive of a heating current and of a voltage measuring device, the voltage being a measure of thermal conductivity of the sample gas.

9. A device according to claim 8 wherein the detector means further comprises an electrically insulating bridge extending laterally through the detector cavity, and the resistive element consists of a thin film resistor supported by the bridge.

10. A device according to claim 9 wherein the bridge is formed of a nitride of boron or silicon.

11. A device according to claim 8 further comprising a second detector including a second resistive element, the second detector being substantially identical to and proximate the first detector and being receptive of a reference gas, the first and second resistive elements being components of a Wheatstone bridge circuit.

12. A gas chromatographic device comprising a body having therein a gas chromatographic column with a gas inlet and a gas outlet, the column having inside column walls with an adsorbent phase being coated thereon, the body further having a detector cavity therein juxtaposed with the gas outlet to be receptive of sample gas from the column, the detector cavity having at least one component associated therewith in the detector cavity to provide detector means for detecting a characteristic of the sample gas, and the detector cavity and the at least one associated component having collective surfaces thereof in the detector cavity with an adsorbent phase being further coated on the collective surfaces whereby the detector means constitutes an integral portion of the gas chromatographic column, the device further comprising injector means juxtaposed with the gas inlet for injecting sample gas into the column, the injector means being formed of at least one injector cavity in the body, the at least one cavity having injector surfaces thereof with an adsorbent phase being further coated on at least a portion of the injector surfaces whereby the injector means constitutes a further integral portion of the gas chromatographic column.

13. A device according to claim 12 further comprising at least one respective valve component in the at least one injection cavity for valving the sample gas into the column, the at least one respective valve component having valve surfaces thereof in the at least one injection cavity with the adsorbent phase being further coated on the valve surfaces.

14. A device according to claim 13 wherein each valve comprises a portion of the body having therein a first cavity and an adjacent second cavity with an inlet portion and an outlet portion, a membrane disposed in the body so as to separate the first and second bodies, a thermally expandable medium filling the first cavity, heating means for heating the medium to expand the medium such that the membrane is caused to distend into the second cavity, and a protrusion extending into the second cavity to a location proximate the membrane such that the outlet portion is closed off from the inlet portion by the distended membrane and open to the inlet portion when the membrane is non-extended, whereby control of the heating means operates the membrane as a gas valve.

15. A device according to claim 14 wherein the membrane is formed of a nitride of boron or silicon.

16. A gas chromatographic device comprising a body having therein a gas chromatographic column with a gas inlet and a gas outlet, the column having inside column walls with an adsorbent phase being coated thereon, the device further comprising injector means juxtaposed with the gas inlet for injecting sample gas into the column, the injector means being formed of at least one injector cavity in the body, the at least one cavity having injector surfaces thereof with an adsorbent phase being further coated on at least a portion of the injector surfaces whereby the injector means constitutes an integral portion of the gas chromatographic column.

17. A device according to claim 16 further comprising at least one respective valve component in the at least one injection cavity for valving the sample gas into the column, the at least one respective valve component having valve surfaces thereof in the at least one injection cavity with the adsorbent phase being further coated on the valve surfaces.

18. A device according to claim 16 wherein the body is formed of at least two wafer members having mating surfaces laminated together with at least one such surface having a serpentine groove therein to define the gas chromatographic column in the body.

19. A device according to claim 18 wherein at least one of the wafer members is formed of single crystal silicon.

20. A device according to claim 16 wherein the adsorbent phase is a liquid phase.

21. A unitary gas chromatographic device comprising a body formed of a multiplicity of wafer members laminated together, the wafer members having mating surfaces with channeling formed therein such as to define in the body a gas chromatographic column having a column inlet and a column outlet and further define in the body an impedance channel having an impedance inlet and an impedance outlet, the impedance channel having a resistance to gas flow similar to that of the column, the body further having therein a carrier gas duct receptive of a source of carrier gas, the carrier duct having a first branch connected to provide a carrier gas flow into the column inlet and a second branch connected to provide a reference gas flow into the impedance inlet, the device further comprising injector means disposed in the body for injecting sample gas into the first carrier gas flow proximate the column inlet, the body further having a detector passage therein having a first terminal connected to the column outlet and a second terminal connected to the impedance outlet, the device further comprising a first valve in the body connected between the first terminal and a vent, a second valve in the body connected between the second terminal and a vent, valve control means for alternatively opening the first and second valves to oscillate between selecting the first flow or the second flow through the passage while simultaneously venting directly the second or first flow correspondingly, while bypassing the passage, detector means disposed in the passage for producing a time varying signal representing a characteristic of the gas in the passage, and processing means receptive of the signal for comparing signals for the first and second flows to present a differential characteristic representing the sample gas.

22. A unitary device according to claim 21 wherein the valve control means and the processing means comprise integrated electronic circuitry contained in the body.

23. A gas injector useful for gas chromatography, comprising a body having therein a plurality of chambers with respective measured volumes, the body further having therein a common gas inlet receptive of sample gas and further having a common gas outlet, the injector further comprising valve means disposed in the body, the valve means being responsive to controller means for selecting one or more of the chambers at a time to be receptive of sample gas from the common inlet, and the injector further comprising pressure means for forcing the sample gas from the one or more selected cavities through the outlet, whereby the total of the respective measured volumes of the selected one or more chambers corresponds to a predetermined volume of sample gas forced through the outlet, and different predetermined volumes are provided by control of the valve means.

24. An injector according to claim 23 wherein the valve means comprises a multiplicity of valves each being disposed between the inlet and a respective chamber and a further multiplicity of further valves each being disposed between each respective chamber and the outlet.

25. An injector according to claim 24 wherein the body further has therein a carrier gas input duct receptive of a source of carrier gas and terminating at a first valve disposed in the body, a sample gas input duct receptive of a source of sample gas and terminating at a second valve disposed in the body, a venting duct commencing at a third valve disposed in the body and being open to disposal, and an output duct commencing at a fourth valve disposed in the body and being adapted for gas communication with a point of utilization, the first and second valves having a first common gas connection in the body with the inlet, and the third and fourth valves having a second common gas connection in the body with the outlet.

26. An injector according to claim 24 wherein each valve comprises a portion of the body having therein a first cavity and an adjacent second cavity with an inlet portion and an outlet portion, a membrane disposed in the body so as to separate the first and second bodies, a thermally expandable medium filling the first cavity, heating means for heating the medium to expand the medium such that the membrane is caused to distend into the second cavity, and a protrusion extending into the second cavity to a location proximate the membrane such that the outlet portion is closed off from the inlet portion by the distended membrane and open to the inlet portion when the membrane is non-extended, whereby control of the heating means operates the membrane as a gas valve.

27. An injector according to claim 26 wherein the membrane is formed of a nitride of boron or silicon.

28. A detector for measuring a characteristic of a sample gas, comprising a body having a passage therein receptive of a first flow consisting of a sample gas mixed into a carrier gas, the passage alternatively being receptive of a second flow consisting of a reference gas, the detector further comprising valve means for selecting between the first and second flows in the passage, valve control means for controlling the valve means to oscillate between selecting the first flow or the second flow through the passage, detector means for producing a time varying signal representing a characteristic of the gas in the passage, and processing means receptive of the signal for comparing signals for the first and second flows to present a characteristic representing the sample gas.

29. A detector according to claim 28 wherein the passage has a first terminal opening receptive of the first flow and an opposite second terminal opening receptive of the second flow, whereby the first and second flows are oppositely directed through the passage, and the valve means comprises a first valve connected to selectively vent gas exiting the passage through the first terminal while the second flow is in the passage, and a second valve connected to alternatively vent gas exiting the passage through the second terminal opening while the first flow is in the passage.

30. A detector according to claim 29 wherein the first and second flows are flowed continuously at similar pressures, so that while the second flow is being vented from the passage through the first valve the first flow is simultaneously being vented directly through the first valve while bypassing the passage, and while the first flow is being vented from the passage through the second valve the second flow is being simultaneously vented directly through the second valve while bypassing the passage.

Description Submit all comments and votes

The present invention relates generally to miniature devices and particularly to unitary gas chromatographic devices.

BACKGROUND OF THE INVENTION

Generally, in gas chromatography ("GC"), a sample to be analyzed is introduced as a pulse of gas in a stream of carrier gas into a chromatographic column. A separation process takes place in the column, and at the end of the column the individual components of the sample will emerge more or less separated in time. The individual components separated by the column are detected by continuously monitoring some physical or chemical property of the effluent.

Ideally, each component in the sample emerges from the column at different times so that, at any one time, the gas flowing into the detector is either all carrier gas or a combination of carrier gas and one of the components of the sample. The detector functions by producing a signal related to the change in the intensity of a given characteristic of the gases flowing through it. As each sample component passes through the detector, the output signal varies from the value it has when the detector is full of carrier gas, with the amount of variation depending on the concentration of the sample component and typically being in the form of a spike or peak on a steady signal. A widely used detector is the thermal conductivity detector (also referred to as a hot wire detector or katharometer) which measures the difference between the thermal conductivity of the pure carrier gas and the mixture of the sample component and the carrier gas.

An injector is also part of a GC system, for introducing the short pulse of a sample gas to be analyzed into carrier gas before the column. Conventional injectors involve the use of a syringe for providing a measured volume of sample.

A miniaturized GC system is disclosed in a report "A Prototype Gas Analysis System Using a Miniature Gas Chromatograph" by J. H. Jerman, S. C. Terry and S. Saadat, Stanford University (June 1, 1980), and in an article "Silicon as a Mechanical Material" by K. E. Petersen, Proc. IEEE 70, 420-457 (May 1982). The techniques of integrated electronic circuit processing are utilized to form the main components of a GC system. A capillary column is formed by etching and laminating wafers of silicon and glass. A valve for the injector comprises a mechanical solenoid plunger and a nickel diaphragm. A volume of sample gas is injected through a capillary by computerized coordination of pressures. A hot wire thermal detector is formed with a thin-film nickel resistor on a thin glass membrane in a cavity.

An improved valve for such a system is disclosed in "A Microminiature Electric-to Fluidic Valve" by M. J. Zdeblick and J. B. Angell, Transducers 87, pp 827-829 (1987). The valve utilizes a sealed cavity filled with a liquid. One wall of the cavity is formed with a flexible membrane which can press against a pneumatic nozzle. When the liquid is heated, it's pressure increases, pushing the membrane toward the nozzle, turning it off.

Although the aforementioned background reflects advancements in miniaturized devices including GC systems, further improvements are quite desirable to increase reliability and precision of operation and also to simplify manufacturability of parts. There also are requirements to reduce even further size, weight, and electrical consumption of instruments for applications such as for aerospace where they are at a premium.

Therefore an object of the present invention is to provide an improved unitary device such as for gas chromatography device, particularly a device of the type utilizing a plurality of wafer members laminated together, having increased flexibility, reliability, speed and precision of operation. Further objects are to provide improved components in such a device, including unique gas chromatographic column structures, sample gas injectors, and detectors.

Another object is to provide an improved gas valve in a unitary body. Yet other objects are to provide an improved gas detector system and to provide a unique structural material for a miniature device.

SUMMARY OF THE INVENTION

The foregoing and other objects are achieved by a unitary gas chromatographic device comprising a body formed of a multiplicity of wafer members laminated together. The wafer members have mating surfaces with channels formed therein such as to define a plurality of gas chromatographic columns in the body. The device further comprises injector means disposed in the body for injecting sample gas into the columns. The columns and injector means should be arranged in the body with substantially minimal connecting channels between the injector means and the columns. The device according to a preferred embodiment further comprises selection means disposed in the body for selecting one or more of the columns at a time to be receptive of sample gas from the injector means.

The selection means preferably comprises one or more gas valves. Each valve comprises a portion of the body having therein a first cavity and an adjacent second cavity with an inlet portion and an outlet portion, a membrane disposed in the body so as to separate the first and second bodies, a thermally expandable medium filling the first cavity, heating means for heating the medium to expand the medium such that the membrane is caused to distend into the second cavity, and a protrusion extending into the second cavity to a location proximate the membrane such that the outlet portion is closed off from the inlet portion by the distended membrane and open to the inlet portion when the membrane is non-distended. This control of the heating means operates the membrane as a gas valve.

According to another embodiment, the body further includes a detector cavity therein juxtaposed with the gas outlet of a gas chromatographic column to be receptive of sample gas from the column. The detector cavity has at least one component associated therewith in the detector cavity to provide detector means for detecting a characteristic of the sample gas. The detector cavity and the at least one associated component have collective surfaces thereof in the detector cavity with the adsorbent phase utilized in the column being further coated on the collective surfaces, whereby the detector means constitutes an integral portion of the gas chromatographic column. Preferably the unitary device further comprises injector means juxtaposed with the gas inlet for injecting sample gas into the column, and the adsorbent phase is further coated on at least a portion of the injector surfaces whereby the injector means constitutes a further integral portion of the gas chromatographic column. Preferably the stationary phase is a liquid phase. In a further embodiment, the body is formed of at least two adjacent wafer members laminated together including a first wafer member with a first surface and a second wafer member with a second surface bonded to the first surface. The first surface has therein a first serpentine groove and the second surface has therein a second serpentine groove in alignment with the first groove so as to define a serpentine channel in the body. Desirably the first and second grooves have semicircular cross sections so as to form the channel with a circular cross section.

A gas injector useful for gas chromatography according to the invention comprises a body having therein a plurality of chambers with respective measured volumes. The body further has therein a common gas inlet receptive of sample gas and further has a common gas outlet. The injector further comprises valve means disposed in the body, the valve means being responsive to controller means for selecting one or more of the chambers at a time to be receptive of sample gas from the common inlet. The injector further comprises pressure means for forcing the sample gas from the one or more selected cavities through the outlet. Thus the total of the respective measured volumes of the selected one or more chambers corresponds to a predetermined volume of sample gas forced through the outlet, and different predetermined volumes are provided by control of the valve means.

In another embodiment, a detector for measuring a characteristic of a sample gas comprises a body having a passage therein receptive of a first flow consisting of a sample gas mixed into a carrier gas, the passage alternatively being receptive of a second flow consisting of a reference gas. The detector also comprises valve means for selecting between the first and second flows in the passage. Valve control means oscillate between selecting the first flow or the second flow through the passage. The first and second flows are oppositely directed alternatively through the passage. Detector means produce a time varying signal representing a characteristic of the gas in the passage. Processing means is receptive of the signal for comparing signals for the first and second flows to present a characteristic representing the sample gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a unitary gas chromatographic device according to the present invention.

FIG. 2 is a perspective view of a paired wafer component of FIG. 1.

FIG. 3 is a magnified cross-section of an embodiment of a column in the component of FIG. 2

FIG. 4 is a magnified cross-section of another embodiment of a column in the component of FIG. 2.

FIG. 5 is an exploded view of a unitary gas chromatographic device according to a further embodiment of the present invention.

FIG. 6 is a cross-section of a gas valve utilized in the present invention.

FIG. 7 is an exploded view of a unitary injector according to the present invention.

FIG. 8 is a schematic diagram of the injector of FIG. 7.

FIG. 9 is an exploded, perspective view of a gas thermal conductivity detector according to the present invention.

FIG. 10 is a plan view of an embodiment for a gas thermal conductivity detector system in a gas chromatographic device according to the present invention.

FIG. 11, consisting of FIGS. 11A and 11B, is a schematic of another embodiment for a gas thermal conductivity detector according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention pertains to miniaturized gas circuitry involving various components and combinations of components in a device that may be fabricated especially by techniques similar to electronic device fabrication. The operable components are controlled electronically such as described in the aforementioned report by Jerman et al. For clarity, components are identified in separate headings below.

Planar Columns

FIG. 1 shows an exploded view of a unitary gas chromatographic device 10 formed of a multiplicity of wafer members 12 that are all laminated together in the actual device. Initially the wafers are in laminated pairs 14, a laminated pair being shown in FIG. 2. One wafer 16 in the pair is made of silicon or the like which can be etched with conventional techniques, such as described in the aforementioned Stanford report and Petersen article, to form a groove 18 therein such as with a rectangular cross-section or with a semicircular cross section as shown in FIG. 3. For example with a boron nitride mask the groove is etched in a (100) plane of silicon with a mixture of hydrofluoric, nitric, and acetic acids (HNA) in proportions 9:75:30 by vol at 22.degree. C. for 10 minutes. The second wafer 20 is preferably glass or quartz with a thermal expansion coefficient similar to that of the first wafer, for example Pyrex (TM) glass. Bonding of wafers 16,20 at their interface 21 is effected by heating them under a small compressive load to 350.degree. C. for 10 minutes with a DC voltage of 1500 V. The structure 14 is, e.g., 6 mm thick by 5 cm.times.5 cm, and has a serpentine channel 22 etched therein which, in a coiled or zig-zag form, may be utilized as a gas chromatographic column. The channel may have a cross sectional dimension D (FIG. 3) between 20 and 220 microns, and a depth of approximately half of these thicknesses, with row separations of 2 to 3 times the cross section. All these dimensions are controllable through the photolithographic and etching process.

Alternatively second wafer 20 also has a groove 22 therein in alignment with first groove 18 to define serpentine channel 22. The second groove should be substantially identical to the first groove, for example to provide a circular channel as depicted in FIG. 4. More broadly the channel may be axisymmetrical as desired for providing suitable inside surface area for adsorbsion. Keys (not shown) may be etched in the wafers 16,20 for alignment, or if one or both of the wafers is glass, as in FIG. 2, the two wafers may be aligned visually. If both wafers are silicon, bonding may be effected as described above with a thin layer of silica between.

With reference again to FIG. 1, a plurality of wafer pairs 14 are stacked and laminated together so as to form a unitary body 24 and to generally align a corresponding plurality of chromatographic micro-columns 22 (four shown). Bonding is effected as described above for each pair of wafers. The columns may be of various dimensions and contain selected stationary liquid phases, as required for simple or complex chromatography. For example, a typical stationary phase may be bonded methyl silicon, or bonded Carbowax (TM), or the like. In a simple form of device 10, planar columns 22 may be linked by connecting channels 26,27,28 directly aligned in pairs to lead perpendicularly through laminated wafers 14. Columns 22 may be connected in series (preferably, as shown) or in parallel by employing columns of different selectivities to optimize complex chromatographic separations. With ten columns and each column being 5 meters long, a total column length of 50 meters or more may be achieved in a single miniature device 5.times.5.times.5 cm.

A heating element 30 conventionally formed of nickel film, with a thermostatic control (not shown) may be formed in at least one end wafer member 32. With such a small device very uniform temperature control of a very long column is quite practical, with less than 10 watt of electrical consumption. Compared with conventional means of column heating this is a reduction of almost two orders of magnitude.

An injector device 34 for injecting a sample gas into a carrier gas duct 36 and thence into columns 22 is also integrated into body 24. A detector 38 is provided in a wafer group 39 at outlet 40 of the columns, and the gas exits through a vent 42. Details for preferable injectors and detectors are described below. The injector and detectors should be connected as close together as practical to the columns to substantially minimize any dead volume between. Similarly all connecting channels between individual columns should be minimized. (For clarity the channels are oversized in FIG. 1.) The laminated structure thus described particularly allows such minimum volumes, a highly desirable goal in gas chromatography to minimize peak broadening. Although described herein for gas, devices within the present invention may be utilized for liquids.

Integrated Miniaturized Valves

According to a further embodiment shown as an example in FIG. 5, gas valves are disposed in a body 24' for selecting one or more of the columns at a time to be receptive of the sample gas. A first injector valve 44 connects injector 34 to a first column 46, and a second injector valve 48 similarly connects the injector to a second column 50. Opening of either valve with the other closed selects a corresponding column 46 or 50 for receiving the sample gas. Opening both valves selects both columns. An intermediate valve 52 is disposed between second column 50 and a third column 54 while another valve 55 bypasses the third column, allowing optional series (tandem) selection of these columns. Other permutations and combinations may conveniently be utilized as desired, such as a single column followed by several columns selectively in parallel. A plurality of detectors 56 with vents 42 may be utilized, with one for each column, to minimize dead volume from each column.

The valves are of the known or desired type for miniature gas devices, for example as taught by Zdbelick et al and shown in FIG. 6. A middle wafer 58 of a laminate 60 has therein a first cavity 62 closed off by a bottom wafer 64. An upper wafer 66 has therein a second cavity 68 adjacent to cavity 62. A membrane 70 is disposed in laminate body 60 so as to separate the first and second cavities. The membrane may be formed as the bottom of a cup-shaped film of aluminum deposited as described in the reference, or may be a film of a nitride or an oxy-nitride of silicon or boron deposited in the cavity. Other layers 72 of aluminum may be used for bonding the boron nitride to wafer 58. Electrical heating element strips 74 of nickel or the like for operating the valve are also deposited adjacent cavity 62 on bottom wafer 64 and connected by electrical leads 76 to a source 78 of electrical current responsive to a controller 80, the electronics being shown schematically in FIG. 6 but preferably incorporated as integrated electronic circuits into the device such as a solid state relay. Upper cavity 68 is divided into an inlet portion 82 and an outlet portion 84 by a protrusion 86 extending from upper wafer 66 to a surface location 88 proxima