A microminiature gas chromatograph (.mu.GC) comprising a least one silicon wafer, a gas injector, a column, and a detector. The gas injector has a normally closed valve for introducing a mobile phase including a sample gas in a carrier gas. The valve is fully disposed in the silicon wafer(s). The column is a microcapillary in silicon crystal with a stationary phase and is mechanically connected to receive the mobile phase from the gas injector for the molecular separation of compounds in the sample gas. The detector is mechanically connected to the column for the analysis of the separated compounds of sample gas with electronic means, e.g., ion cell, field emitter and PIN diode.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to gas chromatography and more particularly 
to micro-instrumentation and the fabrication of sample gas injector valves 
in silicon wafers. 
2. Description of Related Art 
"Chromatography" is from the Greek word for "color writing." It is a method 
used in analytical chemistry to separate and identify the components of 
mixtures. The Russian botanist Mikhail S. Tswett (1872-1919) was the first 
(1903) to employ a general chromatographic technique. Partition 
chromatography was introduced in 1941, paper chromatography in 1944, and 
gas chromatography in 1952. A method of thin-layer chromatography was 
developed for general use in 1958. Since then, many chromatographic 
techniques have been developed that provide for specific needs, e.g., high 
performance, or pressure, liquid chromatography, gel permeation 
chromatography, ion chromatography, and concurrent chromatography. Prior 
art methods have emphasized both sensitivity and speed. 
Column chromatography uses a vertical tube, or column, filled with a finely 
divided solid or liquid, a "stationary phase." A mixture of materials to 
be separated is placed at the top of the tube and is slowly washed down 
with a suitable liquid, eluent or carrying gas, a "mobile phase." As the 
mixture dissolves, each molecule is transported in the flowing liquid or 
carrying gas and becomes adsorbed into the stationary solid or liquid. 
Each type of molecule spends a different amount of time in the column, 
depending on its tendency to be adsorbed. Thus each compound descends 
through the column at a different rate. The various compounds stratify 
over physical distance in the column, as in a parfait. 
Mobile phases may be gases or liquids, and stationary phases are either 
liquids adsorbed on solid carriers or solids. When a liquid stationary 
phase is used, the process is called partition chromatography, since the 
mixture to be analyzed will be partitioned, or distributed, between the 
stationary liquid and a separate liquid mobile phase. Where the stationary 
phase is solid, the process is known as adsorption chromatography. The 
molecules of the mixture to be separated pass many times between the 
mobile and stationary phases at a rate that depends on the mobility of the 
molecules, the temperature, and the binding forces involved. The 
difference in the time that each type of molecule spends in the mobile 
phase leads to a difference in the transport velocity and to the 
separation of substances. 
Commonly used adsorbents are silica gel and alumina, which are powdered 
into particles between 0.05 and 0.2 mm (0.002 to 0.08 in) in diameter for 
optimal flow. Stationary phases with very different properties can be 
obtained; and many different mixtures can be separated if a suitable 
adsorbent is chosen, and the powder is impregnated with a liquid. 
Stepwise, or fractional, elution involves eluting with liquids of 
increasing or decreasing polarities. The emerging liquid eluate can be 
collected automatically in small portions by a fraction collector. Each 
fraction is then analyzed separately. The eluate may then be passed 
through a spectrophotometer that measures the light absorption when a 
specific substance leaves a column. For the analysis of substances still 
in the column, the solid can be carefully pushed out of the column, cut 
into small sections, and treated. 
In thin-layer chromatography (TLC), the stationary phase is a thin layer on 
a glass plate or plastic film. Typical thin layers comprise one of the 
usual adsorbents, such as silica gel or alumina made into a slurry and 
dried in a homogeneous layer on the glass plate. The mixture to be 
separated is first dissolved in a volatile solvent, and a small sample of 
this solution is placed on the thin layer. The solvent is then evaporated, 
and only the mixture to be separated remains in the form of a small spot. 
The plate is placed in an upright position in a jar. A carefully chosen 
developing solvent is then added to the bottom, the atmosphere in the jar 
is completely saturated with the vapor of the eluent, and the dish is 
closed. The liquid rises along the plate by capillarity. When it has risen 
10-15 cm (4-6 in), in 10-20 minutes, the development is stopped and the 
plate is dried. Most chromatograms can be examined under ultraviolet light 
to locate the compounds. However, if the compounds are colorless, the 
plate is sprayed with a special reagent that colors the various compounds. 
Paper chromatography uses a stationary phase of water adsorbed on paper 
and a mobile phase of an organic liquid and is similar to thin-layer 
chromatography. 
Gas chromatography includes gas-liquid chromatography (GLC) and the far 
less common gas-solid (GSC) method. The stationary phase is a liquid on a 
solid support, which is pressed into a narrow, coiled column 1.5-5 m (4-15 
ft) in length. The mobile phase is an inert gas, usually nitrogen, 
hydrogen, helium, or argon, which is passed through a heated column. The 
sample mixture is injected into the column and immediately vaporizes. Its 
constituent substances separate and flow at different rates with the 
carrier gas. A detector is placed at the end of the column, which outputs 
a signal to a recorder in the form of a gas chromatogram having a series 
of detector maximums. Each peak is characteristic of a particular 
substance in the sample gas. 
An important part of each gas chromatograph is its detector. Various types 
have been developed, including the katharometer, the flame ionization 
detector, and the electron capture detector. The flame ionization detector 
can detect a sample as small as 10.sup.-11 grams of material. The electron 
capture detector is as much as 100 times more sensitive than that. As 
such, gas chromatography has become an essential analytical tool in many 
chemical laboratories. 
High performance liquid chromatography, or high pressure liquid 
chromatography (HPLC), is a refinement of standard column chromatography 
and has become, along with GLC, one of the two most commonly used 
separative techniques. In HPLC, the particles that carry the stationary 
liquid phase are uniformly very small, e.g., 0.01 mm/0.0004 in. Thus, the 
stationary phase presents a large surface area to the molecules of the 
sample in the mobile liquid phase. A resistance to input pressure by a 
column filled with such small particles is overcome with a high-pressure 
pump to drive the mobile liquid phase through the column in a reasonable 
time. HPLC offers high resolution and sensitivity. A column of 25-cm 
(9.8-in) length has an overall efficiency of 10,000 plates or individual 
separations. HPLC can resolve a raw urine sample into 200 individual 
components. Its extraordinary sensitivity can be used to detect a 
concentration of one part in one billion of the chemical aflotoxin, which 
is toxic to humans in food concentrations of as little as ten parts in one 
billion. More recent HPLC's use smaller diameter columns (3-5 cm/1.2-2 in) 
that increase the analytic speed and conserve expensive solvents. Some 
units can now perform analyses in one minute or less. 
Gel permeation chromatography is based on the filtering or sieving action 
of the stationary phase. The stationary phase material is selected from a 
set of adsorbents that have pores of uniform size in the range of 20 to 
200 nm. While moving down the column loaded with this type of adsorbent, a 
molecule dissolved in the mobile liquid phase will be excluded from the 
adsorbent if its size is greater than that of the pores. If the molecular 
size is smaller, the molecule will become entrapped. Intermediate-size 
molecules will permeate some pores and not others. The result is a 
separation based on molecular size, with the larger molecules separating 
out first and the smaller molecules last. This technique is used to 
separate and measure the molecular weight of polymers, proteins, and other 
biological substances of high molecular weight. 
Making gas chromatographs smaller has been an objective in the prior art. 
Drew, et al., describe in U.S. Pat. No. 5,313,061, issued May 17, 1994, a 
miniaturized mass spectrometer system. A battery-operated portable unit is 
used in the field to analyze the atmosphere, water, soil, drugs, 
explosives, and other substances. Such patent is incorporated herein by 
reference. Even though such a mass spectrometer system has been 
miniaturized, it is still quite large and not easily carried, e.g., as in 
a shirt pocket. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a microminiature gas 
chromatograph. 
A further object of the present invention is to provide a microminiature 
injector valve system in a silicon wafer for use in a gas chromatograph. 
Briefly, an injector valve of the present invention comprises a flexible 
reed of silicon nitride attached to two silicon wafers that are bonded 
together to form a carrier gas channel with a normally closed side entry 
for a sample gas to pass the flexible reed. A sample gas pressure that is 
higher than the carrier gas pressure opens the valve and allows a mobile 
phase to flow and the separation of sample compounds in a column to 
proceed. A normally open valve is provided for the carrier gas supply with 
samples and also to regulate the pressure pumping of gas through the 
injector valve. It also allows the cleaning of the sample cavity. 
A further advantage of the present invention is that a microminiature 
normally open valve is provided that can be constructed with silicon wafer 
fabrication techniques. An advantage of the present invention is that an 
injector is provided for a microminiature gas chromatograph. 
An advantage of the present invention is that a high temperature injector 
of 350.degree. C. or higher is provided for high resolution result. 
Another advantage of the present invention is that inexpensive 
semiconductor fabrication techniques can be employed to make a 
microminiature gas valve and sample injector.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a microminiature gas chromatograph (.mu.GC) in an 
embodiment of the present invention, referred to herein by the general 
reference numeral 10. The .mu.GC 10 comprises an injector 12, a column 14 
and a detector 16, each and all disposed in and fabricated from respective 
silicon wafers that have been bonded together in an assembly. Such silicon 
wafers are typically two inches in diameter. The column may be constructed 
according to the information provided in United States patent applications 
08/464,020 and 08/465,068, filed Jun. 5, 1995, which are titled, METHOD 
FOR ETCHING MICROCHANNELS IN SILICON WAFERS TO HAVE SEMICIRCULAR CROSS 
SECTIONS and MICROCAPILLARY AND METHOD FOR JOINING SILICON WAFERS IN THE 
FABRICATION OF MICROCAPILLARIES, by the present inventor. Such 
applications are incorporated herein by reference. 
The injector 12 receives a sample gas 18 and a carrier gas 20 for a mobile 
phase. A pump 22 allows a precision amount of the sample gas 18 to be 
injected into a valve 24 that then flows to the column 14. An 
electrical-resistance heater 26 is thermostatically controlled to maintain 
a constant temperature for the injector 12. Similarly, a heater 28 is 
thermostatically controlled to maintain a constant temperature for the 
column 14. 
FIGS. 2A-2C represent the injector 12 in various operational states. A set 
of three silicon wafers 31-33 are bonded together in a stack 34. At the 
interface of wafers 31 and 32 is a silicon nitride membrane that contains 
a power driver volume 38. A port 40 is connected to a pneumatic system 
that provides pulses of air pressure punctuated by exhaust puffs 42. A 
pair of normally open valves 44 and 46 are connected by a pair of channels 
48 and 50 to a precise sample volume cavity 52. The carrier gas 20 enters 
from the bottom through the normally open valves 44 and 46. An outlet 54 
runs past a normally closed valve 56. The sample gas 18 enters the 
normally closed valve 56 from the bottom and is injected by superior 
pressure into the outlet 54 and flows out to the left. 
In FIG. 2A, a pair of silicon nitride reeds 58 and 60 remain open, and a 
silicon nitride reed 62 remains closed. The carrier gas 20 is drawn into 
the cavity 52 through the channels 48 and 50. 
In FIG. 2B, a pressure pulse forces the membrane 36 to bulge down into the 
cavity 52. The pair of silicon nitride reeds 58 and 60 are pressed down 
and closed. The silicon nitride reed 62 remains closed. The carrier gas 20 
in the cavity 52 is forced out the outlet 54 into a mobile phase 60 that 
enters the column 14. 
In FIG. 2C, the sample gas 18 is injected into the normally closed valve 56 
under pressure sufficient to overcome the pressure of the carrier gas 20. 
The reed 62 opens and the sample gas 18 joins the mobile phase 60. 
FIG. 3 shows a normally open valve 70 that is similar to valves 44 and 46. 
A silicon nitride layer 72 is bonded between a pair of silicon wafers 74 
and 76. The nitride layer 72 is suspended as a reed across an outlet 
opening 78 and allows gas to freely pass around it on two sides, because 
the opening 78 is substantially wider than the nitride layer 72. An inlet 
opening 80 narrows to a small port 82 that is normally separated from the 
silicon nitride layer 72. But when a reverse pressure develops and gas 
tries to flow from the outlet 78 to the inlet 80, the silicon nitride 
layer deforms slightly and bulges down to seal against the port 82 and 
thus checks the reverse flow. 
FIGS. 4A and 4B show the normally closed valve 56 in more detail. The 
silicon nitride reed 62 is a layer that is deposited on the wafer 33 and 
undercut by anisotropic etching between an exhaust port (or slit) 84 and a 
sample inlet port 86 to form a groove 88. The partially detached silicon 
nitride can then lip-open and serve as a reed valve. Preferably, the 
silicon nitride is attached on three of four edges such that only the 
fourth edge is free to open with a carrier gas 20, the silicon nitride 
reed 62 lifts up, as in FIG. 4B, to allow the slit. When the pressure of 
the sample gas 18 exceeds the pressure of the sample gas 18 to pass into 
the mobile phase 60. 
In one alternative embodiment, the inlet port is one millimeter square, the 
exhaust port 84 is 200 microns by 50 microns, the layer 62 is 2000 
angstroms thick, and the channel 54 is 500 microns wide and deep. The 
carrier gas 20 is under about 40 psi of pressure. The channel 88 is 
anisotropically etched with an HF buffer solution of ammonium fluoride 
(NH.sub.4 F) and hydrofluoric acid to partially detach the silicon nitride 
layer 62. The silicon wafers 31-33 are anhydrously bonded together. 
Although particular embodiments of the present invention have been 
described and illustrated, such is not intended to limit the invention. 
Modifications and changes will no doubt become apparent to those skilled 
in the art, and it is intended that the invention only be limited by the 
scope of the appended claims.