Gas separation: apparatus – Solid sorbent apparatus – With means regenerating solid sorbent
Reexamination Certificate
1999-08-05
2001-01-09
Spitzer, Robert H. (Department: 1724)
Gas separation: apparatus
Solid sorbent apparatus
With means regenerating solid sorbent
C055SDIG005, C096S101000, C096S154000
Reexamination Certificate
active
06171378
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to chemical analysis (e.g. by gas chromatography), and in particular to a compact chemical preconcentrator formed on a substrate with a heatable sorptive membrane that can be used to accumulate and concentrate one or more chemical species of interest over time and then rapidly release the concentrated chemical species upon demand for chemical analysis.
BACKGROUND OF THE INVENTION
Presently, there is a need for autonomous, portable, hand-held chemical analysis systems for the rapid and sensitive detection of particular chemicals including pollutants, high explosives and chemical warfare agents. Such miniaturized chemical analysis systems, which have been termed “chemical laboratories on a chip”, are currently being developed based on gas chromatography. The requirements for these chemical analysis systems are that they provide a high chemical selectivity to discriminate against potential background interferents which may be present at up to a thousand-fold or more higher concentration, that the chemical analysis be performed on a short time scale (e.g. in a minute or less) and that the chemical analysis be performed with high sensitivity (e.g. at concentrations down to the part-per-billion level). Low electrical power consumption is also needed for field use over a prolonged time period.
The present invention is of a millimeter-sized chemical preconcentrator which can be used with the above miniaturized chemical analysis systems to increase the sensitivity and selectivity with which chemical analysis measurements can be made.
An advantage of the chemical preconcentrator of the present invention is that it can be integrated with other elements of a chemical analysis system in a hybrid or monolithic fashion to provide a substantial improvement in the detectivity of particular chemical species of interest.
A further advantage of the present invention is that a sorptive coating of the chemical preconcentrator can be tailored for chemical selectivity to one or more chemical species of interest and thereby accumulate and concentrate these chemical species from an ambient or sample vapor over time while being relatively insensitive to other chemical species not of interest.
Yet another advantage of the present invention is that the accumulated chemical species of interest can be concentrated in a small area and subsequently released suddenly by thermal desorption to form a sample plug having a narrow temporal width and a relatively high concentration of the chemical species of interest, thereby improving the detectability of each chemical species of interest using the chemical analysis system.
Still another advantage of the chemical preconcentrator of the present invention is that it has a very small heat capacity to allow rapid heating and release of the concentrated chemical species of interest on a time scale of a fraction of a second.
A further advantage of the chemical preconcentrator of the present invention is that it is applicable to different types of chemical analysis systems, including systems based on gas chromatography and systems based on mass spectrometry.
These and other advantages of the present invention will become evident to those skilled in the art.
SUMMARY OF THE INVENTION
The present invention relates to a chemical preconcentrator apparatus (also termed herein a chemical preconcentrator), comprising a substrate having a suspended membrane formed thereon; a resistive heating element disposed on a surface of the membrane; and a sorptive material disposed on at least one surface of the membrane to sorb and concentrate at least one chemical species of interest from a vapor (e.g. a gaseous ambient) over time, with the chemical species being releasable from the sorptive material upon heating of the sorptive material by the resistive heating element. Various types of sorptive materials including microporous materials, sol-gel oxides and polymers can be used in the chemical preconcentrator apparatus, with a particular sorptive material being selected to sorb particular chemical species of interest. Additionally, a chemical modification of the surface of the sorptive material (e.g. a sol-gel oxide) can be used to further enhance sorption of the chemical species of interest.
The substrate used to form the chemical preconcentrator apparatus generally comprises a semiconductor (e.g. silicon or gallium arsenide) or a dielectric (e.g. a glass, crystalline quartz, fused silica, a plastic, a resin or a ceramic). The membrane preferably comprises silicon nitride, although other materials such as polycrystalline silicon, silicon oxynitride and silicon carbide can also be used to form the membrane. The resistive heating element formed on the membrane generally comprises a circuitous metal trace formed from one or more layers of deposited metals including platinum, molybdenum, titanium, chromium, palladium, gold and tungsten. To provide a more uniform heating of the sorptive material, an optional heat-spreading layer (e.g. comprising aluminum or silicon) can be disposed over the resistive heating element. Finally, an optional temperature sensor can be located on the membrane proximate to the resistive heating element to aid in controlling and measuring the temperature during heating of the membrane, with the temperature sensor generally comprising a circuitous metal trace (and in some instances being the resistive heating element itself) which forms a resistive temperature sensor or a thermocouple.
The present invention also relates to a method for forming a chemical preconcentrator apparatus for sorbing a chemical species of interest from a vapor over time and releasing the chemical species of interest upon demand. The method comprises steps for forming a suspended membrane on a substrate; forming a resistive heating element on a surface of the suspended membrane; and coating at least one surface of the suspended membrane with a sorptive material capable of sorbing the chemical species of interest. The step for forming the suspended membrane comprises depositing a film over a top surface of the substrate, and removing material from the substrate underneath a portion of the deposited film. This can be done in different ways depending upon particular embodiments of the present invention. A first method to remove material underneath the membrane comprises one or more etching steps for etching the substrate from its bottom surface. This can be done using either anisotropic wet etching (e.g. with potassium hydroxide) or by reactive ion etching, or by a combination of both types of etching.
A preferred method for etching through the substrate from its bottom surface (i.e. from a backside of the substrate) is to etch through the substrate using an anisotropic dry etching process (e.g. reactive ion etching). This produces an etch opening with substantially vertical sidewalls, thereby minimizing the size of the opening and conserving space on the substrate. In some cases, an additional anisotropic wet etching step may be required to completely remove the substrate material underneath the membrane. Alternately, the substrate can be etched entirely with an anisotropic wet etching process in which case a plurality of angled sidewalls can be formed in etching through the substrate.
Another method for etching through the substrate which is based on surface micromachining process steps is to deposit a sacrificial layer over the substrate, and then deposit a film over the sacrificial layer. A plurality of openings can be formed through the film to expose the sacrificial layer so that the sacrificial layer can be removed, at least in part, through the openings formed in the film (e.g. by etching away the sacrificial layer using a wet etchant comprising hydrofluoric acid, or by dissolving the sacrificial layer in a solvent such as acetone). When this method is used, the film can comprise silicon nitride, silicon (e.g. polycrystalline silicon), silicon oxynitride or silicon carbide. Additionally, the sacrificial layer can comprise silic
Frye-Mason Gregory C.
Manginell Ronald P.
Hohimer John P.
Sandia Corporation
Spitzer Robert H.
LandOfFree
Chemical preconcentrator does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Chemical preconcentrator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Chemical preconcentrator will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2488261