Abstract:
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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to and the benefit of the filing of U.S. Provisional Patent Application Ser. No. 60/763,760, entitled “Method for Improved Preconcentrators”, filed on Jan. 31, 2006, and the specification and claims thereof are incorporated herein by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   The Government has rights to this invention pursuant to Contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy. 

   BACKGROUND 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 zone that can be used to accumulate and concentrate one or more sorbents over time and then rapidly release the concentrated chemical species upon demand for chemical analysis. 
   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. 
   Current gas-phase microanalytical systems typically comprise a gas chromatography column to separate the chemical species, or analyte, in a gas mixture, and a detector to detect the separated species. Such microanalytical systems can also include a chemical preconcentrator. The chemical preconcentrator serves the important function of selectively collecting and concentrating the analyte(s) of interest out of a large gas sample volume on a sorptive material at the inlet of the microanalytical system. In particular, selective analyte preconcentration is an essential step for early-warning, trace chemical detection in real-world, high-consequence environments where a high background of potentially interfering compounds exists. The chemical preconcentrator can deliver an extremely sharp analyte plug to the downstream gas chromatograph by taking advantage of the rapid, efficient heating of the sorbed analyte with a low-heat capacity, low-loss microheater. The very narrow temporal plug improves baseline separations, and therefore the signal-to-noise ratio and detectability of the particular chemical species of interest. Further, with a rapid enough release, there is a greatly reduced need for mechanical means of sample introduction, such as valving. See R. P. Manginell et al., “Recent Advancements in the Gas-Phase MicroChemLab,”  Proc. of SPIE  5591, 44 (2004). 
   Several previous microfabricated chemical preconcentrators have used a heated planar membrane suspended from a substrate as the microheater, wherein the sorptive material is disposed as a layer on a surface of the membrane to sorb the analytes from a gas stream. See U.S. Pat. No. 6,171,378 to Manginell et al., or full wafer thick slats that are in intimate contact with beads having adsorbent disposed thereon and wherein the beads are adjacent to slats that act as the microheater U.S. Pat. No. 6,914,220 to Tian et al. The slats disclosed by Tian et al. are the full thickness of the substrate in order to maximize the surface area of the beads that are coated within the sorbent zone, and therefore lose substantial heat to the substrate to which the slats attach. 
   Referring now to  FIG. 1 , prior art preconcentrator having a heated planar membrane suspended from a substrate wherein the sorptive material is disposed as a layer on a surface of the membrane to sorb the analytes from a gas stream is illustrated. A disadvantage of the design allows for heat loss via conductance from the heat elements through the membrane to the substrate along the entire perimeter of the sorbent. Reducing heat capacity and reducing heat loss between the preconcentrator and the substrate would allow for faster desorption temperature ramps at lower power. 
   Selective sample preconcentration is an essential step for early-warning, real-world, trace analyte detection. We have previously developed a microfabricated planar preconcentrator and three dimensional preconcentrators to address these issues for a wide array of analytes. The thermal efficiency and low heat capacity of these designs make them well suited as a platform support for adsorbent materials. Once analyte is collected on the sorbent zone, integrated thin-film resistive heaters allow for rapid thermal desorption of the sample into an analytical chain for separation and detection. Rapid heating on the order of a second or less is important for it allows the sample pulse to be delivered in a very narrow temporal plug to a gas chromatographic (GC) separation channel. This improves separations, and therefore the signal-to-noise ratio (S/N) and lower-limit of detectability. 
   SUMMARY OF THE INVENTION 
   The present invention provides a millimeter-sized or smaller 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 and provides for reduced energy use. 
   It is an aspect of the present invention to provide a preconcentrator having an improved heat capacity. 
   It is another object of the present invention to provide a preconcentrator having reduced heat loss to adjacent substrate. 
   It is yet another object of the present invention to provide a preconcentrator having improved desportion temperature ramps at lower power. 
   It is a further object of the present invention to provide a preconcentrator having thermal isolation support structures that suspend the sorbent zone of the preconcentrator from the substrate 
   It is a further aspect of the present invention to provide a preconcentrator having thermal isolation support structures that provide thermal isolation of the preconcentrator from the substrate. 
   It is an additional object of the present invention to provide thermal isolation support structures that have a thickness that is less than full subtrate thickness. 
   It is an additional object of the present invention to provide thermal isolation support structures that have a thickness that is between about 90% and 0.3% of the thickness of the full subtrate. 
   It is an additional object of the present invention to provide thermal isolation support structures that have a thickness that is between about 50% and 10% of the thickness of the full subtrate. 
   Additional objects and advantages of the present invention will be apparent in the following detailed description read in conjunction with the accompanying drawing figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying figures which are incorporated in and form part of the specification illustrate embodiments of the present invention and together with the description describe the invention. 
       FIG. 1  illustrates prior art schematic cross section and plan views of existing membrane-isolated preconcentrators. 
       FIG. 2  illustrates a schematic cross section view (A) and plan view (B) of an existing 3-D membrane-isolated preconcentrators is illustrated. 
       FIG. 3  illustrates a cross section view (A) and plan view (B) of preconcentrators in communication with the wafer via a thermal isolation support structure according to one embodiment of the present invention. 
       FIG. 4  illustrates a preconcentrator with thermal isolation support structures according to one embodiment of the present invention. 
       FIG. 5  is a photo of a preconcentrator as illustrated in  FIG. 4 . 
       FIG. 6  illustrates multiple overlapping mask layouts for preconcentrators isolated from the subtrate with thermal isolation support structures according to one embodiment of the present invention. 
       FIG. 7  illustrates a three dimensional view of a preconcentrator with full thickness slat structures as previously described in the art. 
       FIG. 8  illustrates a schematic three dimensional view of a preconcentrator supported by a partial thickness thermal isolation support structure to allow heating at modest power according to one embodiment of the present invention. 
       FIG. 9  illustrates ANSYS finite element prediction of the thermal behavior of 3D preconcentrator with 25 micron wide walls according to one embodiment of the present invention. 
       FIG. 10  illustrates ANSYS finite element prediction of the thermal behavior of 3D preconcentrator with 25 micron wide walls according to one embodiment of the present invention. 
       FIG. 11  illustrates ANSYS finite element prediction of the thermal behavior of 3D preconcentrator with 50 micron wide walls that are full wafer thickness according to one embodiment of the present invention. 
       FIG. 12  illustrates ANSYS prediction of the thermal behavior of a 3D preconcentrator wherein 4.75 V provides 200° C. with 50 micron wide walls, partial thickness according to one embodiment of the present invention. 
       FIG. 13  illustrates ANSYS prediction of the thermal behavior of a 3D preconcentrator with full thickness sorbent zone and reduced thickness thermal isolation support structures is illustrated according to one embodiment of the present invention. 
       FIG. 14  illustrates ANSYS prediction of the thermal behavior of a 3D preconcentrator with full thickness sorbent zone and reduced thickness thermal isolation support, structures is illustrated according to one embodiment of the present invention. 
       FIG. 15  illustrates a preconcentrator having isolation support structures according to one embodiment of the present invention. 
       FIG. 16  illustrates collection of DMMP and desorption with preconcentrator having the thermal isolation support structure shown in  FIG. 15  according to one embodiment of the present invention. 
       FIG. 17  illustrates a chart of calculated stress at the edge of tortuous path preconcentrators having zero, one or more thermal isolation support structures according to one embodiment of the present invention. 
       FIG. 18  illustrates a process for manufacture of a preconcentrator according to one embodiment of the present invention. 
       FIG. 19  illustrates one quarter model of a 3 dimensional sorbent zone and heating element according to one embodiment of the present invention. 
       FIG. 20  illustrates an enlarged sorbent zone as shown in  FIG. 19 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention has been described in terms of preferred embodiments, however, it will be appreciated that various modifications and improvements may be made to the described embodiments without departing from the scope of the invention. 
   Referring now to  FIG. 1 , a schematic cross section (A) and plan view (B) of an existing planar membrane-isolated preconcentrator is illustrated. Heaters are omitted for clarity and sorbent zone  103  is deposed upon the heating element. Heat is lost by thermal conduction through the membrane  102  to the substrate  101  around the entire dashed perimeter  105  which is the interface between the membrane and the substrate  101 . Membranes are typically made of silicon nitride and are between about 0.5-1 um thick 
   Referring now to  FIG. 2  A-B, a schematic cross section view (A) and plan view (B) of an existing 3-D membrane-isolated preconcentrator is illustrated. Heaters are omitted for clarity and adsorbent  203  is deposed upon the heating element. Heat is lost by thermal conduction from the heating element through the membrane  202  around the entire dashed perimeter  211  and  209  to the surrounding substrate  201 . Thermal conductance is proportional to the cross sectional area (e.g. width times thickness) of the conducting structure. In a preferred embodiment, when the sorbent zone is large, as in the case of the 3D preconcentrators, the thermal isolation support structures are ideally thinner than the sorbent zone to reduce heat losses. Alternatively, the sorbent zone and thermal isolation support structures can be the same thickness, provided that are not full-wafer thickness, to reduce power consumption. 
   Referring now to  FIG. 3  A-B, a cross section view (A) and plan view (B) of a preconcentrator formed on the surface of a first portion of a wafer thermally isolated with a thermal isolation support structure is illustrated according to one embodiment of the present invention. (A) illustrates a cross section view of a surface micromachined thermal isolation support structure  303  spanning an air space between the substrate  305  of a wafer and the sorbent zone  301  which is disposed upon the heating element (not shown) which itself is disposed on the substrate. (B) illustrates a planar view of a preconcentrator having a thermal isolation support structure  311  spanning an air space between the substrate  305  and the sorbent zone  313  wherein the thermal isolation support structure  311  is narrower than the sorbent zone  313  to restrict heat loss. Reduced heat capacity and heat loss allow for faster desorption temperature ramps at lower power. For analytes such as semivolatile compounds where collection is very good, smaller collection/desorption area is allowable. Therefore, the smaller adsorption zone area permitted by this system and method will permit even narrower desorption peak widths, further improving GC separations. In a preferred embodiment, an array of thermally isolated preconcentrators are employed to increase collection area. In an alternative embodiment, multiple thermal isolation support structures can provide support and isolation to a high-surface area collection zone. 
   The substrate used to form the chemical preconcentrator apparatus generally comprises a semiconductor (e.g. silicon or gallium arsenide), a dielectric (e.g. glass, crystalline quartz, fused silica, a plastic, a resin or a ceramic) or a combination thereof. The heating element formed on the substrate generally comprises a circuitous metal trace formed from one or more layers of deposited metals including platinum, molybdenum, titanium, chromium, palladium, gold and tungsten. The heating element could also be a semiconductor such as silicon, polysilicon, gallium arsenide for example. 
   Referring now to  FIG. 4  A-B, a cross section view (A) and plan view (B) of a preconcentrator formed on a first portion of a substrate thermally isolated from a third portion of the wafer with a thermal isolation support structure formed on a second portion of the substrate. The thickness of the first portion of the substrate is less than the thickness of the third portion of the substrate which is illustrated according to one embodiment of the present invention. The thickness of the full substrate (third portion of the substrate) is indicated by “t”. Silicon-On-Insulator (SOI) fabrication with reduced-dimension thermal isolation support structures  403  relative to the sorbent zone  401  is illustrated. The SOI substrate comprises a top silicon layer  405  separated from a silicon substrate  411  by a buried oxide layer  409 . A thermal isolation support structure  403  is of a thickness that is less than the thickness t of the full substrate and may span an insulating medium  413  such as air or a membrane that insulates the heating element and sorbent zone from the third portion of the substrate. The thickness of the thermal isolation support structure can be between 90% and 0.5% or between 75% and 90% of said third portion of said substrate. The width of the thermal isolation structure may be the same or different from the preconcentrator. The width of the thermal isolation support structure can range from 100% of the width of the preconcentrator to about 0.01% of the width of the preconcentrator. The first portion of the wafer may have a thickness that is the same or less than the third portion of the wafer. The heating element within a heating zone or sorbent zone is omitted for clarity. 
   Referring now to  FIG. 5 , a photo of a preconcentrator of  FIG. 4  is illustrated wherein the sorbent zone  503  is disposed upon the heating element  507  and is thermally isolated from the substrate and mechanically supported via one or more thermal isolation support structures  505  which span an insulating material  509  such as a membrane or an insulating gas such as air. The dimensions of the active area of the preceoncentrator  501  is approximately 600×1500 micron while the dimensions of the thermal isolation support structure is roughly 160 micron wide by 200 micron long. 
   Referring now to  FIG. 6  A-E, multiple mask layouts for a preconcentrator isolated from the substrate with one or more thermal isolation support structures is illustrated according to one embodiment of the present invention. Each thermal isolation support structure can be made as surface micromachined support structures and/or with a through-wafer etch to allow flow through the sorbent zone and the wafer. Blocks A-B have individually-addressable thermal isolation support structures  601  supporting multiple individually addressable sorbent zones  607 . The sorbent zones can be formed from a sorptive material disposed on a heating element as described previously. The sorptive material can be a microporous material which sorbs and concentrates one or more chemical species of interest from a vapor over time and which releases the chemical species when the sorptive material is heated by the resistive heating element. The sorptive material can comprise a chromatographic stationary phase material, a getter material, a sol-gel material (e.g. a sol-gel oxide which is chemically modified to enhance sorption of the chemical species of interest), or a polymer. Blocks C-D have four thermal isolation support structures  601  in parallel for added coating area of the sorbent zones  607 . The thermal isolation support structures are less than the full thickness of the wafer for example between about 95% of the thickness of the wafer and less. Block E has a lattice sorbent zone  607  isolated from the substrate  605  with multiple thermal isolation support structures  601 . 
   Referring now to  FIGS. 7A-B , a cross section view (A) and plan view (B) of a three dimensional preconcentrator known in the art is illustrated with slat  707  structures  703  that are the same thickness as the full substrate  701 . Preconcentrators with sorbent zone having full substrate thickness slats  703  span an air space  709  have been described by Tian, et al. in U.S. Pat. No. 6,914,220. Finite element modeling of preconcentrators having full substrate thickness slats shows the power losses high and not suitable for portable applications. The increase in power is in part due to the large cross-sectional area of these structures resulting from the structures having a thickness of the full substrate. As a result these structures require 5-10 watts for analyte desorption in a narrow temporal band. 
   Referring now to  FIG. 8  A-B, a cross section view (A) and plan view (B) of a schematic three dimensional according to one embodiment of the present invention preconcentrator thermally isolated and supported by a thermal isolation support structure is illustrated. A thermal isolation support structure  803  having a thickness that is less than the thickness of the full wafer  801  to allow heating of the heating element  805  to release adsorbed molecules or chemicals adsorbed onto the sorbent zone  807  at modest power. For example, these structures require 200-600 mW to raise the temperature at a rate sufficient to allow desorption of adsorbed molecules and compounds within a narrow peak suitable for analysis. Reducing the thickness of the sorbent zone  807  and the thermal isolation support structure  803  reduces power losses. In a preferred embodiment, reducing the thickness of the thermal isolation support structure  803  to be thinner than the sorbent zone  807  allows large sorbent zones to be heated at power low enough to greatly benefit field analysis, say less than 2 W. 
   Referring now to  FIG. 9 , ANSYS finite element prediction of the thermal behavior of 3D preconcentrator with 25 micron wide walls is illustrated. The model was first validated against experimental steady-state and transient heating data of an actual device with 50 micron walls. In this illustration, a silicon nitride membrane provides thermal isolation of a full-thickness sorbent zone. Red indicates the area of the greatest temperature and blue indicates the area of the coolest temperature for  FIGS. 9-14 . 7.1 V provides 200° C. 
   Referring now to  FIG. 10 , ANSYS finite element prediction of the thermal behavior of a 3D preconcentrator with 25 micron wide walls is illustrated. The model was first validated against experimental steady-state and transient heating data of an actual device with 50 micron walls. Addition of a full-thickness silicon strut spanning the air space between the substrate and a preconcentrator similar to the preconcentrator disclosed by Tian, et al. in U.S. Pat. No. 6,914,220 wherein 7.1 V only produces 92° C. 
   Referring now to  FIG. 11 , ANSYS prediction of the thermal behavior of a 3D preconcentrator having 50 micron wide walls that are full-wafer thickness and two isolation support structures that are full wafer thickness is illustrated. 4.75 V provides 200° C. 
   Referring now to  FIG. 12 , ANSYS prediction of the thermal behavior of a 3D preconcentrator is illustrated according to one embodiment of the present invention. A SiN layer, used in this design to prevent analyte from touching the surrounding cold silicon, is omitted from the view, but not the thermal model, to show the sorbent zone and thermal isolation support structure spanning the air space. Heat conduction is the flow of internal energy from a region of higher temperature to one of lower temperature by the interaction of the adjacent particles (atoms, molecules, ions, electrons, etc.) in the intervening space. Heat conductance is proportional to the area of the conducting structure (e.g. width×thickness). By reducing the width and thickness of the sorbent zone and the thermal isolation support structure to about 50 microns in thickness a temperature of 164° C. is achieved with 4.75 V. 
   Referring now to  FIG. 13 , ANSYS prediction of the thermal behavior of a 3D preconcentrator with full thickness sorbent zone and reduced thickness thermal isolation support structures is illustrated according to one embodiment of the present invention. The application of 7.1 V provides 126° C. to a preconcentrator having 25 micron wide walls and 200 micron thick thermal isolation support structures. The thermal isolation support structures are less than full wafer thickness. The substrate has a thickness of greater than 200 um. 
   Referring now to  FIG. 14 , ANSYS prediction of the thermal behavior of a 3D preconcentrator with full thickness sorbent zone and reduced thickness thermal isolation support structures is illustrated according to one embodiment of the present invention. The SiN layer in this illustration is omitted to show the sorbent zone and thermal isolation support structures suspension however, it is not omitted in the simulation. The illustrated thermal isolation support structure is less than full wafer thickness at about 100 micron thick. 7.1V produces 155° C. The wafer is about 400 rm thick. 
   Referring now to  FIG. 15 , membrane preconcentrator having thermal isolation support structures is illustrated according to one embodiment of the present invention. A heater  1507  is placed on the rectangular active area  1505 . The active area  1505  of the device is thermally-isolated from the substrate  1501  by two rectangular thermal isolation support structures  1503  at either side of the active area  1505 . The thermal isolation structure may be of any shape. In a preferred embodiment, the thickness of the structure will be less than the full thickness of the substrate. The thermal isolation support structures are about 160 micron wide by 200 micron long. The thickness has been varied in the SOI method of making the structure used here from 2 micron thick to 5 micron thick. For 2 microns, 1750 K/W was achieved. 
   Referring now to  FIG. 16 , collection of DMMP and desorption with a preconcentrator according to one embodiment of the present invention as shown in  FIG. 15 . Point A is a challenge at 300 sec. Point B is an end to the challenge at 315 sec. Point C is release fire at 400 sec with the heater. Point D is end fire at 405 sec. Point E is recovery at 1 minute past end fire. Point F is Recovery at 2 minutes post end fire. Point G is resonant frequency level without the presence of analyte. The time response of the preconcentrator thermally isolated with a thermal isolation support structure is 25 msec to achieve 200° C. from room temperature. The preconcentrator thermally isolated with the thermal isolation support structure has a thermal efficiency of about 1750 K/W on average for an active area of 0.015 cm2. In contrast, the membrane preconcentrator having the following dimensions has a thermal efficiency of about 1200 K/W on average for an active area of 0.032 cm 2 . By reducing the thermal isolation support structure width from 160 um to less than 160 um, the thermal resistance of the thermal isolation support structure preconcentrator will increase further. 
   Referring now to  FIG. 17 , a chart of mechanical stress at the perimeter of a silicon nitride membrane is illustrated as a function of applied pressure. The results for 1 and 10 micron thick membranes is shown. The failure stress of silicon nitride is about 2000 MPa. Adding thermal isolation support structures made of silicon improve the overall strength of the device. The number ‘ 100  ’ refers to the separation of tortuous path structures in microns in the active sorbent zone. The numbers ‘ 80  ’ or ‘ 60  ’ refer to the length in rm of the added silicon thermal isolation supports. ‘ 4  ’ and ‘ 8  ’ refer to the number of added isolation supports. To prevent analyte from adsorbing on the cold silicon perimeter walls, silicon nitride is utilized. However, the mechanical strength of the device is derived mainly from the silicon thermal isolation support structure and the silicon nitride serves mainly to prevent analyte from approaching cold sidewalls. 
   A SiN membrane of 1 um thickness (solid diamonds) attached to the wafer reaches failure stress at less than 70 psi. The addition of 4 thermal isolation bridges 200 micron thick (i.e., less than full substrate thickness) and having a length of about 80 um which connects the substrate to the preconcentrator decreases the mechanical stress measure while providing acceptable thermal isolation. At about 2100 MPa SiN membranes fail. 
   Fabrication methods known in the art are advanced to allow for precise control of the physical dimensions of the preconcentrators thus formed. Precise dimensional control is important for achieving repeatability in operational characteristics of the device, having significant impact on its manufacturability. In each of the various methods for forming the chemical preconcentrator of the present invention, a number of processing steps are required including processes such as material deposition, photolithography, masking, etching, mask stripping and cleaning which are well-known in the semiconductor integrated circuit (IC) industry and MEMS (microelectromechanical system) industry. Therefore only a limited number of processing steps will be described herein. 
   Referring now to  FIGS. 18A-C , a cross sectional view of a planar preconcentrator according to one embodiment of the present invention is formed according to one fabrication method. In  FIG. 18A , a chip having a suitable substrate  1803  such as silicon with a deposited dielectric  1809  is used as the starting material. A circuitous heater trace  1801  made of metal or semiconductor is patterned on the surface of the dielectric. This will serve ultimately as the resistive heater for the device. The hardmask  1805  is next deposited and patterned on the substrate  1803  such that it is properly aligned to the heater  1801 . This can be accomplished through the use of a semiconductor back side mask aligner or other suitable methods known in the art. Typical hardmask materials include hard-baked photoresist, silicon oxide, silicon nitride, metals, etc. The softmask  1807  is then deposited and patterned, aligning either to the hardmask  1805  via typical frontside alignment techniques, or to the metal via techniques aforementioned. The softmask  1807  can be soft-baked photoresist. Openings  1811  in the combined soft mask  1807  and hardmask  1805  layers are used to define features that ultimately progress through the substrate  1803  and stop on the dielectric  1809 . 
   The first etch step shown in  FIG. 18B  uses the combined mask to etch part way through the substrate  1803 , effectively giving regions  1811  a head start over the ultimately-desired thermal isolation structures  1813  of  FIG. 18C . For silicon substrates  1803 , deep reactive ion etching (DRIE) typical of the Bosch process is used in this step. After this etch step is completed, the softmask  1807  is removed. In the case of a photoresist softmask, this can be accomplished by dipping in acetone or other suitable photoresist strippers. At this point, the partial substrate thickness thermal isolation support structure regions are now open to etching as are the structures already etched in the first etch step. The substrate is then re-inserted into the etch tool such as a DRIE Bosch tool as in  FIG. 18C . The structures partially etched through the substrate progress to the dielectric  1809 , while the thermal isolation support structures  1813  propagate to a depth set primarily by the difference between the substrate  1803  thickness and the first etch depth. After the second etch step, the hardmask  1805  can be removed in a suitable etchant like acetone. 
   In another embodiment, a method of making a preconcentrator thermally isolated and supported with one or more thermal isolation support structures captures” a sacrificial layer within the confines of an inert material. The structure of the preconcentrator is formed on top of the sacrificial layer, overlapping the perimeter of the inert layer to provide anchoring. When the sacrificial layer is removed or “released”, the physical boundary of the structure is defined by the position of the inert layer. Therefore, the boundary conditions for heat loss are fixed from device to device and are not sensitive to over or under etching of the sacrificial layer. It will be obvious to one of ordinary skill in the art that other methods of forming the thermal isolation support structure preconcentrators are possible, including front-side KOH etching. 
   A further advantage is the ability to combine the benefits of surface and bulk micromachining to make improved devices. In one embodiment, the active, thermal isolation support structured supported, heating area of the PC is formed in a surface micromachining approach, according to one embodiment of the present invention where a captured sacrificial layer is used. Like the 3D preconcentrator, an sorbent zone structure can then be formed in the thickness of the wafer by bulk micromachining, aligning the support to the surface micromachined heating area using back-side aligning techniques. The sacrificial layer then serves as an etch stop for the bulk micromachining step, greatly improving the repeatability in device fabrication over earlier methods. As a last step, the sacrificial layer is removed. Sacrificial layers and/or other substrates can be used. 
   As with the 3D preconcentrator, a benefit is the ability to tailor the area of contact between the analyte and the heater and support structures. It was observed that even though the thermal ramp rate of the 3D preconcentrator is orders of magnitude less than the planar device, the desorption peak width is only on the order of a factor of two greater. This implies that the adsorbent support structure is enhancing the collection and release process. The support features can be lithographically defined arbitrarily according to one embodiment of the present invention. Applying surface-and-bulk processing approaches, it is also possible to make direct ohmic contact to the underlying support structure using surface processing techniques. For example, proceeding through the sacrificial layer from the surface thermal isolation support structure to the 3D support, can provide direct electrical contact. Processes typical of the integrated circuit industry would be used to make such contact. The result is direct heating of the support structure, as well as the ability to make diodes in the support structure. 
   Referring now to  FIG. 19 , a sorbent zone and a thermal isolation support structure is illustrated according to one embodiment of the present invention. A thermal isolation support structure  1901  supports the sorbent zone  1905  having a gridlike pattern. A membrane  1907  is suspended from the substrate  1903  and is supported by the sorbent zone  1905 . 
   Referring now to  FIG. 20 , a portion of  FIG. 19  showing a thermal isolation support structure and a sorbent zone  2000  is enlarged. The walls  2003  of the sorbent zone  2000  are fabricated at different depths. The diameter of the pore formed  2009  is about 100 um. A membrane  2005  is supported by the sorbent zone  2000 . The sorbent zone  2000  is connected to the substrate  2007  by a thermal isolation support structure  2001 . Multi-level silicon based mirostructures can be fabricated by methods known in the art. See U.S. Pat. No. 6,930,051. A heating element associated with microstructures provides heat at a desired rate to a desired temperature to allow chemicals that have been sorbed for example from a gas stream onto a chemically sensitive coating or adsorbent that is disposed on the surface of the multi-level silicon based microstructures. 
   Compared with the membrane supported planar preconcentrator, embodiments of the present invention allow for easier integration with other microanalytical components like detectors and GC columns. The ease of integration results from the ability to make flow-through adsorbent supports in the surface or in the bulk that are fluidically coupled to other microanalytical components monolithically. As with the 3D preconcentrator, the preconcentrator thermally isolated and supported by the thermal isolation support structure design is pressure balanced, since the structure is immersed in the fluid it is interacting with, whereas with the planar membrane design, pressure fluctuations could rupture the membrane. 
   The present invention has been described in terms of preferred embodiments, however, it will be appreciated that various modifications and improvements may be made to the described embodiments without departing from the scope of the invention. The entire disclosure of all references, applications, patents and publications cited above and or in the attachments, and of the corresponding application(s) are hereby incorporated by reference.