Abstract:
A temperature programmable microfabricated gas chromatography column enables more efficient chemical separation of chemical analytes in a gas mixture by the integration of a resistive heating element and temperature sensing on the microfabricated column. Additionally, means are provided to thermally isolate the heated column from their surroundings. The small heat capacity and thermal isolation of the microfabricated column improves the thermal time response and power consumption, both important factors for portable microanalytical systems.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to chemical separations in microanalytical systems and, more particularly, to temperature programming of a microfabricated gas chromatography column for efficient separation of gas-phase analytes. 
     Portable, handheld microanalytical systems, which have been termed “chemical laboratories on a chip,” are being developed to enable the rapid and sensitive detection of particular chemicals, including pollutants, high explosives, and chemical warfare agents. These microanalytical systems should provide a high chemical selectivity to discriminate against potential background interferents and the ability to perform the chemical analysis on a short time scale. In addition, low electrical power consumption is needed for prolonged field use. See, e.g., Frye-Mason et al., “Hand-Held Miniature Chemical Analysis System (μChemLab) for Detection of Trace Concentrations of Gas Phase Analytes,”  Micro Total Analysis Systems  2000, 229 (2000). 
     Current gas-phase microanalytical systems are based on gas chromatography (GC). Gas chromatography relies upon the chemical equilibria of analytes between a mobile phase and a stationary phase in a GC column to bring about a temporal separation of analytes in a gas mixture. Chemical equilibria and, therefore, column retention times are strongly influenced by column temperature. Thus, column temperature must be precisely controlled to obtain accurate separations. 
     The goal of a GC analysis is normally to obtain a separation with the required accuracy in the minimum time. Isothermal operation of the GC column can have drawbacks for achieving this goal with certain gas mixtures. If the selected isothermal temperature is too low, early-eluted peaks will be closely spaced whereas more strongly retained analytes will have broad and low-lying peaks and consequent poor detectability. Conversely, the more strongly retained analytes will elute faster at a higher isothermal column temperature, but at the expense of poorer separation and loss of resolution for the early eluting analytes. 
     This general elution problem may be solved by temperature programming of the column. With temperature programming, analysis time can be reduced and the overall detectability of components can be improved. For example, for a given column it is possible to analyze gas mixtures with a broader volatility range in a shorter analysis time with temperature programming. For most analytes, the baseline resolution and peak widths are also improved. In general, temperature programming can comprise a series of changes in the column temperature that can include isothermal and controlled temperature rise segments. As an example of temperature programming, consider temperature ramping. Temperature ramping comprises monotonically increasing the temperature as the gas mixture is passed through the column. Higher volatility analytes in the mixture, which without temperature ramping pass through the column the earliest, still are the first to arrive at the column exit. Temperature ramping only tends to modestly improve the peak widths of these early eluting analytes and enhances baseline resolution somewhat. This is mainly due to the fact that these faster eluting analytes pass through the column before the initial temperature is appreciably increased. On the other hand, less volatile analytes, which in the absence of ramping tend to elute slowly with relatively broad and low-lying peaks, elute more quickly with temperature ramping and are generally improved from the standpoint of baseline resolution and peak width. As a result, analysis time can be improved relative to a low temperature, isothermal elution while retaining resolution. 
     In conventional chromatography, an oven enclosing the GC column is used to effect the temperature program. This process is energy intensive, requiring hundreds of watts of power, and is capable of providing only modest ramp rates of about 25° C./min. These characteristics are adequate for laboratory applications where power is not that limited, and long, 30 meter columns can be used to separate difficult mixtures without the need for faster ramp rates. However, for portable applications, this level of power consumption is unacceptable. Further, given the necessarily shorter length of portable GC columns relative to laboratory instruments, more rapid temperature ramping can compensate for the loss of resolution due to fewer theoretical plates in the portable GC column. Thus, there exists a need for temperature programming of microfabricated GC columns suitable for a portable, energy-efficient microanalytical system. 
     The present invention solves the need for a temperature programmable microfabricated GC column through the integration of a resistive heating element and temperature sensing on microfabricated GC column. Additionally, means are provided to thermally isolate the heated-column from its surroundings. The thermal isolation reduces power losses from the heated zone and reduces column heat capacity, thereby improving the thermal time response and power consumption, both important factors for portable GC applications. The present invention permits rapid, low-power and sensitive temperature programming of the microfabricated GC column and temperature ramp rates that are an order of magnitude faster than conventional GC columns, thereby enabling more efficient chemical separations. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a temperature programmable microfabricated gas chromatography column comprising a substrate, a channel etched in the substrate to separate chemical analytes in a gas mixture, at least one lid disposed on a channel-side of the substrate to seal the channel, and at least one resistive heating element disposed on a least one surface of the substrate to heat the column during the separation. The temperature programmable microfabricated gas chromatography column can further comprise a control board for electrical control of the resistive heating element and fluidic control of the column, means for electrically connecting the control board to the resistive heating element, means for fluidically connecting the control board to the channel, and means for thermal isolation of control board from the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form part of the specification, illustrate the present invention and, together with the description, describe the invention. In the drawings, like elements are referred to by like numbers. 
     FIG. 1 shows a schematic side-view of a temperature programmable microfabricated GC column. 
     FIG. 2 shows a schematic top-view of a GC column showing an integral thin-film resistive heating element with separate temperature sensors. 
     FIG. 3 shows a circuit diagram for control of the resistive heating element. 
     FIG. 4 shows a graph of reproducible temperature ramps as a function of programmed control voltage. 
     FIGS. 5A-5C show chromatograms of a mixture of chemical analytes having a wide volatility range. FIG. 5A shows the chromatogram of an isothermal low temperature elution. FIG. 5B shows the chromatogram of a temperature ramping elution. FIG. 5C shows the chromatogram of an isothermal high temperature elution. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention comprises a temperature programmable microfabricated GC column. In FIG. 1 is shown a schematic illustration of the temperature programmable microfabricated CG column  10 , comprising a channel  11  formed in a substrate  12 , a lid  14  to seal the channel  11 , and a resistive heating element  13  disposed on a surface of the substrate  12  to heat the column  10  during the separation of chemical analytes in a gas mixture. The column  10  can be mounted on a fluidic/electronic control board  15  that can be separated from the substrate by tubes  16  for thermal isolation and fluid coupling to the channel  11 . The electrical controller on the control board  15  can be electrically connected to the resistive heating element  13  by wires  17  bonded to pads  18 . This configuration permits rapid temperature ramping of the column  10  at a low power suitable for portable microanalytical applications. 
     Preferably, the temperature programmable microfabricated GC column  10  can be fabricated by depositing the resistive heating element  13  on a surface of the substrate  12 , etching the channel  11  in the side of the substrate  12  opposite to the resistive heater  13 , and bonding the lid  14  to the etched side of the substrate  12  to seal the channel  11 . Inlet and outlet tubes  16  can be attached through the lid  14  to the channel-side of the substrate  12  and to the control board  15 . The channel  11  can be coated with a stationary phase. Electrical wires  17  can then be bonded to pads  18  on the resistive heating element  13  and the substrate  12 . 
     The substrate  12  can comprise a substantially planar sheet or plate of silicon, glass, ceramic, or other suitable GC column material. For example, the substrate  12  can be a single crystal silicon wafer. 
     Depending on the resistive heating element and substrate materials, a dielectric thin film layer  19  can first be deposited on the substrate  12  to electrically isolate the resistive heating element  13  from the substrate  12 . For example, the insulating layer  19  can be a 0.5 μm-thick layer of silicon nitride deposited by chemical vapor deposition on a silicon wafer  12 . The heating element  13  can comprise a resistive material, such as a refractory metal or lightly doped semiconductor material. The resistive heating element  13  can be a layer deposited by thin-film techniques, such as physical vapor deposition, or by thick-film techniques, such as screen-printing. For example, the resistive heating element  13  can comprise a less than 0.1 μm thick layer of platinum on a thin (e.g., 10-15 nm thickness) titanium adhesion layer. Alternatively, the resistive heating element  13  can comprise a patterned layer having a circuitous or serpentine pattern, multiple heating elements, or a layer of variable cross-section or resistivity to provide for a uniform or tailored temperature over the area of the channel  11 . The bond pads  18  can be gold of about 1 μm thickness. 
     The channel  11  for the GC column  10  can be formed in the substrate  12  by a variety of processes such as those described by Matzke et al., “Microfabricated Silicon Gas Chromatographic MicroChannels: Fabrication and Performance,”  Proceedings of SPIE, Micromachining and Microfabrication Process Technology IV,  3511, 262 (1998) and in U.S. Pat. No. 6,068,684 to Overton, which are incorporated herein by reference. For example, the channel  11  can be a deep high-aspect-ratio channel formed in the side of the substrate  12  by anisotropic wet etching or plasma etching. The high-aspect-ratio cross-section enables close packing of a spiral or serpentine channel  11  to achieve a desired column length. A typical microfabricated silicon-based GC column  10  comprises a 1-meter spiral channel  11  formed on a 1.0-1.5 cm 2  area of a silicon wafer  12 . A typical channel  11  can have a cross-section that is 40-100 μm wide by 300-400 μm deep. The channel cross-section can be varied along the channel length to optimize the separation efficiency along the channel  11  and to enable operation over a wider temperature range. 
     Preferably, the channel  11  can be formed in the substrate  12  opposite the resistive heating element  13  and a lid  14  can be attached to the channel-side of the substrate  12  to seal the open channel  11 . The lid  14  is preferably a thermally insulating material and thin to minimize the heat capacity of the column  10 . The lid  14  can be made from a thin sheet of a material such as glass or polymer. For example, the lid  14  can be a machined Pyrex lid that is anodically bonded to the deep-etched silicon wafer  12 . The Pyrex lid  14  can be less than 1 mm in thickness, and preferably about 250 μm or less in thickness. Inlet and outlet holes can be formed in the lid  14 , for example, by grinding. Following bonding of the lid  14  to the substrate  12 , the channel  11  can be cleaned, for example, with a sulfuric acid/hydrogen peroxide cleaner. Tubes  16  can be capillary tubes that are attached, for example with epoxy, to the inlet and outlet holes in the lid  14  to fluidically connect the channel  11  to the control board  15 . 
     Alternatively, the resistive heating element  13  can be disposed on the channel-side surface of the substrate  12  by depositing a layer of the resistive material on the exposed surface of the lid  14 . An additional resistive heating element (not shown) can be disposed on the side of the substrate  12  opposite the first resistive heater  13  to further increase the column heating rate and heating uniformity. Furthermore, the at least one resistive material layer can be deposited on the surface of the substrate  12  or lid  14  after the lid  14  is bonded to the substrate  12 . 
     Alternatively, the channel  11  can be etched completely through the substrate  12  and sealed by a top lid (not shown) bonded to the top surface and the bottom lid  14  bonded to the bottom surface of the substrate  12 . At least one resistive heating element  13  can be disposed on at least one surface of the substrate  12  by depositing resistive material layers on the exposed surfaces of one or both of the lids. 
     The inside surfaces of the channel  11  can be coated with a stationary phase material to enhance the separation of the chemical analytes of interest in the gas mixture to be analyzed. The stationary phase material can be a polymer having a specific chemical group with the proper physico-chemical interaction to cause separation of the analytes. The channel  11  can be coated with the stationary phase material by pushing a plug of the material through the channel  11  or by filling the channel  11  with a solvent containing the stationary phase material and then applying a vacuum to the end of the channel  11  to dry the solvent out of the channel  11 . The stationary phase can also be applied by gas or liquid phase deposition into the channel  11  prior to bonding the lid  14  to the substrate  12 . Instead of using a stationary phase material to coat the surfaces of the channel  11 , the channel can alternatively be filled with a porous packing material to make a microfabricated packed GC column. 
     The control board  15  can be a printed wiring board (PWB) or a ceramic substrate, such as a low temperature co-fired ceramic (LTCC), that can be connected fluidically to the channel  11  by the tubes  16 . The control board  15  can be electrically connected to the heated column  10  by wirebonding or soldering of fine wires  17  to bond pads  18  on the control board  15  and the resistive heating element  13 . The resulting microfabricated GC column  10  provides advantages compared to conventional GC columns in terms of cost, system size, and power required to heat and temperature control the column. 
     Through the known temperature coefficient of resistance (TCR) of the resistive heating element material, the temperature of the column  10  can be measured. In this mode, the resistive heating element  13  acts as both heater and temperature sensor. Alternatively, a temperature sensor  20  can be fabricated separate from the resistive heating element  13 . Because microfabrication is used throughout, various types of temperature sensors  20  can be used to sense temperature, including diodes, thin film thermistors, thermocouples, and thermopiles. See e.g., W. Menz, J. Mohr, and O. Paul,  Microsystems Technology , Wiley-VCH, Weinheim (2001). 
     FIG. 2 shows a top-view of a resistive heating element  13  and separate temperature sensors  20 , comprising a thermistor material deposited in a serpentine pattern on the upper surface of the substrate  12 . By placing the resistive heating element  13  and temperature sensor  20  in intimate contact with the surface of the channel-containing substrate  12  (by either hybrid or monolithic methods), fine temperature control and time response can be accomplished. 
     A variety of electronic control circuits can be used to control the temperature of the GC column  10 . FIG. 3 shows a control circuit  30  that can be used for operation of the column  10  when the resistive heating element  13  is used to both heat and sense the column temperature. Since the temperature coefficient of resistance of materials is well known, the temperature is equivalent to the resistance of the resistive heating element  13 . The feedback control circuit  30  measures the power (or current) necessary to maintain the resistive heating element  13  at the programmed temperature. A first operational amplifier  31  measures the voltage across the resistive heating element  13 . A second operational amplifier  32  produces a voltage V I  that is proportional to the current I f  through the resistive heating element  13 . Therefore, the output voltage V R  of divider  33  (e.g., an Analog Devices AD 534 chip) is proportional to the resistance of the resistive heating element  13 . Using differential amplifier  35 , V R  can be compared to a programmed voltage V S  that can be provided from a voltage divider  34  or from an external source such as a D-to-A converter. The programmed voltage V S  determines the desired resistance (i.e., temperature) of the resistive heating element  13 . The comparator output of the differential amplifier  35  controls the gate of transistor  36  that feeds back to the resistive heating element  13  to maintain the desired temperature of the GC column  10 . The larger the difference between the divider voltage V R  and the programmed voltage V S , the greater the feedback current I f  that is switched from power supply  37  to the resistive heating element  13 . Other circuits of the type known in the electronic control art can be used to control the resistive heating element  13  with separate temperature sensor  20 . 
     Table I shows heating rates and power requirements for an exemplary temperature programmable microfabricated GC column  10 , comprising an 86 cm long×100 μm wide×400 μm deep channel  11  etched in a 1.3 cm.×1.3 cm.×525 μm thick silicon wafer  12 . The resistive heating element  13  comprised a 30 nm thickness platinum layer deposited on a 10 nm thickness titanium adhesion layer deposited on the surface of the silicon wafer  12  opposite the channel  11 . The channel  11  was sealed by a 500 μm thick Pyrex lid  14  on the channel side of the substrate  12 , opposite the resistive heating element  13 . The column temperature was monitored by a thermistor-type temperature sensor  20  mounted on the silicon wafer  12  and connected to an electronic control circuit. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Heater 
                 Cold Temp 
                 Hot Temp 
                 Time 
                 Heater Power 
                 Heating Rate 
               
               
                 Voltage 
                 (° C.) 
                 (° C.) 
                 (sec.) 
                 (W) 
                 (° C./sec.) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 4 
                 32 
                 50 
                 27.4 
                 0.42 
                 0.65 
               
               
                 7.9 
                 23 
                 100 
                 31 
                 1.7 
                 2.4 
               
               
                 11.8 
                 27 
                 120 
                 14.2 
                 3.8 
                 6.5 
               
               
                 15.7 
                 29 
                 120 
                 7 
                 6.8 
                 13.0 
               
               
                 19.7 
                 38 
                 120 
                 4.5 
                 10.4 
                 20.1 
               
               
                   
               
             
          
         
       
     
     Because the resistive heating element  13  is integrated in a low heat capacity column  10  that is thermally isolated from the control board  15 , heating rates of 0.65° C./sec with 0.42 W of heater power, and 20° C./sec with 10.4 W, can be achieved. These heating rates are about an order of magnitude faster, with at least an order of magnitude less power consumption, than are typical of conventional heated GC columns. 
     To obtain an accurate analysis, a consistent retention time should be obtained for a particular analyte and column condition. In particular, the control circuit  30  should provide a reproducible temperature ramp of the column  10 . FIG. 4 shows a graph of the column temperature, as measured by a thermistor-type temperature sensor  20 , for five separate temperature tests with the same programmed control voltage ramp. A monotonic temperature ramp was reproduced accurately for each test, enabling consistent analyte separations. This reproducibility is easily achieved with the microfabricated GC column  10  of the present invention because of the good thermal contact between the substrate  12  and the resistive heating element  13 . 
     In a typical GC analysis, a plug of the gas mixture to be analyzed is injected into the channel  11  via inlet tube  16 . For example, the gaseous plug can be generated by the rapid release of sorbed chemical species from a chemical preconcentrator, as disclosed in U.S. Pat. No. 6,171,378 to Manginell et al., or by the rapid vaporization of a liquid sample or pyrolyzation of a solid sample by a micropyrolyzer, as disclosed in U.S. patent application Ser. No. 10/035,537 to Mowry, et al., both of which are incorporated herein by reference. The column  10  is then heated by the resistive heating element  13  according to a prescribed temperature program. The analytes in the injected gas mixture are thereby separated with their retention time being dependent on the physico-chemical properties of the channel  11  and the temperature program. The separated analytes can then be removed from the channel  11  through an outlet tube  16  and passed over a detector that measures some property of the gas, such as thermal conductivity. A chromatogram, showing detector response versus retention time, can thereby be recorded. 
     FIGS. 5A-5C show chromatograms of a mixture of chemical analytes having a wide volatility range obtained with both isothermal and temperature-programmed columns. The chromatograms were obtained with a GC column  10  similar to that used to obtain the data in Table 1 and a flame ionization detector was used to detect the separated analytes. 
     As shown in FIG. 5C, an isothermal column may require an operating temperature of as high as 150° C. to obtain an elution of the stronger retained analyte, tributyl phosphate (TBP) in a reasonable retention time. However, with this high isothermal column temperature, the peaks of the early eluting analytes, dimethyl methyl phosphonate (DMMP), diethyl methyl phosphonate (DEMP), and wintergreen (WG), display poor separation. Conversely, as shown in FIG. 5A, at a low isothermal column temperature of 100° C., the higher volatility analytes, DMMP and DEMP, show good separation, but the peaks of the lower volatility analytes are broadened and poorly resolved above the baseline and the TBP does not elute during the two-minute analysis time. The chromatogram in FIG. 5B shows the separation achieved with a temperature program providing a 10 second segment at a column temperature of 100° C., followed by a 30 second temperature ramp to 150° C. (i.e., a heating rate of 1.67° C./sec or 100° C./min). The initial low temperature segment provides good separation of the higher volatility analytes and the temperature ramp segment provides good detectability of the lower volatility analytes. 
     These data show that temperature programming of a microfabricated GC column  10  having an integral resistive heating element  13  can significantly improve separation of some gas mixtures. Furthermore, temperature programming compensates for the limited number of plates available in short-column microfabricated GCs, as compared with long-column conventional laboratory GCs. This microfabricated GC column enables a low-power, efficient portable microanalytical system. 
     The embodiments of the present invention have been described as a temperature programmable microfabricated gas chromatography column. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.