Patent Description:
In GC systems, the amount of time required for a chemical compound to traverse the entire length of a separation column ("column") is known as its retention time. One factor that contributes to the retention time of a chemical compound is the temperature of the separation column. Controlling the temperature of the column precisely from analysis to analysis is beneficial to provide repeatability in the retention time for a particular chemical compound, or analyte. In addition, programmatically changing the column temperature while the sample analytes are migrating through it can advantageously provide shorter analysis time and reduce peak broadening.

Often, columns are heated in known systems using an air convection oven because of its ability to provide a uniform and repeatable thermal environment in a space large enough to accommodate a wide variety of column diameters and lengths. The columns are typically arranged on a support structure that creates an open cylinder. This allows the heated air access over all the column surfaces and results in uniform temperatures across the entire column length. While air convection ovens are useful, their use comes with clear disadvantages. For example, convection ovens require a significant amount of energy and time to heat up, and a significant amount of time to cool down. This leads, of course, to comparatively long cycle times and high power consumption, among other disadvantages. In addition, the ability to do rapid analysis via temperature programmed conditions is limited when using air convection ovens. <CIT> describes a GC column assembly that consists of capillary GC column material, such as fused silica or metal capillary tubing, which is constrained to lie in a flat, ordered, spiral pattern and then encased between two thin opposing surfaces. The resulting column assembly is flat and dimensionally stable. The column assembly can be adapted for chromatographic use by affixing it to the surface of a thermal modulator by means of adhesive force or by mechanical compression, and then by attaching the free ends of the exposed column material to the input and output ports of the chromatographic device. <CIT> relates to a glass-silicon column that includes large area glass, such as a thin Coming <NUM> boron-silicate glass bonded to a silicon wafer, with an electrode embedded in or mounted on glass of the column, and with a self alignment silicon post/glass hole structure. The column includes two outer layers of silicon each bonded to an inner layer of glass, with an electrode imbedded between the layers of glass, and with at least one self alignment hole and post arrangement. The electrode functions as a column heater, and one glass/silicon component is provided with a number of flow channels adjacent the bonded surfaces.

What is needed, therefore, is an apparatus that overcomes at least the drawbacks of known GC column heaters discussed above.

The present teachings are best understood from the following detailed description when read with the accompanying drawing figures. The features are not necessarily drawn to scale. Wherever practical, like reference numerals refer to like features.

It is to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. The defined terms are in addition to the technical and scientific meanings of the defined terms as commonly understood and accepted in the technical field of the present teachings.

As used in the specification and appended claims, the terms 'a', 'an' and 'the' include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, `a device' includes one device and plural devices.

As used in the specification and appended claims, and in addition to their ordinary meanings, the terms 'substantial' or 'substantially' mean to with acceptable limits or degree. For example, 'substantially cancelled' means that one skilled in the art would consider the cancellation to be acceptable.

As used in the specification and the appended claims and in addition to its ordinary meaning, the term 'approximately' means to within an acceptable limit or amount to one having ordinary skill in the art. For example, `approximately the same' means that one of ordinary skill in the art would consider the items being compared to be the same.

In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. Descriptions of known systems, devices, materials, methods of operation and methods of manufacture may be omitted so as to avoid obscuring the description of the example embodiments. Nonetheless, systems, devices, materials and methods that are within the purview of one of ordinary skill in the art may be used in accordance with the representative embodiments. Any "embodiment" or "example" which is disclosed in the description but not covered by the claims should be considered as presented for illustrative purpose only.

Relative terms, such as "above," "below," "top," "bottom," "upper" and "lower" may be used to describe the various elements' relationships to one another, as illustrated in the accompanying drawings. These relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings. For example, if the device were inverted with respect to the view in the drawings, an element described as "above" another element, for example, would now be "below" that element. Similarly, if the device were rotated by <NUM>° with respect to the view in the drawings, an element described "above" or "below" another element would now be "adjacent" to the other element; where "adjacent" means either abutting the other element, or having one or more layers, materials, structures, etc., between the elements. As used herein, an element "disposed over" or "disposed below" another element means the element is "adjacent to" another element. "Directly adjacent" means abutting the other element.

<FIG> is a simplified block diagram of a GC system <NUM> in accordance with a representative embodiment. Many aspects of the GC system <NUM> are known to one of ordinary skill in the art. As such, details of certain known components of the GC system <NUM> are omitted. In certain instances representative examples of known components that may be implemented are noted, but are presented for illustration and are, in no way, intended to be limiting.

The GC system <NUM> comprises a sample inlet <NUM>. The sample inlet <NUM> is fluidically coupled to a contaminant trap <NUM>. The contaminant trap <NUM> is fluidically coupled to a column <NUM>, which may be one of a variety of columns useful in gas chromatography. In an embodiment, the contaminant trap <NUM> may be as described in concurrently filed, commonly owned <CIT>). The contaminant trap <NUM> is a microfluidic contaminant trap configured to trap contaminants in the sample from the sample inlet <NUM> and to prevent the trapped contaminants from reaching the column <NUM>. It is noted that the inclusion of contaminant trap <NUM> is merely illustrative, and the present teachings are contemplated for use in GC systems that do not comprise a contaminant trap, or that do not comprise a microfluidic contaminant trap as described in the application referenced immediately above.

The column <NUM> separates the components of a chemical sample. The column <NUM> may be a capillary column comprising a piece of fused silica or metal tubing (not shown) with a coating on the inner portions of the tubing or packed with particles that interact with the sample from sample inlet <NUM> to separate the components of the chemical sample.

The column <NUM> is provided in contact with a column temperature control apparatus <NUM>, which will be described more fully below in connection with representative embodiments. By virtue of the column temperature control apparatus <NUM>, the retention time for analytes is controlled, while the uniformity of the heating of the column <NUM> is comparatively improved. These and other benefits of the column temperature control apparatus <NUM> are described more fully below in connection with representative embodiments.

The column <NUM> is connected to a detector <NUM>, which detects the presence and frequently the quantity of the components separated by the column <NUM>. Generally, the detector <NUM> is a known GC detector such as a flame ionization detector (FID), a mass spectrometer detector (MSD), a thermal conductivity detector (TCD), an electron capture detector (ECD), a nitrogen phosphorus detector (NPD), a sulfur chemiluminescence detector (SCD), a nitrogen chemiluminescence detector (NCD), a pulsed flame photometric detector (PFPD), a helium ionization detector (HID), or a flame photometric detector (FPD).

The GC system <NUM> also comprises a controller <NUM> and a power source <NUM>. The controller <NUM> may be one of a plurality of controllers (not shown) of the GC system <NUM>, or may be the sole controller of the GC system. Presently, the function of the controller <NUM> with respect to maintaining the heating of the column <NUM> by the column temperature control apparatus <NUM> is described. Other functions of the controller <NUM> or of other controllers are not germane to the present teachings and are not described.

Generally, the controller <NUM> can be implemented in numerous ways (e.g., such as with dedicated hardware) to perform various functions discussed herein. A "processor" is one example of a controller, which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform various functions discussed herein. The controller <NUM> may be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions. Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, microcontrollers, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs).

In various implementations, the controller <NUM> may be associated with one or more storage media (generically referred to herein as "memory," e.g., volatile and non-volatile computer memory such as random-access memory (RAM), read-only memory (ROM), programmable read-only memory (PROM), electrically programmable read-only memory (EPROM), electrically erasable and programmable read only memory (EEPROM), universal serial bus (USB) drive, floppy disks, compact disks, optical disks, magnetic tape, etc.). In some implementations, the storage media may be encoded with one or more programs that, when executed on the controller <NUM>, perform at least some of the functions discussed herein. Various storage media may be fixed within the controller <NUM> or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller so as to implement various aspects of the present teachings discussed herein. The terms "program" or "computer program" are used herein in a generic sense to refer to any type of computer code (e.g., software or microcode) that can be employed to program the controller <NUM>.

The controller <NUM> is configured to receive temperature data from a temperature sensor (not shown in <FIG>), and based on the temperature data, is configured to provide control signals to the power source <NUM>. The power source <NUM> is one of a number of known electrical power sources and is configured to adjust the power of the column temperature control apparatus <NUM> to maintain the temperature of the column <NUM> at approximately a desired temperature.

<FIG> shows an exploded view of a column temperature control apparatus <NUM> (sometimes referred to as "an apparatus") for heating a GC column <NUM> in accordance with a representative embodiment. The column temperature control apparatus <NUM> comprises a first column heating apparatus <NUM> and a second column heating apparatus <NUM>. Notably, the column temperature control apparatus <NUM> of the present embodiment is contemplated for use as the column temperature control apparatus <NUM> in GC system <NUM> described above.

An optional first insulation layer <NUM> is disposed below the first column heating apparatus <NUM> and an optional second insulation layer <NUM> is disposed above the second column heating apparatus <NUM>. The GC column <NUM> is disposed above an upper side <NUM> of the first column heating apparatus <NUM>, and below a lower side <NUM> of the second column heating apparatus <NUM> with the lower side <NUM> of the second column heating apparatus <NUM> opposing the upper side <NUM> of the first column heating apparatus <NUM>. As described more fully below, heat flows through the upper side <NUM> of the first column heating apparatus <NUM> and the lower side <NUM> of the second column heating apparatus <NUM>, which is ultimately transferred to the GC column <NUM>.

Each of the first and second column heating apparatuses <NUM>, <NUM> includes a grommet <NUM> that is used to secure the various layers of the first column heating apparatus <NUM> and the second column heating apparatus <NUM>. Illustratively, the grommet <NUM> comprises stainless steel. Other means of securing the layers are also contemplated, such as brackets, clips etc. The requirements of the grommet or other securing means are that it can tolerate elevated temperatures (e.g. <NUM>) and still maintain sufficient pressure on the first and second column heating apparatuses <NUM>, <NUM>. High temperature metals are the preferred material.

The GC column <NUM> is wound in a substantially spiral manner, and can have portions of the column "stacked" over other portions of the column. Persons skilled in the art would appreciate how a GC column <NUM> may be "stacked" or "wound" to achieve the desired objectives described herein. When stacked or wound, the GC column <NUM> may have a plurality of layers in cross-section. The GC column may also be formed by stacking two or more independent columns on top of each other, each wound in a spiral manner. This, again, would result in the GC column <NUM> having a plurality of layers in cross-section. If one side of the GC column <NUM> is actively heated through contact with a heating element, and another side is not, even if the GC column is in the form of a single-layer Archimedean spiral, the heating of the GC column can be uneven. This non-uniformity can cause thermal gradients to form across the GC column. These effects can be exacerbated when the GC column is wound or "stacked" in a plurality of layers in cross-section. By providing heat to both the "top" and the "bottom" of the GC column <NUM> resting between the substantially flat surfaces of the first and second sides <NUM>, <NUM> of the first and second column heating apparatuses <NUM>, <NUM>, the GC column <NUM> can be more uniformly heated during a sample run. Similarly, by providing active heating on both sides, when the GC column <NUM> is a set of two or more columns on top of each other, each column will be less susceptible to thermal gradients and see a more similar thermal environment.

Additionally, by providing heat to both the "top" and the "bottom" of the GC column <NUM> resting between the substantially flat surfaces of the first and second sides <NUM>, <NUM> of the first and second column heating apparatuses <NUM>, <NUM>, thermal ambient rejection is also significantly improved compared to known GC heating arrangements (e.g., single heater arrangements) because both sides of the GC column <NUM> are temperature controlled surfaces. If GC column <NUM> were only disposed over first column heating apparatus <NUM>, and not "sandwiched" between first and second column heating apparatuses <NUM>, <NUM> as depicted in the representative embodiment of <FIG>, one side of the GC column <NUM> contacts the first surface, while the other is either in contact with first layer of insulation <NUM>, or is simply exposed to air. The first layer of insulation <NUM> is in contact with air so in either case, the ambient temperature of the air impacts the temperature of the portion of the GC column <NUM> not in direct contact with the first column heating apparatus <NUM>. Influence of changes in ambient temperature on the column temperature during an analysis results in shifts in retention time of an analyte that can make an analyte difficult to identify or make an analysis less repeatable. By providing the GC column <NUM> between the first and second column heating apparatuses <NUM>, <NUM>, the thermal environment of the GC column <NUM> can be isolated from the ambient temperature and therefore the retention time of the analytes better controlled.

<FIG> shows the temperature control apparatus <NUM> of <FIG> having the GC column <NUM> disposed thereover (without the second heating apparatus <NUM> or insulation layers <NUM>, <NUM> shown), in accordance with another representative embodiment. As can be appreciated, the first column heating apparatus <NUM> of <FIG> shares certain aspects, details and features common to those of the first column heating apparatus <NUM> described in connection with <FIG> above. Often, such common aspects, features and details are not repeated. As noted above, the GC column <NUM> is oriented in a comparatively flat spiral and is disposed over the upper side <NUM> of the first column heating apparatus <NUM>, and makes thermal contact with the upper side <NUM>. Again, the coiling of GC column <NUM> in representative embodiments may be a substantially planar spiral having one or more "stacked" so that the GC column <NUM> is in thermal contact with the first and second column heating apparatuses <NUM>,<NUM>, which are substantially planar, as described above. In accordance with the depicted representative embodiment, the GC column <NUM> is held in place over the first side <NUM> by GC column supports <NUM> mounted to GC column bracket <NUM>. Illustratively, the GC column supports <NUM> can be constructed of thin strips of metal such as aluminum, nickel, or stainless steel. More generally, the GC column supports <NUM> may be made of a material that is able to support the GC column <NUM> on the upper side <NUM> and tolerate the temperature exposure (up to <NUM>). The GC column bracket <NUM> serves as a structural element for GC column <NUM> and ensures repeatable positioning of the GC column <NUM> on upper side <NUM> for reproducible chromatographic performance. The GC column bracket <NUM> can be mounted in the GC system using one or more of a variety of known elements including, but not limited to, screws, clamps, and magnets (not shown). Notably, the GC column bracket <NUM> and supports <NUM> are contemplated for use alternatively with the second column heating apparatus <NUM>.

<FIG> shows an exploded view of first column heating apparatus <NUM> for heating a GC column (not shown) in accordance with a representative embodiment.

The first column heating apparatus <NUM> comprises a first substrate <NUM>, which is substantially planar. A first spacer layer <NUM> is optionally disposed above or adjacent to the first substrate <NUM>. A first recess <NUM> is provided in the first spacer layer <NUM>, and is configured to receive a first temperature sensor <NUM>.

The first column heating apparatus <NUM> comprises a first heating element <NUM> disposed between an optional first intervening layer <NUM> and an optional second intervening layer <NUM>. The first and second intervening layers <NUM>, <NUM> are generally made from the same material.

While the first heating element <NUM> is shown as a substantially uniform series of traces, it is also contemplated that that the traces may be substantially non-uniform, non-symmetrical, and/or irregular. By way of example, since the outer edge of the first heating element <NUM> is more exposed to the external environment, a decrease in temperature may occur at the outer edge compared to the inner portions of the assembly. By increasing density and/or changing the width of the traces of the first heating element <NUM> at its edge, the power density of the first heating element <NUM> near the outer edge is increased, and the temperature differential between the inner portion and the outer edge of the heating element, manifest in the noted temperature decrease, may be reduced or eliminated. Further modifications of the thickness of the traces may also have desirable properties as described herein. (Notably, the non-uniform, non-symmetrical, and/or irregular distribution of traces, which is described in connection with first heating element <NUM>, is also contemplated for second heating element <NUM>', as described in <FIG>.

First and second intervening layers <NUM>, <NUM> may be selected to act as electrical insulators between the first heating element <NUM> and the first substrate <NUM> and a second substrate <NUM>. Like the first substrate <NUM>, the second substrate <NUM> is substantially planar. The second substrate <NUM> is configured to have the GC column (not shown in <FIG>) in thermal contact therewith. The GC column is in thermal contact with the second substrate <NUM> through either direct contact or within a substantially close proximity to transfer heat. Alternatively, the second substrate <NUM> may be in thermal contact with the GC column despite the presence of an intervening layer (not shown) between the GC column and the second substrate <NUM>. The intervening layer may be, for example, a layer of thermally conductive material to improve thermal conduction to the GC column. Illustratively, the GC column is adjacent to the first side <NUM> of the first column heating apparatus <NUM>, and heat from the first column heating apparatus <NUM> is transferred through the second substrate <NUM> to the GC column. As can be appreciated from a review of <FIG>, the first side <NUM> is substantially planar.

The first and second substrates <NUM>, <NUM> may comprise single layer or multiple layers of the same or different materials. As described more fully below, the first column heating apparatus <NUM> substantially uniformly heats the GC column contacting the second substrate <NUM> at the first side <NUM>.

In known GC heaters, such as air convection ovens, the oven may require large amounts of power (up to 2000W) to allow temperature programming rates of <NUM>-<NUM>/min. By contrast, as described more fully below, the first column heating apparatus <NUM> beneficially provides similar temperature programming rates at substantially less than 100W. In addition, the first column heating apparatus <NUM> allows much faster temperature programming rates (e.g., up to five to ten times faster than a known GC heater with <NUM>% of the power requirement of the known GC heater), resulting in faster chromatographic analyses. Also, where known GC heaters may take six or more minutes to cool from <NUM>° C to <NUM>° C, the first column heating apparatus <NUM> may take less than three minutes allowing for faster cycle times between analyses. The first column heating apparatus <NUM> realizes these improvements in performance by specifying the material properties of the second substrate <NUM> or the first and second substrates <NUM>, <NUM> to be low thermal mass while maintaining mechanical stiffness, small thermal gradients, and resistance to thermal deformation.

As should be appreciated by one of ordinary skill in the art, the "thermal mass" of an object is a measure of its capacity to store thermal energy (i.e., heat). As such, a material that has a comparatively low thermal mass will require less heat in order to change temperature than one of comparatively high thermal mass. As described more fully below, in order to enable faster heating and cooling, the materials selected for the first and second substrates <NUM>, <NUM> and the first column heating apparatus <NUM> have a low thermal mass.

Thermal mass (with units of J/K) is the product of the specific heat of the material, cp, and the mass of the object, m. For convenience, mass can be further specified as the product of the density, ρ, of the material, a surface area, As, and a thickness, t, normal to the surface area. Combining, thermal mass can be expressed as: <MAT>.

Since the surface area of the first column heating apparatus <NUM> is fixed based on the size of the column to be heated, the surface area is viewed as a constant for this discussion. The remaining terms are examined further. The term, ρcp, is also known as the volumetric heat capacity of the material and is an intrinsic property of the material. To minimize thermal mass, this term should be minimized. According to a representative embodiment, materials for the second substrate <NUM> or the first and second substrates <NUM>, <NUM> have a volumetric heat capacity less than approximately <MAT> at <NUM>.

The selection of material for the second substrate <NUM> or the first and second substrates <NUM>, <NUM> is additionally bound by mechanical stiffness, low thermal gradients, and resistance to thermal deformation. These bounds are particularly important in determining the minimum thickness of material required for the second substrate <NUM> or the first and second substrates <NUM>, <NUM>. Along with thermal mass, these are not independent characteristics, so choice of materials is made considering all of them. The ultimate goal is to achieve low thermal gradients across the first side <NUM> of second substrate <NUM> while achieving a relatively low thermal mass for first and second substrates <NUM> and <NUM> to enable faster heating and cooling.

Thermal gradients across the second substrate <NUM> or across the first and second substrates <NUM>, <NUM> result from different parts of the substrates being in different thermal environments. The first heating element <NUM>, for instance, does not have a completely homogenous thermal profile. In addition, the outer edges of the first and second substrates <NUM>, <NUM> will typically have more exposure to the ambient temperature environment. As such, thermal gradients can exist across the first and second substrates <NUM>, <NUM>. Gradients are reduced when the material chosen for the first and second substrates has low resistance to heat flow, that is, a high thermal conductivity, k. It is desirable, therefore, to have a material with comparatively high thermal conductivity, particularly for the second substrate <NUM>, so that the first side <NUM> that touches the GC column is substantially uniform in temperature. According to a representative embodiment, materials for the second substrate <NUM> or the first and second substrates <NUM>, <NUM> have a thermal conductivity greater than approximately <MAT> at <NUM>.

The first and second substrates, <NUM> and <NUM>, provide mechanical structure for the first column heating apparatus <NUM>. Notably, the first and second substrates <NUM>, <NUM> support the relatively non-rigid first column heating element <NUM>, the optional layers <NUM> and <NUM>, as well as the first spacer layer <NUM> and the first temperature sensor <NUM>. Beneficially, materials chosen for the first and second substrates <NUM>, <NUM> are sufficiently stiff to provide adequate support. The stiffness of a material is related to its elastic modulus (or Young's Modulus), E. If a material has a high elastic modulus, then a thinner cross-section is necessary to provide the same stiffness as a material with a lower elastic modulus. It is beneficial, therefore, to have a material with a high elastic modulus so that less (thermal) mass of material is required to achieve adequate stiffness. According to a representative embodiment, materials for the first and second substrates <NUM>, <NUM> have a Young's Modulus greater than approximately <NUM> GPa. In addition to stiffness, the first and second substrates, <NUM> and <NUM> must maintain surface flatness in order to hold the heater and column in direct contact with the first side <NUM>, or in indirect contact with the first side <NUM> (i.e., with the above-mentioned intervening layer (not shown) between the GC column <NUM> and the first side <NUM>). Issues in flatness may occur due to deformation or "buckling" from rapid temperature changes. If large thermal gradients exist in a component such as, for example, when the component is cooled asymmetrically, sections of the component will want to grow due to thermal expansion while other sections will want to remain fixed. In the worst case, this can cause buckling or fracture.

The likelihood of mechanical deformation due to thermal expansion can be minimized by choosing a material with a high thermal conductivity, k, low thermal expansion coefficient, α, or both. A material with high thermal conductivity resists the formation of large thermal gradients within the material. Materials with low thermal expansion do not grow very much even under significant thermal gradients. Choosing materials with a high thermal conductivity, low thermal expansion coefficient, or both, allows for the use of less material (e.g., a thinner piece of it) and therefore less thermal mass while providing adequate resistance to buckling. According to a representative embodiment, materials for the second substrate <NUM> or the first and second substrates <NUM>, <NUM> have a ratio of thermal conductivity to coefficient of thermal expansion greater than approximately <MAT> at <NUM>.

Another consideration in the selection of the material for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM> is the electrical insulating properties of the material. Beneficially, the material is substantially electrically insulating to avoid having to add an additional material in the stack of the first and second column heating apparatuses <NUM>, <NUM> to perform this function.

Finally, it is important to select a material for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM> that is operative in the first and second column heating apparatuses <NUM>, <NUM> at temperatures greater than approximately <NUM>° C.

Table <NUM> presents a summary of some of the factors to be considered in selection of the material for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM>.

In a representative embodiment, the second substrate <NUM> comprises silicon. The silicon may be monocrystalline silicon or polycrystalline silicon. Generally, the silicon layer that forms the second substrate <NUM> has a thickness of approximately <NUM> to <NUM>. Illustratively, the second substrate <NUM> comprises <<NUM>,<NUM>,<NUM>> Si having a thickness of approximately <NUM>. In a representative embodiment, first substrate <NUM> comprises <<NUM>,<NUM>,<NUM>> Si wafer having a thickness of approximately <NUM>, and the second substrate <NUM> comprises two <<NUM>,<NUM>,<NUM>> Si wafers each having a thickness of approximately <NUM> each. It was discovered that the use of two wafers for second substrate <NUM> provides somewhat improved retention time repeatability. Notably, the second substrate <NUM> does not require special polishing or doping. Moreover, and although not essential, the first substrate <NUM> may be made of the same material and to the same specifications as the second substrate <NUM>.

It is noted that the use of silicon for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM> is merely illustrative. More generally, the materials selected for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM> are selected to have a volumetric heat capacity (ρcp) less than approximately <MAT> at <NUM>; a thermal conductivity (k) greater than approximately <MAT> at <NUM>; a ratio of thermal conductivity to coefficient of thermal expansion ( <MAT>) greater than approximately <MAT> at <NUM>; and a Young's Modulus (E) greater than approximately <NUM> GPa.

These physical characteristic are desired in order to achieve faster heating and cooling of the first column heating apparatus <NUM> within several bounds including low thermal mass, mechanical stiffness, low thermal gradients and resistance to deformation. Table <NUM> compares these four characteristics across a range of materials.

Based on the foregoing, the material selected for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM> preferentially has a volumetric heat capacity less than approximately <MAT> at <NUM>. Therefore, copper, alumina, nichrome, stainless steel, nickel, sapphire, silicon nitride, tungsten carbide, beryllium oxide, brass, bronze, aluminum brass, iron, and beryllium are not preferred materials for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM>.

The material selected for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM> should have a thermal conductivity greater than approximately <MAT> at <NUM>. Therefore, Pyrex glass, mica, titanium, quartz glass, gallium arsenide, germanium, boron nitride, zirconium oxide, boron carbide, indium phosphide, niobium, rhenium, and tantalum are generally not preferred materials for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM>.

The material selected for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM> additionally should have the ratio of thermal conductivity, k, to the coefficient of thermal expansion, a , that is greater than approximately <MAT> at <NUM> (at <NUM>° C). Therefore, aluminum, as shown in the table, as well as magnesium, silver, zinc, and gold are not preferred materials for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM>.

The material selected for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM> additionally should have a Young's Modulus greater than approximately <NUM> GPa. Therefore, graphite is not a preferred material for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM>.

Based on the analysis above, illustrative materials that can be used for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM> and meet all of the preferred material characteristics comprise silicon, aluminum nitride, diamond, silicon carbide, tungsten, molybdenum, alloys of tungsten (particularly with copper), alloys of molybdenum (particularly with copper), and combinations thereof.

The first heating element <NUM> can be disposed between the optional first intervening layer <NUM> and the optional second intervening layer <NUM>. The first and second intervening layers <NUM>, <NUM> are generally made from the same material, and each have a second comparatively low thermal mass. Moreover, the first and second intervening layers <NUM>, <NUM> are each made from a material that is electrically insulating. Notably, if the first and second substrates <NUM>, <NUM> are electrically insulating, the first and second intervening layers <NUM>, <NUM> can be foregone. However, if the material can become more electrically conducting at comparatively high temperatures (e.g., silicon), then electrical insulation is needed between the heating element and first and second substrates <NUM>, <NUM> when they are silicon. As such, in a representative embodiment in which first and second substrates <NUM>, <NUM> are silicon, first and second intervening layers <NUM>, <NUM> are needed. Notably, however, in another representative embodiment, rather than including first and second intervening layers <NUM>, <NUM>, the sides of the first and second substrates <NUM>, <NUM> facing the heating element may be coated with a layer of glass or other dielectric to perform this insulating function.

The first heating element <NUM> is illustratively a resistive heating element, such as a wire heater or a foil heater. Other types of heating elements are contemplated. As should be appreciated, the heating element is beneficially quite thin, and thereby does not substantially interfere with the desirably flat nature of each of the layers of the first and second column heating apparatuses <NUM>, <NUM>. With known thin film fabrication methods, such comparatively thin heating elements that are within the purview of one of ordinary skill in the art are contemplated.

Like the comparatively low thermal masses of the first and second substrates <NUM>, <NUM>, the comparatively low thermal mass of the optional first and second intervening layers <NUM>, <NUM> ensures they heat comparatively quickly and will not retain heat very well. As such, the column heating apparatus <NUM> can be heated quickly across its surface, and will not retain heat to the extent as other materials often used in heaters. Again, the former attribute ensures ultimately that the GC column disposed over first side <NUM> of the second substrate <NUM> is heated comparatively very quickly, which improves analysis time. The latter attribute enables the thorough dissipation of heat from the first column heating apparatus <NUM> in a relatively quick and efficient manner enabling faster cycle times and improved retention time repeatability.

In a representative embodiment, the first and second intervening layers <NUM>, <NUM> each comprise mica, which are of sheet silicate (phyllosilicate) minerals. Generally, mica materials are
X<NUM>Y<NUM>-<NUM>Z<NUM>O<NUM>(OH,F)<NUM> in which X is K, Na, or Ca or less commonly Ba, Rb, or Cs; Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.; Z is chiefly Si or Al, but also may include Fe<NUM>+ or Ti. The use of mica for first and second intervening layers <NUM>, <NUM> is merely illustrative, and other materials having similar thermal mass, electrical conductivity, and resistance to mechanical distortion due to rapid temperature change as mica are contemplated. For example, fabrics such as fiberglass, and basalt provide the desired properties.

Generally, the mica layers that form the optional first and second intervening layers <NUM>, <NUM> of the first column heating apparatus <NUM> each have a thickness of approximately <NUM>. More generally, the material selected for the first and second intervening layers <NUM>, <NUM> of the first column heating apparatus <NUM> has an electrical resistivity of approximately <NUM> × <NUM>^<NUM>Ω·m to approximately <NUM> × <NUM>^<NUM>Ω·m, or greater. Since mica has a comparatively low coefficient of thermal expansion (CTE), similar to silicon, it will not expand when heated or cooled and will not suffer from mechanical distortion, Furthermore, the mica is inherently flat and affords intimate contact between the substrate and the heating element. Other materials that could serve as an electrical insulator in place of mica are, for example, aluminum nitride, quartz, glass, silicon carbide, and the fabrics cited above. Compliant materials like the fabrics cited previously need not be as flat as they can be compressed to achieve intimate contact.

First spacer layer <NUM> is optionally disposed over the first substrate <NUM> and beneath the first intervening layer <NUM> of the column heating apparatus <NUM>. The first spacer layer <NUM> is a spacer that accommodates first temperature sensor <NUM>. Illustratively, first spacer layer <NUM> is a compliant material, such as a glass fiber material. The first spacer layer <NUM> beneficially maintains substantially uniform pressure between the optional first intervening layer <NUM> of the first heating element <NUM> and the first substrate <NUM>. Notably, the inclusion of the first temperature sensor <NUM> can compromise the substantially uniform pressure in the absence of the first spacer layer <NUM>. Non-uniform pressure between the first intervening layer <NUM> of the column heating apparatus <NUM> and the first substrate <NUM> can result in a reduction in the overall "flatness" of the first column heating apparatus <NUM>, leading to thermal gradients and "hot spots," and can thus compromise performance of the GC column.

<FIG> shows an exploded view of second column heating apparatus <NUM> for heating a GC column (not shown in <FIG>) in accordance with a representative embodiment. Notably, second column heating apparatus <NUM> is substantially identical to the first column heating apparatus <NUM> described herein, and comprises the same components of the first column heating apparatus <NUM>. As such, common details of the various components of the second column heating apparatus are often not repeated.

The second column heating apparatus <NUM> comprises a third substrate <NUM>', which is substantially planar. A second spacer layer <NUM>' is optionally adjacent to the third substrate <NUM>'. A second recess <NUM>' is provided in the second spacer layer <NUM>', and configured to receive a second temperature sensor <NUM>'.

The second column heating apparatus <NUM> comprises a second heating element <NUM>' disposed between an optional third layer <NUM>' and an optional fourth layer <NUM>'. Third and fourth layers <NUM>', <NUM>' may be selected to act as electrical insulators between the second heating element <NUM>' and the third substrate <NUM>' and a fourth substrate <NUM>'. Like the third substrate <NUM>', the fourth substrate <NUM>' is substantially planar. The fourth substrate <NUM>' is configured to have the GC column (not shown in <FIG>) in direct contact therewith. Alternatively, the fourth substrate <NUM>' may be configured to have the GC column in indirect contact therewith, by having an intervening layer (not shown) between the GC column and the fourth substrate <NUM>'. The intervening layer may be, for example, a layer of thermally conductive material to improve thermal conduction to the GC column.

Notably, in operation the second column heating apparatus <NUM> depicted in <FIG> is rotated <NUM>° (i.e., as depicted in <FIG>) so that the GC column <NUM> is disposed below the lower side <NUM> of the second column heating apparatus <NUM>, and heat from the second column heating apparatus <NUM> is transferred through the fourth substrate <NUM>' to the GC column. As can be appreciated from a review of <FIG>, the second side <NUM> is substantially planar.

The third and fourth substrates <NUM>', <NUM>' may comprise single layer or multiple layers of the same or different materials. As described more fully below, the second column heating apparatus <NUM> substantially uniformly heats the GC column contacting the fourth substrate <NUM>' at the lower side <NUM> of the second column heating apparatus <NUM>.

As alluded to above, the controller <NUM> receives temperature data from the first temperature sensor <NUM>, and based on these data provides control signals to the power source <NUM>. Similarly, the controller <NUM> receives temperature data from the second temperature sensor <NUM>' provided in second column heating apparatus <NUM>, and based on these data provides control signals to the power source <NUM>. Based on the control signals from the controller <NUM>, the power source <NUM> adjusts electrical power to the first and second column heating apparatuses <NUM>, <NUM> to maintain the temperature of the GC column at a substantially constant value or to cause it to change according to some desired and repeatable program.

<FIG> shows an exploded view of first column heating apparatus <NUM> in accordance with another representative embodiment. Many aspects of the first column heating apparatus <NUM> of this embodiment are similar to those described above. As such, many details of various features that are common to those of first column heating apparatus <NUM> of the representative embodiment of <FIG> are not repeated. Notably, the various characteristics of the common elements of the first and second column heating apparatuses <NUM>, <NUM> are the same. <FIG> depicts a representative embodiment of a first column heating apparatus <NUM> comprising a first substrate <NUM> adjacent to a first heating element <NUM>, wherein the optional spacer and first and second intervening layers are omitted.

The first heating element <NUM> is illustratively a resistive heating element, such as a wire heater or a foil heater. Other types of heating elements are contemplated. As should be appreciated, the first heating element <NUM> is beneficially quite thin, and thereby does not substantially interfere with the desirably flat nature of each of the layers of the first column heating apparatus <NUM>. With known thin film fabrication methods, such comparatively thin heating elements that are within the purview of one of ordinary skill in the art are contemplated.

The first column heating apparatus <NUM> also comprises second substrate <NUM> adjacent to the first heating element <NUM>. The second substrate <NUM> is configured to have the GC column <NUM> (not shown in <FIG>) in thermal contact therewith. Illustratively, the GC column is disposed over upper side <NUM> of the second substrate <NUM>, and heat from first heating element <NUM> is transferred through the second substrate <NUM> as described above in connection with the representative embodiments of <FIG> and <FIG>. The first and second substrates <NUM>, <NUM> may comprise single layer or multiple layers of the same or different materials. Through the heat distribution of the second substrate <NUM> described above, the first column heating apparatus <NUM> substantially uniformly heats the GC column contacting the second substrate <NUM>.

In view of this disclosure it is noted that the methods and devices can be implemented in keeping with the present teachings. Further, the various components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. In view of this disclosure, the present teachings can be implemented in other applications and components, materials, structures and equipment needed to implement these applications can be determined, while remaining within the scope of the appended claims.

Claim 1:
A gas chromatography heating apparatus for heating a gas chromatography column (<NUM>) comprising:
a first column heating apparatus (<NUM>), comprising: a first substrate (<NUM>); a second substrate (<NUM>) comprising silicon; and a first heating element (<NUM>) disposed between the first substrate (<NUM>) and the second substrate (<NUM>); and
a second column heating apparatus (<NUM>) comprising: a third substrate (<NUM>'); a fourth substrate (<NUM>') comprising silicon; and a second heating element (<NUM>') disposed between the third substrate (<NUM>') and the fourth substrate (<NUM>');
wherein a lower side (<NUM>) of the second column heating apparatus (<NUM>) is arranged opposite a upper side (<NUM>) of the first column heating apparatus (<NUM>) thereby allowing the gas chromatography column (<NUM>) to be disposed above the upper side (<NUM>) of the first column heating apparatus (<NUM>) and below the lower side (<NUM>) of the second column heating apparatus (<NUM>).