Patent Description:
Precise control of the temperature of the column is, of course, important to the overall performance of the GC measurement. In many column temperature control systems, the temperature sensor does not measure the actual column temperature because it is located away from the column for various reasons. Although it is desirable for the column temperature to be constant along its length, no column heating system provides a completely isothermal environment. For the user of the GC apparatus, it is important that the thermal gradients along the length of the GC column are small and that analytes migrating through the column experience an effective temperature that provides the desired retention characteristics.

<CIT> relates to a plate-type column that includes: a plate-shaped main body; projecting portions protruding from a circumferential edge of the main body; and a fluid-flow passage extending in the main body and the projecting portions. An intermediate portion of the passage is provided in the main body, while each of the end portions of the passage extends from the main body through the projecting portion, with each of the end portions being open to the outside at the tip of the projecting portion.

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

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. Disclosed is an apparatus comprises: a column heating apparatus; a first temperature sensor disposed adjacent to a gas chromatography column; and a second temperature sensor disposed in or above the column heating apparatus. A temperature of the gas chromatography column is altered based on temperature data from the first and second temperature sensors.

The apparatus further comprises a first layer of thermal insulation disposed beneath the column heating apparatus and a second layer of thermal insulation. The second temperature sensor is disposed over, within, or beneath the second layer of thermal insulation.

In accordance with a representative embodiment, the column heating apparatus comprises: a first substrate; a heating element disposed over the first substrate; and a second substrate disposed over the column heating apparatus. The second substrate has a first side and a second side. The second side is configured to have the gas chromatography column in contact therewith. Heat from the column heating apparatus is transferred through the second substrate and substantially uniformly heats the gas chromatography column contacting the second substrate.

In accordance with another representative embodiment, an apparatus comprises: a first temperature sensor disposed adjacent to a gas chromatography column; and a second temperature sensor disposed in or above the column heating apparatus. A temperature of the gas chromatography column is altered based on temperature data from the first and second temperature sensors. The apparatus also comprises a controller configured to receive temperature data from the first and second temperature sensors. A power source is configured to receive control signals from the controller and to adjust electrical power to the column heating apparatus.

In accordance with yet another representative embodiment, a non-transitory computer readable medium storing a program, executable by a controller, for controlling a column heating apparatus, is disclosed. The computer readable medium comprises: a receiving code segment for receiving temperature data from a first temperature sensor and a second temperature sensor; a weighted average code segment for determining a weighted average from the temperature data; a comparison code segment for comparing the weighted average with a current set point; a proportional, integral derivative code segment for determining a temperature error; and a setting code segment for setting a power level to apply to a heating element from the temperature error.

In accordance with yet another representative embodiment, an apparatus for controlling a column heating apparatus is disclosed. The apparatus comprises: a controller configured to receive temperature data from a first temperature sensor and a second temperature sensor. The controller is further configured to execute programming operations. The programming operations comprise: determining a weighted average of temperature data from the temperature data; comparing the weighted average of the temperature data with a current set point temperature; determining a temperature error from the comparison of the weighted average of the temperature data and the current set point temperature; and adjusting a power level to apply to a heating element based on the determined temperature error.

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.

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" the other element. "Directly adjacent" means abutting the other element.

Certain representative embodiments are directed to a gas chromatography (GC) column heating assembly. In accordance with a representative embodiment the GC column heating assembly comprises: a first temperature sensor disposed adjacent to a gas chromatography column; and a second temperature sensor disposed in or above the column heating apparatus. A temperature of the gas chromatography column is altered based on temperature data from the first and second temperature sensors. The system also comprises: a controller configured to receive temperature data from the first and second temperature sensors; and a power source configured to receive control signals from the controller and to adjust electrical power to the column heating apparatus to maintain a temperature of the GC column substantially at a desired value.

<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 is controlled, while the uniformity of the heating of the column <NUM> is comparatively improved. Furthermore, in certain embodiments, the column <NUM> can be cooled in a comparatively thorough manner, ultimately improving repeatability of the retention time of an analyte and analysis cycle time compared to known GC systems. 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 a computer readable medium (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>.

As described more fully below in connection with <FIG> and <FIG>, the controller <NUM> is configured to receive temperature data from at least two temperature sensors (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 "apparatus") in accordance with a representative embodiment. 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 herein.

The column temperature control apparatus <NUM> comprises a column heating apparatus <NUM> configured to have a GC column <NUM> disposed over a surface <NUM>. The GC column <NUM> is contemplated for use as the column <NUM> described in connection with representative embodiments of <FIG>.

The column temperature control apparatus <NUM> additionally comprises a first temperature sensor <NUM> and a second temperature sensor <NUM>. The first temperature sensor <NUM> is disposed in the first column heating apparatus <NUM>. Alternatively, the first temperature sensor <NUM> may be disposed over the first column heating apparatus <NUM>. The first temperature sensor <NUM> is illustratively embedded in the column heating apparatus <NUM>, such as described below in connection with the representative embodiments of <FIG>, and in commonly owned co-pending <CIT> as inventors.

<FIG> depicts two different representative orientations for the second temperature sensor <NUM>. A first layer of thermal insulation <NUM> is disposed beneath the column heating apparatus <NUM>, and a layer of thermal insulation <NUM> is disposed above the GC column <NUM> and the second temperature sensor <NUM>. In one embodiment, the second temperature sensor <NUM> may be disposed over the GC column <NUM> and below the layer <NUM> of thermal insulation as shown. In another embodiment, the second temperature sensor <NUM> may be disposed over the layer of thermal insulation <NUM>. These temperature sensor locations are designed to reflect real-time temperature gradients that are not captured in prior art GC systems.

Preferably, the first temperature sensor <NUM> and second temperature sensor <NUM> are either devices such as a thermocouple, or a platinum resistance thermometer (PRT). The first and second temperature sensors <NUM>, <NUM> must respond quickly enough to detect changes in their thermal environments. Notably, the first temperature sensor <NUM> must provide data to the controller that tracks the relatively rapid temperature changes in the column heating apparatus. Depending on the location of the heating element and its relative location to the second temperature sensor <NUM>, the second temperature sensor <NUM> may be in a thermal environment that experiences slower changes in temperatures. Consequently, temperature changes in the GC column environment may be reflected in measurements from the first temperature sensor <NUM> prior to detection at the second temperature sensor <NUM>.

The first and second layers of thermal insulation <NUM>, <NUM> are made of a material suitable to provide ample thermal insulation without interfering with the performance of the GC system. Illustratively, the first and second layers of thermal insulation <NUM>, <NUM> are made of a glass fabric material having a thickness of approximately <NUM> in. , and can be provided as "blankets" to improve conformance of the first and second layers of thermal insulation <NUM>, <NUM> to the outer surfaces of the column heating apparatus <NUM> and the GC column <NUM> with which they contact. Alternatively, the first and second layers of thermal insulation <NUM>, <NUM> may comprise other types of insulation including, but not limited to fiberglass, glass cloth, basalt, and the like. The material selected for the first and second layers of thermal insulation <NUM>, <NUM> generally needs to provide a sufficient thermal barrier between the column heating apparatus <NUM> and the ambient environment during a GC run, while being capable to be cooled thoroughly and quickly after the GC run.

As described more fully in the present disclosure, the first and second temperature sensors <NUM>, <NUM> provide data to controller <NUM> and a determination is made regarding the power provided by the power source <NUM> to the column temperature control apparatus <NUM>, which comprises the column heating apparatus <NUM>. By controlling the power provided to the column heating apparatus <NUM> based on temperature values from both first and second temperature sensors <NUM>, <NUM>, the temperature of the GC column <NUM> can be more accurately controlled compared to known methods. Notably, by locating the first temperature sensor <NUM> within the or on top of the column heating apparatus <NUM>, comparatively rapid feedback about the temperature of the column heating apparatus <NUM> can be provided to the controller <NUM>. In one embodiment, the first temperature sensor <NUM> is in physical contact with the column heating apparatus <NUM> which is designed to facilitate rapid changes in temperature in response to changes in the power applied to its heating element <NUM> (not shown in <FIG>) contained within the column heating apparatus <NUM>. By contrast, because of the greater relative distance from the heating element <NUM>, the second temperature sensor <NUM> is more thermally isolated. Consequently, changes in temperature originating from power modulation of the heating element will be detected by the second temperature sensor only after temperature information flows across the GC column <NUM> and any intervening layers between the heating element and the second temperature sensor <NUM>.

Moreover, locating the second temperature sensor <NUM> at the interface of the GC column <NUM> and the second layer of thermal insulation <NUM> allows for a measure of the temperature on the side of the GC column <NUM> opposing the side of the GC column adjacent to the surface <NUM>, through which heat from the heating element <NUM> (not shown in <FIG>) of the column heating apparatus <NUM> flows. As such, locating the first and second temperature sensors <NUM>, <NUM> as depicted in the representative embodiment of <FIG> provides a real-time indication of the thermal gradient from the column heating apparatus <NUM> to the side of the GC column <NUM> furthest from the column heating apparatus <NUM>. <FIG> shows an exploded view of the column heating apparatus <NUM> depicting two representative locations of the first temperature sensor <NUM> within column heating apparatus <NUM>. The column heating apparatus <NUM> comprises a first substrate <NUM>, which is substantially planar. A spacer layer <NUM> is optionally disposed over the first substrate <NUM>.

A recess <NUM> is provided in the spacer layer <NUM>, and, in some embodiments, receives the first temperature sensor <NUM>. In other embodiments" the first temperature sensor <NUM> can be located between an intervening layer <NUM> and second substrate <NUM>. While some degree of benefit may be realized by mounting the first temperature sensor <NUM> in other places within the GC column temperature control apparatus <NUM>, locating the first temperature sensor <NUM> in close proximity to the heating element <NUM> is beneficial for heater control.

Preferably, the first temperature sensor <NUM> is located in between the GC column <NUM> and the heating element <NUM>. Illustratively, the first temperature sensor <NUM> may be located between an intervening layer <NUM> and the second substrate <NUM>. Alternatively, the first temperature sensor <NUM> may be located on an "outer" side (i.e., beneath the first substrate <NUM> or above the second substrate <NUM>).

A heating element <NUM> is disposed between an optional intervening layer <NUM> and an optional intervening layer <NUM> (referred to below as intervening layer <NUM>). The intervening layers <NUM>, <NUM> are generally made from the same material. The intervening layers <NUM>, <NUM> may also be selected to act as electrical insulators between the 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. Illustratively, the GC column is disposed over the surface <NUM> of the second substrate <NUM>, and heat from the heating element <NUM> is transferred through the second substrate <NUM> to the GC column. As can be appreciated from a review of <FIG>, the surface <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 column heating apparatus <NUM> substantially uniformly heats the GC column contacting the second substrate <NUM>.

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> of the column heating apparatus <NUM> have a comparatively 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, thermal 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 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 embodiments, 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 surface <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 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 surface <NUM> that touches the GC column is substantially uniform in temperature. According to a representative embodiments, 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 column heating apparatus <NUM>. Notably, the first and second substrates <NUM>, <NUM> provide ample support for the various relatively non-rigid components of the column heating apparatus <NUM> as well as the GC column <NUM> and the second temperature sensor <NUM>. Beneficially, materials chosen for the first and second substrates <NUM>, <NUM> are sufficiently stiff to provide such 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 less of it (e.g., a thinner piece of it) 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 embodiments, 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 heating element <NUM> in contact with the second substrate <NUM>, and the GC column <NUM> in direct contact with the surface <NUM>, or in indirect contact with the surface <NUM> (i.e., with an intervening layer (not shown) between the GC column <NUM> and the surface <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 embodiments, 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>°C.

Another consideration in the selection of the material for the second substrate <NUM>, or the first and second substrates <NUM>, <NUM> is the electrical insulative properties of the material. Beneficially, the material is substantially electrically insulating to avoid having to add an additional material in the stack of the column heating apparatus <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 column heating apparatus <NUM> at temperatures greater than approximately <NUM>.

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. Generally, the silicon layer that forms the second substrate <NUM> is illustratively monocrystalline silicon or polycrystalline silicon, and has a thickness of approximately <NUM> to <NUM>. Illustratively, the second substrate <NUM> comprises <<NUM>,<NUM>,<NUM>> silicon having a thickness of approximately <NUM>. In a representative embodiment, first substrate <NUM> is illustratively monocrystalline silcon or polycrystalline silicon. The first substrate <NUM> may comprise a <<NUM>,<NUM>,<NUM>> silicon wafer having a thickness of approximately <NUM>, and the second substrate <NUM> comprises two <<NUM>,<NUM>,<NUM>> Si wafers having a thickness of approximately <NUM> each. 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. 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>; 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 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> should have 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 preferable 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>. This specification additionally eliminates Therefore, Pyrex glass, mica, titanium, quartz glass, gallium arsenide, germanium, boron nitride, zirconium oxide, boron carbide, indium phosphide, niobium, rhenium, and tantalum are not preferable 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, α, that is greater than approximately <MAT> at <NUM>°C (at <NUM>). Therefore, aluminum, magnesium, silver, zinc, and gold are not preferable 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 preferable 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> comprise silicon, aluminum nitride, diamond, silicon carbide, tungsten, molybdenum, alloys of tungsten (particularly with copper), alloys of molybdenum (particularly with copper), and combinations thereof.

In one representative embodiment, a heating element <NUM> is disposed between the intervening layers <NUM>, <NUM>. The intervening layers <NUM>, <NUM> are generally made from the same material, and each have a second comparatively low thermal mass. Moreover, the 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, then the intervening layers <NUM>, <NUM> may be omitted. However, if the material of the intervening layers <NUM>, <NUM> 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>. As such, in a representative embodiment in which first and second substrates comprise silicon, the intervening layers <NUM>, <NUM> may be needed. Notably, however, in another representative embodiment, rather than including the 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.

In a representative embodiment, the 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 the 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.

The 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 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.

<FIG> shows an exploded view of column heating apparatus <NUM> in accordance with another representative embodiment. Many aspects of the column heating apparatus <NUM> are substantially identical to those of column heating apparatus <NUM> described above in connection with <FIG>. As such, many details of various features that are common to those of column heating apparatus <NUM> in <FIG> are not repeated. Notably, the various characteristics of the common elements of the column heating apparatus <NUM> are the same. For example, when made of the same material (e.g., silicon), the comparative magnitudes of the thermal masses of the first and second substrates <NUM>, <NUM> relative to other components of the column heating apparatus <NUM> are the same as those described above.

The column heating apparatus <NUM> comprises the first substrate <NUM> having heating element <NUM> disposed thereover. Notably, however, the column heating apparatus <NUM> of <FIG> does not comprise spacer and intervening layers <NUM>, <NUM>, <NUM>, which were noted above as being optional.

<FIG> depicts two representative locations of other first temperature sensor in an alternate embodiment of the column heating apparatus <NUM>. In one embodiment, the first temperature sensor <NUM> is disposed over the first substrate <NUM> and beneath the heating element <NUM>. In another embodiment,, the first temperature sensor <NUM> is located between heating element <NUM> and second substrate <NUM>. As noted above, while some degree of benefit may be realized by mounting the first temperature sensor <NUM> in other places within the GC column temperature control apparatus <NUM> locating the first temperature sensor <NUM> in close proximity to the heating element <NUM> is beneficial for heater control.

The column heating apparatus <NUM> also comprises second substrate <NUM> disposed over the heating element <NUM>. The second substrate <NUM> is configured to have a GC column (not shown in <FIG>) in direct contact therewith or indirect contact therewith (i.e., with an intervening layer (not shown) between the GC column and the second substrate <NUM>. Illustratively, the GC column is disposed over the surface <NUM> of the second substrate <NUM>, and heat from heating element <NUM> is transferred through the second substrate <NUM> as described above in connection with the representative embodiments of <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 apparatus <NUM> substantially uniformly heats the GC column contacting the second substrate <NUM>.

The GC column temperature control apparatus <NUM> comprises the first temperature sensor <NUM> disposed within or directly adjacent to a column heating apparatus <NUM> comprising a heating element <NUM> as shown in <FIG> and <FIG>. Locating the first temperature sensor <NUM> within or adjacent to the heating element <NUM> enables comparatively rapid feedback about the temperature of the heating element <NUM> to be provided to the controller <NUM>. Moreover, locating the second temperature sensor <NUM> on the opposite side of the GC column from the heating element captures real-time temperature gradients.

<FIG> shows a simplified block diagram of the controller <NUM> in accordance with a representative embodiment. The simplified block diagram depicts those components of the controller <NUM> that are useful in determining the required power to be provided to the heating element <NUM> in order to maintain the temperature of the GC column, or the immediate surroundings of the GC column, or both, at substantially the desired level. Notably, other components of the controller <NUM>, including other hardware and firmware that do not relate to the temperature control of the GC column, are not shown or described.

The controller <NUM> comprises a proportional-integral-derivative (PID) controller <NUM>. The PID controller <NUM> may be instantiated in software, a microcontroller or programmable logic device (PLD), such as a field programmable gate array (FPGA), or other similar device. The PID controller <NUM> is instantiated with a PID controller algorithm that involves three separate constant parameters, and is often referred to as a three-term control: the proportional, integral and derivative values. The algorithm is presented in the form of software or firmware, or a combination of both. As described more fully below, the PID controller algorithm in the form of a program (instructional code) can be stored in a memory <NUM> or other computer readable medium and can cause the PID controller <NUM> to determine the set point for the power source in order to heat a GC column (e.g., GC column <NUM>) to a desired level. Notably, in representative embodiments in which the PID controller <NUM> is instantiated in software, it may be stored in memory <NUM>.

The controller <NUM> also includes a mathematical processing component or algorithm <NUM>, which is configured to receive temperature data from the first temperature sensor <NUM> and from the second temperature sensor <NUM>, and calculate an improved estimate of the true column temperature than either the first temperature sensor <NUM> or the second temperature sensor <NUM> could provide alone. The algorithm block <NUM> illustratively comprises a processor instantiated in hardware, firmware, or software, or a combination thereof. Alternatively, the algorithm block <NUM> comprises analog circuitry, such as a resistor-pair. In a preferred embodiment, the algorithm is used to determine the weighted average of the two temperature sensors. Calculating the weighted average of the data from the first and second temperature sensors can be effected by multiplying the temperature from the first temperature sensor <NUM> by a value (X) and the temperature from the second temperature sensor <NUM> by a value (<NUM>-X). Notably, the value of X is determined by optimizing ambient rejection and is described more fully below. The weighted average provides an estimate of the temperature near the GC column <NUM>, and by properly selecting the value of X, this estimate can be substantially accurate in the time frame through continuous collection and interpretation of data from the first and second temperature sensors <NUM>, <NUM>. While the weighted average algorithm is described, other approaches to processing the temperature input data and deriving an estimated column temperature are contemplated.

Generally, the algorithm block <NUM> is configured to determine a temperature value that is as close as possible to the actual column temperature over a range of conditions. One condition that will vary in a real-world situation is ambient temperature. The temperature at the first temperature sensor <NUM> and at the GC column <NUM> can vary differently as ambient temperature changes. When using only the value of the first temperature sensor <NUM> as an input, the PID controller <NUM> can only compensate for the effect of ambient temperature shifts near the location of the first temperature sensor <NUM>. Due to thermal resistances in the system, the compensation made will not exactly compensate for the effect of ambient temperature at the GC column <NUM>. This will result in a slight overall change in column temperature. As a sample peak traverses the column it will therefore see a slightly different average temperature and the resulting peak elution time will shift slightly. The column temperature change will usually be a fraction of the ambient temperature change. To maintain repeatable peak elution times (which will allow for the easiest analyte identification) it is optimal to keep this fraction as small as possible. "Ambient rejection" is a term given to describe the relationship between ambient temperature changes and effective column temperature changes and is given by the change in ambient temperature divided by the resulting change in column temperature. Ambient rejection can be either positive or negative. In any case, it is desirable to maximize the absolute value of the ambient rejection. Ambient rejection in a good GC system is usually on the order of <NUM>: <NUM>.

By using a properly selected weighted average of the first temperature sensor <NUM> and a second temperature sensor <NUM>, the PID controller <NUM> can more accurately compensate for ambient shifts at the column because the actual column temperature is better approximated. By measuring the actual column temperature through the elution times of compounds through the GC column <NUM> under varying ambient condition, one can determine how effective including the second temperature sensor <NUM> is in improving ambient rejection.

Beneficially, the value of X is selected so that results in ambient temperature variation having no effect on compound retention. Experimentally, X can be substantially optimized by repeatedly injecting the same sample under the same nominal GC conditions (e.g., oven, inlet, detector temperature and pressure set-points) and observing the shift in retention time or retention index as X and ambient temperature are varied. Retention index, a relative measure of retention for a given analyte, can be used to determine an effective column temperature. The change in ambient temperature divided by the change in effective column temperature gives the ambient rejection of the thermal system.

The controller <NUM> comprises a temperature set-point module <NUM>, which provides the current power set point to the PID controller <NUM>. The algorithm block <NUM> provides the weighted average value from the most recent calculation to the PID controller <NUM>. The PID controller <NUM> calculates the difference between the set-point temperature from the temperature set-point module <NUM> and the weighted average value to determine a temperature error. The PID controller algorithm adds P times the temperature error, adds I times the integral of the temperature error over the time since the last temperature data were received from the first and second temperature sensors <NUM>, <NUM>, and then adds D times the derivative of the temperature error. There are various means of determining useful values of P, I and D for each iteration of calculating the temperature error and, ultimately, determining the value of the power applied by the power source <NUM> to the heating element <NUM>.

The temperature error is the instantaneous error, and thus, is independent of what it was before, or what it will be after. The integral, however, is the running sum of all of the temperature errors since a particular point in time, such as when the zone was turned on, or when the temperature was near the set-point. The derivative is based on the current measurement and one or more of the previous measurements. In one embodiment, the derivative could be the change in temperature between the most-recent temperature error and the previous temperature error, measured a fixed time apart, divided by the time difference between the determinations of the two temperature errors. However, more sophisticated means of calculating the derivative may be needed, and are contemplated by the present teachings, to reduce the effects of noise. Notably, while the derivative can be computed from the difference between two measurements offset in time, this simple technique may be too sensitive to noise in the measurements and may give less than desirable control. There are numerous known ways to calculate a more noise-immune value for the derivative. For example, a least-squares fit of a function to multiple data points could be performed, and the derivative calculated from this function. Even though the multiple data points cover a period of time leading up to the current calculation, the computed derivative would be the estimate for a particular time within that period, for example, the middle of the period. Beneficially, the estimate of the derivative should be made for a time as close as possible to that of the most recent temperature measurement, either by using a small number of data points to fit the function, or by evaluating the function at a time corresponding to the time of the most recent measurement.

Once the PID controller <NUM> determines the new power level for the power source <NUM> to provide to the heating element <NUM>, the power source <NUM> applies the new power level. Additional data are then gathered from the first and second temperature sensors <NUM>, <NUM> and the process is repeated. Generally, the process is repeated prior to beginning the next measurement. Specifically, the interval between iterations (the elapsed time between temperature measurements) needs to be short enough to match the speed of the thermal response of the thermal zone. As is known, the thermal response represents a delay between when the applied power changes, and the time this change is sensed in the first and second temperature sensors <NUM>, <NUM>. The thermal response results, inter alia, from the combined effects of thermal resistance between the heater and sensor, and various thermal masses associated with the system. A fast system will have low thermal resistance or low thermal mass, or both.

Each time the temperature is measured, the PID calculation is performed and the heater power is adjusted to this new value. Of course, this can all be done in analog circuitry, in which case everything is continuous rather than discreet, and there is no interval.

<FIG> shows a flow-chart of a method <NUM> of controlling a temperature of a GC column in accordance with a representative embodiment. The method <NUM> is illustratively implemented in connection with the embodiments described above in connection with <FIG>. Notably, the method <NUM> may be carried out a number of ways through the hardware, software or firmware of the controller <NUM>. In a representative embodiment, non-transitory computer readable medium storing a program is provided in the controller (e.g., in memory <NUM>). This program includes code for effecting the method. In each part of the method below, different aspects of the code are disclosed. Such code is readily determined by one of ordinary skill in the art, and is not repeated in the interest of clarity of description of the present embodiments.

At S401, the method comprises measuring temperatures. As noted above, temperature measurements are made by the first and second temperature sensors <NUM>, <NUM>. A receiving code segment is provided in computer readable medium for receiving temperature data from the first temperature sensor <NUM> and the second temperature sensor <NUM>.

At S402, the column temperature estimate is determined in the controller <NUM> as described above. In a preferred embodiment, a weighted average code segment is provided in computer readable medium for determining the weighted average temperature from the temperature data.

At S403, a comparison is made between the column temperature estimate and the current set point at the controller <NUM>. A comparison code segment is provided in computer readable medium for comparing the column temperature estimate with a current set point temperature. Based on this comparison, a temperature error is determined at the PID controller <NUM>. A proportional, integral derivative code segment is provided in computer readable medium for determining a temperature error.

At S404, as described above, the PID algorithm determines the new power level required to apply to the heating element <NUM>. A setting code segment is provided in computer readable medium for setting a power level to apply to a heating element from the temperature error.

At S405, as described above, the power source <NUM> adjusts the power level applied to the heating element <NUM> based on the power level input from the PID controller. An adjusting code segment is provided in computer readable medium for adjusting the power level to apply to a heating element <NUM> based on the temperature error.

As shown, the process is repeated beginning at S401. As mentioned, this depends on how fast the thermal zone responds. Illustratively, thermal zones of representative embodiments are serviced <NUM> times per second. As is known, servicing includes measuring the temperature; using the temperature data to perform a PID calculation in the PID controller <NUM>, and providing control signals to the power source <NUM> to change the power provided to the column temperature control apparatus <NUM> in GC system <NUM> described above.

Claim 1:
A temperature control system, comprising:
- gas chromatography column temperature control system, comprising:
a column heating apparatus (<NUM>),
a capillary gas chromatography column (<NUM>),
(i) a first temperature sensor (<NUM>) located within or directly adjacent to the column heating apparatus (<NUM>), and
a second temperature sensor (<NUM>) disposed above the column heating apparatus (<NUM>); or
(ii) a first temperature sensor (<NUM>) disposed adjacent to the gas chromatography column (<NUM>), and
a second temperature sensor (<NUM>) disposed in or above the column heating apparatus (<NUM>);
- a controller (<NUM>) configured to receive temperature data from the first and second temperature sensors (<NUM>, <NUM>) and to output control signals based on the temperature data from the first and second temperature sensors (<NUM>, <NUM>); and
- a power source (<NUM>) configured to receive control signals from the controller (<NUM>) and to adjust electrical power to the column heating apparatus (<NUM>) to alter the temperature of the gas chromatography column (<NUM>), characterized in that
the apparatus (<NUM>) further comprises a first layer of thermal insulation (<NUM>) disposed beneath the column heating apparatus (<NUM>) and a second layer of thermal insulation (<NUM>) disposed above the gas chromatography column (<NUM>), the second temperature sensor (<NUM>) being disposed over, within, or beneath the second layer of thermal insulation (<NUM>).