Abstract:
A thermal conductivity detector (TCD) includes a detector cell body having a plurality of fluid cavities, at least one detector element associated with each of the plurality of fluid cavities, and a control circuit associated with each of the at least one detector elements, wherein the control circuit varies the power to the at least one detector element to maintain the at least one detector element at a constant temperature. Power compensation and temperature compensation are also provided to minimize temperature variation of the body of the TCD cell.

Description:
BACKGROUND 
       [0001]    Gas chromatography (GC) is used to separate and detect different compounds in a sample mixture. One of the common methods for performing gas chromatography uses columns to separate the sample gas into its constituent compounds. The interior surface of the column is typically an inert material that is coated with, or has adsorbed onto it, a material referred to as the “stationary phase.” The sample mixture is introduced into the column through a sample inlet device preferably in what is referred to as a “plug” and is transported through the column using an inert carrier gas, which is referred to as the “mobile phase.” When the sample gas encounters the stationary phase, the different components in the sample gas are attracted differently to the stationary phase, causing the different components in the sample gas to travel through the system at different speeds. Separation occurs by the differential retardation of sample components through interaction with the stationary phase as they are driven through the column by the mobile phase. Each sample component will have a characteristic delay between the time it was introduced into the chromatographic system and the time that it is detected after it elutes from the separation column. This characteristic time is called its “retention time.” Some minimum amount of difference in retention time allows differentiation of sample components chromatographically. One or more detectors at the exit of the column detect the different compounds when they elute from the column and provide an output signal proportional to amount of the sample component. The different components are shown as “peaks” on a chromatogram where the height and area beneath the peak corresponds to the amount of the compound. 
         [0002]    A thermal conductivity detector is widely used to provide the output signal referred to. In a simple form, a thermal conductivity detector includes a cell having an electrically heated element suspended in a cavity. As an example only, the element may be a filament, or another heated structure. As the output from the column flows through the cavity, the rate at which heat flows from the heated element to the wall of the cavity varies with the thermal conductivities of the gases in the cavity. The thermal conductivity of the carrier gas differs from the thermal conductivities of the sample gases, and the thermal conductivities of the sample gases mixed with carrier gas vary with the concentration of the sample gas in the carrier gas. Means are provided for deriving a signal that varies with the rate of heat flow. Accordingly, an output signal of the cell has a baseline value when carrier gas is flowing through its cavity and peaks when the concentrations of the respective sample gases are flowing through the cavity. 
         [0003]    A common design for a thermal conductivity detector cell uses multiple elements. Configurations for a thermal conductivity detector cell include four heated elements, or two heated elements and two fixed resistors or one heated element and three fixed resistors. The heated elements and resistors are connected together in a bridge circuit, such as a “Wheatstone Bridge” and powered symmetrically in which two heated elements, or one heated element in the case of a detector cell having fixed resistors, are located in a sample gas stream and the remaining two, or one, heated elements are located in a reference gas stream. In the case of a system with a single heated element the carrier and reference gas may be alternately switched over the heated element. The output is taken across the bridge and indicates the difference between the resistance of the sample element and the resistance of the reference elements due to variation in the thermal conductivity of the gas mixture passing over the elements. 
         [0004]    However, in a common TCD design the presence of the sample gas changes the temperature of the element and disrupts the thermal balance of the system. The change in temperature of the element is a potential cause for several adverse effects including changes in the characteristics of the sample element and changes to the nature of the sample, which can skew the results of the analysis. Therefore, it would be desirable to maintain a thermal conductivity detector element at a constant temperature and to maintain the power input to the detector constant and maintain the temperature of the detector body constant. 
       SUMMARY 
       [0005]    According to an embodiment, a thermal conductivity detector (TCD) includes a detector cell body having a plurality of fluid cavities, at least one detector filament associated with each of the plurality of fluid cavities, and a control circuit associated with each of the at least one detector filaments, wherein the control circuit varies the power to the at least one detector filament to maintain the at least one detector filament at a constant temperature. 
         [0006]    Other embodiments of the thermal conductivity detector having compensated constant temperature elements will be discussed with reference to the figures and to the detailed description of the preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0007]    The invention will be described by way of example, in the description of exemplary embodiments, with particular reference to the accompanying figures. 
           [0008]      FIG. 1A  is a schematic diagram illustrating an embodiment of a detector cell. 
           [0009]      FIG. 1B  is a planar view of the detector cell of  FIG. 1A . 
           [0010]      FIG. 2  is a schematic diagram illustrating a detector circuit that can be used to control the temperature of a sample element and a reference element and generate an output in the detector cell of  FIGS. 1A and 1B . 
           [0011]      FIG. 3  is a block diagram illustrating an embodiment of a power compensation circuit that can be used with the detector circuit of  FIG. 2 . 
           [0012]      FIG. 4  is a block diagram illustrating an embodiment of a temperature compensation circuit that can be used with the detector circuit of  FIG. 2 . 
           [0013]      FIG. 5  is a block diagram illustrating a simplified gas chromatograph, which is one possible device in which the embodiments of the thermal conductivity detector may be implemented. 
           [0014]      FIG. 6  is a flow chart illustrating the operation of an embodiment of the detector circuit of  FIG. 2 . 
           [0015]      FIG. 7  is a flow chart illustrating the operation of an embodiment of the power compensation circuit of  FIG. 3 . 
           [0016]      FIG. 8  is a flow chart illustrating the operation of an embodiment of the temperature compensation circuit of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    While described below as used in a thermal conductivity detector having particular characteristics, the thermal conductivity detector having a compensated constant temperature element can be used in any thermal conductivity detector having one or more elements and where it is desirable to precisely control the temperature of an element, or elements, the amount of power supplied to the detector and the temperature of the detector. 
         [0018]    As will be described below, the thermal conductivity detector having a compensated constant temperature element can be used to precisely control the temperature of the sample element the power supplied to the detector and the temperature of the detector. 
         [0019]      FIG. 1A  is a schematic diagram illustrating an embodiment of a detector cell  100 . The detector cell  100  generally includes a body  102  into which a pair of cavities to  104  and  106  are formed. In an embodiment, the body  102  can be fabricated from a planar structure such as silicon, into which the cavities  104  and  106  are formed. The cavities  104  and  106  can be, for example, etched into the silicon, or can be formed using other methods. In this example, the cavity  104  is referred to as a sample cavity and the cavity  106  is referred to as a reference cavity. Although omitted from  FIG. 1A , the output of a gas chromatograph column can be provided to the sample cavity  104 , while a carrier gas can be provided as a reference gas to the reference cavity  106 . 
         [0020]    The detector cell  100  also includes, in this example, variable resistances  110 ,  120 ,  130 , and  140 . The variable resistances  110 ,  120 ,  130 , and  140  have a characteristic such that the resistance changes monotonically with temperature. The variable resistances  110 ,  120 ,  130 , and  140  can be formed in the body  102  as etched structures, and are also referred to as detector filaments. A detector filament located in the sample cavity  104  is referred to as a sample filament and a detector filament located in the reference cavity  106  is referred to as a reference filament. 
         [0021]    As used in this description, the term “filament” is used to describe a particular type of heated element. However, the term “filament” is not intended to be limiting. Any heated element can be used according to the principles of the thermal conductivity detector having a compensated constant temperature element described herein. In this example, the sample cavity  104  and the reference cavity  106  each have two variable resistances, but this is not a requirement. A glass lid can be secured over the silicon structure, thus forming the body  102 , the sample cavity  104  and the reference cavity  106 . It should be mentioned that other structures can be used to form the detector cell  100 , so long as at least one variable resistance is located in the sample cavity  104  and one variable resistance is located in the reference cavity  106 . It should further be noted that a single variable resistance can be used in a system in which the sample and reference gasses are alternately switched across the element. 
         [0022]    The variable resistances can be formed as described above, or can be other resistive structures, so long as the resistance of each of the variable resistances  110 ,  120 ,  130 , and  140  intrinsically vary in a regulated way as a function of the amount of power provided to the variable resistances  110 ,  120 ,  130  and  140 . In this example, the variable resistances  110  and  120  can be referred to as sample filaments, or sample elements, and the variable resistances  130  and  140  can be referred to as reference filaments, or reference elements. In an embodiment, the power supplied to at least one sample filament is adjusted to maintain constant temperature of the sample filament that is exposed to sample gas. 
         [0023]    In this example, the flow of the reference gas through the detector cell  100  is illustrated using the arrows  112  and  114  and the flow of the sample gas through the detector cell  100  is illustrated using the arrows  116  and  118 . However, this flow direction is arbitrary. 
         [0024]      FIG. 1B  is a planar view of the detector cell  100  of  FIG. 1A . The variable resistances  110  and  120  are located in the sample cavity  104  and the variable resistances  130  and  140  are located in the reference cavity  106 . As a reference gas passes through the reference cavity  106 , the reference gas envelops the variable resistances  130  and  140 . A reference gas can be, for example, a carrier gas such as helium, hydrogen, nitrogen, etc. Similarly, as a sample gas, which includes a carrier gas and a sample material, flows through the sample cavity  104 , the sample gas envelops the variable resistances  110  and  120 . As the output from the column (not shown) flows through the sample cavity  104 , the rate at which heat flows from the sample filament  110  varies with the thermal conductivities of the gases in the sample cavity  104 . The thermal conductivity of the carrier gas differs from the thermal conductivities of the sample gases, and the thermal conductivities of the sample gases mixed with carrier gas vary with the concentration of the sample gas in the carrier gas. 
         [0025]    In accordance with an embodiment of the thermal conductivity detector having a compensated constant temperature element, as the sample gas envelops the sample filament  110 , the temperature of the sample filament  110  will change. As will be described below, the amount of power provided to the sample filament  110  when a sample is present will be changed proportionally with the change in the temperature of the sample filament. In this manner, the temperature of the sample filament  110  remains constant. 
         [0026]      FIG. 2  is a schematic diagram illustrating a detector circuit  200  that can be used to control the temperature of a sample element and a reference element and generate an output in the detector cell  100  of  FIGS. 1A and 1B . The detector circuit  200  includes a reference circuit  202  and a sample circuit  204 . The reference circuit  202  includes the reference filament  130  arranged in a bridge circuit with fixed resistances  206 ,  208  and  212 . The arrangement of resistances is commonly referred to as a “Wheatstone Bridge,” or bridge  205 . The fixed resistances  206 ,  208  and  212  are located outside of the detector cell  100  ( FIG. 1A ) and can be discrete resistances, resistors, or other resistive elements. The sample circuit  204  includes the sample filament  110  and fixed resistances  226 ,  228  and  232 . The circuit arrangement of the sample circuit  204  is also referred to as a Wheatstone Bridge, or bridge  207 . The fixed resistances  226 ,  228  and  232  are located outside of the detector cell  100  ( FIG. 1A ) and can be discrete resistances, resistors, or other resistive elements. 
         [0027]    The reference circuit  202  also includes an operational amplifier (op-amp)  220 . The inverting input of the operational amplifier  220  is connected between the fixed resistance  208  and the reference filament  130  via connection  214 . The non-inverting input of the operational amplifier  220  is connected between the fixed resistance  206  and the fixed resistance  212  via connection of  216 . 
         [0028]    The sample circuit  204  includes an operational amplifier  240 . The inverting input of the operational amplifier  240  is connected between the fixed resistance  228  and the sample filament  110  via connection  234 . The non-inverting input of the operational amplifier  240  is connected between the fixed resistance  226  and the fixed resistance  232  via connection  236 . 
         [0029]    The output of the reference circuit  202  on connection  222  is stable only when the ratio of the resistances  206  and  212  is the same value as the ratio of the resistances  208  and  130 . Similarly, the output of the sample circuit  204  on connection  242  is stable only when the ratio of the resistances  226  and  232  is the same value as the ratio of the resistances  228  and  110 . 
         [0030]    The operational amplifier  240  provides a feedback signal via connection  242  to control the amount of power supplied to the fixed resistances  226 ,  228 ,  232  and the sample filament  110 , to keep the bridge  207  balanced and keep the resistance value of the variable resistance  110  constant. This maintains the sample filament  110  at a constant temperature. The operational amplifier  220  provides a feedback signal via connection  222  to control the amount of power supplied to the fixed resistances  206 ,  208 ,  212  and the reference filament  130  to keep the bridge  205  balanced and keep the resistance value of the variable resistance  130  constant. In this manner, the detector circuit  200  maintains the sample filament  110  at a constant resistance and at a constant temperature. 
         [0031]    As a sample gas envelops the sample filament  110 , the temperature of the sample filament  110  will change. By varying the power supplied to the bridge  207  by the output of the operational amplifier  240 , the temperature of the sample filament  110  is kept constant. By varying power to the bridge  207 , only the variable resistance  110  will change in resistance. The power output of the operational amplifier  240  on connection  242  is controlled by the signal on connections  234  and  236  so as to maintain the sample filament  110  at a constant temperature. In this manner, the power (i.e. the voltage signal) on connection  242  becomes a measure of the thermal conductivity of the material passing over the sample filament  110 . 
         [0032]    The output of the reference circuit  202  is provided via connection  222  to the non-inverting input of a differential amplifier  250 . The output of the sample circuit  204  on connection  242  is provided to the inverting input of the differential amplifier  250 . The differential amplifier  250  determines the difference between the output of the reference circuit  202  and the sample circuit  204  and provides a signal on connection  252 . 
         [0033]    The reference filament  130  is exposed only to reference gas and remains at a constant temperature. Because the reference filament  130  remains at a constant temperature the signal on connection  252  is dependent upon the difference in the temperature between the sample filament  110  and the wall of the sample cavity  104  within which the sample filament  110  is located. The amount of energy transferred from the sample filament  110  to the cavity wall is dependent on the thermal conductivity of the gas that is located between the sample filament  110  and the wall of the sample cavity  104 . The output of the detector circuit  200  on connection  252  is the difference between the energy used to balance the reference filament  130  and a sample filament  110 . The signal on connection  252  is representative of the thermal conductivity of the sample enveloping the sample filament  110 . 
         [0034]      FIG. 3  is a block diagram illustrating an embodiment of a power compensation circuit  300  that can be used with the detector circuit  200  of  FIG. 2 . It is desirable to keep the temperature of the wall in the sample cavity  104  ( FIG. 1A ) constant. This can be accomplished by keeping the total power supplied to the detector cell  100  constant. 
         [0035]    The power compensation circuit  300  includes a heating element that is located in the vicinity of the sample filament  110 . The heating element can be any heating element located in the vicinity of the sample filament  110 . In this example, the detector cell  100  ( FIGS. 1A and 1B ) includes additional variable resistances  120  and  140 . Therefore, for simplicity of illustration, the heating element is illustrated as the variable resistance  120 . However, the heating element can be either one, or a combination of the variable resistances  130  and  140 . Alternatively, the heating element need not be one of the variable resistances in the detector cell  100 , but can be any heating element located in the vicinity of the sample filament  110 . 
         [0036]    The power compensation circuit  300  also includes an operational amplifier  332 , the output of which on connection  334  is connected to a heater resistance  336 . The heater resistance  336  is connected via connection  338  to the variable resistance  120 . 
         [0037]    The power compensation circuit  300  also includes a multiplier  304 . The multiplier  304  receives as a first input signal a current derived by dividing the voltage across the resistor  228  ( FIG. 2 , the voltage between nodes  242  and  234 ) by the resistance value of the resistor  228 . The multiplier  304  also receives as input the voltage across the sample filament  110  ( FIG. 2 , the voltage between nodes  234  and  244 ). The output of the multiplier  304  on connection  308  is a signal representing the power consumed by the sample filament  110 . The signal representing the power consumed by the sample filament may alternatively come from other locations in the detector circuit  200 . 
         [0038]    The power compensation circuit  300  also includes a multiplier  316 . A first input to the multiplier  316  is a current signal derived by dividing the voltage across the resistance  336  (the voltage between nodes  334  and  338 ) by the value of the resistance  336 . Another input to the multiplier  316  is a voltage signal representing the voltage across the variable resistance  120  (the voltage between nodes  338  and  342 ). The output of the multiplier  316  on connection  318  is a signal representing the power consumed by the variable resistance  120 , which is the resistance of the heater. 
         [0039]    The signal representing the power consumed by the variable resistance  120  on connection  318  is provided to a gain element  322 . The gain element  322  is configurable to adjust the amount of power that is provided to the variable resistance  120  (the heater) so that the amount of power added back to the detector cell  100  can be adjusted based on factors such as, for example, the location of the variable resistance  120  with respect to the sample filament  110 , the flow rate through the sample cavity  104  and the reference cavity  106 , the design of the detector cell, and other factors. 
         [0040]    The power consumed by the sample filament  110 , which is indicated on connection  308 , is added to the output of the gain element  322  by the adder  326 . The output of the adder  326  is a signal representing a variable component of the total power provided to the detector cell  100 . This power signal is provided to the inverting input of the operational amplifier  332 . A constant DC power source  344  provides a setpoint voltage, Vsetpoint, to the non-inverting input to the operational amplifier  332 . The operational amplifier  332  and the heater resistance  120  provide a servo loop that will keep the power signal on connection  328  constant. 
         [0041]    As mentioned above, when the sample envelops the sample filament  110 , the temperature of the sample filament  110  varies. To keep the temperature of the sample filament  110  constant, the power supplied to the sample filament  110  is changed by the detector circuit  200  as described above in  FIG. 2 . However, to keep the temperature of the detector cell  100  constant, in an embodiment, it is desirable to keep the total amount of power supplied to the detector cell  100  constant. The power compensation circuit  300  adds or reduces an equivalent amount of power back to the detector cell  100 , via the variable resistance  120 , based on the amount of power that was changed to the sample filament  110  by the detector circuit  200  as a result of the sample material passing the sample filament  110 . 
         [0042]      FIG. 4  is a block diagram illustrating an embodiment of a temperature compensation circuit that can be used with the detector circuit of  FIG. 2 . It is desirable to keep the temperature of the wall in the sample cavity  104  ( FIG. 1A ) constant. This can be accomplished by keeping the total power supplied to the detector cell  100  constant. 
         [0043]    The temperature compensation circuit  400  includes the variable resistance  140  implemented as a temperature sensor. The variable resistance  140  is arranged in a bridge circuit with fixed resistances  406 ,  408  and  412 . The arrangement of resistances is commonly referred to as a “Wheatstone Bridge,” or bridge  405 . The fixed resistances  406 ,  408  and  412  are located outside of the detector cell  100  ( FIG. 1A ) and can be discrete resistances, resistors, or other resistive elements. 
         [0044]    A DC voltage source  432  is coupled to the bridge  405  via connection  426 . The variable resistance  140  senses the temperature in the reference cavity  106 , which is also a close approximation of the temperature in the sample cavity  104 . This is also a close approximation of the temperature of the body  102  of the detector cell  100  ( FIG. 1A ). 
         [0045]    The temperature compensation circuit  400  includes an operational amplifier (op-amp)  420 . The inverting input of the operational amplifier  420  is connected between the fixed resistance  408  and the temperature sensor  140  via connection  414 . The non-inverting input of the operational amplifier  420  is connected between the fixed resistance  406  and the fixed resistance  412  via connection of  416 . The output of the operational amplifier  420  is coupled to the variable resistance  120 , which is implemented as a heater element in this embodiment. 
         [0046]    The operational amplifier determines the difference in the value of the signals on connections  414  and  416  and provides a difference signal on connection  422 . The difference signal controls the amount of heat generated by the variable resistance  120 . The heat generated by the variable resistance  120  (the heater) is thermally coupled to the variable resistance  140  (the sensor). The amount of heat provided by the variable resistance  120  is based on the temperature difference between the variable resistance  140  and a reference value provided on connection  416 . In this manner, thermal coupling between the variable resistance  120  and the variable resistance  140  maintains the variable resistance  140 , and the body  102  of the detector cell  100 , at a stable temperature. 
         [0047]      FIG. 5  is a block diagram illustrating a simplified gas chromatograph  500 , which is one possible device in which the embodiments of the thermal conductivity detector may be implemented. The gas chromatograph  500  includes a means of introducing a sample. A sample can be introduced via any of several devices known to those skilled in the art. For example, a sample may be introduced via a sample valve  504  which receives a gaseous sample of material to be analyzed via connection  502  and provides the sample via connection  508  to the inlet  512  of a gas chromatograph. The inlet  512  is connected to a chromatographic column  516  via connection  514 . A control processor  522  can be coupled to a flow control module  518 , via connection  524  to control the flow from the inlet  512  to the chromatographic column  516 . 
         [0048]    The output of the chromatographic column  516  is directed to a detector  526  via connection  523 . The detector  526  can include the detector cell  100 , the detector circuit  200 , the power compensation circuit  300  and the temperature compensation circuit  400 , described above. The output signals from the detector  526  are displayed and/or stored digitally and/or recorded mechanically with a plotter to provide a record  532  of the analytical run. 
         [0049]      FIG. 6  is a flow chart illustrating the operation of an embodiment of the detector circuit  200  of  FIG. 2 . The blocks in the flowchart can be performed in the order shown or out of the order shown, or can be performed in parallel. In block  602 , a flow of reference gas is introduced to the detector cell  100  ( FIG. 1A ). In block  604  a flow of sample gas is introduced to the detector cell  100  ( FIG. 1A ). 
         [0050]    In block  606 , a change in the temperature of the sample filament  110  is detected by the detector circuit  200  as described above. In block  608 , the detector circuit  200  changes the amount of power supplied to the sample filament  110  to maintain the sample filament  110  at a constant temperature. 
         [0051]      FIG. 7  is a flow chart illustrating the operation of an embodiment of the power compensation circuit  300  of  FIG. 3 . The blocks in the flowchart can be performed in the order shown or out of the order shown, or can be performed in parallel. In block  702 , a flow of reference gas is introduced to the detector cell  100  ( FIG. 1A ). In block  704  a flow of sample gas is introduced to the detector cell  100  ( FIG. 1A ). 
         [0052]    In block  706 , a change in the temperature of the sample filament  110  is detected by the detector circuit  200  as described above. In block  708 , the detector circuit  200  changes the amount of power supplied to the sample filament  110  to maintain the sample filament  110  at a constant temperature. 
         [0053]    In block  712 , the amount of power consumed by the sample filament is determined. In block  714 , the amount of power consumed by the heating element (the variable resistance  120 ) is determined. In block  718 , the total power consumed by the sample filament and the heating element is determined. In block  722 , the total power is compared against a reference power level (Vsetpoint). In block  724 , sufficient power is added to or removed from the detector cell, via the heating element, to maintain the total power supplied to the detector cell at a constant level, thereby maintaining the detector cell at a constant temperature. 
         [0054]      FIG. 8  is a flow chart illustrating the operation of an embodiment of the temperature compensation circuit  400  of  FIG. 4 . The blocks in the flowchart can be performed in the order shown or out of the order shown, or can be performed in parallel. In block  802 , a flow of reference gas is introduced to the detector cell  100  ( FIG. 1A ). In block  804  a flow of sample gas is introduced to the detector cell  100  ( FIG. 1A ). 
         [0055]    In block  806 , a change in the temperature of the sample filament  110  is detected by the detector circuit  200  as described above. In block  808 , the detector circuit  200  changes the amount of power supplied to the sample filament  110  to maintain the sample filament  110  at a constant temperature. 
         [0056]    In block  812 , the temperature of the reference channel  106  ( FIG. 1A ) is detected by the variable resistance  140  ( FIG. 4 ). In block  814 , the power provided to the variable resistance  120  ( FIG. 4 ) is varied based on the temperature of the reference cavity  106  to control the amount of heat generated by the variable resistance  120 . The heat generated by the variable resistance  120  is thermally coupled to the variable resistance  140  to control the temperature of the body  102  of the detector cell  100 , thereby maintaining the detector cell at a constant temperature. 
         [0057]    The foregoing detailed description has been given for understanding exemplary implementations of the invention and no unnecessary limitations should be understood therefrom as modifications will be obvious to those skilled in the art without departing from the scope of the appended claims and their equivalents. Other devices may use the thermal conductivity detector having a compensated constant temperature element described herein.