Abstract:
A concentrator and sensor assembly are disclosed that use phased heaters to multiply the concentration levels that can be eluted, relative to operation with a single interactive element having a sorbent material to increase the concentration of desired gas constituents at a detector. This is accomplished here by providing two or more interactive concentrator elements that are selectively heated in a time phased sequence so that each of the interactive elements becomes heated and desorbs gas constituents into the sample fluid stream at substantially the same time that an upstream concentration pulse, produced by heating one or more upstream interactive elements, reaches the interactive element. This produces a multiplication effect that significantly increases the concentration of the gas constituents at the detector, thereby increasing the effective sensitivity of the detector.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to gas monitoring devices, and more particularly to methods and apparatus for detecting and identifying various gas constituents in a sample gas stream and/or determining the concentrations of such constituents. 
     The detection of gases and vapors at low concentrations is often difficult due to limitations in the sensitivity of detector devices and measurement instruments. The process of detecting various constituents within a gas sample at low concentrations can be greatly enhanced if the constituents can be concentrated prior to detection. One approach for concentrating selected constituent gases is described in “Quartz Crystal Gas Monitor With Gas Concentrating Stage”, Kindlund et al, Sensors and Actuators, 6 (1984) pp. 1-17. Kindlund et al. suggest providing a gas concentrator in front of a detector to increase the concentration of the desired gas constituents at the detector. The gas concentrator of Kindlund et al. includes a thick organic sorbent layer that is coated on the walls of a cavity. When cool, the sorbent layer adsorbs the desired gas constituents from the gas sample flowing through the cavity. A heating pulse is then applied to the sorbent layer, causing the adsorbed constituents to desorbs into the cavity to produce a short concentration pulse. The concentration pulse is conducted to a quartz crystal gas monitor that ultimately registers the presence of the constituent. 
     A limitation of Kindlund et al. is that typically sorbent materials can only accumulate a limited amount of gas constituents. Thus, the concentration pulse produced when the sorbent layer is heated is also limited, thereby limiting the effective sensitivity of the detector. What would be desirable, therefore, is a concentrator and/or sensor assembly that can further increase the concentration level of desired gas constituents at the detector to produce a detector of increased effective sensitivity. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes many of the disadvantages associated with the prior art by providing a concentrator and sensor assembly that use phased heaters to increase or multiply the concentration levels beyond those that can be achieved by a single interactive element having a sorbent material. Generally, this is accomplished by providing two or more interactive elements that are selectively heated in a time phased sequence so that each of the interactive elements becomes heated and desorbs constituent gases into the sample fluid stream at substantially the time that an upstream concentration pulse, produced by heating one or more upstream interactive elements, reaches the interactive element. As can be seen, this produces a multiplication effect that can significantly increase the concentration of the gas constituents at the detector, thereby increasing the effective sensitivity of the detector. 
     In a first illustrative embodiment, a concentrator is provided for concentrating one or more constituents in a sample fluid stream. The concentrator preferably has two or more interactive elements spaced along and exposed to the sample fluid stream. Each of the interactive elements include an interactive substance that adsorbs and desorbs selected constituents of the sample fluid stream, depending on the temperature of the interactive element. Two or more heater elements are provided, with each heater element in thermal communication with a corresponding interactive element. 
     A controller energizes the heater elements in a time phased sequence. The controller preferably energizes the heater elements such that each of the corresponding interactive elements become heated and desorb selected constituents into the sample fluid stream at substantially the time at which an upstream concentration pulse, produced by one or more upstream interactive elements, reaches the interactive element. It is contemplated that a large number, N, of interactive elements may be used to achieve the desired multiplication of concentration of constituent gases in the concentration pulse by a factor N. 
     The resulting concentration pulse may then be provided directly to a detector for detection and analysis. The detector may be a thermal conductivity detector, discharge ionization detector, or any other type of detector such as those commonly used in gas chromatography. More preferably, however, the resulting concentration pulse is first provided to a separator. The separator separates selected gas constituents of the resulting concentration pulse into individual constituent components. The detector may then detect the concentration of each constituent that elutes from the separator. 
     The heater elements are preferably formed from a resistive material having a common resistance and length along the flow direction. As such, the controller can equally energize the heater elements by providing an equal voltage, current, or power pulse to each heater element. The voltage, current, or power pulse may have any desired shape including a triangular shape, a square shape, a bell shape, or any other shape. The shape or height of the voltage, current, or power pulse may even be chosen to produce a temperature profile that only desorbs selected gas constituents from the sorbent material. 
     It is also contemplated that the length of the heater elements may increase along the sample fluid stream. The length of each heater element may be increased, relative to the upstream heater elements, by an amount that corresponds to the expected increased length of the concentration pulse of the upstream heater elements caused by diffusion. To match this diffusion effect for best utilization of the growing concentration wave in the concentrator, the length of each of the heater elements may be similarly increased to produce the same resistance, thereby tailoring equal voltage, current, or power pulses to be used for each heater element to achieve equal temperature profiles. Alternatively, all heater elements may have the same length as the N-th element, so that the controller may provide equal voltage, current, or power pulses, suitably phased in time, to all heater elements to result in equal temperature profiles. 
     It is also contemplated that the two or more interactive elements need not be separate elements, but rather may be formed from a single interactive layer. Two or more heater elements may then be in thermal communication with different portions of the interactive layer. This configuration may simplify the manufacture of the concentrator. 
     The present invention also contemplated a number of methods. In one illustrative method, a sample fluid flow or stream is provided using a pump, thermal convection, or the like. The sample fluid stream is allowed to pass over two or more interactive elements (or an interactive layer) until the interactive elements adsorb one or more constituents from the sample fluid stream and reach equilibrium. Thereafter, the two or more interactive elements are heated in a time phased sequence. 
     Preferably, an upstream interactive element is first heated, which causes the upstream interactive element to increase in temperature and to desorb selected constituents into the sample fluid stream to produce a first concentration pulse that is carried by the sample fluid stream downstream toward a downstream interactive element. Thereafter, the downstream interactive element is heated as the first concentration pulse reaches the downstream interactive element. This causes the downstream interactive element to desorb selected constituents into the sample fluid stream and at least partially overlap the first concentration pulse to produce a larger concentration pulse that is carried by the sample fluid stream further downstream. The larger concentration pulse has an increased concentration level of the selected constituents than that of the first or second concentration pulses. It is contemplated that any number of downstream interactive elements may be heated in a like manner to produce an even further increased concentration level at the output of the concentrator. 
     After the concentrator provides a desired concentration pulse, selected constituents may be separated to provide one or more individual constituent components. The concentration of the individual constituent components may then be sensed and analyzed as desired. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 is a schematic diagram of a first illustrative sensor apparatus according to the present invention; 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a graph showing illustrative heater temperatures, along with corresponding concentration pulses produced at each heater element; 
     FIG. 4 is a graph showing a number of heater elements having lengths to match the expected increased lengths of the concentration pulses due to diffusion; 
     FIG. 5 is a graph showing a concentration pulse that reaches a 100% concentration level; 
     FIG. 6 is a schematic view of a sensor assembly in accordance with the present invention; 
     FIG. 7 is a schematic view of another sensor assembly in accordance with the present invention; 
     FIG. 8 is a timing chart showing the operation of the sensor assembly of FIG. 7; and 
     FIG. 9 is a simplified layout of an integrated circuit that includes a concentrator, a separator, and a sensor in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram of a first illustrative sensor apparatus in accordance with the present invention. The sensor apparatus is generally shown at  10  and includes a substrate  12  and a controller  14 . It is contemplated that the controller  14  may or may not be incorporated into substrate  12 . 
     The substrate  12  preferably has a number of thin film heater elements  20 ,  22 ,  24 , and  26  positioned thereon. While only four heater elements are shown, it is contemplated th at any number of heater e elements may be provided, preferably between one hundred and one thousand. Heater elements  20 ,  22 ,  24 , and  26  may be fabricated of any suitable electrical conductor, stable metal, or alloy film, s such a s a nickel-iron alloy sometimes referred to as permalloy, with a composition of eighty percent nickel and twenty percent iron; platinum, platinum suicide, and polysilicon. The heater elements  20 ,  22 ,  24 , and  26  are preferably provided on a thin, low-thermal mass, low-in-plane thermal conduction, support member  30 , as best shown in FIG.  2 . 
     The substrate  12  also preferably has an accurately defined channel  32  for receiving the sample fluid stream. The channel  32  is preferably fabricated by selectively etching the silicon substrate  12  beneath support member  30 . The process of forming channel  32  may be similar to that used to form the microbridge system illustrated in U.S. Pat. No. 4,944,035 to Aagardl et al., which is incorporated herein by reference. The channel includes an entry port  34  and an exhaust port  36 . 
     The sensor apparatus also preferably includes a number of interactive elements inside channel  32  so that they are exposed to the sample fluid stream. Each of the interactive elements is preferably positioned adjacent, i.e. closest possible contact, to a corresponding heater element. For example, and referring, to FIG. 2, interactive elements  40 ,  42 ,  44 , and  46  are preferably provided on the lower surface of support m-ember  30 , and adjacent to heater elements  20 ,  22 ,  24 , and  26 , respectively. The interactive elements may be formed from any number of films commonly used in liquid or gas chromatography, such as silica gel or active carbon. 
     In one embodiment, the interactive elements are formed by passing, a stream of material carrying the desired sorbent material through channel  32 . This provides an interactive layer throughout the channel. If separate interactive elements are desired, the coating, may be selectively “developed” by providing a temperature change to the coating, via the heater elements. After the coating is developed, a stream of solvents may be provided through channel  32  to remove the coating everywhere except where the coating has been developed, leaving only the sorbent material that is adjacent the heater elements. 
     Controller  14  preferably is electrically connected to each of the heater elements  20 ,  22 ,  24 ,  26 , and detector  50  as shown. The controller  14  energizes the heater elements  20 ,  22 ,  24 , and  26  in a time phased sequence (see bottom of FIG. 3) such that each of the corresponding interactive elements  40 ,  42 ,  44 , and  46  become heated and desorb selected constituents into the sample fluid stream at precisely the time when an upstream concentration pulse, produced by one or more upstream interactive elements, reaches the interactive element. It is contemplated that any number of interactive elements may be used to achieve the desired concentration of constituent gases in the concentration pulse. In the embodiment shown, the resulting concentration pulse is provided to detector  50  for detection and analysis. Detector  50  may be a thermal conductivity detector, discharge ionization detector, or any other type of detector such as those typically used in gas or fluid chromatography. 
     FIG. 3 is a graph showing illustrative heater temperatures, along with corresponding concentration pulses produced at each heater element. As indicated above, the controller  14  may energize the heater elements  20 ,  22 ,  24 , and  26  in a time phased sequence. Illustrative time phased heater temperatures for heater elements  20 ,  22 ,  24 , and  26  are shown at  60 ,  62 ,  64 , and  66 , respectively. 
     In the example shown, the controller  14  (see FIG. 1) first energizes the first heater element  20  to increase its temperature as shown at  60 . Since the first heater element  20  is thermally coupled to the first interactive element  40 , the first interactive element desorbs selected constituents into the sample fluid stream to produce a first concentration pulse  70 . The sample fluid stream carries the first concentration pulse  70  downstream toward the second heater element  22 , as shown by arrow  72 . 
     The controller  14  next energizes the second heater element  22  to increase its temperature as shown at  62 . Since the second heater element  22  is thermally coupled to the second interactive element  42 , the second interactive element also desorbs selected constituents into the sample fluid stream to produce a second concentration pulse. The controller  14  energizes the second heater element  22  such that the second concentration pulse substantially overlaps the first concentration pulse  70  to produce a higher concentration pulse  74 , as shown. The sample fluid stream carries the larger concentration pulse  74  downstream toward the third heater element  24 , as shown by arrow  76 . 
     The controller  14  then energizes the third heater element  24  to increase its temperature as shown at  64 . Since the third heater element  24  is thermally coupled to the third interactive element  44 , the third interactive element  44  desorbs selected constituents into the sample fluid stream to produce a third concentration pulse. The controller  14  energizes the third heater element  24  such that the third concentration pulse substantially overlaps the larger concentration pulse  74  provided by the first and second heater elements  20  and  22  to produce an even larger concentration pulse  78 , as shown. The sample fluid stream carries this larger concentration pulse  78  downstream toward the “Nth” heater element  26 , as shown by arrow  80 . 
     The controller  14  then energizes the “Nth” heater element  26  to increase its temperature as shown at  66 . Since the “Nth” heater element  26  is thermally coupled to the “N-th” interactive element  46 , the “N-th” interactive element  46  desorbs selected constituents into the sample fluid stream to produce an “N-th” concentration pulse. The controller  14  energizes the “N-th” heater element  26  such that the “N-th” concentration pulse substantially overlaps the larger concentration pulse  78  provided by the previous N- 1  interactive elements, as shown. The sample fluid stream carries the “N-th” concentration pulse  82  to either a separator or a detector, as more fully described below. 
     As indicated above, the heater elements  20 ,  22 ,  24 , and  26  may have a common length. As such, the controller can achieve equal temperatures of the heater elements by providing an equal voltage, current, or power pulse to each heater element. The voltage, current, or power pulse may have any desired shape including a triangular shape, a square shape, a bell shape, or any other shape. An approximately square shaped voltage, current, or power pulse is used to achieve the temperature profiles  60 ,  62 ,  64 , and  66  shown in FIG.  3 . 
     FIG. 4 is a graph showing a number of heater elements having lengths to match the expected increased length of the concentration pulses due to diffusion. It is recognized that each of the concentration pulses may tend to reduce in amplitude and increase in length when traveling down the channel  32  due to diffusion. To accommodate this increased length, it is contemplated that the length of each successive heater element may be increased along the sample fluid stream. For example, the second heater element  102  may have a length W 2  that is larger than the length W 1  of the first heater element  100 . Likewise, the third heater element  104  may have a length W 3  that is larger than the length W 2  of the second heater element  102 . Thus, it is contemplated that the length of each heater element  100 ,  102 , and  104  may be increased, relative to the adjacent upstream heater element, by an amount that corresponds to the expected increased length of the concentration pulse of the upstream heater elements due to diffusion. 
     To simplify the control of the heater elements, the length of each successive heater element may be increased to produce the same overall heater resistance between heater elements, thereby allowing equal voltage, current, or power pulses to be used to produce similar temperature profiles. Alternatively, the heater elements may have different lengths, and the controller may provide different voltage, current, or power pulse amplitudes to the heater element to produce a similar temperature profile. 
     FIG. 5 is a graph showing a concentration pulse  110  that achieves a 100% concentration level. It is recognized that even though the concentration pulse  110  has achieved a predetermined concentration threshold, such as 100%, the concentration of the corresponding constituent can still be determined. To do so, the detector may detect the concentration pulse  110 , and the controller  14  may integrate the output signal of the detector over time to determine the concentration of the corresponding constituent in the original sample. 
     FIG. 6 is a schematic view of an illustrative sensor assembly in accordance with the present invention. The sensor assembly may include a solenoid pump  120 , a sample fluid stream  122 , a concentrator  124 , a separator  126 , a detector  128 , and a controller  130 . At the request of the controller  130 , the solenoid pump  120  preferably draws a sample from a flue gas stream  132  through one way valve  134 . The controller  130  may then direct the solenoid pump  120  to provide a sample fluid stream, at a desired pressure, to concentrator  124 . 
     Concentrator  124  preferably includes two or more interactive elements that are in communication with the sample fluid stream. Concentrator  124  also preferably includes two or more heater elements that are in thermal communication with the interactive elements. When energized, each heater element heats a corresponding interactive element, causing the interactive element to desorb selected constituents into the sample fluid stream. As described above, controller  130  preferably energizes the heater elements in a time phased sequence to provide an increased concentration pulse. 
     The sample fluid stream carries the concentration pulse to separator  126 . Separator  126  separates selected constituents of the concentration pulse and provides the separated constituents to detector  128 . Detector  128  provides a signal to the controller  130  indicating the concentration level of each constituent. The controller  130  may determine the actual concentration level of each constituent in the original gas sample by dividing the sensed concentration level by the concentration amplification provided by the sorbent material of each interactive element and the multiplier effect provided by the phased heater arrangement. 
     FIG. 7 is a schematic view of another illustrative sensor assembly in accordance with the present invention. FIG. 8 is a timing chart showing the operation of the sensor assembly of FIG.  7 . The sensor assembly is generally shown at  150 , and may include a pump  152 , a gas preheater  154 , and a microbridge type integrated circuit chip  156 . The microbridge type integrated circuit includes a channel  158 , a number of heater elements  160   a ,  160   b ,  160   c , and  160   d , a separation heater  162 , and a detector  164 . Each of the heater elements  160   a ,  160   b ,  160   c , and  160   d , the separation heater  162 , and the detector  164  are preferably provided on a support member that extends over the channel  158  (e.g. see FIG.  2 ). Interactive elements (not explicitly shown) are placed in the channel  158  and in thermal communication with each of the heater elements  160   a ,  160   b ,  160   c , and  160   d.    
     The microbridge type integrated circuit chip  156  also preferably includes a heater control block  166  and a number of energizing transistors  168   a ,  168   b ,  168   c ,  168   d , and  170 . The heater control block  166  can individually energize each of the heater elements  160   a ,  160   b ,  160   c , and  160   d , by activating the corresponding energizing transistor  168   a ,  168   b ,  168   c ,  168   d . Likewise, the heater control block  166  can energize the separation heater  162  by turning on transistor  170 . Heating or cooling block  169  (FIG. 7) complements preheater  154  in maintaining an average or overall temperature that is optimal for operation of the sensor assembly. 
     A sensor assembly control block  180  directs the overall operation of the sensor assembly. Sensor assembly control block  180  first asserts a flow control signal  190  to pump  152 . The flow control signal  190  is shown explicitly in FIG.  8 . In response, pump  152  draws a sample from flue  182  and provides the sample, at a desired pressure, to preheater  154  and eventually to channel  158 . Preheater  154  preheats and the heater maintains the sample gas at optimal operating element temperature and thus helps to prevent loss of sample due to condensation and to increase the amount of constituents that can be accumulated in each of the interactive elements. 
     The sample fluid stream passes down channel  158  for a predetermined time period  192  until the interactive elements reach a state of substantially saturation of adsorption of one or more constituents from the sample fluid stream and reach equilibrium. Thereafter, the sensor assembly control block  180  notifies heater control block  166  to begin heating the heater elements in a time phased sequence. The heater control block  166  first provides a first heater enable signal  194  and a separation heater enable signal  196 , as better shown in FIG.  8 . The first heater enable signal  194  turns on transistor  168   a , and the separation heater enable signal  196  turns on transistor  170 . Transistor  168   a  provides current to the first heater element  160   a , causing the first heater element  160   a  to increase in temperature. This heats the corresponding interactive element, which desorbs one or more constituents into the sample fluid stream in the form of a first concentration pulse. The first concentration pulse is carried downstream toward the second heater element  160   b  by the sample fluid stream. This process is repeated for the 3rd, 4th and N-th element as follows: 
     The heater control block  166  then provides a second heater enable signal  198 , which turns on transistor  168   b . Transistor  168   b  provides current to the second heater element  160   b , causing the second heater element  160   b  to increase in temperature. This heats the corresponding interactive element, which desorbs one or more constituents into the sample fluid stream in the form of a second concentration pulse. Preferably, the heater control block  166  times the second heater enable signal  198  such that the second concentration pulse substantially overlaps the first concentration pulse. Both the first and second concentration pulses are carried downstream toward the third heater element  160   c.    
     The timing of the second heater enable signal  198  relative to the first heater enable signal  194  may be established by prior calibration. More preferably, however, the heater control block  166  senses the resistance of the second heater element  160   b . It is recognized that the resistance of the second heater element  160   b  will begin to change when the first concentration pulse arrives at the second heater element  160   b  because the first concentration pulse is typically hotter than the sample fluid stream. Once a predetermined resistance change is sensed in the second heater element  160   b , the heater control block  166  may energize the second heater element  160   b  via transistor  168   b . The remaining heater enable signals may be likewise controlled. 
     The heater control block  166  then provides a third heater enable signal  200 , which turns on transistors  168   c . Transistor  168   c  provides current to the third heater element  160   c , causing the third heater element  160   c  to increase in temperature. This heats the corresponding interactive element, which desorbs one or more constituents into the sample fluid stream in the from of a third concentration pulse. Preferably, the heater control block  166  t times the third heater enable signal  200  such that the third concentration pulse substantially overlaps the first and second concentration pulses. The first, second, and third substantially overlapping concentration pulses are carried downstream toward the “Nth” heater element  160   d.    
     The heater control block  166  then provides ant “Nth” heater enable signal  202 , which turns on transistors  168   c . Transistor  168   c  provides current to the “Nth” heater element  160   d , causing the “Nth” heater element  160   d  to increase in temperature. This heats the corresponding interactive element, which desorbs one or more constituents into the sample fluid stream in the form of an “Nth” concentration pulse. Preferably, the heater control block  166  times the “Nth” heater enable signal  202  such that the “Nth” concentration pulse substantially overlaps the previously generated concentration pulses. The resulting concentration pulse is carried downstream to the separator heater  162 . The separator heater, in conjunction with the channel  158 , separates selected constituents in the concentration pulse into individual constituent components. The individual constituent components may include one or more compounds, depending on a number of factors including the sample gas provided. 
     Transistor  170  then energizes the separation heater  162 , which separates the various constituents into individual components, as described above. The separated constituents are carried downstream to detector  164  by the sample fluid stream. The detector  164  may be a thermal conductivity detector, discharge ionization detector, or any other type of detector such as those commonly used in gas chromatography. The detector  164  preferably senses the concentration levels of each individual constituent component, and provides a corresponding signal to amplifier  210 . Amplifier  210  amplifies the detector output signal and preferably provides the detector output signal to a data processing unit for analysis. It is contemplated that the heater control block  166  may provide a detector enable signal  212  to enable the detector only when the individual constituent components are present. 
     FIG. 9 is a simplified layout of an integrated circuit that includes a concentrator, a separator, and a detector in accordance with the present invention. The integrated circuit preferably includes a channel  250  that traverses back and forth across the chip. A first part of the channel  250  has a number of heater elements  252  extending thereover on a support member, as described above. Interactive elements (not explicitly shown) are positioned in the channel  250  adjacent each of the heater elements. While only one column of heater elements  252  is shown, it is contemplated that each of the channel legs  254   a-h  may have a column of heater elements  252 . In a preferred embodiment, there are between one hundred and one thousand heater elements spaced along channel  250 . 
     A second downstream portion of the channel  250  has a separation heater  260  extending thereover. The separation heater helps separate the various constituents in the concentration pulses provided by the heater elements  252 . Finally, a detector  264  is provided over the channel  250  downstream of the separation heater  260 . The detector preferably senses the concentration of each of the separated constituent components provided by the separator. 
     Because the concentrator, separator, and detector are provided on an integrated circuit, other conventional electronic circuits can be easily integrated therewith. In the embodiment shown, a phased heater control block  270  and amplifier  272  are fabricated on the same substrate. Chemical sensors, especially chemical microsensors as described, potentially afford many attractive features such as low cost, high sensitivity, ruggedness, and (in the case of microsensors) small size. 
     Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that the teachings found herein may be applied to yet other embodiments within the scope of the claims hereto attached.