CA1249334A - Selective chemical detection by energy modulation of sensors - Google Patents

Abstract

SELECTIVE CHEMICAL DETECTION BY
ENERGY MODULATION OF SENSORS

ABSTRACT OF THE DISCLOSURE
A portable instrument for use in the field in detecting, identifying, and quantifying a component of a sampled fluid includes a sensor which chemically reacts with the component of interest or a derivative thereof, an electrical heating filament for heating the sample before it is applied to the sensor, and modulating means for continuously varying the temperature of the filament (and hence the reaction rate) between two values sufficient to produce the chemical reaction. In response to this thermal modulation, the sensor produces a modulated output signal, the modulation of which is a function of the activation energy of the chemical reaction, which activation energy is specific to the particular component of interest and its concentration. Microprocessor means compares the modulated output signal with standard responses for a plurality of components to identify and quantify the particular component of interest. In particular, the concentration of the component of interest is proportional to the amplitude of the modulated output signal, while the identifying activation output energy of the chemical interaction indicative of that component is proportional to a normalized parameter equal to the peak-to-peak amplitude divided by the height of the upper peaks above a base line signal level.

Description

SELECTIVE CHE~IC~L DETECTION BY
ENERG~ MODULATION OF SENSORS

aAC~GROUND OF THE I~VENTION
The present invention relates to analytic devices and, more particularly to devices for detecting the presence of chemicals in fluids such as air or other gaseous streams.
The device has particular application to the identification of unknown components of a fluid sample, such as to~ic, haz-ardous, or other chemicals.
There are many situations where rapid identification of a chemical component of a fluid sample is necessary. For ex-ample, in chemical processing it is frequently necessary tomonitor a product gas or vapor or effluent from a plant, in medical diagnosis, it is frequently essential to rapidly de-termine the concentration of gases in specimen samples of blood or exhaled air, in the analy~ical laboratory, one fre-633~4 quently encounters the need to measure the concentration ofone gaseous chemical in the presence of another and this situation is created in ~he gas chromatographic detector.
There are many other instances where it is necessary or de-sirable to rapidly and selectively determine the concentra-tion of gases.
It is currently possible to anatyze substances selec-tively using expensive and cumbersome analytic equipment.

For such purposes it is usually necessary to ob~ain a sample of the gas and send it to a laboratory for a remote anal-ysis. This is a costly and time consuming process.
Semi-portable versions of more powerful laboratory equipment have been commercially introduced in recent years.
But such instruments have certain inherent limitations.
Gas chromatographic devices cannot operate in a continuous real-time monitoring mode. Infrared analyzers require a del-icate optical system with a rather long absorption path, which contributes to their bulk, weight and unwieldiness.
Furthermore, such instruments must usually be operated, and their results interpreted, by well-trained professionals.
~ any existing sensors are incapable of detecting chem-ical components in low concentrations, particularly where the component is substantially non-reactive. In issued sritish Patent Nos. 2,155,184 and 2,184,244, there is disclosed a sensor which catalytically reacts the 33~

component of interest to produce a chemically active deriv-ative product which can be readily sensed. But that device, and most semi-portable or field-usable devices are not selec-tive, but are rather specifically designed for detection of a particular chemical component, and are not designed to both detect and identify an unknown component.
Detectors have been developed utilizing an array of electrochemical sensors, each operated in one or more prede-termined modes or conditions, the co!lective responses being analyzed to provide identification of one of a number of gas-es. Such detectors are disclosed in United States Patent No.
4,670,405, issued January 20, 1987. But such de~ices are still capable of identifying only a relatively few compo-nents, unless a large number of sensors are used, thereby rendering the device more expensive and complicated and less suitable for portable field use.
Virtually all prior detection devices utilize a sensor which produces a steady-state output signal which changes when the chemical/physical environment changes. The design goal of such devices is generally to eliminate sensitivity to all but one environmental parameter or chemical, thus pro-ducing a useful monitoring and measurement tool ~or that pa-rameter or chemical. But the practical achievement of this goal is extremely difficult. For example, pressure t.ansduc-ers may be susceptible to temperature variations and methane sensors tend to respond to most hydrocarbons. Expensive measures must frequently be taken to minimize this cross sensitivity.
Typical chemical sensors are defined as devices that change an output characteristic (e.g., current, voltage, absorbence, resistance, fluorescence, size, etc.) when ex-posed to the chemical of interest. The change in response is usually examined at equilibrium or steady-state and the magnitude of the response is related to the concentration.
Great care is taken tO make sure that the device is designed so that the response only occurs when the chemical of inter-est is present. This steady-state approach does not provide sufficient data to resolve hundreds or perhaps thousands of chemicals that may be present in a sample using a single sen-sor, instrument, or sensor array.
U.S. Patent No. 4,399,6~4 discloses a gas measuring method wherein a metal-oxide gas sensor is sequentially heat-ed and cooled during exposure to a sample gas. The patent discloses that during such thermal cycling a continuous, con-

2~ centration-dependent, unique signature for different gas con-centrations is produced. This signature comprises a ratio of two samples of the sensor output signal taken at predeter-mined times during the thernal cycle. This signature yields sufficient information to identify the gas concentration by comparison to standard signatures for known concentrations.

~3 But because the signatures developed are concentration-dependent, they cannot be used to identify an unknown compo-nent of the sample. The patent implies that the method disclosed may be used to identify an unknown gas, but it gives no explanation of how such an identification could be effected.
S~RY OF THE INVENTION
It is general object of the present invention to pro-vide an improved detection device and method which avoids the disadvantages of prior devices and methods, while afford-ing additional structural and operating advantages.
~ n important object of the invention is the provision of a method of identifying an unknown component of a fluid sample, which method is uniquely suitable for field application.
In connection with the foregoing object, another object of the invention is the provision of a method of determining ~he concentration of the identified chemical.
~ nother object of the invention is the provision of a ~9 method of the type set forth which is capable of identifying a large number of chemical components by the use of a single sensing means.
In connection with the foregoing objects, it is another object of the invention to provide a method of the type set forth which utilizes a sensor in a dynamic mode to determine a dynamic or chemical reaction related parameter.
Another object of the invention is the provision of a detection device which incorporates the method of the forego-ing objects.
Yet another object of the invention is the provision of such a detection device which operates by energy modulation of the sensing means.

Another object of the invention is the provision of a sensing device of the type set forth, which is of simple and economical construction and characterized by small, compact size.
Still another object of the invention is the provision of a detection method and apparatus of the type set forth which affords rapid and selective determination of the iden-tity and concentration of chemical components.
It is another object of the invention to provide a de-tection method and device of the type set forth, which per-mits detection of very low concentration levels of chemical components.
These and other objects of the invention are attained by providing a method for identifying a component of a fluid sample, comprising the steps of: exposing the fluid sample to a sensing unit have an energy input and adapted for inter-action with the component to produce a response, the in-_7~ 3 ~ ~

teraction having a parameter which varies with the interactingcomponent, modulating the energy input to produce a modulated response proportional to the parameter, and measuring the parameter from the modulation of the response characteristic for identifying the component. In other words, the incoming chemical is reacted, the extent of the reaction being modulated in a cyclical or other-wise regular pattern and this extent of reaction is followed by the sensing means. The information produced in this modulated record is sufficient to provide the identity and concentration of the chemical entering this sensing means.
In a further embodiment, the invention provides an instrument for identifying a component of a fluid sample which comprises a sensing means having an energy input and adapted for interaction with the component to produce a response, with the interaction having a parameter which varies with the interacting component, a means for introducing the fluid sample to the sensing means, a means for modulating the energy input to produce a modulated response proportional to the parameter, and a means for processing the modulated response to measure the parameter for identifying and/or quantifying the component.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
DESCRIPTION OF THE FIGURES
For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawings -8~

a preferred embodiment thereo~, from an inspection of which, when considered in connection with the following de~crip-tion, the invention, its construction and operation, and many of its advantages should be readily understood and appreciated.
FIG. I is a block diagram of a detection apparatus con-structed in accorda~ce with and embodying the features of the present invention;
FIG. 2 is a further block diagram of the sensing appara-tus, showing a particular type of sensing unit and modulator in greater detail;
FIG. 3A is a graph of the modulation of the heating el-ement temperature;
FIG. 3a is a graph of the modulated output from the sensing unit, on the same time base as FIG. 3A when the in-put gas is 200 ppm cyclohexane in air; and FIG. 4 is a graph of the parameter h/a vs. pseudo-activation energy for formation of electrochemically active compounds on a Rh filament~ illustrating .he characteristic pseudo-activation energy for a number of different chemicals.
DESCRIPTION OF THE PREFERRED E~eODl~ T
In its broadest aspect, the present invention involves the generation of a large amount of information or data about a sample medium with the use of a single sensing appa-ratus by the technique of modulating the sensor signals.
~ore particularly, the invention resides in energy modula-~2~
g tion of an interaction between the sensor and the components to be detected, thereby producing a modulated output signal from the detector, and using this modulation information to derive a parameter (related to the kinetic or thermodynamic characteristics of a chemical or chemical reaction) which pa-rameter can be used to determine the identification and con-centration of the component of interest. While it is possible to use a number of different types of sensors and modulation means for ascertaining different parameters spe-cific to a chemical of interest, the preferred embodiment de-scribed below utilizes thermal modulation of anelectrochemical sensor signal for determining a kinetic pa-rameter representative of the "activation energy" of a chem-ical reaction with air for the chemical being detected, identified, and quantified by the sensor system (modulator and sensor).
Referring to FIGS. I and 2, there is illustrated a de-tector, generally designated by the numeral 10, constructed in accordance with and embodying the features of the present invention. The detector 10 includes a gas mixer 11, an air inlet 12 and another inlet which is coupled through a suit-able valve to either an air inlet 13 or a sample gas inlet 14. By this arrangement, either ambient air containing chem-ical components to be detected can be emitted directlv to the gas mixer 11, or laboratory samples to be identified can -- l o--be mixed in the gas mixer 11 with air to a desired con-centration range prior to analysis by the modulator/sensor.
It will be appreciated that the gas mixer 11 is optional and that if desired ambient air or other source of a sample to be detected could be coupled directly to the remainder of the detector 10.
The gas mixer 11 has an outlet 15 which is coupled to the inlet of a sensing unit 20. More particularly7 the sens-ing unit 20 includes an electrochemical sensor 21 coupled to a potentiostat 22 for regulating electrode potentials andperforming electro-oxidation or electro-reduction of the chem-icals that enter the sensor. The sensing unit 20 also in-cludes a heating filament 23 for heating the gas sample before it is admitted to the electrochemical sensor 21. The filament 23 serves not only as a heater, but preferably also acts as a catalytic or chemical reactor. The filament 23 may be of a suitable material, e.g. noble metals like Pt, Pd, Rh, Au, Ir, or other catalyst, depending upon the types of chemical components to be detected, and the particular electrochemical sensor 21 being used. In an experimental model of the invention, the filament 23 is formed of a noble metal, such as Rh, but it will be appreciated that non-noble ~etal catalysts such as tungsten or molybdenum could also be used. Further, any "microcatalytic" reactor capable of

3~

producing repeatable and rapid ~e.g. more rapid than sensor response) modulation can be used.
The fi!ament 23 is coupled to the modulator 30 which in-cludes a power supply 31, a function generator 32 and a cur-rent amplifier 33. ~lore particularly, the power supply 31 is coupled to both the function generator 32 and ~he current amplifier 33. The function generator 32 produces an output signal of predetermined waveform, such as a sawtooth wave, which is applied through the current amplifier 33 to one ter-minal of the filament 23. The other terminal of the fil-ament 23 is connected through an ammeter 34 to the power supply 31. A voltmeter 35 may be connected across the termi-nals of the filament 23. It will be appreciated that the current through the filament 23 and therefore the temper-ature thereof, is modulated by the output signal from the function generator 32, as will be explained more fully below.
The sample gas exits the electrochemical sensor 21 and ?asses through a flow meter 36 and a pump 37 to a suitable venting hood (not shown) or the like. This provides safe discharge of any chemicals that may be toxic or hazardous.
The electrochemical sensor 21 produces an electrical output signal which is produced by the potentiostat 22 and rea~ by an elect onic processor 40, which may include a mi-croprocessor circuit. Preferably, the processor 40 includes 3~
( -12-a comparator 4i which receives the output signal from the sensing unit 20, and which is also coupled to a suitable memory 42, such as a semiconductor memory. Standard response parameters for a plurality of different chemical components are stored in the memory 42. The modulation of the filament 23 causes a correspondin~ modulation of the output signal from the electrochemical sensor 21 to produce a characteristic output response parameter. This response parameter is compared in the comparator 41 with the standard response parameters stored in the memory 42, and if a match is detected a suitable indication of the identity and concentration of the detected chemical component is pro-duced in an indicator 43, which may be of any desired type.
For example, the indicator 43 may produce a readout on a dig-ital display, such as a CRT or other type of display.
Referring now also to FIGS. 3A and 3B, the operation of the detector 10 will be explained by way of example, in con-nection with the detection of cyclohexane. For this pur-pose, the electrochemical sensor 21 is a CO sensor, and the filament 23 is a Rh filament. ~ir contaminated with 230 ppm cyclohexane is passed over the filament 23 and thence to the sensor 21. Preferably, the modulator 30 is capable of vary-ing the temperature of the filament 23 between ambient and about 1500 C, but the actual range of variation will be de-termined by the output signal frorn the function generator 32.
The filarrlent 23 produces a pyrolysis reaction of cyclohexane in accordance with the react iOIl cyclohexane + air (20% oxy~en~ = CO + products.
At low temperatures, e.g., less than about 200 C, little or no pyrolysis of cyclohexane occurs, i.e., the reaction rate is very low at this temperature and, therefore, the electrochemical sensor 21 reads zero. But as the temper-ature is raised, this reaction begins to proceed at an appre-ciable rate, and the sensor 21 responds to the increase in CO concentration.
The usual kinetic expression f~r the rate of CO produc-tion is d[CO~/dt - r [cyclohexane] [air] [C]
where [C] is the concentration of catalvst, usually taken to the first power, and r is the rate constant. The concentra-tion of air or cyclohexane can be taken to any power. The rate constant can be written A -E/kT
where A is a pre-exponential factor, T is the absolute tem-perature, k is Boltzmann's constant, and E ;s the activation energy for the reaction.
In this case, the function generator 32 produces a saw-tooth output waveform, which results in a sawtooth 3~33~

modulation of the filament temperature in accordance with the waveform 50 in FIG. 3A, the temperature undergoing one complete cycle in about 40 seconds. The temperature cycles between a low point 51 of about 600 C and a high point ~2 of about 1000 C. This modulation of the filament temper-ature continuously varies the rate of CO production to pro-duce a modulated output signal from the sensor 21, indicated by the waveform 60 in FIG. 3B. Line 61 in FIG. 3B desig-nates the background or base line level, i.e., the out?ut produced by the sensor 21 in response to pure air, the actu-al pure air response signal being indicated by a portion 62 of the waveform. As the gas sample bearing -he cyclohexane contaminant is admitted to the sensor 21 its response builds up and approaches a steady state level indicated by the right-hand portion of the waveform 60. As can be seen, this response is a modulated signal 63 which varies between upper peaks 64 and lower peaks 65. The peak-to-peak amplitude of the signal 63 is a - b, where a is the distance between the base line 61 and the upper peak 64, and b is the distance be-0 tween the base line 61 and the lower peak 65.lt can be seen from the kinetic expression for the rate of GO production, above, that the rate of CO production and, therefore, the sensor output signal, will be proportional to the cyclohexane concentration if the concentration of air and catalyst are held virtually constant. Also, it can be seen that a concentration-independent parameter is the rate of CO production divided by the cyclohexane concentration, which is d[CO]/[cyclohexane]dt = r [air] [C]
and is a constant at constant concentration and temperature.
Thus, from the aforementioned expression for the rate con-stant r, it can be seen that the temperature change in the filament produces a changing CO concentration that is deter-mined by the pre-ex?onential factor ~ and the activation en-ergy E. This reaction rate constant r is very specific forchemical reactions. Thus, the thermally modulated CO concen-tration divided by the cyclohexane concentration is propor-tional to the activation energy characteristic for the pro-duction of CO from cyclvhexane over a heated Rh filament. se-cause the CO concentration divided by the cyclohexane concentra-tion is independent of the cyclohexane concentration, this infor-mation can be used to identify the contaminant as cyclohexane.
This information is expressed by the normalized param-eter h/a, where h - a - bJ i.e., the peak-to-peak amplitude of the waveform 60 divided by the magnitude or height of the upper peaks 64. The magnitude of the height of the upper peaks 64 is found to be proportional to the cyclohexane con-centration. It has also been found that the parameter h/a is proportional to a pseudo-activation energy in kcal/mol for a number of chemical components studied, including ammonia, acrylonitrile, cyclohexane, methane, toluene and benzene, as illustrated in FIG. 4. It has also been found that the peak-to-peak amplitude h of the signal response waveform 60, as well as the height "a" of the upper peaks 64, is proportional to the concentration of the chemical component being detected.
Thus, the processor 40 operates on the output waveform 60 from the sensing unit 20 to determine the quantity h and the parameter hJa, and compares this parameter with standard parameters (i.e. the pseudo-activation energies) stored in the memory 42 to identify the contaminant as cyclohexane and to register the concentration thereof.
In the preferred embodiment just described, the catalyt-~
ic surface of the filament 23 is separate from the electro-chemical sensor 21. But it will be appreciated that the principles of the present invention could also be utilized in a reaction scheme wherein the temperature of a semi-conductor sensor is modulated to produce a-cata!vtic reac-tion, and then the same surface is used as the gas detector.
`~hile, in the preferred embodiment, thermal modulation of an electrochemical C~ sensor has been described for de-tecting hydrocarbons, it will be appreciated ~that the principles of the present invention apply to other types of sensors and other types of modulation of other types of in-~eractions. Thus, for example, benzene could be detected by ~2~

modulating the photon energy input to a photoionization de-tector for measuring the ionization potential of the interac-tion. Infrared radiation input to a thermopile detector could be modulated to measure the infrared absorption coeffi-cient for detecting chemicals which are strong infrared ab-sorbers, such as methane. Similarly, thefmal energy input to a thermionic ionization detector could be modulated to measure ionization potential. Another alternative would be the modulation of a chemical reagent, e.g., ozone, in a chemi-luminescence detector for measuring related kinetic pa-rameters, e.g., rate order. Such a technique might be use-ful in detecting nitric oxide, for example. Another technique could involve the use of magnetic field modulation with a microwave detector for measuring magnetic energy lev-els of electrons with unpaired spins, which technique could be used for detecting odd molecules with unpaired electrons.
ln general, all that is necessary is to provide a means (e.g. energy input) to chemically or otherwise modulate the interaction of the chemical to be detected in the sample and ~hen a means to detect the modulated signal. Then one is able to determine the specific kinetic or thermodynamic pa-rameters that describe the situation and this provides the selective information desired to identify and quantify the chemical of interest.

L~ 3 A significant aspect of the invention is that it pro-vides selective identification of a large number of chemical components utilizing a detector with a minimal number of parts, resulting in a detector with wide application which can be conveniently miniaturized for portability and field use. Furthermore, it will be appreciated that the detector of the present invention can be designed to produce unambig-uous output indications so that it can be used by non-skilled personnel.
While the present invention has been described in terms of operation wi~h gaseous samples, it will be appreciated that the principles of the invention could also be applied to analysis of chemicals in a liquid medium.

Claims (20)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for identifying a component of a fluid sam-ple, comprising the steps of: exposing the fluid sample to a sensing unit having an energy input and adapted for interac-tion with said component to produce a response, said interac-tion having a parameter which varies with the interacting component, modulating the energy input to produce a modulat-ed response proportional to said parameter, and measuring said parameter from the modulation of the response for iden-tifying and/or quantifying said component.

2. The method of claim 1, wherein said fluid sample is a gas or vapor.

3. The method of claim 1, wherein said sensing unit is electrochemical.

4. The method of claim 3, wherein the modulating step includes the step of modulating an operational condition of said interaction.

5. The method of claim 4, wherein the operational con-dition modulated is temperature.

6. The method of claim 5, wherein the temperature is continually varied between two temperatures sufficient to produce a significant interaction with said component.

7. The method of claim 6, wherein said temperature is varied between 600° C and 1000° C.

8. The method of claim 1, wherein said sensing unit is electrochemical and the energy input is thermal, said param-eter being proportional to the activation energy of the chem-ical reaction of said sensing unit with said component.

9. An instrument for identifying a component of a flu-id sample, said instrument comprising: sensing means having an energy input and adapted for interaction with said compo-nent to produce a response, said interaction having a param-eter which varies with the interacting component, means for introducing the fluid sample to said sensing means, means for modulating said energy input to produce a modulated re-sponse proportional to said parameter, and means for process-ing the modulated response to measure said parameter for idenifying and/or quantifying said component.

10. The instrument of claim 9, wherein said modulating means includes means for modulating the temperature of said interaction.

11. The instrument of claim 10, wherein said sensing means includes a heating filament of Pt, Rh, Ir, Au, or Pd or an alloy thereof disposed for exposure to the fluid sam-ple, said modulating means including control means coupled to said heating filament for continuously varying the temper-ature thereof.

12. The instrument of claim 9, wherein said sensing means includes an electrochemical sensor and said energy in-put is thermal for producing a chemical reaction with said component.

13. The instrument of claim 12, wherein said kinetic parameter is proportional to the activation energy of said chemical reaction.

14. The instrument of claim 13, wherein said electro-chemical sensor responds to a derivative product of said chemical reaction.

15. The instrument of claim 9, wherein said processing means includes a microprocessor.

16. The instrument of claim 15, wherein said process-ing means including means for providing a plurality of stan-dard responses including a standard response for identifying said component, and means for comparing the produced re-sponse with at least one standard response to identify and quantify said component.

17. An instrument for identifying a component of a flu-id sample and its concentration, said instrument comprising:
sensing means having an energy input and adapted for chem-ical reaction with said component or a derivative thereof to produce a response, said chemical reaction having a param-eter which varies with the concentration of said component or a derivative thereof, means for introducing the fluid sam-ple to said sensing means, means for modulating said energy input to produce a modulated response having an oscillating waveform proportional to said parameter, and means for pro-cessing the oscillating waveform to measure said parameter for identifying said component and determining the concentra-tion thereof.

18. The instrument of claim 17, wherein said waveform oscillates between upper and lower peaks with respect to a base line, said waveform having a normalized parameter (a -b)/a which is indicative of said component wherein a is the distance from the base line to the upper peaks of the wave-form and b is the distance between the base line and the low-er peaks of the waveform.

19. The instrument of claim 18, wherein the quantity a is proportional to the concentration of said component.

20. The instrument of claim 17, wherein said modulat-ing means includes means for varying the temperature of said chemical reaction.