CA1219910A - Method of and system for real time differential pulse detection (rdpd) - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An electrochemical detection cell which includes, in the order named, a stack of components constituted by a first membrane, an electrode carrier having reference and counter electrodes thereon, a membrane support, an electrode carrier having a sensing electrode thereon, and a permselective membrane Whatman filter papers are positioned between the sensing electrode and both the counter electrode and reference electrode, these being positionable in a central rectangular opening of the membrane support. The cell allows air or gas ambient to address the membranes and effects application of an electro-lyte to space between the electrodes by, for example, a wick which extends between an electrolyte-containing reservoir and the Whatman filter papers. A system for detecting a gaseous agent uses the aforementioned detection cell coupled with circuitry capable of applying a fixed DC voltage bias to the reference electrode and superimposing a train of DC
voltage pulses on the fixed bias. Further circuitry is coupled to and responsive to signals from the sensing electrode.
This circuitry determines the difference between the sensing electrode signal at the end of each pulse minus that just before each pulse. The difference is proportional to the concentration of the test gas in the atmosphere adjacent to the sensor, and is a more specific and sensitive indication of the test gas concentration than prior art sensors.

Description

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MET~OD OF AND SYSTEM FOR REAL TIME
DIFFE~ENTIAL PULSE DETECTION
(RDPD) BACRGROUND OF T~E INVENTION

The invention relates to a method o~ gas detection using real time differential pulse detection (nRDPDn), to an electronic sygtem for car~ying ou- ~he method, anZ to an electrochemical detection cell which can be used in the system~ ~e~l time differential pulse 10 dete~tion (RDPD) is a modification of differential pulse voltam~atry (nDPVn).
Rnown analytical systems using conventional voltammet~y maintain an accura~e potential between the sensing and reference elec~od~s of an electrochemical 15 sensing cell w~ich may include, in addition to the sensing and ref~rence electrodesl a counter eleckrode.
Examples of prior art electrochemical detection cells, which include a sensing electrode, a reference electrode and a coun~er elec~rode are illustrated in 20 respective U.S~ Letters Patents Nos. 3,776,832 and 3,925,183 to Oswin et al~ en~itled respectively "Elec-trochemica~ De~ection Cell" and "Gas Detecting and Quantita~ive Measuring Device" and is~uad respectively ~s~æ

Dec~mber 4, 1~73 and December 9, 1975. The electrodes are of a type which musk be operated in a con~en~ional - voltammetry mode and r consequently, one cannot realize the advantages of RDPD techniques.
Another example of a known electrochemical detec tion cell, which includes an anode, a cathode and a referPnce elec~rode is disclosed in U.S. Letter~ Patent No. 4,201,634 to Stetter en~i~led ~Method of Detection of Hydrazine" and issued May 6, l980. In this l0 instance, the sensing or working electrode includes a noble metal catalyst capable of electrooxidation of hydrazine bonded to a hydrophobic material. The electrodes are so configuxed that detection c lls constructed as proposed have the same shor~comings as 15 those disclo-~ed in the ~etters Patents to Oswin et al., supra; electi~ity and sensitivity is limit~d.
Addi~ional examples of ele~roch2mical det~ction cells are disclosed and illustrated in the U.S~ Letters Patents identified as follow~:
20 No. 4,040,805 Nelms et al. August 9, 1977 No. 4,04B,041 Da~id et ~l. September 13, 1977 No. 4,227,984 Dempsey et al~ October 14, 1980 No. 4,235,097 Rring et al. ~ovember 25, 1980 No. 4,271,l2l Diller et al. June 2, l98l, A detailed discussion of the theory, techniques and waveforms used in conventional differential pulse voltammetry as used in laboratories can be found in Sawyer et al., Experimental Electrochemistry for Chemists, pages 280, 389-394, John Wiley & Sons, New 30 york, ~ew York [l974) and in Skoog et al, Fundamentals of Analytical Chemistry, 3rd Edition, pages 492-494, Sanders College, Philadelphia, ~ennsylvania (1976). In this known technique, a linearly increasing D.C.

voltage ramp is imposed on th~ sensing electrod , with respect to a xeference electrode, and a small D.C.
pulse is superimposed on the ramp. The di~ference in current at the end o~ the pulse and be~ore the pulse is 5 utili~ed to determine the D.C. level at which a partic-ular electroactive species, if present, can be detected.
Of interest as general background are a number of publications identified as follows:
Oldham et al./ ~ , Vol. 38, No. 7, June 1966, pages 867-872; Klein et al., ~Improved Differential Puls~ Polarography", Electro-anal~tical Chemistry and Interfacial Electrochemistry, Vol. 61, No. 1, May 10, 1975, pages 1-9. Christie et 15 al., ~Cons~an~ Potential Pulse Polarography", nalytical Chemls~ry, Vol. 48, No. 3~ March 1976, pages 561-564. Parry et al., "E~aluation of Analytical Pulse Polarography~, ~nalytical Chemistr~, Vol. 37, No. 13 December 1965, pages 1634-1637; and 5chmidt et al., 20 Modern Polarographic Metho~s, Academic Press, New York, New York (1963~o A number of ~lectronic systems ha~e been developed and disclosed which effect processing of signals developed from elec~rochemical detection cells, i~cluding the Letters Patent to David et ~l., cupxa.
25 Examples of such syst~ms are shown and discussed in U.S. Letters Patents identified as follows:
No. 3,420,764 Schlein Janua~y 7, 1969 No. 4,253,847 Matson et al. March 3~ 1981 No. 4,321,322 Ahnell March 23, 1982.
The known methods, techniques and systems may be characterized as providing a limited specificity and sensiti~ity.

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S MMARY OF THE INVENTIO

An object o~ the present invention is to provide a method of gas d~tection using real time differential pulse detec~ion (RDPD~ which ha~ improved speci~icity 5 and s2nsitivity as compared to con~en~ional voltammetry.
An additional o~ject of the pxesent invention is to provide an electronic system especially useful for carrying cut the method.
Another object o~ ~he present invention is to provide an electrochemical detec~ion cell which can readily be associa~ed with and form part of the electronic system.
A ~urther objact is to provide an electronic 15 syste~ for detecting gas which can easily be carried abou~ by an individual and can work in various orien tations.
The improvements in specificity and sensitivity of the present invention in its variou aspects are a 20 result of two modifications o E the prior art. First, the bias o the sensing electrode of a sensor is periodically pulsed, ovexcomlng the mass transfer limitation3 which lLmited the sen~itivity of prior art systems. Second, differen~ial signal processing is 25 used, rather than the D.C. processing used in prior systems, r~sulting in importan~ specificity improva-ments. A major aspect of the present invention includes modification of the configurations o:E prior art sensors so they are compatib~e with pulsed 3n electronics.
In its sy~tem aspect, ~hese objectives are achieved in accordance with the present invention by ~z~

providing a sys-tem which include~ a con~uctive connec~
tion between a counter electrode of an electrochem~cal detection cell and the output terminal o~ a first operational amplifiex, which has its invertiny (-) 5 input terminal conductively connected to the reerence electrode of the cell. The inverting input terminal and the output terminal of the first operational amplifier may be capacitively connected to ensure amplifier stability. The other, a non-inver~ing ~+) 10 input terminal of the first operational amplifier is connected to a dynamic biasing networ~ which provides an additive or ~ubtractive D~C. pulse train, fed via a first analog switch, that is superimposed on the ixed background~bias D.C. voltage~ The dynamic 15 biasing network includes adjustable potentiometers or voltage dividers, active circuit components and a first analog switch. The working or sensing elec~rode of the electrochemical de~ection cell is ~oupled, via a resis~or, to the inverting (-) input terminal of a 20 second operational ~mplifier, its o~her, non-inverting (~3 input terminal being connec~ed to a point of reference potential~ The output terminal o the second operational amplifier is coupled to a buffered sample-a~d-hold circuit, which i~cludes a storage 25 capacitor and an analog switch, via a processing circuit. The processing circuit includes second and third analog switches conductively connected to respective storage capacitors which are coupled ~o respec~ive inputs of a fuxther opera~ional amplifier, 30 one via another operational amplifier. The ~our analog switches are operatively associated with a synchronized control circuit which opens and closes them in a predetermined sequence. As a result the further ~L99~

operational am~lifier, which acts as a subtracting amplifier, produces a series o~ output pulses which are stored via a sample-and-hold buffering stage having its output coupled to an alarm circuit via a thre~hold 5 comparator. Fur~her data proce~sing analog and/or digital may be provided from here.
In its apparatus (detection cell) aspect, the objects are achieved by providing an electrochemical detection cell which includes, in the order named, a 10 stack of components constituted by a first membrane, an electrode carrier having reference and counter elec-trodes thereon, a membrane support, an electrode carrier having a sensing electrode thereon, and a permselective membrane. Wha~man filter papers or 15 layers o~ another inert hydrophilic insulator are positioned between the Rensing electrode and both the counter and reference elec~rodes, these being position-able in a central rec~angular opening of the membrane support. These components are operatively associated 20 with structures which allow air or gas ambient to address the membranes, effect the application of an electrolyte to space between the electrodes by, for example, a wic~ which extends between an electrolyte containing reservoir and the Whatman ilter paper-~.
In its method aspect, the objects are achieved by providing an electrochemical deteckion cell having a sensing electrode, a counter electrode and a reference electrode, applying a fixed D.C. voltage bias to the reference electrode, superimposing at leas one 30 differential ~.C. voltage pulse on the selected fixed D.C. voltage applied to the reference electrode, and determining difference bPtween signals produced from the sensing electrode during a period before 9~

superimposiny the di~erential D.C. voltage pulse and in a period before terminakion of the cli~erential D.C.
voltage pulse, while subjecting the sensing electrode to an environment in which the gas to be de~ected may 5 be pre~ent.
The step of superimposing at least one differen-tial D.C. voltage pulse train on the fixed D.C. voltage bias involve~, in a prefarred realization of the method, superimposing a fixst train of differential 10 D.C. voltage pulses thereon, supplying a second train of pulses a~d a third train of pulses both synchronized with the first train of pulses and having ~he same ~e~-f ;f ,'cr, rate, and using respectively the second and the third train of pulses to control the sampling of 15 signals from ~he sensing elec~rode in periods before start and beore termina~ion of each differential D.C.
pulse of the ~irst train, and detexmining the difference between the signals produced during ~he periods b2fore ~he respective diferen~ial D.C. voltage 20 pulses and those produced during the periods before termlnation of the respective d.ifferential D.C. voltage pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. lA is a graphical represantation of potential 25 applied to a sensing electrode of a detecting cell plotted against time, illustrating conventional volt~m-met~y as practiced in a laboratory.
Fig. lB is a gxaphical representative of siynal currents plotted against sensing electrode potential of 30 a detecting cell, illustrating conventional voltammetry as practiced in a laboratory.

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Fig. 2A is a graphical representation o~ the voltage bias applied to a sensing electrode o~ a detecting cell and current measuring periods plotted against time, .illustrating diferenkial pu1~8 vol~arn-5 metry again as practiced in a laboratory.
Fig. 2B is a graphical repre entation of signal currents plotted against se~sing electrode potential of a detecting cell, illustrating di~ferential pulse voltammetry as practiced in a laboratory.
Fig. 3 is a chart derived ~xom the electromotive series of elements indicating for exem~lary redox couples theoxetical rela~ive electrode potentials determing whether a couple will undergo an oxidation or a reduc ion reaction.
Fig. 4 is a pictorial view of a disposable module which i~corporates two gas detecting electrochemical detection cells t one being shown exploded, constructed in accordance with the present inven~ion and constitut-ing the sensor of same in its system aspect.
Fig. S is a graphical representation of an exemplary voltage bias applied to a sensing electrode of a detecti~g cell plot~ed against ~ime, illus~rating ~eatures o gas detection method using real time differential pulse detection in accordance with the 25 prese~t invention in its method aspect.
Fig. 6 i5 a pictorial vi w of the disposable module illustxated in Fig. 4 in association with a casing which also houses an alarm unit.
Figs. 7A-7C axe graphical representations respec~
30 tively of sensing electrode bias, background current and faradaic current which is to flow in the sensor shown in Fig~ 4 during opera~ion ~hereof.

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Figs. 8~-8C are graphical reprPsenta~ions respsc-tively of sensing electrode bias, standard prior art sensor electxode bac~ground current and background current in ~he sensor shown in ~.ig. 4.
Fig. 9 is a simplified, schematic, block diayram of an electroaic system for detecting gas using real time, differential pulse de~ection in accordance with an exemplary embodiment o~ the present invention in its system aspect.
Fig. 10 is constituted by a series of graphical representations of waveforms at various points in the electronic system shown in Fig. 9, these - represen~a~ions being helpful in understanding the operation thereofO

Beore turning to the details of the present invention as illu~trated, a few brief remarks ar~ in order, reference being made to various o~ the drawing figures t to provide an understanding of the background ~0 of the invention i~ its sys~em, apparatus and method aspects.
Real ~ime differential pulse detection according to the pres~nt invention is a modif ication of differential pulse voltammetry (DPV3. Both specificity 25 improvements and sensitivity gains on the order of 1,000 times have been realized in analytical instruments where conve~tional vol~ammetry has been replaced wl~h DPV for reasons mentioned below.
Analytical instruments u~ilizing conventional 30 voltammetry maintain an accurate potent.ial between the sensing and reference electrodes of an electrochemical -- --3~

sensing cell. The potential is increas~d linearly over time, as illustrated by line V in Fig. lA. The current I, which is proportional to the concentration of any species reacting at the potential on the qensing S elec~rode, is ~hen plotted vs. the potential of the sensing electrode, as illustrated in Fig. lB. Minia ture detectors~ which are sometimes carried by indi-viduals, utilizing conventional voltammetry cannot scan the electrode bias due to the extremely large 10 capacitive b ckgxound currents that would d velop at the high surface area gas sensing electrodes when its bias is scanned. Instead, the sensing electrode bias is set at a fixed potential slightly ~bove ~hat where the gas to be detected undergoe a redox reac~ion. The 15 sensing electro~e of a detec~or designed to detect gas P, as illustrated in Fig. lB, would be set at E2 The current a~ this point, minus ~he bacXground, is propor~ional to the conce~ratio~ of gas B in the airO
The major drawback of this prior art technique is that 20 all gases oxidized balow E2~ that is a~ E3 and E4 for example, will act as interferents. For exam~le, gas A
will act as an interferent to gas B in this example.
If gas ~ i~ a typical gas ~o be detected, there are a ~ignificant number of gasses which ac~ like A in mos~
25 environments.
In prior art laboratory differentlal puls~ voltam-metry, as illustrated in Fig. 2A, a series of small pulses P are superim~osed on a linaarly increaslng voltage ramp R. The current is measured just before 30 the individual pulses and just before the end of the individual pulses as illustrated, respectively, as i~
and ip. The DPV signal is the difference between the average current at the end of a pulse P and that before ~2~
~ 11 --the pulse. ~herefor~, a curren~ vs voltage plot fox a DPV experiment yields a series of peaks in~tead o plateaus, as illustrated by the dashed lines in Fig. 2B. The peaks roughly coincide with the hal~ wave S potentials of conventional voltammetry. ~he specificity advantage of DPV becomes obvious when one ex~mines Fig. 2B. The signal current at E2 is proportional only to the background and gas ~ in the DPV mode. Gas A does not interfera since it is 10 included in both the current before and at the e~d of the pulse. The signal current at E3 has, in e~fect, been cancelled. The solid curves A and B, and the dashed curves A an~ B, in Fig. ~B represent respec-tively co~ventional and known DPV signals. The 15 improvement in sensitivity is due to decreased concentration gradients at the ~lectrode surface. The ~.
co~2ntration gradient of the elec~roactive species at the electrode surface i5 th~ primary phenomenon lLmiting the sensitivity of most conventional 20 voltammetry experiments. The concentration gradient is reduced i~ DPV, since the elec~roactive species is only deple~ed 20-300 milliseconds per second in a typical application.
The presen~ invention involves RDPD, whioh in some 25 respects is s~milar to DPV. The major difference bet~een the two are that (1~ the pulses are superimposed on a fixed bias in RDPD instead of a linearly increa ing vol~age ramp, and (2) the RDPD
signals are continuously sampled, held, and updated.
30 Real time signal monitoring is accomplished since the update process occurs af~er each pulse. The fixed bias would be set at a small voltage, typically from about 5m~ to about lOOmv, below E2 in Fig. 2B in a RDPD

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detector designed to detect gas B. A pulse o~ from about 10 msec. to about 200 me3~;, ~or example, a 20 msec. pulse of approximately 20mv would be, superimposed periodically on the continuous ~ixed bias, S in accordance with the present invention. The RDPD
signal would be qual to the difference between the average current during the last 2 to 50 msec. of th~
pulse minus the average curren~ during the last 2 to 50 ~se. before the pulse. Detectors based on RDPD
10 display most of the sensitivity and specificity advantage~ of analytical DPV instru~ents. In addition, the background signal deviation associated with temperature fluctuations of detectors based on conventional voltamme~ry are eliminated, since both 15 sampled currents io and ip will vary equally wi~h tempera~ure.
Re~erring to Fig 3.~ which is known from the Let~ers Patent ~o. 3,776,~32 to Oswin et al., supra, it is obvious that if one were ~o apply a po~ential of 20 1.0V vs. the standard hydrogPn alectrode (SHE) to a voLtametric cell in order to detect NO, C2H5O~ and CO
may also be oxidized and a¢~ as interferen~s. In practicing the present i~vention~ how~ver, one would apply a fixed bia~ between the sznsing and re~erence 25 electrodes, for example tO .90V as shown in FigO 5, slightly lower than potential of the redox coupla to be detected. A small ~ddi~ional pulse of approximately .20V would be superimposed on the fixed potential such that the sum of the fixed potential and pulse would be 30 greater than the half wave potential of the species to be detected (including overpo~en~ial). The current would be measured before each pulse and toward the end of each pulse as represented by io and ip, in Fig. 5.

.. . .. . . .

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~ 13 -The output signal would be the difer~nce b2tween the wo measures or ip-io.
Since msst interferen~s are oxidized equally at the fixed background bias B and the bias B ~ P, most 5 interferents would not be included in the siynal (io-ip). Only substances oxidized between 0.90~ and 1.10V would be oxidized if a pulse were applied. The sensitivity would also be increased through the utili-zation of the RDPD technique.
A disposable module is illustrated in Fig. 4, the module including a pair of electrochemical detPction cells constructed in accordance with the present inve~tion. The two cells are operationally associated with a molded cell housing 10 made of a plastic, one o 15 the cells ll being shown fixed to the outside of the 'housing 10 by a plurality of screws 12. The other cell is shown for the puxpose o~ illustration in an exploded configu~a~ion so as to show the relations~ip of its various componen~s. It is to be unders~ood ~hat.for 20 most applicationst a singLs cell would be provided and used. A pair of cells i5 provided for some applica-tions.
A pair of electroly~e-containing reservoirs, only one reservoir 13 being vislble ~hroush a brok2n a~ay 25 portion o~ a side wall, are provided within the cell housing 10. The reservoirs communica~e with respec~ive apertures 14, lS which extend through a top portion of th~ housing 10 and allow one to place an appropriate electrolyte i~ the respec~i~e reservoirs. When the 30 disposable module is being carried by a user or is in operation, it is to be understood that the apextures l~, lS are closed by appropriate plugs. The re ervoir 13 is defi~ed in part by a wall 16, which is ~æ~
partially visible through a broken away portion of the side wall of the housing 10. The wall 16 also constitutes the back wall of an ambient chamber 17 having its bottom defined by a portion of the housing 10. The chamber 17 is in fluid communication with the~atmosphere or other gaseous ambient via at least one aperture 18 which eY-tends throuyh -the bo-ttom of the housing 10. The aperture 13 is shown somewha-t laryer relative to other features for the sake of clarity, it ma~
be constituted by an aperture having a diameter of .02 inch.
The exploded electrochemical detection cell, as shown in Fig. 4, includes a series of members held in stacked relationship over the chamber 17, when the cell is assembled for operation. The depth of chamber 17 is shown larger relative to other features for the sake of clarity.
It may, in fact, be very sha]low, for example .1397 cm (--.055 inch--) in depth in a practical embodiment. It can be formed by a recess in a surface of the housing 10, the wall 16 being integral with the housing 10. The members include an apertured rectangular gasket 20 which is positioned about the open front of the chamber 17. The cell has, in the order named, in stacked relationship a polypropylene ~PPE) membrane 21 which allows passage of 2 while not allowing a large amount of interferent gases to pass, an electrode carrier 22 which may be realized as a porous polytetrafluoro-ethylene (PTFE) membrane or the like, two sheets of Whatman filter paper or another suitable hydrophilic insulator 23, 24, a membrane support 25, a further Whatman filter paper 26, an electrode carrier 27 which may be realized as a porous polytetrafluoroethylene (PTFE) or polypropylene mab~ ~

membrane or the like, a permselective membrane 2~ made, for axample, of a molecular sieve or alumina impregnated with a reactive material such as potassium permanga~ate, an apertured rectangular gasket 29, a 5 metal frame 30, and one or more grille members 31, 320 The membrane ~, when made as suggested above, is especially useful when one wishes to detec~ C0, while excluding passage of H2S, alcohols and several other ga~es.
The electrode carrier 27 suppor~s or is integral with a sensing electrode 33, the sensing electrode being preferably of a noble metal, for example, gold or of carbon, the noble metai or carbon being supported by and having been applied to a substrate of nickel or the 15 like. The elec~rode 33 is desirably in the form of mesh or scr~en and could be a pure noble metal. The electrode carrier 22 supports a pair of spaced-apart electrodes 34, 35, these electrodes constitutin~
respectively the reference electrode and counter 20 electrode of ~he electrochemical detection cell of the present inven~ion. The re~erence elec~rode 34 and counter electrode 35, like the se~ing elec~rode 33, are preferably made of a noble metal, such as gold or platinum, or of carbon a~d have the form o~ a mesh or 25 scree~. Alternatively, the reference and counter electrodes could be compo ite electrodes fabricated from a finely divided conductive powder and an inert plastic binder, the conduc~ive powder being, for example r metal or carbon powder. The reference c~C~( r, f cr e~ ~ro~/~
30 el~ctrode 34, the coun~e~e~e~trode 35 and the sensing electrode 33, b~cause of ~heir cons~ruc~ion, present a large surface to the materials with which they come in contact, particularly th2 electrolyte and gas from the . .

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environment with which the ~ensing electrode 33 comes in contact with via the permselective membrane 28 and the grille members 31, 32. In ordex to pro~ide electrical connec~ions to the respective electrodes 33, 5 34 and 35, these electrodes are respectively provided with upwardly extending electrode portions 36, 37 and 38 an~ which may ~ ad~antageously be extension~ of the electrodes proper and be respective meshes.
~ach of the stacked members 20-22, 25, and 27-32 10 are provided the vicinity o~ thaix edges with a plural-ity of the align~d apertures 39, six being shown for purposes of illustration for each, through which respecti~e screws 40 are to extend, the housing 10 including a correspondi~g and alig~ed plurality of 15 threaded bores 41 which are to receive the screws 40 when the components are in assemhled condition.
Th~ membrane ~upport 25 is an insulating member and is provided on its upper-most edge with an insul-?ting plate or extension 42 th reof which is fixed 20 thereto or made i~tegral therewith in a conventional fashion. Thret3, spaced apart, elec~rical contacts 43, 44 and 45 extend upwardly from the insulating plate or extension 42 and through the insulating plate or exten~ion 42 so a~ to provide respective contact areas 25 beneath the insulating plate or extension 42 which contact respec ively with the upwardly extending respective metal portions 38, 36 and 37 of the respec-tive counter electrode 35, the sensing electrode 33 and the reference electrode 34, when the exploded cell i5 30 in its assembled condition. The contacts 43~45 are yre7~crR bi~f pre~r~ r made by the same conductive m~terial as the portions 38, 36 and 37 of the electrodes and, if -3~

desired, may be fixed thereto by a co~ductive epoxy resin containing the same metals.
~ n aper~ure 46 ex~ends through the hou~ing 10 i~to the reservoir 13 and has therein a wick 47 which, when 5 the members are assembled, extends through a passagew~y defined by respective aligned apertures 48 in the gasket 20, the memhrane 21, the electrode support 22 and the membrane support 25 so as to supply electrolyte from the reservoir 13 to the Whatman filter papers 23, 10 24 and 26 between the sensing electrode 33 and the reference and counter elec~-rodes 34, 3S.
A urther aperture 50 extendQ through the housing lO and into th~ reservoir 13, the ap~rture 50 defining a passageway, when aligned with apertures 51 15 in th~ membrane 21 and the ga ket 20, between the space between the electxodes 33, 34, 35 via a ~lot ~ in th~
top por~ion of the m~mbrane suppor~ 25 ~0 to allow gas produced ts move ~he r~servoir 13 thereinto.
As illustrated in Fig. 4, the disposable module 20 includes between the two electrochemical sen~ing cells a portio~ shown as an elongated, longitudinal protrus-ion 52 which i~cludes wi~hin it a batte~y for powering the electronic system of the present inve~io~. The battery wi~hin the por~ion of ~he module behind the protrusion 52 i5 conductively connected to the elec-tronic systam of the present invention via elec~rical contacts 54, 55. The electronic portion of the invention, as illustrated in some detail in Fig. 9, is physically locat~d on the top portion of the cell 30 housing 10 and is elactrically connected to the elec-trical contacts 43-45; 54 and 55. The contacts 54, 55 also are connected to supply power to a s~cond elec-tronic system located on the top portion of the .

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lg housing 10 and which corresponds to that of Fig. 9, but is set to detect a dif~erent gas or is present as a redundant system to assure a high degree o~ relia-bili~y, this second electronic system being associated S with the cell 11 via three contact~ (unnumbered) which correspond to the contacts 43-45.~
The cell housing 10, with its associated elec-trical contacts 43 45, and 54-55, is shown in Fig. 6, with the apertures 14, 15 being visible. The 10 electronic system, including a small buzzer or other audible alarm, i~ carried within the casing generally designated by the numeral 56. The casi~g 56 includes a pair of side walls 57, 58, a backwall 59 and a front wall 60. The fron~ wall 60 does not ex~end longitud-15 inally to the same extent as the side walls 57, 58 andthe bac~ wall 59 so as to provide an ope~ing 61 having a var~ical ex~ent uch tha~ when ~he disposable module housing 10 is inserted in~o ~he casing 56, the outer-m~r~ grille member 32 of the one cell and the 20 corr~sponding grille member of the other cell 11, as ~ell as the protruding portion 52 uf the cell housing lO which contains ~he battery for ~he elec-tronic system/ will not be covared. The casing 60 is open at the bottom ~o that ~he cell housing 10 can be 25 easil~ inserted into it ro~ the bott~m with its respective electrical contacts 43-45 and corresponding ones associated with the cell 11 and contacts 54, 55 coming into contact with appropriate connectors associated with the two respective elec~ronic systems 30 which are carried within the casing 60. The top wall 62 of the casing 56 includes a grill work 63, behind which a~ appropriate buzzer or audible alarm (not visible) for sounding an alarm signal is provided.

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~ 19 --An alligator clip 64 is fixe.dly connected with an upstanding tab 65 of the casin~ 56 CO a- to provide a means connecting the alaxm u~lit, constltued by the casing 56 and the disposable cell module within it, to 5 the shirt pocket or the like of a user. The cell housing 10 is removably held within the casin~ 56 by a pair of detents, only one detenk 66 being visible on one of the side surfaces of the cell housing 10, a corresponding detent being and on its coxresponding 10 ou~wardly facing side surface, which is not visible.
The detents 66 cooperate with a pair of de~ents which are present on opposed, inwardly facing side surfaces of the casing 56, only one groove 67 baing visible.
The real tLme differen~ial pulse detection (RDPD) 15 sensor shown in FiyO 4 allows one ~o use the differential pulse technique ~o ~mprove both the specifici~y and sensitivity of electrochemical cells adapted to detec~ variou species in a gaseous e~ironment~ ~he conskruc~ion of the sensor is similar 20 to tho e described in the U.S~ Letters Patents o Oswin et al., supra, and Stetter, supra, with the exceptio~ of the axrangement of par~s and electrode diference~; ~he metal electrodes 33-35 are configured ln such a way to provide low re~ista~ce and a very low 25 dual layer capacitance, ar~ spaced close toge~her. The sensing electrode 33 is sandwiched between a gas permeable membrane 28 and a hydrophilic insulator, in the form of Whatman filter papers 23, 24 and 26, soaked with the electroly~e, so the sensor can be operated in 30 any oxientation or dur.ing movement. The filter papers 23, 24 and 26 are positioned between the sensing elec~rode 33 and each of the re~exence electrode 34 and the counter electrode 35 allowing effective application ~2~9~

o~ the electroly~e while maintaininy close spacing.
The polypropylene membrane 21 serves the purpose o~
protecting the reference electrode 34 from interferen~s. The metals prefe.rably used to construct 5 the sensing electrode 33 can be operated at much higher potentials than mercury, allowing one to detect both oxidizable and reducible gases, unlike the sensor disclosed in the Letters Patent to David et al., supra.
One should desirably use concentrated solution of a 10 s.xong acid or ba e or a salt as the electrolyte. Of course, these materials should be selected and used in such concentration taking into account expec~ed operating conditions and safety fa~tors, so as to assure that the materials will not - ~ out of 15 solu~ion while using a solution which is as concentrated as pra~ticable. Amony the many materials which can be used for the electrolyte are aqueous solutions of ~2SO~ ~O~ and RC1.
The advantages of the RDPD sensor become ob~ious 20 after examining the two types of current which flow thru an electrochemical sen or operating in the RDPD
mode. Pigs~ 7A-7C depict characteris~ics of the RDPD
s~nsor of ~he present inven~ion in which cyanide is oxidized on a silver or gold sensing elec~rode. The 25 sensing electrode potential with respect to the refer-ence electrode is periodically pulsed from zero mV to 50 mV, as shown in Fig. 7A. Charging the sensing electrode to its new potential resul~s in a posi~ive spike at the beginning of each pulse and a negative 30 spike at the end of each pulse, as shown in Fig. 7B.
~his current is governed by the ollowing ~quation, Schmidt et al., supra:

~z~g~

5 i~t) = ~ E~ exp r~ ~ where ~1) R L RCJ

i(~) = background current at time t E = bias pulse height R = cell resistanc t = time after ini~iation of bias pulse C = dual layer capacitance o~ the ~ensing electrode~

The capaci~ive background curre~t occurs in both the absence or presence of the gas ~o be detected. If 15 the gas to be detected is pre~e~t, the Faradaic signal current shotnn in Fig~ 7C is superimposed on the back grou~d capacitive c~rrent of Fig. 7B. The signal current is described by the followiny equation Parry et al., supra:

~ 20 i~t) = nFAC ~ ~ - ~ , where (2 ; i(t) = agent signal current at time t n = number of electrons trans~erred per molecule o~ gas 25 F - Faraday's constant A = . sensing electrode ac~ive area C = agent ~oncentration in the electrolyte adjacent to the electrode D = difusion coe~ficient of thP agent in the electrolyte t = time a~er initiation of the pulse ~ - exp ( nF~E/2RT) .~E = DPV bias pulse height f--R - gas constant T - temperature (K).

~ he DPV signal is the di~erence current, ~hat i5 ip-io~ Therefore mos~ inter~eren~s are removed when 5 ~he electronics system subtracts ip - io.
Gas sensors described to date, such as those cited in U.S. Lett~rs Patents to Oswin et al., supra, and to S~etter, supr~ are unsuitable for use in thç FDPV mode fsr several reason~. The primary reason involves poor 10 sensitivity due to ~he nature of the gas sensing electrodes described in the literature. Equation (2), supra, states that the signal level is inversely proportional to the square root of the t~me after the pulse begin~. Sinee ~he signal is measured at the end 15 of the pulse, maxi~um 5en5itivity is achieved by minimizing the pulse widtho The minimum size of ~he pulse width is de~ermined by the time requir~d for the ~apacitive background current to decay, sinc~ it masks ; th~ Faradaic gas signal until it decays to nearly zero.
20 Unfortunately, electrodes used in prior art gas sensors exhibit a very slow background curren~ decay, which severaly limits their sensiti~ity. Their extraordinary large ~lectrode sur~ace area c~use a slow background current decay due to the large R and C terms in equa 25 tion ~1), supra.
The background current problem o~ prior art electrodes used in the RDPD mode is overcome in this invention by using noble metal screen electrodes or the like and plaeing them in relation to the other parts as 30 described above. This decreases the R and C terms described in equa~ion (1), supra, of the sensor several orders of magnitude, and results in the improvement in ..9~ ~

the background signal shown in E'ig. 8C as compared ~o the standard bacXground current shown in Fiq~ 8B. The background signal improvement allows one to u~ilize shorter pul~es, which increases the sen~itivlty as 5 described in equation ~2)~
The use of the proposed waveform shown in Fig. 8A
instead o the prior art waveform shown in the U.S.
Letters Patent to David et al., supra, enhances sensi-tivity in two ways. First, the shorter pulse allows 10 measurement of the gas signal before it has decayed to the degree in the UOS. Letters Patent to David et al., supra. Second, the Rhort pulse, typically about 50 msec. long, is only repeated once every second l50 msec. on-950 msec. of~3, in contrast to the 200 msec.
15 on 200 m~ec. off cycle used i~ the prior art. It has been established in the literature that the sensitivity decrea es a the pul~e on-off ratio increa~es.
I~ Fig. 9, a suitable, exempla~y elec~ro~ic system for real time, differential pulse detec~ion, which is 20 particularly useful when~ associated w1th the det~c~ion cell shown in Fig. 4 is illustrated. The real tLme, differential pulse detection electronic system, as illustrated in Fig~ 9 includes a control circuit 68 having ~hrae syncronizing outpu~sO a 25 functio~ generator 69 responsive to one outpu~ from the control circuit 68 a~d ha~ing one output, a detection cell 70, which corresponds to the cell shown in exploded configuration and Fig. 4, and a transducing network (po~entiostat) 71 which generates voltage 30 and/or current signals propor~ional to the amount of agent reacting at the sensing electrode 33O The output from ~he transducing networX 71 is coupled to an analog processing circuit 72, which is controlled by two of the outputs ~rom the control circuit 6 a, ~nd has it9 output coupled to a bu~fered, sample-and-hold CiXGUlt 73~ The output from the qample-and-hold circuit 73 is fed to a threshold circuit 74 which, in 5 turn, has its output fed to an audible alanm circuit 75 which may be cons~ituted by a buzzer, and to a visible alarm circuit 116, which may be constituted by a light-emitting~diode IL.E.D.).
In a practical embodiment, the control circuit 68, 10 the function generator 69, the transducing network 71 and the analog p-ocessing circuit 72 may be realized as a miniaturized cixcui~ which can be posi~ioned adjacent to a top portion of the cell housing 10 (Fig. 43 and in ~lectrical connection with contacts 43-45 and 54,55 15 (Fig. 4), while the buffered, sample~and-hold circuit 73, the alarm circuits 75 and 116, and the threshold circuit 74 are de~irably carxied separa~ely within the casing 56 (Fig. 6). It is to be appreciated that the output of a second analog processing circuit, ~o associated with the second detecting cell 11 (Fig. 4), could have its output also connected to the buffered, sample-and-hold ci~cuit 73 which would serve to sound the alarm circuit 75 and energize the alarm 116 were aither system to produce an output from the respective 25 analog processing circuits. Of course, the buffered, sample-and-hold circuit 73, the ala.rm circult 75 and the threshold circuit 74 could be realized as part of a single disposable unit with the control circuit 68, the function genera~or 69, the transducing networ~ 71 and 30 the analog processing circuit 72 were a single real time differential pulse detection system, rather than two such systPms, utilized.

The power for the circuit illustrated in Fig. 9 is obtained from a batte.ry pack (not illustrated~ which includes a ground midpoint, a V+ voltage terminal and a V- vol~age terminal. A 4K ohm resis~or 76, a ~irst S voltage reference 77, a second voltaye reference 78 and a 4K ohm resistor 79 are connected in serie~ with one another in the order named be~ween the V+ and the V-terminal~, ~he reerence point (ground) being provided b~tw~en the a~ode and cathode of the respective voltage 10 references 77 and 78. The voltage references 77 and 78 provide for a con~tan~ steady D.C~ voltage across the same. A irst voltage di~ider, constituted by the series connec~ion of a lR ohm resistor 80, a 10~ ohm potentiometer 81 and a 1~ ohm resistor 82 connected in 15 series, is connected across the voltage references 77, 78. ~ second voltage divider constituted by the ~eries conn~ction of a 1~ ohm resistor 83, a lOR ohm potentiometer 84 and a 1~ ohm resistor 85 is connected in parallel with the voltage refexences 77, 78. The ~o wiper of the potentiometer 81 is connected to the non-inverting (+) terminal of an operational amplifier 86, arranged in a non-inver~ing amplifier, which has its inver~ing I-) terminal connected to ground via a lOR ohm re~istor 87 and its output 25 terminal connected to its lnverting terminal via a 20R
ohm resistor 88~ The opera~ional amplifier ~6, with its a-~sociated circuit components, is configured to buffer and to amplify the voltage supplied from the potentiometex 81.
The wiper of the potentiometer 84 is connected to th~ non inverting t+) terminal of an operational amplifier 89 which is connec~ed as a unity gain buffer ~2~9~

and has its output terminal directly connected to its inverting terminal.
The output terminals of the operational amplifiers 86 and 89 are coupled to ~he inver~i~g ( ) terminal of 5 a further opera~ional amplifier 90 arranged as a summing, in~erting ampli~ier, the output termlnal of the operational amplifier 86 being connected via a lOR
ohm resistor 91, while the output terminal of the operational amplifier 89 is connected via a s~ries 10 connection of a controlled analog switch 92 and a lOK
ohm resistor 93. The non-inverting (+) termlnal of th~
operational amplifier 90 i-~ connected to ground via a SK ohm resistor 94 and its output terminal is connected to its in~erting input termlnal via a lOK ohm 15 resistor 95. A synchronized ou~put from the control circuit 68, having the ~orm of the waveform illu trated as waveform C in Fig. 10, is coupled to the enabling input of the analog switch 92.
Th~ final s~age, operational amplifier 90, of the ` ~0 unction generator 69 has its output terminal connected directly to the non-inverting (+3 input terminal of an operational amplifier 96, the irst stage of the transducing network (potentiosta~) 71. The ou~put terminal of the ope~ational amplifier 96 is con~ected C~ h ~er e/ee f r~/e 25 to the _ 35 of the detection cell 70.
The inverting t-) input terminal of the opera~ional a~plifier 96 is co~nected to the reference electrode 34 of the dçtection cell 70. The output and inverting input terminals of ~he operational amplifier ~6 being 30 couplsd together via a .1 ~f capacitor 97, so as to reduce ~he possiblity of the operational amplifier 96, and its associated circuitry undesirably going into oscillation.

~g~o The detection cell 70, as illustrated in ~ig. 9, includes a sensing electrode 33, which i3 al50 shown in Fig. 4, is coupled to the inverting ~-) tenmi~al of an operational amplifier 98, via a 33 ohm resi~kor 99 5 which provides noise reduction. The non~:Lnverting (~) terminal of the operational ampliier 98 is connected to grou~d, the output termlnal thereof being connected to its inver~ing ~ ) inpu~ terminal via 1 2.2K ohm resis~or 100~ The resistor 100 can be considered, in 10 association with the other cirouit components connected to the operational amplifier 98, to effect a current-to-~oltage convsrsion, the resistor 100 determining the conversion gain. The output terminal of the ; operational amplifier 98 is connec~d via a 5K ohm 15 resistor 101, to the respective input terminals of controlled analog switches 102 and 103, which are the input stag~s of the analog processing circuit 72. Tha re~pective controlling inputs to the analog switches lQ2 an. 103, are obtained from the control circuit 68 20 via connPctions ~hereto, the respec~ive controlling inputs being constit~ted by signals corresponding to th~ waveforms shown in Fig. 10 as letter ~ and letter B, respec~ively. The respective output terminals from the analog switches 102, 103 are respec-25 tively connected to the u~grounded side of .l~fcapacitors 104 and 105. The opposite plates of the capacitors 104 and 105 being connected to ground. The ungrounded sides of the respective capacitors 104, 105 are connected respectively to the non-inverting (~) 30 input terminals o~ respective operational amplifiers 106 and 107, the respective output terminals and inverting (-) input terminals of the respec~ive amplifiers 106 and 107 being connected via respec~ive ~L9~

22K ohm resistors 108 and 1091 The output te~minal o~
the operatisnal amplifier 106 is connected to the inverting (-) input terminal o~ the opera~ional ampli~ier 107 via a 22R ohm resistor 110. The inver~-- 5 ing (-) input terminal o~ the operational amplifier 106 is connected to ground via a 22X ohm resistor 111.
The output terminal of the operational amplifier 107, which constitutes the output ~rom the analog processing circuit 72, is connected to a con-10 trolled analog switch 112 which ~orms the input stageof the buffered~ sample-and-hold circui~ 73. The enabling input to the controlled analog switch 112 is obtained rom he control circuit 68 ~ia the connection thereto which also provides the ~ame enabling input lS signal to the con~rolled analog swi~ch 103, this signal being show~ a~ a waveform B in Fig. 10. The output from ~hs analog switch 112 is fed ~o the unground plate o~ a .lf~f capacitor 113, its other plate being grounded. The capacitor 113 has its ungrounded plate 20 connec~ed to the non-i~Yerting (+) iApUt terminal of an operational amplifier 114~ the inverting (-) input terminal of the operational amplifier 114 being direct-ly connected to ~he outpu~ ~erminal ~hereo~, which constitutes the ou~pu~ from the buffered, sample-and-25 hold circuit 73. The output terminal from theopera~ional amplifier 114 is connected ~o a firs~ input terminal of a threshold comparator 115 which has its other input terminal connected to a wiper of a potent-iometer 116 which, as shown, i~ connected between the 30 V~ and V- terminals of the ba~tery pack power source.
The pOtentiQm~ter 116 could be connected in parallel with the voltage references 77, 78 for better regulation. The output from ~he threshold comparator 115, which cons~itutes the output of the threshold circuit 74, is con~ected to the audible alarm circuit 75, illustrated as a buzzer. ~he output o~ the thre.~hoLd comparator llS is also ~ed to a visible alarm 5 circuit 117, illustrated as a light emitting diode (L.~.D.) 116 which may be pow~red from a switched power source or in some cases, an oscillator which would oause the L.E.D. 116 to blink on and of~. The operational amplifiers 86, 89, 90, 96, 98, 106, 107 and 10 114 can be realiz~d from respective one-quarters of two quad CMOS OP AMPS e.g. Intersil's ICL 7641 circuit module~.
Referring further to Fig. 9, reference bPing made additionally to Fig. 10, a brief discussion of the set 15 up ~nd operative sequence of the present invention is to be discuss~d. The user irs~ de~ermines which gas is to be detected, and selec~s ~he appropriate electrolyte which is placed i~ the electrolytÆ
reservoir 13 (Fig. 4). This electroly~e, because of 20 the wick 47 which ~xtends from ~he reservoir 13 and ~o physical contact wi~h ~he ~hatman filter papers 23, 24 and ~6 (Fig. 4), ~ransfers some elec~rolyte into space between the se~ing electrode 33 and each o the counter electrode 35 and the reference electrodP 34.
One then selec~s, before placing the system into field operation, an appropriate fixed D.C. voltage bias by suitably adjusting the wiper of the potentiometer 81 so tha~ the D.C. voltage at the non-inverting input to the operational amplifier 86 in Fig. 9 has a selected 30 ~teady level which results in a steady, given D.C.
vol~age appearing on the inverting input ~erminal of the operational amplifier 90, which has its output directly connected to tAe non-inverting input terminal . . ~ .

~ 30 -of the operational ampli~ier 96. The output t~rminal of the operational amplifier 96 and its inverting input ~erminal are directly res~ec~iv~ly connected conductively to the ~ ~ ~nc~ electrode 35 and the ~-e~e r~ ,7 a ~ f r Clc~c 5 ~rw=~h~ ~e~o~e 34 of ~he detection cell 70, as pointed out aboveO One then selects ~he magnitude of the diffarential D.C. pulse which is to be superimposed on a representation of the fixed-background-bias D.C.
by adjusting the wiper o~ the potentiometer 84. One 10 may also set the sensitivity of the system by adjusting the wiper of the potentiometer 116 of the threshold circuit 74 90 ~hat a given output from the buSfered, sample-and-hold circuit 73 must be presen~ before the alarm circui~s 75 and 116 respond. The system is then 15 ready for operation under the control and synchronism of the control circuit 68 which may be raalized as a timing circuit which includes a master oscillator, counters and logic circuits, so as to produce the appropriate co~trol puls~s, shown as waveform A-C in ~o Fig. 10, in ti~ed, synchxonized sequence.
The con~rol circuit 63, when turned on by user as sugges~ad by the STAR~ inpu~ produc~s a train of short, for example ten msec. puls~s r the pulses being shown as wavaform A ln Fiy. 10, the individual pulses acting to 25 enable ~he analog switch 102 which, when enabled, effects a transfer of the output signal from the operational amplifier 98 of the transducing network 71 to charge the s~orage capaci~or 104 during the duration of the enabling pulse. This results in a voltage 30 across the capacitor 104 which corresponds to the current produced from ~he transducing network 71 which responds to output from the sensing electrode 33 ~Figs. 4, 9I the detection ceLl just prior to the ~2~9~

superposition of the di~ferential D.C. pulse on the aforeme~tioned and already presen~ ~ixed~background~
bias D.C. voltage fed to ~he inverting inpu~ terminal o~ th~ operational amplifier 90 and provided ~rom the 5 operational ampli~ier 86. In timed relationship with ~he individual pulses shown in waveform A in Fig. 10, the pulse control circuit 68 produces, at the trailing edge of each of these pulses, respective control pulses of, for example, 40 msec. lengths having the wave~orm C
10 illustrated in Fig. 10 which enables the analog gate 92, causing a repreqentation of the D~C. voltage output from the operational amplifier 89 to be super-imposed on ~he fixed-background-bias D.C. pote~tial output ~rom the operational amplifier 86 as set by the 15 placement of the wiper of the potentiometer 81. As a resul , the ref~renc~ electrode 34 exhibi~s an increase in vol~age thereof for the duratio~ of the individual control pulses illustrated as waveform C in Fig. 10.
About 10 msec. befoxe the respective enabling pulses 20 applied to the analog switch 92 ends, the control circuit 68 produces a further 10 msec. enabling pulse which i9 applied to the respective analog switches 103 and 112 so as to enable these respective analog - switches and in effect, tra~s~ex the difference between : 25 the stored signals appearing across the capacitors 104 and 105 to the buffered sample-an~-hold circuit 73 via the operatio~al amplifier 107 and, in particular, to the capacitor 113.
As pointed out above, the capacitors 104 and 105 30 are identical in size, therefore the respective volt-ages which appear on these respective capacitoxs correspond respectively to currents produced from the transducing network 71 which respactively correspond to . . .

~2~L9~

the voltages produced from the detection cell 70 just before the beginning and just be~ore the end o~ each o the pulses sho~n in Fig. 10 as waveform C and which are used to enable the analog switch 92.
The sig~als from th~ operatiunal ampli~ier 107 are thus periodically stored on the capacitor 113 and the production thereof, at least over a given special level, indicates the presencP of an agent to be detec-tPd, such as a gas, and which has been detected by the 10 detection cell 7U. The bufered, sample-and-hold circui~ 73, in effect, holds the highes~ level it receives from the operational amplifier 107, which output is fed to the operatio~al ~mplifier 114 which has its output coupled to the threshold circuit 74, in 15 particular, to one input o~ the thrashold comparator llS, which has it~ other input connected to the adjustable threshold level ~etting potentiometer 116. Whenever the output from the sperational æmpli~ier 114 exceeds th~ voltage supplied from the 20 potentiometer 116, the threshold comparator 115 provides an output which enables ~he alarm circuits 75 and 116 or one of them if only one is present, these alarm circuits being sho~n as a bu~zer and an L.E.D.
In instances where t~e ~oE~D~ alarm circuit 116 is 25 provided, it is turned on by an ou~put from ~he threshold comparator 115 t bli~king in the cas2 an oscillator is used to supply the power to the L.E.D.
via a controlled switch (not sho~n) responsive to output from the threshold comparator 115. The ou~put 30 fxom the amplifier 114 could be used alternatively or in addition to effecti~g the turning on of the alarm circuits 75 a~d 116, to provide a direct analog readout ., .

~9~
~ 3 -to a meter or the like or be a~sociated with digital storage and/or display and/or furthe~ proces~ing.
I~ is ~o be understood that the foregoing descrip~
tion, and accompanying illustra~ions have not been se~
5 by way of limitation, bu~ by way of example. It is to be appreciated that numerous other embodiments and variants a~ possible, without departing from the 5cope and spirit of the present inven~ion, its scope being defined by the appended claLms.

Claims (24)

THE EMBODIMENTS OF THE INVENTION IN WHICH AM EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A system for detecting a gas in an environment comprising an electrochemical detection cell having a reference electrode, a counter electrode and a sensing electrode; said reference electrode and said counter electrode spaced apart from each other and from said sensing electrode, an electrolyte in contact with said reference electrode, counter electrode and sensing electrode, a membrane having a first surface exposed to said environment for passing said gas in said environment through said membrane to said sensing electrode, an operational amplifier having an output terminal, an in-verting input terminal and a non-inverting input terminal, said output terminal being coupled to said counter electrode, said reference electrode being coupled to one of said input terminals; means for applying a fixed D.C. voltage bias to the other of said input terminals; means for applying at least one differential D.C. voltage pulse to said other input terminal to superimpose same on said fixed D.C. voltage vias to provide a corresponding differential D.C. voltage pulse on its output terminal, and means coupled to said sensing electrode and responsive to current therefrom for determining difference in Faradaic currents produced during a period before application of the differential D.C. voltage pulse and a period before termination thereof as an indication of presence of a given gas.

2. A system according to claim 1, wherein said reference electrode is coupled to said inverting input terminal of said operational amplifier, and said means for applying a fixed D.C. voltage bias and said means for applying a differential D.C. voltage pulse are coupled to said non-inverting input terminal.

3. A system according to claim 2, further including means for adjustably setting the fixed D.C. voltage bias.

4. A system according to claim 3, including means for adjustably setting amplitude of said differential D.C. voltage pulse.

5. A system according to claim 2, further including means for adjustably setting the amplitude of said differential D.C. voltage pulse.

6. A system according to claim 1, wherein said means coupled to said sensing electrode and responsive to current therefrom includes first storage means for storing a signal representative of current from said sensing electrode during a period before application of said differ-ential D.C. voltage pulse and second storage means for storing a signal representative of current from said sensing electrode during a period before termination of said D.C.
voltage pulse.

7. A system according to claim 6, wherein said first storage means and said second storage means comprise respective first capacitance means and second capacitance means, the signals being voltage signals.

8. A system according to claim 6, including means coupled to said first storage means and to said second storage means and responsive to the signals therefrom for determining the difference therebetween.

9. A system according to claim 8, wherein said first storage means and said second storage means comprise respective first capacitance means and second capacitance means, the signals being voltage signals.

10. A system according to claim 8, including alarm means, said alarm means being coupled to said means coupled to said first storage means and to said second storage means.

11. A system according to claim 10, including sample-and-hold circuit means for holding the highest signal representing the difference current, said sample-and-hold circuit means being coupled between said means coupled to said first storage means and to said second storage means, and said alarm means.

12. A system according to claim 10, wherein said alarm means comprises means for producing an audible signal.

13. A system according to claim 12, wherein said alarm means comprises means for producing a visual signal.

14. A system according to claim 10, wherein said alarm means comprises means for producing a visual signal.

15. A system according to claim 1, wherein said electrochemical detection cell comprises at least one further membrane positioned between said counter electrode and a gas-containing chamber and between said reference electrode and the gas-containing chamber.

16. A system according to claim 1, wherein said reference electrode, said counter electrode and said sensing electrode are meshes.

17. A system according to claim 1, wherein said sensing electrode is a mesh and said reference electrode and said counter electrode are composite electrodes.

18. A system according to claim 17, wherein said composite electrodes are of finely divided conductive powder and an inert plastic binder.

19. A system for detecting a gas in an environment comprising an electrochemical detection cell having a reference electrode, a counter electrode and a sensing electrode, said reference electrode and said counter electrode spaced apart from each other and from said sensing electrode an electrolyte in contact with said reference electrode, counter electrode and sensing electrode, a membrane having a first surface exposed to said environment for passing said gas in said environment through said membrane to said sensing electrode, means for applying a fixed D.C.
voltage bias to said counter electrode for supplying electrical current through said electrolyte to said sensing electrode to maintain a predetermined potential at said reference electrode, means for superimposing at least one differential D.C. voltage pulse on said fixed D.C. voltage bias, and means coupled to and responsive to Faradaic signals from said sensing electrode for determining the difference between Faradaic signals produced during a period before the at least one differential D.C. voltage pulse is super-imposed on the fixed D.C. voltage bias and during a period before termination of the at least one differential D.C.
voltage pulse.

20. A system according to claim 19, wherein said means for superimposing at least one differential D.C. voltage pulse comprises means for superimposing a first train of differential D.C. voltage pulses on the fixed D.C. voltage bias, and said means responsive to signals from the sensing electrode includes: (1) means for producing a further train of pulses and an additional train of pulses both synchronized with the first train of pulses and having the same repetition rate and being timed to effect sampling of signals from the sensing electrode in periods before start and before termination of each differential D.C.
pulse of the first train and (2) means responsive to the sampled signals for determining the difference between those produced during the periods before the respective differential D.C. voltage pulse and those produced during the periods before termination of the respective differential D.C. voltage pulses.

21. A system according to claim 20, including sample-and-hold circuit means for storing the difference signal.

22. A system according to claim 20, including at least one alarm circuit responsive to the difference signal.

23. A method for detecting a gaseous agent in an ambient with an electrochemical detection cell having a sensing electrode, a counter electrode and a reference electrode, said reference electrode and said counter electrode spaced apart from each other and from said sensing electrode and having an electrolyte in contact with said reference electrode, counter electrode and sensing electrode, comprising the steps of exposing said ambient to one side of a membrane capable of passing said gaseous agent, exposing said sensing electrode with the gas passing through said membrane, applying a fixed D.C. voltage bias to the counter electrode for supplying electrical current through said electrolyte to said sensing electrode to maintain a pre-determined potential at said reference electrode, super-imposing at least one differential D.C. voltage pulse on the fixed D.C. voltage bias and determining the difference between Faradaic signals produced from the sensing electrode during a period before superimposing the differential D.C.
voltage pulse and in a period before termination of the differential D.C. voltage pulse.

24. A method according to claim 23, wherein the step of superimposing at least one differential D.C.
voltage pulse on the fixed D.C. voltage bias involves superimposing a first train of differential D.C. voltage pulses thereon, supplying a second train of pulses and a (Claim 24 cont'd....) third train of pulses both synchronized with the first train of pulses and having the same repetition rate, and using respectively the second and the third train of pulses to affect sampling of signals from the sensing electrode in periods before start and before termination of each differential D.C. pulse of the first train, and determining the difference between the signals produced during the periods before the respective differential D.C. voltage pulses and those produced during the periods before termination of the respective differential D.C. voltage pulses.