Patent Description:
There is significant ongoing development of low cost, disposable electrochemical sensors for use in the electroanalysis of an environmental sample outside a laboratory. For this purpose, the electrochemical sensor is typically interfaced with a portable field instrument in a system which enables the electrochemical sensor to be operated amperometrically. The analysis provides rapid results and facilitates instant decision-making but may be undertaken in challenging environmental conditions which cause inter alia moisture ingress and contamination of sample areas and of electrical contact pins. Moreover contamination or moisture ingress can result from agitation of the sample through careless handling, condensation due to poor maintenance or as a consequence of the operator having wet or dirty hands. The presence of moisture on the electrical contact pins can cause a short circuit and lead to a significant delay in the use of the electrochemical sensor or even the return of the electrochemical sensor to the manufacturer for servicing or replacement.

Electrochemical sensors of the type disclosed in <CIT> have been developed with an overall area of working electrode which is small. Typically the working electrodes are in a dimensional range (<NUM>-<NUM> micron) which is sufficient for them to be considered to be microelectrodes. In these electrochemical sensors, a reagent formulation is dried on the microelectrode surface to provide the chemical components essential for specific ion electroanalysis. Once the electrochemical sensor is immersed in a test solution, dissolution takes place to give rise to natural convection of the reagent formulation from the surface into the bulk solution.

Microelectrodes have a number of advantages over macroelectrodes including faster mass transport rates, lower ohmic drop and improved diffusion provided that the gap between adjacent microelectrodes is sufficient to ensure diffusional independence. For electrochemical sensors of this type however, moisture ingress and contamination can be particularly troublesome because the microelectrodes are typically incorporated onto a relatively small substrate and the distance between the electrical contacts and from the microelectrodes to the electrical contacts is therefore small.

The precise composition of an environmental sample is often unknown. There may be chemical or microbiological toxins associated with the environmental sample of which the operator is unaware. The direct handling of the electrochemical sensor for disposal is therefore potentially hazardous to the operator.

An instrument for detecting the concentration of chlorine and chlorne ions in water (the "ChlordioXense" instrument) is disclosed in the "<NPL>).

The present invention seeks to improve the performance of an electroanalytical instrument (eg a portable field instrument).

The present invention provides an electroanalytical instrument for determining by amperometry (eg chronoamperometry) the presence or quantity (eg concentration) of an analyte (eg an oxidant of interest) in an aqueous sample as defined in claim <NUM>.

By virtue of a discrete mounting for electrical contact pins being mounted outside the sealed housing, the access to and cleaning of the electrical contact pins does not compromise the water-sensitive electroanalytical components such as the potentiostat housed in the sealed housing. This enables an operator to inspect, clean and dry the electrical contact pins straightforwardly in order for the instrument to be maintained operational.

In a preferred embodiment, the mounting for electrical contact pins is mounted detachably on the distal end of the elongate pivotal arm.

By virtue of its detachability in this embodiment, the mounting for electrical contact pins can be easily removed and replaced to extend the life of the electroanalytical instrument.

In a preferred embodiment, the mounting for electrical contact pins comprises a flexible polymer body which sealingly encapsulates the electrical contact pins such that the electrical contact pins are protuberant from a front face of the body. The flexibility of the polymer body advantageously allows movement of the electrical contact pins so as to provide sufficient contact force between the electrical contact pins and the electrical contacts on the superior part of the electrochemical sensor.

The electrochemical sensor may be mounted in a slot in the anterior wall of the pivotal forearm. The inferior part of the electrochemical sensor may be outside the slot and the superior part of the electrochemical sensor may be inside the slot exposed through a window in the anterior wall.

Preferably in use, a slanted part of the front face of the flexible polymer body biassingly abuts the electrochemical sensor (eg through the window in the anterior wall).

In a preferred embodiment when the elongate pivotal arm is in the fully flexed position, the receptacle prevents the pivotal forearm from pivoting to the fully open position. Particularly preferably when the elongate pivotal arm is in the fully flexed position, the receptacle constrains the pivotal forearm to a partially open position sufficient to permit the electrochemical sensor to dismount from the anterior wall of the pivotal forearm. For this purpose, a part of the anterior wall may extend below the rim of and into the receptacle.

This embodiment allows partial opening to enable the electrochemical sensor to be safely released into the receptacle for disposal without exposing the operator to the electrochemical sensor or the electrical contact pins.

In a preferred embodiment, the elongate pivotal arm pivots restrainedly between an extended position (eg the fully extended position) and a flexed position (eg the fully flexed position).

By virtue of the elongate pivot arm pivoting restrainedly, the descent of the electrochemical sensor into the receptacle is retarded and the immersion of the electrochemical sensor in the aqueous sample is therefore controlled. This has demonstrable benefits for the dissolution of the reagent formulation from the electrochemical sensor and the resultant measurements. It also minimises the risk of splash contamination.

Preferably the proximal end of the elongate pivotal arm is equipped with (eg configured into or attached to) a rotary sleeve which is mounted on an elongate shaft extending from the sealed housing, wherein the rotary sleeve and elongate shaft are sealingly spaced apart by a seal (eg a seal ring).

The frictional effect of the seal may be sufficient to ensure that the elongate pivotal arm pivots restrainedly with the advantages referred to hereinbefore.

In a preferred embodiment, the elongate pivotal arm pivots about a first axis between a fully extended position and a fully flexed position and the pivotal forearm pivots about a second axis between a fully open position and a fully closed position, wherein the first axis and second axis are substantially perpendicular.

The perpendicular axes of this embodiment serve to ensure advantageously that the pivotal forearm is unlikely to open during pivoting of the elongate pivotal arm.

Typically the pivotal forearm pivots medially between the fully open position and the fully closed position.

An interior compartment of the sealed housing may house a computer which interfaces with the potentiostat and provides a display.

The pivotal forearm may be retained in the fully closed position by a retaining catch. The retaining catch may be magnetic. The retaining catch may comprise a first part mounted on the anterior wall of the pivotal forearm cooperable with a second part mounted on the distal end of the elongate pivotal arm. The first part and second part may constitute a male and female part.

The electroanalytical instrument may further comprise a temperature probe extending inferiorly from an inferior face of the elongate pivotal arm whereby when the elongate pivotal arm is in the fully flexed position the temperature probe is immersed in the aqueous sample in the receptacle.

Preferably the electroanalytical instrument further comprises a switch which in response to an operating position of the elongate pivotal arm switches on the potentiostat, wherein the operating position is at or near to the fully flexed position. The switch may be a magnetically-operated switch (eg a reed switch) or an optically-operated switch.

In this embodiment, the switch ensures advantageously that potential is only applied to the electrical contact pins by the potentiostat when the electrochemical sensor is immersed in the aqueous sample and not (for example) when the elongate pivotal arm is an extended position for the pivotal forearm to be opened to clean the electrical contact pins.

The electroanalytical instrument may further comprise a removable lid on the receptacle. This may be useful to contain the aqueous sample and electrochemical sensor for disposal.

The receptacle may be configured to prevent overfill of an aqueous sample. For example, the receptacle may be configured to facilitate overflow to an overflow compartment when a threshold volume of aqueous sample is breached. This ensures a consistent volume of aqueous sample in the receptacle for electroanalysis.

For this purpose, the receptacle may be equipped with a notched wall which divides the internal chamber into a sample compartment and an overflow compartment. When a threshold volume of aqueous sample is breached in the sample compartment, there is an overflow of the aqueous sample to the overflow compartment.

The reagent formulation may include chemical components essential for specific ion electroanalysis such as a chemical reagent and a buffer. The chemical reagent may be a reductant essential for electroanalysis of a specific oxidant of interest. The reagent formulation is typically dried on the electrode surface. Once the electrochemical sensor is immersed in the aqueous sample, dissolution takes place to give rise to natural convection of the reagent formulation from the surface into the bulk sample.

Typically the reference electrode, counter electrode and at least one working electrode are microelectrodes.

In a preferred embodiment, the electrochemical sensor comprises:.

The first conductive track may be between the second conductive track and the third conductive track.

The array of apertures may be fabricated in the non-conductive layer by a mechanical, chemical or physical removal technique such as ablation (eg photoablation) or etching. The array of apertures may be fabricated in the non-conductive layer by screen printing.

Each aperture may have a substantially regular shape. Typically the apertures are uniformly shaped. Each aperture may be substantially circular or non-circular (eg rectangular or square). Preferably each aperture is substantially circular.

The array may adopt any suitable pattern (eg cubic or rectangular). The array may comprise <NUM> to <NUM> apertures, preferably <NUM> to <NUM> apertures, more preferably <NUM> to <NUM>, most preferably about <NUM> apertures.

Preferably each aperture has a dimension (eg diameter) in the range <NUM> to <NUM> (eg about <NUM>).

Each aperture may be elongate (eg linear). Each elongate aperture may be substantially parallel to the first, second and third conductive track (eg vertical).

Preferably each elongate aperture is substantially perpendicular to the first, second and third conductive track (eg horizontal).

In a preferred embodiment, each aperture of the array of apertures is substantially rectangular (eg a microband). For example, each aperture may be microscopic in width (eg about <NUM> microns) and macroscopic in length.

In a preferred embodiment, the array of apertures is a substantially rectangular array.

In a preferred embodiment, the electrochemical sensor further comprises:
a fourth conductive track deposited axially onto the substrate layer, wherein the first, second, third and fourth conductive track are in a parallel mutually spaced apart relationship, wherein on the fourth conductive track near to the second end of the substrate layer is a carbon deposit whereby the third and fourth conductive tracks constitute a pair of working electrodes, wherein the first and second conductive tracks are flanked by the third and fourth conductive tracks, wherein each of the first, second, third and fourth conductive tracks terminates near to the first end of the substrate layer in an electrical contact, wherein the non-conductive layer is deposited on the first, second, third and fourth conductive tracks and is fabricated to fully expose each electrical contact near to the first end of the substrate layer, to fully expose the carbon deposit on the second conductive track near to the second end of the substrate layer, to fully expose the first conductive track near to the second end of the substrate layer and to partially expose discrete working regions of the carbon deposits of the third and fourth conductive tracks through an array of apertures, wherein the reagent formulation is deposited on or near to the surface of either or both of the pair of working electrodes.

Each aperture may be elongate (eg linear). Each elongate aperture may be substantially parallel to the first, second, third and fourth conductive track (eg vertical).

Preferably each elongate aperture is substantially perpendicular to the first, second, third and fourth conductive track (eg horizontal).

The non-conductive layer may be fabricated by a known deposition or growth technique such as printing (eg screen printing, silk screen printing, ink-jet printing or thick film printing), casting, spinning, sputtering, lithography, vapour deposition, spray coating or vacuum deposition. Preferably the non-conductive layer is fabricated by screen printing. The non-conductive layer may be composed of a non-conductive ink.

Each conductive track may be fabricated by a known deposition or growth technique such as printing (eg screen printing, silk screen printing or thick film printing), casting, spinning, sputtering, lithography, vapour deposition, spray coating or vacuum deposition. Each conductive track may be composed of an inert metal such as gold, silver or platinum. Each conductive track may be composed of a conductive ink such as silver or silver/silver chloride ink. The conductive ink may be printable.

The substrate layer may be a sheet or strip. The substrate layer is typically composed of an insulating polymer. The substrate layer may be composed of polyester, polycarbonate or polyvinyl chloride.

The carbon deposit on each conductive track may be deposited by known techniques such as printing (eg screen printing, silk screen printing, ink-jet printing or thick film printing), sputtering, lithography, vapour deposition, spray coating or vacuum deposition. The carbon deposit may be composed of inert carbon such as graphite, glassy carbon or pyrolytic carbon.

The aqueous sample may be potable water, recreational water, process water or waste water (eg industrial waste water).

Typically the analyte is an oxidant of interest and the quantity of the oxidant of interest is its concentration. For this purpose, the reagent formulation includes a reductant.

Preferably the oxidant of interest is one or more of the group consisting of chlorine dioxide, chlorine, chlorite, hypochlorite, free chlorine, total chlorine, ozone, peracetic acid, hydrogen peroxide and monochloramine. Particularly preferably the oxidant of interest is free chlorine (and optionally total chlorine).

The reductant may be an iodide such as an alkali metal iodide (eg potassium iodide), N, N-diethyl-p-phenyldiamine (DPD) or tetramethylbenzidine (TMB).

The reagent formulation may further comprise one or more additives such as a buffer, gelling agent, thickening agent, wetting agent or stabiliser. Typical additives are one or more of the group consisting of sodium phosphate, potassium phthalate, sodium carbonate, disodium EDTA, hydroxylethylcellulose and polyvinylpyrrolidone. The reagent formulation may incorporate an acidic salt (eg sodium hydrogen sulphate) which in use reduces the pH to about <NUM>.

The reagent formulation may take the form of a reagent layer. A reagent layer advantageously permits the redox reaction between the oxidant of interest and the reductant to occur intimately in situ.

The reagent formulation may be deposited and dried onto or near to the surface of either or both of the pair of working electrodes to form the reagent layer.

The reagent layer may include a porous matrix. The reagent layer may include a porous matrix impregnated with the reductant. The porous matrix may comprise polyvinylpyrrolidone and/or hydroxyethylcellulose. The reductant may be impregnated in the porous matrix by printing or microdosing.

In a preferred embodiment, the reagent formulation includes tetramethylbenzidine (TMB), a phosphate buffer and polyvinylpyrrolidone.

Viewed from a further aspect the present invention provides an electroanalytical instrument for determining by amperometry (eg chronoamperometry) the presence or quantity (eg concentration) of an analyte (eg an oxidant of interest) in an aqueous sample comprising:.

Viewed from a yet further aspect the present invention provides an electroanalytical instrument for determining by amperometry (eg chronoamperometry) the presence or quantity (eg concentration) of an analyte (eg an oxidant of interest) in an aqueous sample comprising:.

The present invention will now be described in a non-limitative sense with reference to Examples and the accompanying Figures in which:.

<FIG> and <FIG> are front and rear perspective views respectively of an embodiment of the electroanalytical instrument of the invention. The electroanalytical instrument may be used to determine the concentration of an analyte of interest in an aqueous sample. Measurements are made by chronoamperometry and for this purpose, the instrument may be used in conjunction with an electrochemical sensor of the type disclosed in <CIT> which is commercially available. Such an electrochemical sensor is planar and consists of an inferior part where there is a reference electrode, a counter electrode and two working electrodes which are dosed with a reagent formulation for electroanalysis and a superior part where each of the reference electrode, counter electrode and working electrodes terminates in an electrical contact. For the sake of simplicity, an electrochemical sensor <NUM> is shown in the Figures without any detailed structure.

The electroanalytical instrument comprises a sealed housing <NUM> which is compartmentalised. An exterior compartment defines a receptacle <NUM> for the aqueous sample which contains the analyte of interest. An interior compartment of the sealed housing <NUM> houses a computer which provides a display <NUM>. Other interior compartments of the sealed housing <NUM> house the electrical and electronic components necessary for electroanalytical measurements.

The receptacle <NUM> is equipped with a notched wall <NUM> which divides the internal chamber into a sample compartment 2b and an overflow compartment 2a. When a threshold volume of aqueous sample is breached in the sample compartment 2b, there is an overflow of the aqueous sample to the overflow compartment 2a.

An elongate pivotal arm <NUM> is mounted pivotally at a proximal end on the sealed housing <NUM>. A distal part <NUM> of the elongate pivotal arm <NUM> houses a potentiostat. A mounting <NUM> for four electrical contact pins <NUM> is mounted sealingly on a distal end of the elongate pivotal arm <NUM>.

A pivotal forearm <NUM> is joined pivotally to the distal end of the elongate pivotal arm <NUM>. The electrochemical sensor <NUM> (not shown in <FIG> and <FIG>) is mounted in a slot <NUM> in an anterior wall <NUM> of the pivotal forearm <NUM> and extends inferiorly to the elongate pivotal arm <NUM>. The inferior part of the electrochemical sensor <NUM> is outside the slot <NUM> and the superior part of the electrochemical sensor <NUM> is inside the slot <NUM> exposed through a window <NUM> in the anterior wall <NUM>.

The elongate pivotal arm <NUM> pivots between a fully extended position (see <FIG>) and a fully flexed position (see <FIG>). In the fully flexed position, the inferior part of the electrochemical sensor <NUM> is immersed in the aqueous sample in the receptacle <NUM>. The pivotal forearm <NUM> pivots medially between a fully open position (see <FIG>) at which the electrical contact pins <NUM> are exposed and a fully closed position (see <FIG>) at which the superior part of the electrochemical sensor <NUM> is in operative contact with the electrical contact pins <NUM>. The axes about which the elongate pivotal arm <NUM> and pivotal forearm <NUM> pivot are perpendicular. When the elongate pivotal arm <NUM> is in the fully flexed position and the pivotal forearm <NUM> is in the fully closed position as shown in <FIG>, the electrochemical sensor <NUM> interfaces the electrical contact pins <NUM> and the aqueous sample.

As shown in <FIG>, the proximal end of the elongate pivotal arm <NUM> is configured into a rotary sleeve <NUM> which is mounted on an elongate shaft <NUM> extending from the sealed housing <NUM>. The rotary sleeve <NUM> is retained on the elongate shaft <NUM> by a retaining bracket <NUM>. The rotary sleeve <NUM> and elongate shaft <NUM> are sealingly spaced apart by a seal ring. The frictional effect of the seal ring ensures that the elongate pivotal arm <NUM> pivots restrainedly between the fully extended position and the fully flexed position. This slows the descent of the electrochemical sensor <NUM> into the receptacle <NUM> and the immersion of the electrochemical sensor <NUM> in the aqueous sample is controlled. The benefits of this are demonstrated in the Example hereinafter.

The mounting <NUM> for the electrical contact pins <NUM> is detachable from the elongate pivotal arm <NUM> and is shown detached in <FIG> (front view), 5b (rear view) and 5c (side view). The mounting <NUM> comprises a flexible polymer body <NUM> which sealingly encapsulates the electrical contact pins <NUM> such that the electrical contact pins <NUM> are protuberant from a front face of the body <NUM> (see <FIG>). The electrical contact pins <NUM> are disposed in such a way as to be able to apply a potential to the electrical contacts of the reference electrode, counter electrode and working electrodes of the electrochemical sensor <NUM>. The electrical contact pins <NUM> are electrically connected at the rear to the potentiostat by a connector <NUM> (see <FIG>). To enable the mounting <NUM> to be mounted flush with the distal end of the elongate pivotal arm <NUM>, opposing corners of the flexible polymer body <NUM> are equipped with a socket <NUM>, <NUM> to receive a threaded fastener.

When the pivotal forearm <NUM> is fully closed, the electrochemical sensor <NUM> can be forcibly inserted into slot <NUM> (see <FIG>). A slanted part <NUM> of the front face of the polymer body <NUM> abuts the electrochemical sensor <NUM> through the window <NUM> (see <FIG> and <FIG>). By virtue of the flexibility of the polymer body <NUM>, the abutment of the slanted part <NUM> and the electrochemical sensor <NUM> is biased so as to retain the electrochemical sensor in the slot <NUM>.

The pivotal forearm <NUM> is retained in the fully closed position by a magnetic catch. A male part 5a of the magnetic catch is mounted on the anterior wall <NUM> of the pivotal forearm <NUM> and is cooperable with a female part 5b of the magnetic catch mounted on the distal end of the elongate pivotal arm <NUM>.

When the elongate pivotal arm <NUM> is in the fully flexed position (see <FIG>), the receptacle <NUM> constrains the pivotal forearm <NUM> to a partially open position. To achieve this, a part of the anterior wall <NUM> extends below the rim of and into the receptacle <NUM>. in the partially open position, the slanted part <NUM> and the electrochemical sensor <NUM> are no longer in abutment and the electrochemical sensor <NUM> dismounts freely from slot <NUM> into the receptacle <NUM> without exposing the operator to the electrochemical sensor <NUM> or the electrical contact pins <NUM> (see <FIG>).

The electroanalytical instrument further comprises a reed switch or optical switch which in response to an operating position of the elongate pivotal arm <NUM> switches on the potentiostat. The operating position is at or near to the fully flexed position.

A temperature probe <NUM> extends inferiorly from an inferior face <NUM> of the elongate pivotal arm <NUM>. When the elongate pivotal arm <NUM> is in the fully flexed position, the temperature probe <NUM> is immersed in the aqueous sample in the receptacle <NUM>.

An experiment was carried out to determine whether the manner in which electroanalysis was carried out could have an effect on the performance of an electrochemical sensor.

A <NUM>/L free chlorine solution was prepared and tested using a single batch of ChlorosenseR electrochemical sensors. Free chlorine readings were taken at immersion times* of <NUM> and <NUM> seconds.

The results are presented in the Table below and in <FIG>.

Claim 1:
An electroanalytical instrument for determining by amperometry the presence or quantity of an analyte in an aqueous sample comprising:
a sealed housing (<NUM>) which is compartmentalised, wherein the sealed housing (<NUM>) has an exterior compartment defining a receptacle (<NUM>) for the aqueous sample and interior compartments;
an elongate pivotal arm (<NUM>) mounted pivotally at a proximal end on the sealed housing (<NUM>), wherein the elongate pivotal arm (<NUM>) houses a potentiostat;
a pivotal forearm (<NUM>) joined pivotally to a distal end of the elongate pivotal arm (<NUM>), an electrochemical sensor (<NUM>) capable of being mountable in or on an anterior wall (<NUM>) of the pivotal forearm (<NUM>) to extend inferiorly to the elongate pivotal arm (<NUM>); and
(A) a mounting (<NUM>) for electrical contact pins (<NUM>) which is mounted sealingly on the distal end of the elongate pivotal arm (<NUM>), or
(B) electrical contact pins (<NUM>) on the distal end of the elongate pivotal arm (<NUM>),
wherein the electrical contact pins (<NUM>) are electrically connectable to the potentiostat to apply a potential to the electrochemical sensor (<NUM>),
wherein the elongate pivotal arm (<NUM>) is configured to pivot between a fully extended position and a fully flexed position whereat an inferior part of the electrochemical sensor (<NUM>) may be immersed in the aqueous sample in the receptacle (<NUM>) and wherein the pivotal forearm (<NUM>) is configured to pivot between a fully open position whereat the electrical contact pins (<NUM>) are exposed and a fully closed position whereat the electrical contact pins (<NUM>) are configured for positioning in operative contact with the contacts on the superior part of the electrochemical sensor (<NUM>) whereby when the elongate pivotal arm (<NUM>) is in the fully flexed position and the pivotal forearm (<NUM>) is in the fully closed position the electrical contact pins (<NUM>) are configured to interface with the electrochemical sensor (<NUM>) and the aqueous sample.