Electrochemical detector system
FIELD OF THE INVENTION 
     The present invention relates to an electrochemical detector and to a method for detecting one or more electroactive species. 
     BACKGROUND OF THE INVENTION 
     In the past electrochemical detector techniques have been widely used for on-line voltammetry and low resolution liquid chromatography. However despite the rapid advances that have been made in the column technology of high pressure liquid chromatography, a serious limitation to development and use of this method has been the lack of a suitable detector system. High pressure liquid chromatography involves the resolution of substances which are only present in the eluting solution in trace amounts. The requirements of an electrochemical technique to detect such trace eluents are (1) high sensitivity (2) detectors with ultra-low cell volume (3) a wide range of response (4) large linear range of operation (5) little influence of experimental parameters, e.g. temperature, flow rate etc. and (6) long term stability and reliability. 
     SUMMARY OF THE INVENTION 
     The present invention provides an electrochemical cell and controller unit which fulfill most of the above requirements and in addition overcome many of the conventional problems encountered in on-line voltammetry. 
     According to the present invention we provide an electrochemical detector having an electrochemical thin-layer flow cell adapted to operate on the wall-jet principle and comprising a sensor electrode, a solution inlet line terminating in a fine nozzle, the said nozzle being located normally to but displaced from a surface of the sensor electrode, at least one solution exit line, at least one other electrode and means comprising a controller unit for applying a periodically changing voltage to the sensor electrode so that a first part of each cycle of the applied voltage functions as a detecting potential and second part of each cycle of the applied voltage functions as a cleaning potential. 
     According to the invention we also provide a method for detecting one or more electroactive species in a flow stream by means of the said electrochemical detector. 
     In a preferred embodiment of the invention an inlet solution impinges as a jet normally at the centre of a planar surface of a disc-shaped sensor electrode, the electrode consisting of high density vitreous carbon. 
     The cell design of the present invention is based on the socalled wall-jet principle which gives very high efficiency in terms of mass transfer to the electrode irrespective of cell volume and thus allows detection of substances down to the sub-nanogram (10 -9  g) range. The application of the wall-jet principle to an electrochemical flow cell was first disclosed by Yamada J. and Matsuda H. in J. Electroanal. Chem. 44 189 (1973). A wall-jet electrode in-line with a high pressure liquid chromatography column was first disclosed by the present inventor and another in J. Chromat. Sci. 12 747 (1974). Although such an arrangement was used successfully to detect electroactive species only present in the column effluent in trace amounts, the electrochemical cell had to be frequently dismantled in order to clean the sensor electrode surface by mechanical means e.g. polishing. Products of electrolysis tend to accumulate at the sensor electrode surface which, unless removed, unfavourably affect the working of the sensor electrode and the accuracy of the results obtained. The present invention provides a means for continuously removing products of electrolysis from the sensor electrode surface thus maintaining a clean working surface for long periods of time. A periodically changing voltage is applied to the sensor electrode so that a first part of the applied voltage functions as a detecting potential and a second part of the applied voltage functions as a cleaning potential. A stable, reliable and continuously operable electrochemical detector suitable for commercial application is thus provided. 
     In a preferred embodiment of the invention a reference electrode and a tubular counter electrode are located radially to the disc electrode, the tubular counter electrode being contained in a solution exit line. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     A particular embodiment of the invention and method by which it is to be performed will now be described with reference to the accompanying drawings wherein: 
     FIG. 1 is a schematic diagram of the invention. 
     FIG. 2 is a detail from FIG. 1 to show the working of a wall-jet electrode, 
     FIG. 3 is a diagrammatic representation of a square wave voltage potential. 
     FIGS. 4 and 5 are chart recordings with respect to examples 1 and 2 respectively. 
     FIG. 6 is a table illustrating the electroactivity of a range of organic functional groups which can be detected by the electrochemical detector of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As shown in FIG. 1 an electrochemical flow cell 1 has an inlet line 2, a sensor electrode 3, an exit line 4, a tubular counter electrode 5 and a reference electrode 6. The inlet line 2 terminates in a fine nozzle and is located normally to the centre of a planar surface of the disc sensor electrode 3. The distance between the fine nozzle and the sensor electrode surface forms the axial limit of a cylindrical space 8 which includes an &#34;effective cell volume&#34;. The sensor electrode 3 is held in position by means of a screw mount 9. The cylindrical space 8 and hence the effective cell volume can thus be varied by fine adjustment of the screw mount 9. With a disc sensor electrode 3 mm in diameter the &#34;effective cell volume&#34; could be varied from less than 1 μl to 100 μl (10 -6  l). In place of a disc-shaped sensor electrode a ring or a split-ring electrode 3, as indicated by dotted lines in FIG. 2, may be employed. 
     The tubular counter electrode 5 and the reference electrode 6 are located radially to the disc electrode 3 and diametrically opposed to each other. The reference electrode 6 contacts the solution through a porous junction 10 and the tubular counter electrode 5 is located in exit line 4, both of which are necessarily downstream of the sensor electrode 3. An alternative to the electrode mode shown in FIG. 1 is a two electrode mode consisting of a sensor electrode and reference electrode only. The provision and mode of use of reference and counter electrodes is per se known. In an alternative embodiment of the invention the reference electrode 6 is contained in a second solution exit line either with or without a counter electrode present. In any of these electrode modes the potential across the cell is controlled and monitored by a controller unit 11 which is connected to a recorder unit e.g. a pen recorder 12. 
     The sensor electrode 3 is preferably made of high density vitreous carbon due to its wide operating potential range, typically from +1.3V to -1.0V vs. calomel reference electrode, and its relative freedom from surface film effects. 
     Conventional electrode materials such as platinum, gold or impregnated graphite may alternatively be employed. 
     The operation of the electrochemical cell 1 and controller unit 11 will now be described. 
     The solution, for example the effluent from a high performance, high pressure liquid chromatography column, enters the cell 1 through the inlet line 2 and is forced by that pressure through the fine nozzle 7 to form a jet of solution. This jet impinges at the centre of the disc sensor electrode 3 and spreads radially over the surface of the electrode in a thin film as shown in FIG. 2. 
     An electrochemical cell, operating on the wall-jet principle as just described, includes an &#34;effective cell volume&#34; and a &#34;dead cell volume&#34;. The &#34;effective cell volume&#34; is contained in the cylindical space 8 as stated above, whilst any solution downstream of the sensor electrode is contained in the &#34;dead cell volume&#34; and has no effect on the sensitivity of the cell. After flowing over the sensor electrode surface the solution passes into the cylindrical cell volume where it makes contact with the reference electrode 6 via the porous junction 10 and then leaves the cell via the tubular counter electrode 5. 
     To control and monitor the potential across the cell 1 the controller unit 11 applies a periodically changing voltage to the sensor electrode 3 such that a first part of the applied voltage functions as a detecting potential and a second part of the applied voltage functions as a cleaning potential. Any products of electrolysis which have accumulated at the sensor electrode surface during a detecting step are thus re-oxidised or re-reduced during a cleaning step and the sensor electrode surface area is maintained at a substantially constant value. The voltage is applied either in the conventional potentiostatic configuration employing the three electrode system in which the reference electrode 6 is used to monitor and control the potential of the sensor electrode 3, or in the standard two electrode system. In either system the amplitude and polarity of the detecting potential may be varied. The sensor electrode 3 may therefore operate as a cathode for the detection of electroreducible species or as an anode for the detection of electrooxidisable species. 
     The controller unit includes a switching circuit so that the periodically changing voltage applied to the sensor electrode may have a square wave voltage formation which may be regarded as a periodic voltage, which may be varied in amplitude, added to a variable d.c. voltage reference level. An example of such a square wave is shown in FIG. 3 in which voltage V is measured along the ordinate and time t along the abscissa. The amplitude (a) and (b) of each polarity of the square wave may be independently varied according to the electroactive species under investigation and what other species are present. In FIG. 3 if the electroactive species is electro-oxidisable then (a) is the detecting potential and (b) is the cleaning potential. The polarity of the cleaning potential is selected so as to ensure that any products of electrolysis which may have accumulated at the electrode during the detecting potential, for example by electro adsorption, are (in this example) rereduced and desorbed from the surface. The electrode surface is thus cleaned and ready for the next detecting step. Alternatively, depending on the species under investigation, sufficient desorption from the electrode surface may occur if the cleaning potential is equal to zero or even of the same polarity as the detecting potential provided that (b) does not exceed (a). 
     Similarly, in FIG. 3 time components (c) and (d) of each potential level of the square wave may be independently altered in order to achieve the optimum working conditions for the species under investigation. The current is only measured for a short period, for example the period (e) corresponding to the time required for one cycle of the local mains voltage supply, (usually 50 Hz or 60Hz), towards the end of the detecting potential level. The principal of this measuring technique is well established and arises from the need to discriminate the faradaic current due to reduction or oxidation of the measured species from the capacitative double layer charging current which is intially large but decays rapidly. 
     Typical applications for which the electrochemical detector is suitable are the continuous monitoring of trace electroactive materials in water, continuous flow anodic stripping voltammetry and organic electroanalytical chemistry. Examples of electroactive materials which can be detected by the present invention include heavy metals such as mercury, lead, copper and cadmium, as well as electroactive organic species such as phenols. In continuous flow anodic stripping voltammetry a small amount of mercuric ions is added to the inlet solution and the metal ion i of interest is simultaneously deposited as an amalgam in a thin mercury film on the surface of the carbon electrode. During the subsequent electrolytic dissolution of the film the anodic current is used to measure the initial concentration of the metal ion i in the solution. 
     FIG. 6 is a table illustrating the electroactivity of a range of organic functional groups which can be detected by the electrochemical detector of the present invention. The following examples are illustrative of its application in combination with a high performance, high pressure liquid chromatography column having duct means for an effluent from the column to enter the solution inlet line of the electrochemical cell. 
     EXAMPLE 1 
     5 μl of a solution containing 10 -2  g/l of (1) methyl-4-hydroxybenzoate in 20% MeOH/Water V/V (containing 0.1M KNO 3 ) and 5 μl of a solution containing 10 -2  g/l of (2) n-propylhydroxybenzoate in 20% MeOH/Water V/V (containing 0.1M KNO 3 ) were injected into the head of a 50 cm ODS &#34;Permaphase&#34; (Registered Trade Mark) column and eluted at a flow rate of 1.2 ml/min. The effluent from the column was fed into the inlet line of the electrochemical cell and a square wave voltage at a frequency of 5 Hz was applied to a high density vitreous carbon sensor electrode. Using a detecting potential of +1.2 V vs. Ag/AgCl for 160 ms of each cycle and a cleaning potential of -0.2 V for the remaining 40 ms of each cycle, both compounds (1) and (2) were detected. A distinct peak was recorded by the pen recorder for each compound as shown in a chart recording illustrated in FIG. 4 in which current is measured along the ordinate and time along the abscissa. 
     EXAMPLE 2 
     5 μl solutions of each of (1) 4-methoxy-phenol, (2) 4-aminophenol, (3) 4-nitrophenol and (4) 4-chlorophenol each in MeOH/Water 5% V/V (containing 0.1 MKNO 3 ) at a concentration of 10 -2  g/l were injected into the head of a 50 cm ODS &#34;Permaphase&#34; (Registered Trade Mark) column and eluted at a flow rate of 1.0 ml/min. The effluent from the column was fed into the inlet line of the electrochemical cell and a square wave voltage at a frequency of 5Hz was applied to a high density vitreous carbon sensor electrode. 
     Using a detecting potential of +1.1 V vs. Ag/AgCl for 160 ms of each cycle and a cleaning potential of -0.2 V for the remaining 40 ms of each cycle, each of the phenol derivatives (1,2,3 and 4) were detected. A chart recording, having current measured along the ordinate, and time along the abscissa, made by the pen recorder for the present example is illustrated in FIG. 5.