AN ELECTROCHEMICAL SAMPLING DEVICE

Abstract: An electrochemical sampling device comprising a flow through electrochemical cell (1) containing a porous work electrode (16) through which the cell directs the flow of a sample solution from a peristaltic pump for deposition in the electrode of analyte from the sample. The porous electrode can be formed from reticulated vitreous carbon (RVC) or can include a conductive polymer to deposit analyte by complexing therewith. In the case of an RVC porous electrode deposition is preferably effected by electrode position and the electrode can be precoated with mercury or other metals to enhance deposition. A dual porous electrode version of the cell is also disclosed in which the upstream electrode can be used to release a reagent to enhance deposition at the downstream electrode. The upstream electrode can also be used to remove interferents for example oxygen, or as a means of collecting a different analyte to the downsteam electrode.

Description: "AN ELECTROCHEMICAL SAMPLING DEVICE" TECHNICAL FIELD This invention relates to analytical sampling of solutions for analytes and in particular to an electrochemical sampling device. BACKGROUND ART A most crucial step in analytical sampling is the procruing of a quantity of analyte ' which is representative of the amount contained in the sample. Advances in analytical instrumentation have provided sensitive, selective methods capable of determining ultra trace levels of analyte even in very complicated samples. However, the problems involved in conventional analytical sampling, including sample contamination, altering the chemical form (changing speciation) and changes in concentration due to adsorption or desorption effects during transportation and storage often introduces a substantial error to the analytical result. Furthermore, conventional sample collection, in particular for liquids, can be relatively cumbersome involving collection of many litres of sample to ensure it is representative. Such an operation can be very expensive and, if dealing with toxic solvents or solutes, can also be dangerous. Electrodeposition, wherein the product of an electrochemical reaction is insoluble in the solvent or is adsorbed onto the electrode surface, has been used by various workers in analytical chemistry. Electrogravi etric analysis, electrochemical stripping analysis and in-situ sampling on graphite followed by neutron activation analysis are known methods which using relatively simple instrumentation and under appropriate conditions can attain 100% deposition efficiency and selectivity can be controlled by adjusting the potential of the working (deposition) electrode. The use of electrodeposition as a sampling means also allows the 9 analyst to retain some information about the form of the analyte in solution. Hitherto electrodeposition techniques have, however, required samples to be collected and transported to the laboratory so that difficulties of the type mentioned above are encountered. It is an object of this invention to provide an electrochemical sampling device which will overcome or at least ameliorate the above disadvantages. DISCLOSURE OF INVENTION Accordingly, this invention consists in an electrochemical sampling device comprising a flow through electrochemical cell containing a porous working electrode through which the cell directs the flow of a sample ■ solution for desposition in the electrode of analyte from the sample. The porous electrode is preferably formed from reticulated vitreous carbon (RVC) or in another preferred form can include a conductive polymer to deposit analyte by co plexing therewith or electrodeposition or electrochemically induced polymerisation. For preference an RVC porous electrode cartridge is used and deposition is effected by electrodeposition. The cartridge is removeable and can be precoated with various materials and preferably amalgam forming metals for example mercury or copper to enhance electrodeposition. It is preferred that a reference electrode is positioned in the .cell downstream of the porous electrode. In an alternate embodiment-the electrochemical cell includes a second porous working electrode disposed such that the sample solution flows through one porous electrode and then the other. Both porous working electrodes can be disposed upstream of the reference electrode or one porous work electrode can be disposed upstream of the reference electrode and the other disposed downstream. In a preferred form the upstream porous electrode can be used to release a reagent to enhance deposition at the downstream electrode. The release of reagent is preferably controlled electrochemically by the application of a potential to the upstream porous electrode or the first electrode may be used to remove interferent eg. oxygen or simply as a means of collecting different analytes on the two different cartridges. The sample solution is preferably pumped through the electrochemical cell by a pump of known type and the potential is applied between the reference and porous work electrode using a potentiostat of substantially conventional construction. A major advantage of the sampling device according to this invention is that the analyte can be collected in the porous electrode cartridge and thereafter transported and stored with minimum degradation problems. In addition, cartridge volume is small compared to the volume of sample solution from which the analyte is collected. Thus, transportation and storage problems can largely be overcome. In this regard It will also be apparent that the relatively simple nature of the sampling device of this invention allows it to be embodied in a portable instrument so that sampling can be effected by collecting analyte in the cartridge on site. This allows transportation and storage problems to be further avoided. It will be apparent that the electrode cartridge can thus be transported, stored and when convenient analysed in the laboratory. The steps of stripping the analyte from the cartridge and analysing the analyte can be combined if electrochemical stripping techniques such as voltammetric or potentiometric stripping analysis are used. These analyses can be performed in the electrochemical cell according to this invention or in another suitable cell. Alternatively, the analytes can be stripped from the cartridge using chemical oxidation or a suitable solvent and subsequently analysed using known analytical techniques. The selectivity of the sampler may be controlled either chemically or electrochemically. Thus, suitable procedures allow information on speciation to be retained. It will also be appreciated that the sampling device of this invention is particularly suited to automation since few control steps are required in the analyte collection procedure. Embodiments of this invention will now be described with reference to the accompanying drawings and an example. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a sectional elevation of a flow through electrochemical cell forming part of the invention; Figure 2 is a view similar to Figure 1 of another embodiment of the electrochemical cell; and Figure 3 is a view similar to Figures 1 and 2 of a further embodiment of the electrochemical cell. BEST MODE-FOR CARRYING OUT THE INVENTION As shown in Figure 1 a flow through electrochemical .cell 1 has a cylinderical body 2 with an axially extending bore 3. One end of bore 3 has an enlarged portion 4 which terminates in a still larger internally threaded portion 5. The other end of bore 3 terminates in an enlarged internally threaded portion 6. Two threaded holes 7, 8 extend from the outside of body 2 and communicate with bore 3. Hole 7 is fitted with a screw in Ag/AgCl reference electrode 9 of known type and a screw in platinum wire auxiliary electrode 10 is fitted into hole 8. This electrode can also be a platinum coil or RVC or conductive plastic or carbon cloth material and be wrapped around the cartridge described below. An end cap 11 attaches, to body 2 by means of a threaded extension 12 which engages threaded portion 5. An o-ring 13 provides a seal between end cap 11 and body 2. End cap 11 has an axially extending bore 14 which has an enlarged portion 15 at threaded end 12 of the cap. Enlarged portion 15 of the cap and enlarged portion 4 of the body form a cylinderical chamber into which a removeable porous work electrode cartridge 16 fits. The outer end of bore 4 terminates in an internally threaded portion 17 for connection with a sample supply line (not shown) . A threaded hole 18 extends obliquely into end cap 11 and communicates with enlarged portion 15 via a small aperture 19. Threaded hole 18 receives a screw-in electrode fitting 20 from which a platinum wire extends through aperture 19 to make electrical contact with porous electrode 16. An end cap 21 attaches to the other end of body 2 by means of a threaded extension 22 which engages threaded portion 6. An o-ring 23 provides a seal between end cap 21 and body 2. End cap 21 has an axial bore 24 which communicates with bore 3 of the body and terminates at the outer end of the cap 21 in an internally threaded portion 25 for connection to a sample discharge line (not shown) . The porous electrode cartridge may be formed from a reticulated vitreous carbon (RVC) cylinder with a plastics protective coating on the exterior surface of the cylinder. RVC is commercially available from E.R.G. of Oakland California USA in block form. The electrode cartridges are cut from these blocks and the coating is applied by heat shrinking. Various porosities of RVC are . available and porosity is measured in terms of the number of pores per inch (ppi). • The electrochemical cell is connected with a potentiostat with a facility for current monitoring via an auxiliary electrode and the sample is pumped through the cell by means of a peristaltic pump. Both of these devices are of known construction and are therefore not illustrated or described in detail. The RVC electrode cartridge 19 forms the working electrode of the cell and the potential is applied between the cartridge and reference electrode 9 and auxiliary elextrode 10 is connected with the potentiostat (not shown) for current measurement in the known manner. The electrode cartridge is kept at ground and the potential varied by adjusting the potential at reference electrode 9. The potential is usually varied in the range of from +2 volts to -2 volts. The sample solution is pumped through cell 1 by means of a peristaltic pump (not shown) acting on a supply line (not shown) connected to end cap 11 through threaded portion 17. It will thus be apparent that in the Figure 1 embodiment the reference electrode 9 is positioned downstream of the porous electrode or cartridge 19. In addition, the cartridge 19 is elongate and the sample flows substantially longitudinally through the electrode. In use a sample solution is pumped through the cell ' and a potential appropriate to the analyte being deposited is applied. As the solution passes through the porous electrode the analyte is electrodeposited in the cartridge. Once the analyte is preconcentrated on to the cartridge a variety of analytical techniques can be used to determine the amount of analyte collected and hence concentration of analyte in the sample. These include direct voltammetric stripping analysis, electrochemical stripping analysis, chemical stripping analysis, and potentio etric stripping analysis. It has been found that using a suitable flow rate and potential a 100% electrodeposition of analyte in the cartridge can be achieved. For a cylinderical porous electrode or cartridge 10 mm in diameter and 18 mm long of 10 pores per inch porosity, the flow rate should be between 0.6 mL/ in and 2.0 mL/min and optimally less than 1.0 mL/min. It has also been found that if the RVC cartridge is precoated with mercury, copper or some other amalgam forming metal the electrodeposition for some species is enhanced and flow rates can be increased whilst maintaining 100% deposition. Scanning electron micrographs show that a mercury coating applied to the RVC cartridge in a stationary electrochemical cell is evenly distributed as spheres on the electrode surface and that up to 1 mg can be deposited on the cartridge described above without altering pore sign signi icantly. Experiments have shown that using a cartridge precoated with mercury Cu 2+ can be preconcentrated from nitrate or acidified chloride media at levels of from 20 ppb to 1.0 ppm with 100 - 5% recovery. Investigations of the effect of porosity on electrodeposition have found that the percentage increased with increasing electrode surface area or decreasing porosity. Optimum RVC porosity was found to be 10 ppi or less. Example An RVC cartridge as described above was pretreated electrochemically in 0.5M NaN0 3 by applying a potential of -0.9V to a Ag/AgCl reference electrodge for 2 minutes and sweeping at 2mV/sec to a potential of +1.0V and holding at this potential for 2 minutes. The cartridge was precoated with mercury in a stationary electrochemical cell and then fitted to a flow through electrochemical cell of the type described above. A sample of industrial effluent containing ppb level Pb was obtained. The Pb was deposited on the precoated RVC cartridge by passing 320 mL of sample (flow rate 1 mL/min) through the cell. The lead content was analyzed by: (i) Direct ASV. A differential pulse stripping waveform was employed and using a calibration curve, a value of 105ppb Pb 2+ was obtained. (ii) Electrochemical stripping and AAS. Lead was stripped from the cartridge by applying 0.0 V vs Ag/AgCl and flushing at 0.1 mL/min for 20 minutes then 2 mL/mion for ten minutes. • A value of 108 ppb Pb 2+ was obtained.. (iii) Chemical stripping - AAS. Lead was stripped using 8M HN0_(3 mL) - six flushes. A value of 108 ppb was obtained. These results compared well with a value of 90 ppb obtained using an evaporation/AAS procedure. It is likely that some lead is lost during the sample pretreatment in this method. Figure 2 shows an electrochemical cell 1 similar to that of Figure 1 except that provision is made for a second porous work electrode cartridge 16. The same reference numerals as Figure 1 are used to denote the same features. In the Figure 2 embodiment two end caps 11 are employed and the bore 3 has enlarged portion 4 at each end so as to form with end caps 11 two cylinderical chambers for two cartridges 16. It will be apparent that the second cartridge 16 is disposed such that the sample solution flows through one porous electrode cartridge and then the other. In addition the reference electrode 9 is in the centre of the cell so that one of the porous electrode cartridges 16 is disposed upstream of the reference electrode and the other electrode cartridge 16 can be pretreated so as to release a reagent to enhance deposition at the downstream electrode. The release of reagent can be controlled electrochemically and an example of a suitable reagent is Hg 2+. Mercury is deposited on the RVC cartridge in- the manner described above for the Figure 1 embodiment and is released electrochemically by the application of a potential with respect to the coated upstream electrode cartridge. Figure 3 shows a further embodiment of the electrochemical cell in which provision is made for a second porous electrode cartridge 16 with both cartridges disposed upstream of the reference electrode. Again the same reference numerals are used to denote the same features. Provision is made of the second upstream cartridge by fitting a body extension 2A between end cap 11 and body 2. The body extension has an axial bore 3A with an enlarged portion 4A at each end. The enlarged portion 4A with corresponding enlargements 4 and 15 cylinderical cavities for cartridges 16 on the upstream side of reference electrode 9. The upstream electrode cartridge 16 can be used to release reagent or provide for additional deposition as described for the Figure 2 embodiment. The electrochemical cells of the Figure 2 and 3 embodiments require a dual electrode potentiostat. This can be of substantially conventional construction. In the case of the Figure 3 embodiment the electrode cartridge closest to the reference electrode 9 is kept at ground whilst the potential of the reference electrode is adjustable. The second remote or upstream cartridge electrode has a potential applied thereto with reference to the other electrode cartridge. The use of the electrochemical cells of the Figure 2 and 3 embodiments in this invention is otherwise identified to that described above for the Figure 1 embodiment. The foregoing describes specific embodiments of this invention and it will be appreciated that alternate embodiments are possible. For example, the flow through electrochemical cell can be arranged such that the flow is achieved by gravitational force. This con iguration is particularly suitable for a cell that is submersible in the sample solution. In one arrangement of this type the cell is contained in a vertically disposed column with an inlet at its top and a reservior at the bottom. The column is submerged in the sample which flows into the inlets through the cell and into the reservior under gravitational force. In addition the porous work electrode cartridge can include a conductive polymer to deposit analyte by complexing therewith. Suitable polymers include polypyrrole, polythiophene, polyphenol polymers and derivatives of the above.

Claims: CLAIMS 1. An electrochemical sampling device comprising a flow through electrochemical cell containing a porous working electrode through which said cell directs the flow of a sample solution for deposition in the electrode of analyte from the sample. 2. A sampling device as claimed in claim 1 wherein said electrode is formed from reticulated vitreous carbon. 3. A sampling device as claimed in claim 1 or claim 2 wherein said electrode includes a conductive polymer to deposit analyte by complexing therewith. 4. A sampling device as claimed in claim 2 wherein the porosity of the electrode is less than 10 pores per inch. 5. A sampling device as claimed in claim 2 wherein deposition is effected by electrodeposition. 6. A sampling device as claimed in claim 5 wherein said porous electrode is precoated with an amalgam forming metal to enhance electrodeposition. 7. A sampling device as claimed in any one of claims 1 to 6 wherein said porous electrode comprises a removeable cartridge in which the analyte is deposited. 8. A sampling device as claimed in any one of claims 1 to 7 wherein said porous electrode is elongate and said sample solution flows longitudinally through the electrode. 9. A sampling device as claimed in any one of claims 1 to 8 wherein the sample flows through said cell at a rate in the range from 0.6 mL/min to 2.0 mL/min. 10. A sampling device as claimed in any one of claims 1 to 9 wherein a potential in the range of from +2 volts to -2 volts is applied between a reference electrode and the porous electrode. 11. A sampling device as claimed in any one of claims 1 to 6 wherein a reference electrode is positioned in said cell downstream of said porous electrode. 12. A sampling device as claimed in any one of claims 1 to 10 wherein said cell includes a second porous working electrode disposed such that said sample solution flows through one porous electrode and then the other. 13. A sampling device as claimed in claim 12 wherein both said porous electrodes are disposed upstream of a reference electrode. 14. A sampling device as claimed in claim 12 wherein gne of said porous electrodes is disposed upstream of a reference electrode and the other electrode is disposed downstream. 15. A sampling device as claimed in any one of claims 12 to 14 wherein the upstream porous electrode releases reagent to enhance deposition at the downstream electrode. 16. A sampling device as claimed in claim 15 wherein the release of reagent is controlled electrochemically by the application of a potential with respect to the upstream porous electrode. 17. A sampling device as claimed in claim 16 wherein mercury is released from the upstream cartridge. 18. An electrochemical sampling device substantially as described with reference to Figure 1 or Figure 2 or Figure 3 of the accompanying drawings.

Inventors: WALLACE GORDON GEORGE, WALLACE, GORDON, GEORGE
Applicants: WALLACE GORDON GEORGE, WALLACE, GORDON, GEORGE
Application year: 1986
Main classification: C25B11
IPC codes: G01N27