WO2003019133A2

WO2003019133A2 - Capillary free voltammetric flow cell

Info

Publication number: WO2003019133A2
Application number: PCT/IL2002/000704
Authority: WO
Inventors: Vladimir Dozortsev
Original assignee: Ionguard Ltd.
Priority date: 2001-08-23
Filing date: 2002-08-25
Publication date: 2003-03-06
Also published as: WO2003019133A3; AU2002328138A1

Abstract

The present invention provides a voltammetric apparatus (10) comprising a) an electroanalytical cell comprising a mercury meniscus electrode as a working electrode (14), a working electrode channel (14) a counter electrode (18), a reference electrode (16), and a measurement channel (24); b) a purification container in connection with the cell for purifying contaminants from the working electrode; and c) a means for delivering the resulting purified mercury from the purification container to the working electrode. The present invention further provides a method for trace element detection comprising the steps of i) providing a voltammetric apparatus comprising a) an electroanalytical cell, the cell comprising a mercury meniscus electrode as a working electrode, a reference electrode, a working electrode channel, and a measurement channel; b) a purification container in connection with the cell for purifying contaminants from the working electrode; and c) a means for delivering the resulting purified mercury from the purification container to the working electrode; ii) introducing a first sample into the measurement channel; iii) delivering mercury from the bottom of the working electrode channel towards the measurement channel, thereby forming a mercury meniscus in contact with the measurement channel, thereby forming the mercury meniscus electrode; and iv) performing at least one measurement, thereby detecting the trace elements in the sample.

Description

CAPILLARY FREE NOLTAMMETRIC FLOW CELL

FIELD OF THE INVENTION

[001] The present invention generally relates to a Noltammetric apparatus for trace element detection, and more particularly to an improved Noltammetric apparatus which is based on a static dropless mercury electrode, in which the mercury can be purified and reused.

BACKGROUND OF THE INVENTION

[002] Electrochemical detectors and Noltammetric cells are known in the field and have been used successfully for the analysis of trace elements in the laboratory. The electrode cell comprises of a working electrode, an auxiliary (counter) electrode and a reference electrode, which are intended to establish and maintain a constant potential relative to the working electrode. In many cases the working electrode becomes polluted with time and as a result the measured signal is decayed or shifted.

[003] In order to avoid such poisoning, a Dropping Mercury Electrode has been used. US 3,922,205 describes the basic structure of a polarografic cell. An automated polarographic cell is described by C.Ν. Yarnitzky (Analytical Chemistry, Vol. 57 No. 9, Aug. 1985, p. 2011-2015). US 4,260,467 describes a dropping mercury electrode, which comprises a reservoir for liquid mercury, a mercury capillary at the outlet end of which mercury drops are formed and a valve for selective air-purging passage of mercury from the reservoir to the inlet end of the capillary. US 4,138,322 discloses a structure of a shielded dropping mercury cathode.

[005] In general, there are two types of measurement techniques based on dropping mercury electrode: Direct Voltammetry (DN), and Stripping Noltammetry (SN). In the direct method mercury continuously flows via a glass capillary tube and the measurement is performed while the drop exists in its place, at the tip of the electrode. The used mercury is collected and periodically removed. The average drop lifetime ranges between 3 to 15 seconds, depending on the capillary used. This method is not satisfactory for very low concentration analysis. The Stripping

Noltammetric method consists of a two-step process: The first step is a pre-concentration process where electroactive ions are accumulated on the mercury drop surface by a reduction or adsorption process. This stage is followed by a stripping step in which the material species are stripped out rapidly, at their Red-Ox potential and the resulting current is measured and related to the original solution.

This method requires the mercury drop to remain for an elongated period of time (few minutes), detrained by the time needed for the accumulation stage. In this case, Static Mercury Dropping Electrode (SMDE) is needed. SMDE is a glass capillary tube provided with mechanical arrangements to "freeze" the drop at the tip of the capillary tube long enough to perform the analysis.

[006] All commercial Noltammetric apparatuses known in the art for the last decades, which use mercury as a working electrode, are characterized by a mercury drop, which is formed at the end of a glass capillary tube, the tip of which is exposed to the sample solution. The unique physical properties and surface tension between the mercury and the glass enable repeatability and stability of the drop, thus provide the means for simple and trivial methods for renewing the electrode surface forming new drops.

[007] However, such cells are not satisfactory and have several drawbacks: i) working electrode made of glass capillary tubes cannot resist some chemicals such as hydro-fluoric acid; ii) the mercury "feed and freeze" apparatus is complicated; and iii) mercury capillary tubes get affected by impurities existing in the sample, particles tend to clogged the capillary tube while surface active materials tend to be adsorbed on the internal surface of the capillary tube causing irreproducible drops and poor stability. Therefore, the capillaries must be replaced frequently, involving human exposure to the mercury. The contact with mercury can cause serious health and ecological hazards of which even the previously mentioned improved voltammetric cell is not free. These drawbacks are common to all the mercury voltammetric cells of the prior art.

[008] Since solid electrodes cannot be used directly and since the use of mercury is beneficial in many aspects, various attempts were made to overcome this problem by using solid working electrodes that undergo, prior to analysis, an electroplating process, during which a thin film of mercury is formed on the surface of the electrode. Yet, this electrode also suffers from many drawbacks as the thin film of mercury is even more sensitive to surfactants than the mercury drops, and additionally, high amount of very toxic ionic mercury is released to the drain.

[009] Moreover, the accumulation step in the Stripping Noltammetry method calls for fresh ions to be provided to the surface of the electrode, for example by intensive mixing. However, proper agitation requires high volume and cumbersome measuring cell with many mechanical parts. A voltammetric cell in which intensive agitation of the sample has been achieved by its pumping via a small diameter flow throw channel is described by van den Berg (Analytica Chimica Acta 346 (1997) p. 101-111). Yet this apparatus has many limitations and drawbacks. The small diameter of the measuring tube requires tiny mercury drops that can not be produced in a simple way, a complicated mechanism of drop formation is described in order maintain repeatability. Even so flow rate (or tube diameter) is limited in order not to cut off the mercury drop from the glass capillary. At the end of the analysis mercury drops flow to the drain with the sample.

[0010] There is therefore a need to provide a voltammetric apparatus and particularly a voltammetric apparatus based on mercury electrode, which is free of the above described drawbacks which will be simple, reliable, safe, user-friendly, inexpensive to make and suitable for automation.

SUMMARY OF THE INVENTION

[OOl ljIn one embodiment, the present invention provides a voltammetric apparatus comprising an electroanalytical cell comprising a working electrode wherein the working electrode is a mercury meniscus electrode, a counter electrode, and a reference electrode; a purification container in connection with the cell for purifying contaminants from the working electrode; and a means for delivering the resulting purified mercury from the purification container to the working electrode.

[0012] In one embodiment, the cell further comprises a working electrode channel for delivering mercury to the working electrode. [0013]In one embodiment, the cell further comprises a measurement channel in connection with the working electrode channel. In another embodiment, the measurement channel is a flow channel.

[0014] In one embodiment, the mercury meniscus electrode is formed by delivering mercury from the bottom of the working electrode channel towards the measurement channel, thereby forming a mercury meniscus in contact with the measurement channel; thereby forming the mercury meniscus electrode.

[0015] In one embodiment, the measurement channel comprises an inlet for introducing a sample to the measurement channel, an outlet for disposing the sample from the measurement channel to a drain, and an outlet to the purification container for collecting contaminated mercury. In another embodiment, the purification container further comprises a separation reservoir for separating contaminated mercury from the sample, thereby preventing the release of the mercury to the drain.

[0016] Furthermore, in another embodiment, the present invention provides a method for trace element detection comprising the steps of providing a voltammetric apparatus comprising an electroanalytical cell, the cell comprising a working electrode wherein the working electrode is a mercury meniscus electrode, a counter electrode, a reference electrode, a working electrode channel, and a measurement channel; a purification container in connection with the cell for purifying contaminants from the working electrode; and a means for delivering the resulting purified mercury from the purification container to the working electrode; introducing a first sample into the measurement channel; delivering mercury from the bottom of the working electrode channel towards the measurement channel, thereby forming a mercury meniscus in contact with the measurement channel, thereby forming the mercury meniscus electrode; performing at least one measurement, thereby detecting the trace elements in the sample.

[0017]In one embodiment, the method further comprising the steps of introducing a second sample to the measurement channel, thereby cutting off the mercury meniscus.

[0018] In one embodiment, the method further comprising the step of disposing the sample from the measurement channel through the outlet.

[0019] In one embodiment, the method further comprising the step of separating used mercury from the sample before disposing the sample to the drain. In another embodiment, separating comprises delivering mercury from the sample to the purification container through a separation reservoir, thereby preventing the release of the mercury to the drain.

[0020]In one embodiment, the cell further comprises a cell housing. In another embodiment, the cell housing is made of a chemically inert plastic material. In another embodiment, the inert plastic material is Teflon, peek, or acrylic compound.

[0021]In one embodiment, the counter electrode is a platinum, gold or glassy carbon electrode. [0022]In one embodiment, the working electrode further comprises a platinum wire for providing an electrical current to the working electrode. In another embodiment, the potential of the working electrode is in the range of +0.2 to -1.6 N, in relation to the reference electrode.

[0023] In one embodiment, the means for delivering the purified mercury from the purification container to the working electrode comprises a pump. In another embodiment, the pump is a peristaltic pump. In another embodiment, the pump is a diaphragm pump.

[0024] In one embodiment, introducing the sample to the measurement channel comprises the use of an external pump. In another embodiment, the pump is a peristaltic pump. In another embodiment, the pump is a diaphragm pump.

[0025]In one embodiment, the present invention further comprising a potentiostat in communication with the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which:

Fig. 1 a vertical cross-section schematic illustration of a voltammetric apparatus, constructed in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention provides a voltammetric apparatus comprising a) an electroanalytical cell comprising a mercury meniscus electrode as a working electrode, a counter electrode, and a reference electrode; b) a purification container in connection with the cell for purifying contaminants from the working electrode; and c) a means for delivering the resulting purified mercury from the purification container to the working electrode. The present invention further provides a method for trace element detection comprising the steps of i) providing a voltammetric apparatus comprising a) an electroanalytical cell, the cell comprising a mercury meniscus electrode as a working electrode, a reference electrode, a working electrode channel, and a measurement channel; b) a purification container in connection with the cell for purifying contaminants from the working electrode; and c) a means for delivering the resulting purified mercury from the purification container to the working electrode; ii) introducing a first sample into the measurement channel; iii) delivering mercury from the bottom of the working electrode channel towards the measurement channel, thereby forming a mercury meniscus in contact with the measurement channel, thereby forming the mercury meniscus electrode; and iv) performing at least one measurement, thereby detecting the trace elements in the sample.

[0028] The present invention relates to novel a capillary free voltammetric apparatus for trace element detection. In one embodiment, the voltammetric apparatus uses mercury without the need of a dropping mercury electrode. In another embodiment the voltammetric apparatus uses a mercury meniscus electrode.

[0029] In one embodiment, the present invention provides an electroanalytical cell for the detection of trace elements, which is based on stripping voltammetry methods, known in the art as the most sensitive methods. Since the cell does not comprise a capillary glass and does not use mercury drops, it is free of the well-known drawbacks, which characterize the cells that currently exist in the market. These prior art cells comprise glass capillaries, which can be clogged by mercury and also use mercury drops, which are unstable and irreproducible. As contemplated herein, in one embodiment, the apparatus of the invention is based on a vertical mercury capillary tube, wherein the mercury is flowing from beneath to the top and forms a mercury meniscus, which is the area of the measurement. The formed mercury meniscus is reproducible and has a consistent size.

[0030] Reference is now made to Fig. 1, which is a schematic representation of a vertical cross-section of a voltammetric apparatus [10], constructed in accordance with one embodiment of the present invention. Voltammetric apparatus [10] comprises a cell housing [12], a working electrode [14], a reference electrode [16], an auxiliary (counter) electrode [18], and a container connected to a pump [32], which contains a mercury reservoir [20].

[0031] In another embodiment of the invention, the cell comprises a general means for purifying the contaminated mercury, such as and without being limited to, those described in Provisional Patent Application No. 60/254041. In one embodiment of the invention, the cell body housing [12] is made of a chemically inert plastic material. In another embodiment the chemically inert plastic is Teflon, peek or acrylic compound.

[0032] In one embodiment, the sample is introduced into the cell through an inlet [22] on the left side of a measurement channel [24]. In another embodiment, the diameter of the measurement channel [24] is in the range of 0.5 mm to 2.5 mm. In another embodiment, the diameter of the measurement channel is in the range is 0.7 - 1 mm.

[0033] In one embodiment of the invention, the reference electrode [16] is connected to the measurement channel. In another embodiment, the reference electrode [16] is connected to the measurement channel through a bore drilled from the top of the cell body, to the measurement channel. In one embodiment of the invention, the auxiliary electrode [18] is, for example without limitation, of a platinum wire. In another embodiment, the diameter of the platinum wire is in the range of 0.5 to 1.0 mm. In another embodiment, the platinum wire is sealed via the drilled bore reaching the measurement channel. In one embodiment, proper conductivity between the reference electrode and the auxiliary electrode is provided. In anothr embodiment, the distance between the reference electrode and the auxiliary electrode should not exceed 15 mm in order to provide proper conductivity between the two electrodes. In one embodiment, the cell further comprises a working electrode channel [14]. In another embodiment, the inside diameter of the working electrode channel is in the range of 0.1-1 mm. In another embodiment, the inside diameter of the working electrode channel is in the range of 0.5-0.8 mm.

[0034] In one embodiment, the novelty of the invention is that the working electrode is sealed through a vertical drilled bore reaching the measurement channel from the bottom and forming a mercury meniscus [28]. Through the cross section of the bore, the mercury meniscus [28] is exposed to the sample solution flowing above, and the measurement is performed on the mercury meniscus [28].

[0035] In one embodiment, a platinum wire [26] is inserted to the working electrode channel [14], thereby provides an electrical connection with the working electrode. In one embodiment the outlet of the flow-channel [24] is in connection with the mercury reservoir [20], wherein the contaminated mercury is separated from the sample and then stored and regenerated, for example without being limited to, according to the method described in US Provisional Patent 60/251,041. The tested sample is discharged via the measurement channel outlet [30] to the drain.

[0036] The present invention provides, in one embodiment, the principle of operation of the voltametric apparatus: the previously degassed sample solution, which contains, for example without limitation, an electrolyte, is introduced continuously to the cell through the inlet [22] by an external pump, which in one embodiment, is a peristaltic pump, and in another embodiment is a diaphragm pump. As the sampling step is completed, the peristaltic pump [32], which is connected to the reservoir [20], starts to deliver mercury from the reservoir [20] into the measurement channel [24] via the working electrode channel [14]. The pump continues to work until the electrode channel is filled by the released mercury. At this point, the sample pumping starts again and the mercury excess is cut off and removed by the sample solution stream.

In one embodiment, the mercury is collected in a reservoir [20]. Thus, in each measurement, a reproducible mercury meniscus [28] is formed in the cross-section of bore [14]. The mercury meniscus [28] is exposed to the sample solution and acts as a working electrode. In one embodiment, voltammetric scanning measurements are performed. After the measurementes are completed, the peristaltic pump [32] delivers purified mercury from the mercury reservoir [20] to the working electrode channel [14] and the above cycle is restarted.

[0037] The major advantage of the cell of the invention is that the cell provides high reproducibility of working electrode surface area without complicated mechanical and electronic means, in contrast to the existing capillary-based conventional techniques. The high stability of the working electrode permits both prolonged accumulation stage and high sample flow rates. As a result, the sensitivity of the determinations, which is highly important in trace analysis, is increased. Low dead volume of the cell of the invention enables the use of a small sample volume and eliminates the memory effect. The cell construction is simple and reliable, due to the elimination of the glass capillary. In addition, theoretically, the cell of the invention has an unlimited lifetime since it does not have to be replaced due to clogged glass capillaries or inconsistent working electrode size.

[0038] The term "working electrode" refers to the place where the reaction of interest occurs. In one embodiment of the invention, the working electrode is a mercury meniscus electrode that is reproduced prior to each test.

[0039] The term " auxiliary electrode" or "counter electrode" refers to an electrode paired with the working electrode, through which a current equal in magnitude and opposite in sign to the current passing through the working electrode, is passed. The auxiliary electrode can be made, for example without limitation, of metals like platinum or carbon-based materials or of any other conductive material that are not affected form the sample solution.

[0040] The term "reference electrode" refers to a specially designed electrode that maintains a constant potential in reference to the sample solution through all the measurement cycle. In one embodiment, the reference electrode is silver/silver chloride electrode. Silver/silver chloride electrode is available from Ercon, Inc. ( areham, Mass), Metech Inc. (Elverson, Pa), E.I. du Pont de Nemours and Co. (Wilmington, Del), Emca-Remex Products (Montgomery Ville, Pa), or MCA Services (Melbourne, Great Britain). Silver/silver chloride electrodes illustrates a type of reference electrode that involves the reaction of a metal electrode with a constituent of the sample or body fluid, in this case chlorine ion.

[0041] The term "trace element detection" refers to applying an electrical potential between the working electrode and the reference electrode or the auxiliary/reference electrode, and measuring the resulting current, which is a function of the concentration trace in the sample. In one embodiment, the potential of the working electrode is selected in the range between +0.2 to -1.6V in relation to Ag/AgCI reference electrode, which is about 0.2V versus normal hydrogen electrode.

[0042] It will be appreciated by persons skilled in the art that the embodiments of the present invention is not limited by what has been particularly shown and described hereinabove, and that numerous modifications, all of which fall within the scope of the embodiments of the present invention, exist. Rather the scope of the invention is defined by the claim that follows:

Claims

What is claimed is:

1. A voltammetric apparatus comprising: an electroanalytical cell comprising a working electrode wherein said working electrode is a mercury meniscus electrode, a counter electrode, and a reference electrode; a purification container in connection with said cell for purifying contaminants from said working electrode; and a means for delivering the resulting purified mercury from said purification container to said working electrode.

2. The apparatus of claim 1, wherein said cell further comprises a working electrode channel for delivering mercury to said working electrode.

3. The apparatus of claims 2, wherein said cell further comprises a measurement channel in connection with said working electrode channel.

4. The apparatus of claim 3, wherein said mercury meniscus electrode is formed by delivering mercury from the bottom of said working electrode channel towards said measurement channel, thereby forming a mercury meniscus in contact with said measurement channel; thereby forming said mercury meniscus electrode.

5. The apparatus of any of claims 1-4, wherein said cell further comprises a cell housing.

6. The apparatus of claim 5, wherein said cell housing is made of a chemically inert plastic material.

7. The apparatus of claim 6, wherein said inert plastic material is Teflon, peek, or acrylic compound.

8. The apparatus of any of claims 1-7, wherein said counter electrode is a platinum, gold or glassy carbon electrode.

9. The apparatus of any of claims 1-8, wherein said working electrode further comprises a platinum wire for providing an electrical current to said working electrode.

10. The apparatus of any of claims 1-9, wherein the potential of said working electrode is in the range of +0.2 to -1.6 V, in relation to said reference electrode.

11. The apparatus of any of claims 1-10, wherein said means comprises a pump.

12. The apparatus of claim 11, wherein said pump is a peristaltic pump.

13. The apparatus of claim 11 , wherein said pump is a diaphragm pump.

14. The apparatus of any of claims 3-13, wherein said measurement channel is a flow channel.

15. The apparatus of any of claims 3-14, wherein said measurement channel comprises an inlet for introducing a sample to said measurement channel, an outlet for disposing said sample from said measurement channel to a drain, and an outlet to said purification container for collecting contaminated mercury.

16. The apparatus any of claims 1-15, wherein said purification container further comprises a separation reservoir for separating contaminated mercury from said sample, thereby preventing the release of said mercury to said drain.

17. The apparatus of any of claims 15-16, wherein said introducing comprises the use of an external pump.

18. The apparatus of claim 17, wherein said pump is a peristaltic pump.

19. The apparatus of claim 17, wherein said pump is a diaphragm pump.

20. The apparatus of any of claims 1-19, further comprising a potentiostat in communication with said cell.

21. A method for trace element detection comprising the steps of: providing a voltammetric apparatus comprising an electroanalytical cell, said cell comprising: a working electrode wherein said working electrode is a mercury meniscus electrode, a counter electrode, a reference electrode, a working electrode channel, and a measurement channel; a purification container in connection with said cell for purifying contaminants from said working electrode; and a means for delivering the resulting purified mercury from said purification container to said working electrode; introducing a first sample into said measurement channel; delivering mercury from the bottom of said working electrode channel towards said measurement channel, thereby forming a mercury meniscus in contact with said measurement channel, thereby forming said mercury meniscus electrode; performing at least one measurement, thereby detecting said trace elements in said sample.

22. The method of claim 21, further comprising the steps of introducing a second sample to said measurement channel, thereby cutting off said mercury meniscus.

23. The method of any of claims 21-22, wherein said measurement channel comprises an inlet for introducing said sample to said measurement channel, an outlet for disposing said sample from said measurement channel to a drain, and an outlet to said purification container for collecting contaminated mercury.

24. The method of any of claims 21-23, further comprising the step of disposing said sample from said measurement channel through said outlet.

25. The method of claim 24, further comprising the step of separating used mercury from said sample before disposing said sample to said drain.

26. The method of claim 25, wherein said separating comprises delivering mercury from said sample to said purification container through a separation reservoir, thereby preventing the release of said mercury to said drain.

27. The method of any of claims 21-26, wherein said cell further comprises a cell housing.

28. The method of claim 27, wherein said cell housing is made of a chemically inert plastic material.

29. The method of claim 28, wherein said inert plastic material is Teflon, peek or acrylic compound.

30. The method of any of claims 21-29, wherein said counter electrode is a platinum, gold or glassy carbon electrode.

31. The method of any of claims 21-30, wherein said working electrode channel further comprises a platinum wire for providing an electrical current to said working electrode.

32. The method of any of claims 21-31, wherein the potential of said working electrode is in the range of +0.2 to -1.6 V, in relation to said reference electrode.

33. The method of any of claims 21-32, wherein said means comprises a pump.

34. The method of claim 33, wherein said pump is a peristaltic pump.

35. The apparatus of claim 33, wherein said pump is a diaphragm pump.

36. The method of any of claims 21-35, wherein said measurement channel is a flow channel.

37. The method of any of claims 21-36, wherein introducing comprises the use of an external pump.

38. The method of claim 37, wherein said pump is a peristaltic pump.

39. The apparatus of claim 38 wherein said pump is a diaphragm pump.

40. The method of any of claims 21-39, further comprising a potentiostat in communication with said cell.