WO2004109256A2

WO2004109256A2 - Deposition and detection of zinc and other metals in solution

Info

Publication number: WO2004109256A2
Application number: PCT/US2004/017823
Authority: WO
Inventors: Miklos Gratzl; William Rozakis; Gautam Nithyanand Shetty
Original assignee: Case Western Reserve University
Priority date: 2003-06-06
Filing date: 2004-06-07
Publication date: 2004-12-16
Also published as: WO2004109256A3

Abstract

A system for deposition and subsequently detecting an analyte, such as zinc ions, in an aqueous solution (12), includes a working electrode (14). All or a portion of the analyte in a test sample of the solution is selectively deposited on the working electrode by pulsing a voltage across the working electrode in the form of a plurality of short pulses each having a duration which is sufficiently short that electrochemical splitting of water is substantially avoided. The deposited analyte, in the form of zinc metal, is subsequently stripped from the working electrode. In one method, the deposited metal is replaced in a redox-type reaction with a detectable species, such as copper, which has reaction kinetics that allow stripping of the replacement species from the electrode at a slower rate than is the case with zinc.

Description

DEPOSITION AND DETECTION OF ZINC AND OTHER METALS IN SOLUTION

BACKGROUND OF THE INVENTION

[0001] This application claims the benefit of U.S. Provisional Application

Serial No. 60/417,149, filed October 9, 2002, U.S. Provisional Application Serial No. 60/137,134, filed May 28, 1999, and U.S Application Serial No. 09/980,089, filed May 30, 2000, as PCT/US00/14805, the specifications of which are incorporated herein in their entireties by reference.

FIELD OF THE INVENTION

[0002] The invention relates to a method of depositing and subsequently detecting metals in solution which have a deposition potential which is in the range at which water splits. In particular, it relates to a method for detection of low levels of zinc in microliter size samples, and will be described with particular reference thereto.

DISCUSSION OF THE ART

[0003] Routine analysis of the chemical composition of fluids is important in a wide range of fields, including clinical diagnosis, food and drug industries, industrial process control, and environmental studies.

[0004] For detection of trace amounts of metal ions in solution, a variety of analytical techniques have been developed, including electrodeposition and electrochemical stripping analysis, using either a rotating electrode or array of electrodes; atomic absorption spectroscopy (AAS); and inductively coupled plasma-mass spectroscopy (ICP-MS).

[0005] Electrochemical stripping techniques offer a number of advantages over other methods including simplicity of equipment, operating costs, and ease of portability of equipment. Hydrodynamic electrochemical techniques with enhanced convective mass transport exhibit a number of advantageous voltammetric characteristics. The relative contribution of mass transport limitations with respect to electron kinetics is less pronounced. (Bard, A.J.; Faulkner, L.R.: Electrochemical Methods; John Wiley, (1980)). Steady state conditions (where the current is independent of potential scan direction and time) are attained quickly. Thus, measurements can be carried out with high precision. In addition, at steady state, double layer charging is not a factor.

[0006] Traditionally, to obtain well-defined convective mass transport, a rotating electrode system, such as a rotating disc or ring-disc electrode is used to generate convective flow in the test solution. Hydrodynamic methods also play an important role in electrochemical preconcentration techniques, such as stripping voltammetry or potentiometry, where enhanced mass transport allows for efficient extraction of the analyte onto the surface of the electrode. Preconcentration of heavy metal trace elements is particularly useful in the analysis of food, environmental, and biological samples, because of the large useful concentration range, and the simpler, portable and less expensive instrumentation (Bersier, P.M., et al. , 119 Analyst 219-32 (1994)). Potentiometric stripping techniques have been used successfully in the determination of lead in blood samples (Jagner, D., et al., 6 Bectroanalysis 285-91 (1994)), in gasoline (Jagner, D., et al., 267 Anal. Chim. Acta. 165-69 (1992)), and of heavy metals in tap water (Jagner, D., et al., 278 Anal. Chim. Acta 237-42 (1993)).

[0007] In potentiometric stripping analysis, an oxidizing agent, added to the sample, is used for the stripping of the deposited analyte from the electrode surface. In voltammetric stripping analysis, an anodic voltammetric scan is applied. Potentiometric stripping has advantages over voltammetric stripping in that it is unaffected by dissolved oxygen present in the sample, and does not require sophisticated anodic scanning instrumentation, since the potential is detected in time. (Jagner, D. et al. 278 Anal. Chim. Acta 237-42 (1993)). However, the potentiometric method has a number of disadvantages. For low sample concentrations, the fast stripping rate requires a very high real time data acquisition rate. Also, reproducible hydrodynamic conditions are more important than in anodic stripping, since the driving force of the oxidation is diffusion controlled mass transport. The detection limit of hydrodynamic techniques can be further reduced (i.e., improved) by sinusoidally modulating the rotation speed of the electrode (Miller, B. and Bruckenstein, S., 46 Anal. Chem. 2026-33 (1974)).

[0008] Electrochemical techniques have recently been developed for the detection of analytes in small samples. U.S. Patent No. 6,043,878 to Gratzl, et al. discloses methods of detecting reagents in microliter-sized samples. A droplet of a solution to be tested in placed inside a hydrophobic ring, mounted to a suitable substrate. A ring or disk electrode is used to make electrochemical measurements on the droplet. The electrochemical reactions are limited by the rate of mass transport towards the surface of the electrode. Convection in the sample is achieved by blowing air or inert gas on the droplet to rotate the sample droplet. The technique allows detection of a variety of electroactive species and has applications in many areas, including clinical measurements, semiconductor development, the fuel industry, corrosion, quality control, and process monitoring.

[0009] To detect metal ions in solution, the metal is plated onto the electrode and then stripped electrochemically using potentiometric or voltammetric stripping techniques. This method can be used for metal ions which have a deposition potential which is within the water window, i.e., the potential range in which water is neither electrochemically reduced (to evolve hydrogen gas) nor oxidized (to liberate oxygen gas). However, metals having a deposition potential outside this window are not readily detected by such techniques because at the potentials needed for plating or stripping, water in the solution rapidly splits into H⁺ and OH-. The H⁺ is then reduced to hydrogen and evolves at the cathode as gas bubbles. The detection or the plating of the metal is impeded by the hydrogen evolution. The current applied to the solution is used up primarily by hydrogen evolution and thus little or no preconcentration of the metal on the electrode takes place.

[00010] The present invention provides a new and improved method for deposition and/or detection of electroactive species with deposition potentials outside the water window, which overcomes the above-referenced problems and others.

SUMMARY OF THE INVENTION

[00011] In accordance with one aspect of the present invention, a method of detecting an analyte in a solution which includes water is provided. The analyte has a deposition potential at which water tends to split electrochemically. The method includes depositing at least a portion of the analyte from the solution on to an electrode by pulsing a voltage across the electrode. The pulse has an amplitude and duration which is such that electrochemical splitting of water is substantially avoided. The deposited analyte can then be detected.

[00012] In accordance with another aspect of the present invention, a method of determining a concentration of at least one analyte ion in a solution is provided. The method includes pulsing a voltage across an electrode, the voltage being at a potential which is outside the water window and which causes the analyte to deposit on the electrode. The deposited analyte is indirectly detected, including one of (a) potentiometrically stripping the deposited analyte with an oxidizing agent and determining the time taken for the stripping to complete, and (b) redissolving the deposited analyte, the charge generated in the redissolution process used to deposit a replacement species on one of the electrode and a secondary electrode and stripping the replacement species from the one of the electrode and the secondary electrode. The concentration of the at least one analyte ion in the solution is determined from at least one of (i) a current flowing in a circuit comprising the one of the electrode and the secondary electrode during stripping, and (ii) a time for stripping to complete.

[00013] In accordance with another aspect of the present invention, a method of determining a concentration of at least one analyte ion in a solution is provided. The method includes pulsing a voltage across an electrode, the voltage being at a potential which is outside the water window and which causes the analyte to deposit on the electrode. The deposited analyte is indirectly detected, including one of (a) potentiometrically stripping the deposited analyte with an oxidizing agent and determining the time taken for the stripping to complete, and (b) indirectly, by addition of a species with which the deposited analyte undergoes a redox reaction, the added species being deposited onto one of the working electrode and a secondary electrode, and then stripping the species from the one of the electrode and the secondary electrode. The concentration of the at least one analyte ion in the solution is determined from at least one of (i) a current flowing in a circuit comprising the one of the electrode and the secondary electrode during stripping, and (ii) a time for stripping to complete.

[00014] In accordance with another aspect of the present invention, a system for determining a concentration of at least one analyte ion in a solution is provided. The system includes a container which contains a sample of the solution. A working electrode is disposed within the container, on which the analyte is selectively deposited. The system further includes a counter electrode and a source of an electrical potential which forms an electrical circuit with the working electrode and the counter electrode, the source of electrical potential applying a pulsed voltage to the working electrode for depositing the analyte from the solution on to the working electrode. A plating electrode is disposed within the container. Means are provided for selectively applying a voltage to the plating electrode independent of the working electrode for plating out at least one interfering species in the solution on the plating electrode.

[00015] In accordance with another aspect of the present invention, a method of depositing a metal ion from a solution which includes water, the metal ion having a deposition potential at which water splits electrochemically is provided. The method includes depositing at least a portion of the metal ion from the solution in the form of its corresponding metal onto a substrate by pulsing a voltage across the substrate, the pulse having an amplitude and a duration which are such that water splitting is substantially avoided and a layer of the metal is produced on the substrate.

[00016] An advantage of at least one embodiment of the present invention is that zinc and other metals are detectable in solution.

[00017] Another advantage of at least one embodiment of the present invention is that the method is amenable to detection of electroactive species in microliter-sized samples.

[00018] Another advantage of at least one embodiment of the present invention is that it enables the detection of electroactive species in trace amounts without the need for convective transport.

[00019] Still further advantages of the present invention will be readily apparent to those skilled in the art, upon a reading of the following disclosure and a review of the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[00020] FIGURE 1 is a circuit diagram of a system for deposition of an analyte in a test solution in accordance with the present invention;

[00021] FIGURE 2 is a plot of applied voltage for deposition of the analyte in the system of FIGURE 1 ;

[00022] FIGURE 3 is a plot of applied voltages for a second embodiment of a method for deposition of an analyte using the system of FIGURE 1 ;

[00023] FIGURE 4 is a circuit diagram for a second embodiment of a system for deposition and detection of an analyte in a test solution in accordance with the present invention;

[00024] FIGURE 5 is a circuit diagram for a first embodiment of a detection system;

[00025] FIGURE 6 is a schematic diagram of a third embodiment of a system for deposition and detection of an analyte in a test solution in accordance with the present invention;

[00026] FIGURE 7 is a side view of a fourth embodiment of a system for deposition and detection of an analyte in a test solution in accordance with the present invention;

[00027] FIGURE 8 is a top plan view of a fifth embodiment of a system for deposition and detection of an analyte in a test solution in the presence of interfering ions in accordance with the present invention;

[00028] FIGURE 9 is a side sectional view of the system of FIGURE 8;

[00029] FIGURE 10 is a plot of current vs. voltage for a voltammetric scan of an electrode during stripping of copper deposited as a substitute for zinc, the zinc having been deposited from a test solution at a concentration of 1 ppm zinc; [00030] FIGURE 11 is a plot current vs. voltage for a voltammetric scan of an electrode during stripping of copper deposited as a replacement for zinc deposited from 10 and 100 ppm solutions, respectively, and showing a background scan for an electrode prior to zinc or copper deposition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[00031] The term "analyte" is used herein to refer to an electroactive species, such as a metal ion, which undergoes an electrochemical reaction when a current or voltage is applied to a solution containing the analyte. While the analyte is described with particular reference to metals and their ions, such as zinc, it will be appreciated that the term also encompasses other electroactive species, including, but not limited to Mg²⁺, Al³⁺, Fe²⁺, Cd²⁺, Tl⁺, Sn²⁺, Pb ⁺, and the like.

[00032] The invention is described with particular reference to the deposition (plating) of a metal, from a sample of a solution to be tested, on the surface of an electrode. The deposition is optionally followed by electrochemical stripping (dissolution) of the metal from the electrode. This allows a measurement of the concentration of the metal in the solution to be made. It is to be understood, however, that each of these steps may be carried out independently. The method is suited to the detection of one or more analytes in solution, and is particularly suited to the detection of analytes in trace amounts, e.g., at parts per million (ppm), or even parts per billion (ppb), or less.

Methods of Deposition

[00033] With reference to FIGURE 1, one embodiment of a system for deposition of an analyte in solution is shown. The system includes a container 10, which holds a sample 12 of the solution under test. A working electrode 14 and a counter electrode 16 are in electrical contact with the sample 12. In the case of zinc and similar metals, the working electrode may be formed from a material which is relatively inert to hydrogen evolution, such as carbon (e.g., pyrolytic carbon, graphite, glassy carbon, or diamond-like carbon) or noble metal, such as platinum, gold, or the like. The counter electrode may be formed from stainless steel, platinum, gold, carbon, or the like. A voltage source 18 is connected across the counter electrode 16 and working electrode 14 by suitable wiring 20 to form a cell in which the solution completes the circuit. The voltage source can be a DC source, such as a battery or plurality of batteries. A means for pulsing the voltage 22, such as an analog switch or waveform generator in the circuit allows the voltage to be pulsed by switching the voltage on and off intermittently. During the deposition step, a layer 23 of the analyte (not to scale) is deposited on the working electrode 14 in the form of the corresponding metal. At low analyte concentrations, the analyte may be deposited as islands, rather than a contiguous layer, as shown.

[00034] While FIGURE 1 illustrates a system in which the counter electrode is positioned in the test solution 12, it is to be appreciated that the counter electrode may be in contact with a second electrolyte, which is spaced from the test solution by a salt bridge or selectively permeable membrane, such that species generated at the counter electrode 16 do not pass into the test solution.

[00035] The system is particularly suited to the detection or concentration of metal ions having a deposition potential which is outside (typically beyond) the water window, i.e. in a range at which water splits electrochemically. Examples of such metal ions include, but are not limited to Zn²⁺, Mg²⁺, Ai³⁺, Fe²⁺, Cd²⁺, Tl⁺, Sn²⁺, Pb^{2 +}, and the like. For example, water and zinc undergo reactions as follows.

2 H⁺ + 2 e →H₂0 E₀ = 0V (1)

Zn²⁺ + 2 e- →Zn E₀ = -0.762V (2)

E₀ is defined as the standard reduction potential at a pH for which the expression is true.

[00036] In general, if voltages outside the "water window" are employed (i.e., voltages in the range at which water splits, which can be either above or below the water window), evolution of hydrogen (or oxygen, when the voltage is below the water window) tends to occur at the expense of analyte deposition or the current due to the analyte is lost in the current that goes into evolution of hydrogen from electrochemical splitting of water. By employing a voltage pulsing technique, splitting of water is minimized to such an extent that it does not significantly interfere with the deposition process (by this it is meant that less than 10% of the current is taken up by hydrogen evolution, more preferably, less than 1 %). Ideally, there is no visible hydrogen evolution. [00037] While not fully understood, it is suggested that a short duration pulse does not provide sufficient time for water to be reduced and for hydrogen to evolve at the electrode surface. The faster reaction kinetics of metals, such as zinc, allow the reduction of zinc ions to the metal and deposition on the electrode 14 to proceed at a faster rate than water splitting, despite the relatively larger concentration of water than metal ions. The zinc ions are able to exchange ions with the working electrode at a substantially faster rate than water is able to split completely to form ions and then for hydrogen to evolve. The metal can thus be deposited within the duration of the pulse with minimal to zero interference from hydrogen evolution.

[00038] In particular, the pulsing technique is suited to the deposition of analytes with a deposition potential which is more negative than the potential at which water splits and hydrogen evolves (e.g., those with an E₀ of from -0.1V to -2.8V versus hydrogen evolution at 0V). While the method is described with particular reference to these elements having a deposition potential beyond the potential at which hydrogen evolution begins to occur, it will be appreciated that the method can also be used for the detection of analytes having a deposition potential which is lower (more positive) than that of water (i.e., outside the water window), including, but not limited to ions of Cu, Sn, Ag, and the like, alone or in combination with analytes having a deposition potential within the water window.

[00039] The length V of the deposition pulse is preferably selected to be sufficient for the selected analyte to plate out while not long enough for visible evolution of hydrogen, or other species in solution. The optimal length of the pulse is thus dependent on the reaction kinetics of the analyte to be detected. Where the analyte deposits quickly (i.e., has relatively fast kinetics), the duration of the pulse can be shorter. In one embodiment, pulses of duration t 'of from about 200 microseconds to about 2 milliseconds, e.g., about 1 millisecond, are applied at intervals t of from about 100 microseconds to about 50 milliseconds, more preferably, at least 5 milliseconds, e.g., about 10 milliseconds. FIGURE 2 illustrates pulses of duration t'of 1 millisecond at intervals f of 10 milliseconds. If the time f'is too long, evolution of hydrogen may be observed during the pulse and if it is too short, then the analyte tends to deposit relatively slowly, or not at all. If the time t between pulses is too short, evolution of hydrogen may be observed during the pulse. In one embodiment, the pulse length t', and interval between each pulse t, remain constant throughout the deposition process. Alternatively, the pulse length t and interval between each pulse t are modified for dynamic modulation of the diffusion layer. This is suited to a deposition process without the need for convection. I one embodiment, the pulse intervals are of sufficient duration that the diffusion layer relaxes prior to the application of the next pulse.

[00040] In addition to the kinetics of the target analyte, the optimal pulse duration and interval between pulses may also be dependent on the concentration of the analyte in the solution being tested and on the pH of the solution. Accordingly, experiments may be conducted to establish a suitable pulse duration and interval for a particular analyte in an expected concentration range at a selected pH prior to conducting deposition and stripping of the same analyte in a test solution.

[00041] The optimal pulse voltage is one which is sufficient for deposition to occur. For example, for deposition of zinc at a given pH, a voltage of about -0.762 V (versus hydrogen evolution potential at 0 V) or higher, more preferably, in the range of about -1V to -2V, is generally sufficient. While higher (more -ve) applied voltages can be used, this is generally not necessary because once limiting current is reached, the current does not increase, even on increasing the overpotential (applied voltage). Moreover, with lower voltages, the double layer does not become charged more than is appropriate. As a result, the discharge time for the double layer is reduced, and thus the time between pulses t can be reduced.

[00042] In another embodiment, which is particularly suited to plating, rather than detection applications, higher potentials may be beneficial to overcome a resistive electrode, i.e., an electrode in which the potential distribution is non- uniform, due to material resistance. The non-uniform potential distribution results in a non-uniform current distribution where areas with higher potential have higher current densities than others. Using pulses with higher potential allows a uniform current distribution, provided that the highest and lowest potential values both deposit the metal at limiting current.

[00043] The optimal length of the pulse f is also dependent, to some degree, on whether a non-convective system is used or whether convection is employed. When the voltage pulse length is short enough, a convective system is generally not needed. Such a system is not mass-transport limited, but is limited only by electrode kinetics. In any event, the pulse should be of sufficient length to allow the metal to deposit. Where more than one metal is present in solution, faster pulsing tends to be selective towards the metal with the faster reaction kinetics, and can thus be used for selective deposition, as described in greater detail below.

[00044] The optimal pulse length is also dependent to some degree, on the pH and temperature, the above values for zinc being most applicable to deposition at ambient temperatures (15-25°C).

[00045] The use of an analog switch 22 allows the DC voltage source to be switched on and off intermittently, for a short duration. This allows for a pulse to be applied for the short time V that the switch is closed. The pulse duration f'can be controlled by controlling how long the switch remains closed. The switch enables deposition of the analyte during the short time that it is closed without visible hydrogen evolution. The switch also inhibits the deposited metal from redissolving in the solution when there is no voltage pulse because at that time, the current loop is open circuited. At shorter pulsing times, a waveform generator is optionally used in place of or in addition to a switch.

[00046] In one embodiment, a simple analog switch 22 is used to switch on and off the DC power supply 18. The time between pulses t is preferably of sufficient length to ensure that the double layer formed at the electrode surface during the pulse is discharged before the next pulse is applied. The switch 22 can be controlled by a digital output line 24 of a data acquisition system 25, e.g., a computer controlled potentiostat or other computer controlled device, which is used subsequently in the detection stage, as described in greater detail below.

[00047] In another embodiment, the double layer is actively discharged. This allows the time t between pulses to be reduced. The discharge can be achieved by applying a pulse, or several pulses, of a small amplitude and of opposite polarity between pulses, as shown in FIGURE 3. The discharge pulse is preferably of short duration t", preferably shorter in duration than the deposition pulse. The discharge pulse can be applied just before or just after the deposition pulse. The discharge pulse can dissolve some of the deposit. Accordingly, the length t" of the discharge pulse is selected to minimize dissolution of the deposit while ensuring discharge of the double layer.

[00048] In another embodiment, the duration between pulses is such that the double layer is not allowed to discharge. This way, current from subsequent pulses contributes to Faradaic current and very little to charging the double layer.

[00049] In another embodiment, the means for pulsing 22 includes a waveform generator in place of or in addition to the analog switch. The waveform generator generates the deposition and/or the discharge pulses, preferably both. For example, as shown on FIGURE 3, the waveform generator 22 produces a periodic deposition pulse, followed by, or preceded by a discharge pulse. The corresponding current generated is shown in hatched lines. At short enough pulse duration t', the discharge pulse may not be necessary since the voltage across the double layer approximates the applied voltage pulse.

[00050] In place of a pulsing technique, it is also contemplated that the analyte may be deposited by conventional techniques, such as using a galvanostat 26, as illustrated in FIGURE 4. In this embodiment a current is forced through the test solution, to ensure that water does not split. Such techniques, however, are less effective since specificity is not as high as with the above-described pulsing techniques.

[00051] The time taken for depositing the analyte varies upon a number of factors, including the concentration of the analyte in the solution to be tested. In one embodiment, where the concentration of the analyte is about 1-10 ppm, the deposition time is from about 1-10 minutes, to allow for accurate and reproducible results, although shorter and longer deposition times are also contemplated.

[00052] In one embodiment, only a portion of the analyte is deposited and detected. The detected amount is then related to the actual amount in solution, for example, by using a calibration curve which is generated using stock solutions of known concentration, preferably having analyte concentrations close to or within the range to be detected.

[00053] In another embodiment, the deposition time is sufficient for all (or essentially all- i.e., 95% or more) of the analyte in the solution to be deposited. If the analyte is exhaustively deposited in this way, calibration-free measurements are possible, avoiding the need for calibration curves to be performed, but it can be time consuming.

[00054] Particularly where voltammetric scanning stripping methods are used (see below), it is desirable to deposit a layer of analyte on the working electrode which is a monolayer (i.e., a single layer of atoms) in thickness, or less for fast oxidation. More preferably, the coverage of the electrode is less than about 70%, more preferably, about 50%. Accordingly, where high concentrations of analyte in the test solution are to be expected, it may be desirable to dilute the test solution prior to analysis, particularly where all, or essentially all, of the analyte is to be deposited. Where sustained electrolytic redissolution is used, thicker layers of deposited analyte are feasible. This has advantages in that it permits absolute (calibration free) determinations in convective systems, such as a rotating sample system (RSS), with a very low ratio of sample volume to sample surface area.

[00055] Where the test solution is a body fluid, such as blood, it may be desirable to pre-treat the sample with a buffering agent before deposition of the analyte. The buffer is preferably one which liberates the zinc from any complexes with which the zinc is associated and standardizes solution parameters, such as pH. Suitable buffers include HCI, HF, HN0₃ and the like, which may be present at a concentration of about 0.1-2M in the test solution. For samples which are low in ionic concentration, such as environmental samples, a supporting electrolyte may be added, such as NaCI or KN0₃

Methods of Detection a) Direct Methods of Detection

[00056] In one embodiment, voltammetric stripping is used to detect the deposited metal. In this method, a voltage scan is performed and the current detected. A peak in the current is seen at the voltage at which the analyte is stripped. By integrating the area under the peak, the total charge can be determined and from this value, the amount of deposited analyte is calculated using Faraday's Law. Alternatively, a rough estimate of the analyte concentration can be determined from the peak height. The scan can be pulsed to inhibit the splitting of water outside the water window, in a similar manner to that described for deposition of the metal. However, the polarity of the voltage pulses is opposite to that used for depositing the analyte.

[00057] However, metals with fast kinetics, such as zinc, tend to strip off so quickly that detection of current tends to be difficult. This can be overcome, to some degree, by increased sampling of the stripping current, generally on the order of megasamples/second to gigasamples/second (10⁴ to 10⁹ samples/second). By way of example, where stripping pulses of 1 millisecond pulse duration are used, about 10-100 current samples, or more during the pulse period are generally sufficient to provide an accurate determination of the stripping current in the case of zinc or similar metals with fast reaction kinetics. Once the stripping step is complete, the sampled current values are plotted to provide a continuous stripping voltammetry current.

[00058] As an alternative to voltammetric scanning stripping, sustained electrolytic redissolution can be used. In this method, a selected voltage is maintained which is sufficient to cause the deposited analyte to redissolve. The current is measured as a function of time and the charge calculated. This is used to determine the concentration of analyte, as discussed above. As with other embodiments, voltage pulsing can be used. The voltage may thus be in the water window (voltage range at which water slits electrochemically), but because of the short pulse duration, dissolution of analyte proceeds rather than splitting of water (i.e., less than 10% of the current is taken up by hydrogen evolution). The current is detected when the switch is closed (i.e, during the pulse), for example, by using a high sampling frequency. The sample currents can then be put together to provide continuous stripping current data. As with other methods, the detected current is converted to a measure of the analyte concentration in the test solution.

[00059] FIGURE 5 shows a suitable arrangement for detection of the stripping current, where similar elements are numbered with a prime. The working electrode 14 is moved to a fresh electrolyte 12" and is electrically coupled with detection equipment 25'. For example, a data acquisition system 25' can be used to control the pulsing, via a digital input/output line 24', where used, of the stripping voltage, and also serves to acquire the current data. In the embodiment shown in FIGURE 5, a potentiostat 25" is used to apply the stripping voltage and to acquire data. In this embodiment, a three electrode system, which includes a reference electrode 28, in addition to the working electrode 14 and counter electrode 16', is used to provide a three electrode system, although it will also be appreciated that it is possible to use a counter electrode to serve as both the counter and reference electrodes. While separate systems may be used for the deposition and stripping steps, with only the working electrode 14 being transferred from the deposition system to the stripping system, it is also contemplated that the same system may be used for both deposition and stripping.

[00060] Such schemes can be used for selective detection of metals (i.e., the detection of more than one analyte in the test solution). For example, in the deposition step, pulses of long enough duration to plate/deposit two or more metals with faster kinetics than hydrogen evolution are used. The pulse polarity is then inverted and a shorter pulse is used, of short enough duration f to strip off substantially only the metal with the fastest kinetics. The detection current is preferably sampled relatively fast (sample time f^{am le} is on the order of nanoseconds). Using a fast sampling (short f^amP'^e), the stripping current is measured. Then, the metal with the next fastest kinetics is stripped, using a longer pulse time t and so forth. This technique is most effective where there is no redox exchange between the target analyte(s) and other cations in the sample solution. The pulse durations f and t can be seen as kinetic fingerprints of the respective metal ions.

[00061 ] Alternatively, where there is more than one analyte which is desired to be detected, the analytes can be selectively deposited. For example, pulses f'of a duration short enough to deposit only the fastest depositing analyte A_n are used to deposit that analyte. The analyte I _Λ is then stripped off the electrode and the stripping current measured. In a second deposition procedure, the fastest and second fastest analytes A₁ and A₂ are deposited, using a slightly longer pulse duration t'. The two analytes are then stripped together and the stripping current measured. By deducting the stripping current of the analyte A, from the result, the charge associated with the second analyte A₂can be calculated. This procedure can be repeated for any number of analytes, with each deposition process resulting in the deposition one more analyte. This method is amenable to use with both direct and indirect methods of detection. [00062] In all the above detection methods, particularly the scanning methods, the measured stripping current values are preferably adjusted to account for the effects of capacitive currents and background currents, for example, by running a blank sample and subtracting the current measured from the stripping current. The blank test may be carried out by pulsing voltage to the electrode in an electrolyte without deposited analyte and detecting the capacitive current and the current due to any other interfering species present.

b) Indirect Methods of Detection

(i) Substitution Method (Detection by Proxy)

[00063] In this indirect detection method, the plated metal is substituted/exchanged with a detectable (proxy) species which is more readily detected, either by pulsing techniques or by other conventional techniques. The proxy species can be another metal ion. It preferably has a reduction potential E₀ which is within the water window (i.e., in a potential range at which water does not split and/or has reaction kinetics which cause it to react more slowly, allowing the progress of the stripping current to be measured more readily, for example, without the need for a high speed data acquisition system. A redox reaction may be used to replace the plated metal with the detectable species. For example, to detect plated zinc, a metal, such as copper, is used as the proxy species, from which zinc can be detected by proxy. Copper tends to plate on the working electrode 14 at the expense of zinc. The zinc dissolves into solution as Zn²⁺ ions. The following redox reaction would drive itself:

2Cu⁺ + 2e^" → 2 Cu E₀₁ = + 0.518V (3) Zn²⁺ + 2e- E₀₂ = - 0.762V (4)

The potential E₀ = E₀₁ -E₀₂ = 1.28V will drive the above two reactions.

[00064] In one embodiment, the working electrode which has been plated with an analyte, such as zinc, is immersed in a solution containing ions of the proxy species, such as Cu⁺ or Cu²⁺ ions. The copper ions may be added to the test solution from which the zinc has been plated. Alternatively, the test solution is replaced with a fresh solution containing the copper ions. The concentration of the proxy species in this solution is preferably substantially in excess, such that all of the deposited target analyte is displaced from the working electrode and replaced with the proxy species, and mass transport problems with the proxy species are minimized. Copper deposits on to the working electrode, replacing the zinc. Where the proxy ions are directly in contact with the working electrode, there are no ohmic overpotentials involved. The redox reaction is impeded only by the activation overpotential (i.e., the kinetics between the depositing species and the electrode), and is self-driven- i.e. no external potential is required to drive the redox exchange.

[00065] The plated copper on the working electrode is then subject to a detection method, such as stripping, e.g., a voltammetric stripping technique. As for stripping the analyte itself, this method may include scanning the voltage in the range of the stripping voltage (e.g., about 200-300 mV for copper stripping, vs. silver/silver chloride as a reference) and measuring the stripping current. The charge is obtained by integrating the current. The total charge can be related to the quantity of copper which is stripped using Faraday's constant; one Faraday (96,487 coulombs) of charge being equivalent to one gram equivalent of the proxy species for a monovalent ion, half that amount for a divalent ion, and so forth. Since the copper is present in an equimolar amount to the zinc which has dissolved at the expense of Cu²⁺ deposition (if Cu²⁺ ions are used as the proxy species, or twice the molar amount of zinc, if Cu⁺ ions are used as the proxy species), a measurement of the amount of copper on the electrode can be used to provide a quantitative measurement of the concentration of zinc in the original test solution. This is effective even when low concentrations are to be measured, e.g., where the analyte is deposited in the form of a monolayer, or less.

[00066] Instead of scanning the voltage and detecting the current at the peak corresponding to stripping of the detectable species, the voltage may be kept constant (or pulsed) at or about the voltage at which the detectable species is stripped, and the current measured.

[00067] In another embodiment a system employing two cells 30, 32 is employed, as shown in FIGURE 6. A first cell 30 holds the electrode 14, on which the analyte (zinc in the illustrated embodiment) is deposited in a first step with a switch 34 connecting the working electrode with a circuit which includes a source of potential 18 and a counter electrode 16. It will be appreciated that this first step may alternatively be carried out in a separate cell, similar to that shown in FIGURE 1.

[00068] In a second step, the working electrode 14 remains in the test solution 12, or the test solution may be replaced with another electrolyte. The first cell 30 is connected electrochemically by a salt bridge 36 to the second cell 32. The salt bridge preferably includes chelators for the proxy species, such as Cu chelators, e.g., ethylenediaminetetraacetic acid (EDTA) or ethylene glycol-(β- aminoethylether)N,N,N',N'-tetraacetic acid (EGTA). This avoids diffusion of the proxy species into the sample solution. The second cell includes a proxy electrode 38 (essentially, a second working electrode), which is formed from an inert material, such as those described for the working electrode 14. The proxy electrode 38 is preferably of larger surface area than the working electrode 14. The proxy electrode 38 is in contact with a second electrolyte 40, which contains the proxy species (copper ions in the illustrated embodiment) in a molar amount which is preferably far in excess of the plated zinc. The electrodes 14, 38 are electrically connected by suitable wiring 42, which may be either short circuited between the electrodes, or driven by a voltage source 44. For the second step, the switch 34 is positioned such that the working electrode 14 is connected with a second circuit which includes voltage source 44 and proxy electrode 38. The voltage source drives the redox reaction by compensating for the significant ohmic drop and voltage drops across contact and connecting wire resistances that the redox reaction driven by E₀ would otherwise have to overcome. Specifically, the voltage source preferably has a voltage V, where:

V = activation overpotential +concentration overpotential + IR drop in solution + IR drop in wires 42, etc. (5)

[00069] The potential from the power source 44 offsets these potential barriers and voltage drops across the resistances. However, the voltage from the voltage source should be carefully matched so that it does not drive the deposition of copper even after zinc dissolution is complete. As the deposited zinc layer 23 dissolves in the first electrolyte 12, copper is deposited on the proxy electrode 38. [00070] In a third step, the copper is stripped from the proxy electrode 34, and the stripping current measured, as discussed above. The equivalent amount of plated zinc is then determined from the copper stripping current.

[00071] In another embodiment, the Zn signal is amplified by the indirect methods of voltammetric stripping. As with the techniques described above, a proxy species is used to displace the zinc from the electrode. The proxy species is one which has at least two valence states, such as copper (Cu⁺ and Cu²⁺). In this embodiment, a solution containing Cu⁺ as the proxy species is introduced to the zinc-plated electrode. Two Cu⁺ ions replace each Zn, which goes into solution as Zn²⁺. These Cu⁺ ions, which deposit as Cu, are then stripped off as Cu⁺ ions, thus resulting in a doubling of the current. This serves to amplify the signal, which is particularly useful when low concentrations of the analyte are to be detected.

[00072] In an alternative embodiment, a suitable stripping voltage is applied to the deposited copper which strips the copper in the form of Cu⁺ but not as Cu²⁺. The liberated Cu⁺ ions are dissolved in an electrolyte solution which is preferably initially free of the detectable species (i.e., all the dissolved Cu ions are from the electrode). The dissolved Cu⁺ ions are then redeposited on the electrode, for example by reversing the polarity of the electrode. In a further step, the voltage is reversed and raised to a potential sufficient to strip the redeposited copper from the electrode as Cu²⁺. The stripping current is measured in this second copper stripping step as discussed above. In this way, every Zn originally deposited results in the liberation of 2 Cu⁺ions, which in turn, leads to the liberation of 2 Cu²⁺ ions in the second copper stripping step. This doubles the current generated, making detection easier.

[00073] In all the above detection methods, particularly the scanning methods, the measured stripping current values are preferably adjusted to account for the effects of capacitive currents, for example, by running a blank sample and subtracting the current measured from the stripping current. The blank test is preferably carried out in the same manner as that for the test solution, except that the analyte absent. (i) Substitution Method

[00074] In another embodiment of an indirect detection method, an oxidizing agent capable of oxidizing the deposited metal is delivered at a rate which enables potentiometric stripping to proceed at a detectable, controlled rate. In this analysis method, an oxidizing agent, added to the sample, is used to strip of the deposited analyte from the electrode surface. Potentiometric stripping has advantages in that it is unaffected by dissolved oxygen present in the sample, and does not require sophisticated anodic scanning instrumentation, since the potential is detected in time. Suitable oxidizing agents include Sn⁴⁺, Hg²⁺, and the like.

[00075] For example as shown in FIGURE 7, the electrode 14, with deposited metal layer 23 is contacted with an electrolyte 50, which, in the illustrated embodiment, is in the form of a droplet 52 of liquid. The electrode 14 is illustrated in this embodiment as a ring electrode, although disk and other shapes of electrodes are also contemplated. The droplet is contained within a sample container 54 on a substrate 56 in the form of a plate. The substrate 56 is constructed from a material such as Pyrex®, which is non-reactive towards the chemicals under investigation. Its surface is preferably flat, but may optionally be indented to hold the sample droplet.

[00076] The sample container 54 is constructed of a material that is non- reactive toward the chemicals under investigation and which maintains the droplet in a bounded area on the surface of the substrate. Where the droplet is primarily hydrophilic, such as water, the sample container 54 preferably includes a hydrophobic ring, such as an annulus of silicone elastomer. The annulus 54 helps to position the droplet, such that the droplet is well defined in shape, preferably hemispherical. In an alternative embodiment, the substrate 56 may be formed of a hydrophobic material or coated with a hydrophobic layer (not shown). A hydrophilic disk is defined on the substrate of a suitable size for supporting the droplet. The droplet is contained on the hydrophilic area by the surrounding hydrophobic area.

[00077] The counter electrode 14, which is also optionally used in the deposition step, is also formed on the substrate 56, within the annulus 54. Alternatively the counter electrode is located in a separate solution 40, which is connected with the test solution by a junction hole 64 in the substrate, which is filled with a membrane or gel 66, or by a conventional salt bridge arrangement.

[00078] The electrode 14 is connected by suitable wiring to a computer- controlled potentiostat 25 or other suitable potential maintaining equipment. The potentiometer may apply voltages for the deposition step as well as making measurements in the detection step. A reference electrode 28 is also formed on the substrate within the annulus or may be located in the solution 40 connected with the droplet 52 by the junction 66. The circuit is completed with the potentiometer, working and counter electrodes, and the droplet of electrolyte. A delivery device 70 delivers the oxidizing agent into the electrolyte.

[00079] For controlled delivery of accurate concentrations of oxidizing agent, a diffusional burette 70 is used as the delivery device to deliver the oxidizing agent at a slow rate. This method is particularly suited to small sample sizes and/or low analyte concentrations (e.g., ppm, or less). A suitable diffusional microburette is disclosed in provisional application Serial No. 60/417,149, which is incorporated herein in its entirety by reference. In one embodiment, shown in FIGURE 7, the diffusional burette 70 includes a body 72 which contains a solution 74 of the oxidizing agent. A narrow tip 76 of the burette is fitted with a permeable membrane 78, which is permeable to the oxidizing agent. The membrane can be formed from a material which is permeable to the oxidizing agent. Alternatively or additionally the membrane has a plurality of fine bores (not shown) formed therethrough, each one of approximately 10 microns in diameter. An excimer laser, or other suitable means is used to form the fine bores in the membrane with accuracy and precision. In place of or in addition to a membrane, the oxidizing agent solution is held in the body within a substrate material (not shown), which may be in the form of a gel or a porous ceramic. The substrate allows a controlled rate of delivery through the tip.

[00080] The oxidizing agent diffuses from the tip through the membrane and into the electrolyte 50 (which may be the test solution from which the analyte has been plated, or a different electrolyte). Using a burette 70 as described, an extremely slow and controlled rate of delivery of the oxidizing agent can be achieved. For example, oxidizing agent delivery rates can be selected to allow finite detection times of in the range of a few seconds to several minutes and are suited to the potentiometric stripping of zinc, and other metals.

[00081] In an alternative embodiment, the oxidizing agent may be delivered to the droplet using a diffusional microburette, as described in U.S. Patent Application Serial No.09/980,090, filed on November 28, 2001 , which is incorporated herein in its entirety by reference. Such a microburette could be located below the substrate and connected with the droplet by a junction hole similar to hole 66.

[00082] In one method, detection of the deposited analyte proceeds by delivering the oxidizing agent to the electrolyte 52 at a constant rate or known or precalibrated rate. The potentiostat 25 maintains the working electrode 14 at a selected voltage, relative to the reference electrode 28. A voltage is generated between the working electrode 14 and the counter electrode 16 as the analyte is stripped. Data acquisition device 24 detects the time over which the voltage is maintained. When the voltage drops, this is indicative that all of the analyte has been stripped from the working electrode. The time taken for the voltage to change is dependent on the amount of analyte to be stripped, and hence can be correlated to the concentration of analyte in the test solution.

[00083] Stirring of the droplet can be achieved, if desired, for example, using a flow of gas, such as air, nitrogen, or other inert gas, from a suitably positioned tube or tubes 90. A source of air (not shown), such as a cylinder of compressed air or pump is connected with the tube for providing a stream of air which blows gently onto the droplet, causing rotation. The air has a velocity component which is tangential to the sample boundary, thus providing circular momentum for rotation. Preferably, the tube is arranged tangential to the droplet boundary. Alternatively, vibration of the tip of the burette with a vibrator 94 can serve both to mix to droplet and to increase homogeneity of the oxidizing agent solution in the burette 70. In yet another embodiment, electrode 14 is rotated.

[00084] With reference now to FIGURES 8 and 9, a system suited to detection of zinc or other analyte species in the presence of interfering metal ions is shown. In the case of blood samples, these interfering metals can include Pb, Cd, Fe, Cu, and the like. The system is suited to use with both direct and indirect detection methods, as described above. One or more plating electrodes 100, 102 in the form of disks, ring electrodes, or the like, are used to plate out one or more, and preferably all interfering species prior to deposition and detection of the analyte, zinc in the illustrated embodiment. The plating electrodes 100, 102 may be formed from carbon, or other material on which hydrogen does not evolve readily. Preferably, the area of the plating electrode(s) is larger than that of the working electrode 14 so that the interfering species are plated out relatively quickly. The plating electrodes are used to plate out some, and preferably most or all of the types of metals and other electroactive species which may interfere with zinc deposition and/or subsequent detection. These interfering ions are preferably exhaustively plated, i.e., all or substantially all of the interfering species present in solution is plated out. In a preconditioning step, a deposition voltage is selectively applied to one or more of the plating electrodes at below the deposition potential of zinc using a source of a potential 25 such as a computer-controlled potentiostat (using suitable switches to direct the applied voltage as appropriate), or a different voltage source. The electrode 14 is open circuited during this stage, so that no deposition occurs on the working electrode. Once the interfering metals have plated out, the voltage to the plating electrode(s) is discontinued. A voltage is then selectively applied across the working electrode to deposit zinc using the voltage source 18. In one embodiment, in the case of an RSS realization, the preconditioning electrodes 100, 102 and zinc sensing electrode 14 are placed toward the edge of the sample droplet 52, where the diffusion layer has been found to be thinner. In another embodiment, the working electrode is placed closer to the axial center of the droplet to take advantage of hydrodynamic flow patterns within the droplet. The counter electrode 16 may be placed in a separate solution, as shown, or formed on the substrate, as for the embodiment of FIGURE 7.

[00085] The sensor system thus described and method of operation have a wide variety of applications including the detection of zinc in medical or biological samples, for example, for detecting zinc or other metal ion concentrations in patients' body fluids or for analysis of industrial or environmental samples containing zinc.

[00086] Another application is in zinc plating. In this application, the deposition is carried out by pulsing outside the water window (more negative potential, in the case of zinc). Obviously, the stripping step is not needed. Since pulse durations are small, the diffusion layer would propagate about 1 micron or less. This property is useful but not limited to plating of small structures. For example, for a 1 millisecond pulse, the diffusion layer is about 1 micron in thickness, assuming the plating potential is maintained for the duration of the pulse. The working electrode takes the form of a substrate onto which a layer of zinc is desired to be plated. Typically, the concentration of zinc or other ions in solution will be relatively high, and can be up to the saturation level although any suitable concentration can be used. At least a portion of the metal ion from the solution is deposited in the form of its corresponding metal onto the substrate by pulsing a voltage across the substrate, the pulse having an amplitude and a duration which are such that water splitting is substantially avoided and a layer of the metal is produced on the substrate. The thickness of the deposited layer can be accurately controlled, for example, by controlling the overall deposition time.

[00087] Plating by the deposition techniques described herein requires substantially less energy than conventional zinc plating techniques. It also results in a much more uniform layer of zinc, reducing the "dogboning" effect typically found in conventional plating techniques where distribution is non-uniform. This has particular benefits in applications such as semiconductor applications were thin layers of reproducible thickness of zinc are desired in accurately placed regions of a semiconductor chip.

[00088] The method also has applications in studies where a large potential window is desirable.

[00089] Another example is as an electrochemical cleaning tool. By reversing the anode and cathode, the working electrode can be electrochemically cleaned. The pulses are too short to result in oxygen evolution, thus the problem of oxide formation from the evolved oxygen is minimized.

[00090] To improve the specificity to an analyte where interfering ions are present, both the kinetic and potential signature of the analyte are optionally used to select a pulse amplitude and a pulse duration which favor the deposition of the selected analyte. [00091] Without intending to limit the scope of the invention, the following Examples demonstrate its effectiveness for zinc detection in aqueous solutions.

EXAMPLES

Example 1

[00092] An experimental system as shown in FIGURE 1 is used for detection of zinc in a test solution. The system employs a 0-20V DC power supply, obtained from Hewlett Packard Company, 2850 Centerville Road, Wilmington, Del. 19808- 1610, an analog switch, such as a DG 411 , DG 611 , or DG 201 HS series analog switch obtained from Vishay Intertechnology, Inc., 63 Lincoln Highway, Malvern, PA 19355-2120 USA. A data acquisition board from National Instruments Corporation, 11500 N. Mopac Expwy, Austin, TX 78759-3504 is used for closing and opening the switch. The working electrode (cathode) is formed from platinum and the counter electrode (anode) from steel/copper wire. Zinc plating from the test solution containing 1 ppm zinc ions by weight is carried out using 1 millisecond pulses at about 4.9V, at intervals of 10 milliseconds. No hydrogen evolution is observed. The zinc deposition is continued for approximately 3 minutes. No convection is used.

[00093] Copper sulfate solution at a concentration of 0.5M is then added to the sample and the solution kept stagnant for six minutes before conducting stripping voltammetry.

[00094] An Osteryoung stripping voltammetric scan is then carried out between 600 mV and -300mV. FIGURE 10 shows a plot of current generated, in Amperes, vs. voltage over this range. A clear peak is shown at about +50mV corresponding to copper stripping. The area under the peak can be used to determine the copper deposited and hence the zinc concentration in the original test solution.

Example 2

[00095] Example 1 is repeated but with test solutions containing 10 ppm and 100 ppm by weight zinc ions, respectively. FIGURE 11 shows a plot of the current vs scanning voltage for these two test samples. However, since the deposition time in each case was relatively large (7 minutes) saturation occurs. This explains why the 100 ppm sample does not have an area which is 10 times that of the 10 ppm sample. Thus, for quantitative measurements, a shorter deposition period, sufficient to deposit only about a monolayer (single atom thickness) or less is preferred. A blank scan is carried out on the working electrode prior to deposition of zinc to allow the background effects to be subtracted. Peaks for both Cu²⁺ and Cu⁺ can be seen. By limiting the deposition time, a single Cu²⁺ peak can be obtained, making calculations easier. Example 2, however, demonstrates the viability of the technique, if suitable deposition times are selected.

Claims

Having thus described the preferred embodiments, the invention is now claimed to be:

1. A method of detecting an analyte in a solution which includes water, the analyte having a deposition potential at which water splits electrochemically, the method including: depositing at least a portion of the analyte from the solution on to an working electrode by pulsing a voltage across the working electrode, the pulse having an amplitude and a duration which are such that water splitting is substantially avoided; and detecting the deposited analyte.

2. The method of claim 1 , wherein the analyte is selected from the group consisting of Zn²⁺, Mg²⁺, AI³⁺ _, Fe²⁺, Cd²⁺, Tl⁺, Sn²⁺, Pb²⁺, and combinations thereof.

3. The method of claim 2, wherein the analyte includes Zn²⁺.

4. The method of claim 3, wherein for detection, the deposition pulse duration is less than about 10 milliseconds.

5. The method of claim 1 , wherein the pulse duration is selected such that less than 10% of current generated by the pulse is used by the splitting of water.

6. The. method of claim 1 , wherein the step of pulsing a voltage includes applying voltage pulses at intervals sufficient for a capacitive double layer formed on the working electrode to discharge.

7. The method of claim 6, wherein the step of pulsing a voltage includes applying voltage pulses at intervals of at least about 100 microseconds.

8. The method of claim 1 , wherein the step of pulsing a voltage includes applying voltage pulses for a sufficient time to deposit all of the analyte present in the solution.

9. The method of claim 1 , wherein the step of detecting the deposited analyte includes electrochemically stripping the deposited analyte from the working electrode.

10. The method of claim 9, wherein the step of stripping the detected analyte from the working electrode includes: applying a stripping voltage to the working electrode; detecting a current generated as the analyte is stripped; and determining a concentration of the analyte in the solution from at least one of (a) a measure of the current and (b) a time for stripping to be completed.

11. The method of claim 10, wherein the stripping voltage is applied in the form of a plurality of pulses which are each of sufficiently short duration to inhibit electrochemical splitting of water.

12. The method of claim 11 , wherein the stripping voltage is applied in the form of pulses of opposite polarity to the deposition pulses and/or is applied at a potential at which the analyte undergoes oxidation.

13. The method of claim 9, wherein the step of detecting the deposited analyte includes stripping the analyte using an oxidizing agent in solution.

14. The method of claim 13, wherein the oxidizing agent is selected from the group consisting of Sn ⁺ and Hg²⁺.

15. The method of claim 13, wherein the oxidizing agent is introduced to the working electrode by diffusion and the method further includes: measuring the time for a detectable change in an electrical property of the working electrode to occur which change corresponds to completion of the analyte stripping; and determining a concentration of the analyte in the solution from the measured time.

16. The method of claim 9, wherein the step of stripping the detected analyte from the working electrode includes: depositing a proxy species on one of the working electrode and a proxy electrode using a charge generated by the deposited analyte as it is dissolved in solution as metal ions; stripping the proxy species from the one of the working electrode and the proxy electrode; and determining a concentration of the analyte in the solution from at least one of (a) a measure of the current flowing during the step of stripping the proxy species and (b) a time for stripping the proxy species to be completed.

17. The method of claim 16, wherein the step of stripping the proxy species from the one of the working electrode and the proxy electrode includes applying a voltage to the one of the working electrode and the proxy electrode and detecting the current flowing in a circuit comprising the one of the working electrode and the proxy electrode.

18. The method of claim 16, wherein the proxy species has a first valence state and a second, higher valence state and wherein the step of depositing the proxy species includes depositing the proxy species from the first valence state and wherein the step of stripping the proxy species includes stripping the proxy species in the form of the second valence state, causing an amplification of the current signal.

19. The method of claim 1 , wherein the solution includes first and second analytes, and the method includes: a) depositing the first analyte on the working electrode using voltage pulses of a first duration; b) stripping the first analyte from the working electrode; and c) after steps a) and b), depositing the second analyte on the working electrode or on a separate electrode using pulses of a second, longer duration.

20. The method of claim 1 , wherein the solution includes first and second analytes, and the step of detection includes: a) stripping the first analyte using pulses of a first duration; and b) after step a) stripping the first analyte using pulses of a second, longer duration.

21. The method of claim 1 , further including: exhaustively plating out interfering species on an plating electrode prior to the step of depositing.

22. A method of determining a concentration of at least one analyte ion in a solution, the method comprising: (A) pulsing a voltage across an working electrode, the voltage being at a potential which is outside the water window and which causes the analyte to deposit on the working electrode; and

(B) indirectly detecting the deposited analyte, including one of:

(a) potentiometrically stripping the deposited analyte with an oxidizing agent and determining a time taken for the stripping to complete, the concentration of the at least one analyte ion in the solution being determined from time taken for the stripping to complete; and

(b) redissolving the deposited analyte, the charge generated in the redissolution process being used to deposit a detectable species on one of the working electrode and a proxy electrode, and stripping the replacement species from the one of the working electrode and the proxy electrode, the concentration of the at least one analyte ion in the solution being determined from at least one of:

(i) a current flowing in a circuit comprising the one of the working electrode and the proxy electrode during stripping of the replacement species, and

(ii) a time taken for the stripping to complete.

23. A system for determining a concentration of at least one analyte ion in a solution, the system comprising: a container which contains a sample of the solution; a working electrode disposed within the container; a counter electrode; a source of an electrical potential which forms an electrical circuit with the working electrode and the counter electrode, the source of electrical potential applying a pulsed voltage to the working electrode for depositing the analyte from the solution on to the working electrode, optionally including a switch for interrupting the current path and simulating a pulsed potential source; a plating electrode; means for selectively applying a voltage to the plating electrode independent of the working electrode for plating out at least one interfering species in the solution.

24. A method of depositing a metal ion from a solution which includes water, the metal ion having a deposition potential at which water splits electrochemically, the method including: depositing at least a portion of the metal ion from the solution in the form of its corresponding metal onto a substrate by pulsing a voltage across the substrate, the pulse having an amplitude and a duration which are such that water splitting is substantially avoided and a layer of the metal is produced on the substrate.

25. The method of any one of claims 1-22 and 24 further including: open circuiting the current loop in the interval between deposition pulses to minimize oxidation and dissolution of the deposited metal.

26. The method of any one of claims 1-22 and 24, further including: using an analog switch to create a pulsed voltage from a substantially constant voltage DC source.

27. The method of any one of claims 1-22 and 24, further including: sampling current or its voltage equivalent at intervals of microseconds or less, e.g., in the submicrosecond range to measure stripping of a metal with very fast kinetics