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What is ASV?

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Anodic Stripping Voltammetry (ASV) is an analytical technique that specifically detects heavy metals such as Arsenic, Cadmium, Lead, Mercury and others. Heavy metals are usually toxic to humans and animals (and plants in some cases). ASV does not detect the light metals such as Sodium, Potassium, Aluminium, Calcium and Magnesium. It cannot detect inorganic elements such as Sulphur, Iodine, Bromine and Phosphorus or anions such as Sulphate, Chloride, Nitrate etc.

ASV (Anodic Stripping Voltammetry) essentially works by electroplating certain metals in solution onto an electrode. This concentrates the metal. The metals on the electrode are then sequentially stripped off, which generates a current that can be measured. The current (milliamps) is proportional to the amount of metal being stripped off. The potential (voltage in millivolts) at which the metal is stripped off is characteristic for each metal. This means the metal can be identified as well as quantified.

ASV Chart

This method was first described in the 1920s. JAROSLAV HEYROVSKY won the Nobel Prize for chemistry in 1959 for developing it. It is therefore older than Atomic Absorbtion (AAS) or ICP (Induced Coupled Plasma) and is essentially a European invention. The method has found widespread acceptance, but only in specialist laboratories that need to analyse metals in the low part per trillion (nanogram per litre) range, without needing to concentrate the sample.

The original methods used liquid Mercury as the electrode. These devices are called Hanging Mercury Drop devices (HMD). Some devices allow the Mercury to form drops continuously; a mini analysis is carried out as each drop forms. These are called Dropping Mercury Electrodes (DME). DME electrodes consume significant amounts of liquid mercury.

Another way around the problem was to use 3 electrodes. The working electrode is where the metal is plated and stripped off, the counter electrode measures the current flow and the important reference electrode that is used to ensure the potential of the working electrode is maintained correctly. This helps to virtually eliminate the effects of the electrical field building up on the working electrode. This system is now used on HMD systems as well because it allows much lower detection limits to be achieved.

The Cogent Environmental system is a new type of 3 electrode device. Instead of liquid mercury as the electrode, this device uses a glassy carbon electrode that is plated with a very thin film of Mercury. (Mercury Thin Film Electrode, MTFE) This is carried out at the beginning of an analytical run and lasts for between 10 and 30 subsequent analyses. The Mercury is contained as a salt in the supporting buffer used. This means only a very small amount of Mercury is used and ensures the operator never comes into contact with liquid Mercury. The amount of Mercury used per analysis is measured in parts per trillion. If however the analysis is for Arsenic, Selenium or Mercury, a Gold electrode can be used, removing the need for any Mercury.

What is CSV and how does it differ from ASV?
Cathodic Stripping Voltammetry (CSV) is essentially the opposite of ASV. In ASV the potential is changed in the negative to positive direction. In CSV the potential is changed in a positive to negative direction. The following graphic displays this concept.

What is the difference between Linear, Square Wave, and Differential Pulse sweep types?
The preceding graphical display shows the difference between the ASV and CSV methods, but it is also in essence a linear sweep. The appearance of the sweep is that of a staircase, but if the steps are made infinitesimally small, the appearance is that of a line, thus the term Linear Sweep meaning that the potential is changed in a linear fashion over time.

The following graphic displays the parameters of a Differential Pulse Sweep. In a differential pulse sweep, the step size, pulse amplitude or height, pulse width, and rest width are all variable factors that can be used to increase sensitivity of a particular application. The VAS software provided with the instrument has a mathematics function that will calculate recommended values, or they can be modified independently.

The differential pulse potential wave form consists of small pulses (of constant amplitude) superimposed upon a staircase wave form. In the example waveform shown, the current would be sampled at two points during the pulse. One measurement is taken at the beginning of the pulse, and one at the end of the pulse; the difference between these two values being the result that is displayed (or recorded as data).

Differential Pulse Waveform

In Square Wave Voltammetry, the potential wave form consists of a square wave of constant amplitude superimposed on a staircase wave form. The current is measured at the end of each half-cycle, and the current measured on the reverse half-cycle (ir) is subtracted from the current measured on the forward half-cycle (if). This difference current (if – ir) is displayed as a function of the applied potential (again recorded as data). The parameters of this waveform are displayed in the following graphic.

Square Wave Waveform

Square Wave Waveform Analysis

To analyze a sample, the metals must be in solution. For effluent and water the metal is already in solution (though some sample types may still require a digestion), but soil samples need to have the metal extracted.

The liquid sample is added to supporting electrolyte (buffer) to ensure the oxidation states of the metal ions are optimized for electrochemistry. This also dilutes the sample, which removes many of the potentially interfering compounds (cf immunoassay methods). Another component of the buffer removes any dissolved oxygen in the sample that would interfere with the analysis.

The analysis proceeds by initially plating the working electrode with Mercury or Gold (some methods are done on solid electrodes and this step may not be required). Several quick runs with a standard are performed to stabilize the Mercury or Gold film and to confirm the analyzer is working correctly. The diluted sample is then added to the cell and the working electrode is given a negative potential relative to the reference electrode. The value can be varied depending on which metals are to be analyzed. The negative potential attracts the positive metal ions to it, where electrons combine with the metal ions to produce the metal. The use of the Mercury film enhances the process as when the metal ion is reduced to the metallic state, it forms an amalgam with the Mercury, which stabilizes it during the stripping phase. Mercury on glassy carbon also has a high over-potential relative to Hydrogen. This means the potential can be set that allows metals such as Zinc to be plated onto the electrode, without producing hydrogen gas. Hydrogen is very reducing and will interfere with the subsequent stripping. The potential is then held for around 60 seconds (up to 300 in some applications) while the metal accumulates on the electrode, effectively concentrating the metal in the sample onto a small area. Not all of the metal in solution is reduced onto the electrode, but the plating time selected is long enough to reduce sufficient metal onto the electrode to give a good signal.

During the plating process, the sample is mixed at high speed. This ensures that the metal ion concentration at the electrode/sample interface is the same as the concentration in the bulk sample. By mixing the sample, the major factor that pulls the ions to the working electrode is the negative potential and not diffusion, convection or other random movement in the sample. This also helps prevent a capacitive build up on the electrode where a layer of positive ions shield the negative electrode from other ions in the sample. By ensuring the negative potential is the dominant factor during the analysis, the reproducibility of the analysis is dramatically improved. An added bonus is the complex mathematical formula used to calculate the amount of metal deposited for a given time at a given potential is simplified.

The potential is then allowed to become less negative and the metals re-oxidize (or are stripped from the electrode), which generates electrons (2 for each Cu atom, 3 for As etc). Each metal will strip from the electrode at a specific potential, which allows for identification of a metal. The data can be plotted to give a graph of current against potential. This graph is called a voltammogram.

The rate at which the potential is changed is called the sweep rate and is another variable that can be altered to optimize an analysis. The faster the sweep rate (mV/sec) the better the resolution and better detection limits. This is because at high sweep rates, the metals on the electrode quickly strip off from the electrode, giving a narrow peak on the voltammogram. A slow sweep rate allows the metal to strip off slowly, giving a broader peak, which is more variable in size. However, this slow sweep can separate two metals that have similar stripping potentials. By applying different waveforms to the sweep, stripping potentials can be shifted, which is useful when 2 metals of interest strip at a similar potential.

The generation of electrons is measured by the counter electrode as a current produced in the cell. The current in micro or nano amps is proportional to the metal concentration on the electrode. As each metal strips from the electrode, a graph (voltammogram) is produced showing a series of peaks corresponding to current (metal concentration) at specific potentials. By selecting a potential “window” where a specific metal is expected to appear, ASV can be used to identify and quantify the metal concentration in the sample.

Quantification

The calibration curves for individual metals can be linear over 2 orders of magnitude. Most ASV instruments can therefore use a single concentration of standard to analyses samples between 10ppb and 1,000ppb. The calibration curve also has a characteristic gradient which is useful for initial QC of the instrument performance. This method is called calibration curve comparison.

As with all analytical methods there can be interferences. The matrix and presence of other metals or substances can change the potential at which a metal strips from the electrode. Certain metals have similar stripping potentials so a slight shift in stripping potential can cause peak overlap. (similar to using GC or HPLC.) For this reason the analysis is always run with a standard of the metal of interest to identify the exact stripping potential.

To minimize the effects of interference, methods have been developed that use specific buffers that are best suited to various matrixes and metals together with a procedure known as standard additions. The instrument is first calibrated using a known concentration of the metal of interest in specific buffer. The concentration selected should be in the same order of magnitude as the expected concentration of metal in the sample. The current produced should match the expected value for that instrument. If it does the system passes the initial QC check.

The sample is then analyzed and an initial metal concentration calculated. A small volume of a known concentration of the standard is then added to the sample and it is re-analyzed. A second, small volume of the same standard is then added to this sample and it is analyzed again. The three results are compared. As ASV produces a linear calibration curve over 2 orders of magnitude, the results should also produce a linear curve with a gradient similar to that expected for the target metal. For simple shifts in the line such as a slight curve, parallel line or slight divergence, a simple calculation can take the analytical result and convert it to a compensated result. The VAS software supplied with the instruments can compensate for more complex shifts in the calibration curve.