Anodic Stripping Voltammetry (ASV) predates Atomic Absorbtion (AAS) and Induced Coupled Plasma (ICP) techniques, the mainstay of laboratory heavy metal analysis. The technique uses electrochemistry to obtain very high sensitivity, typically detecting part per billion to part per trillion concentrations. Stripping voltammetry and its related electrochemical techniques were the only methods available for trace metal analysis before the introduction of ICP-MS.
The original devices were very large and complicated to use. The advent of modern integrated circuits and micro-fabrication techniques has revitalised this methodology. The system QROS uses is the size of a small paperback book, weighs the same and is powered by a small 9V battery
A set of 3 electrodes completes the instrument, which is capable of part per billion detection limits in just 60 seconds.
No gases, flames, degassing, fume hoods or expensive maintenance is needed. Just plug in, turn on and start analysing. Results for metals in water are obtained in a few minutes, including sample preparation.
Contaminated Land Applications
Stripping Voltammetry is ideal for site work where low detection limits are required. For soil analysis, samples will need a simple acid extraction that extracts the available metals, but water can be analysed directly. One of the biggest advantages of this technique is that salt (sodium), aluminium or chloride does not affect the result. In fact the system needs some salt in the solution to make it work. For ICP and AAS methods, salt and aluminium can seriously interfere with the signal generated, causing significant inaccuracies, especially at low concentrations of the target metals.
For most contaminated sites, XRF techniques are sensitive enough to classify the soil, but the domestic limits for Hg and Cd are too low for the XRF to detect. ASVcan easily reach these limits. Hg and Cd are also readily soluble in the extraction acid because usually they are pollutants and are not chemically bound into natural soil types, unlike arsenic, chromium and nickel containing minerals
The above trace is from a soil extract that has been scanned for Zn, Cd, Cu and Pb. The red line is a Cd, Pb, Cu standard containing 100 parts per billion of each metal. The scan took 30 seconds.
Zn, Pb and Cu are often the cause of soils failing the WAC test because of their H14 ecotoxicity status. Unfortunately, conventional analysis cannot differentiate between soluble and insoluble forms of the metal. WAC testing is the only way to determine the soluble portion of any metals present.
For practical purposes, a WAC test takes between 10 and 15 days before a result is given. For an on site operator, 10 days is about 9.5 days to long. ASVcan be used to indicate if the sample will fail the WAC test due to metals.
A quick screen for the solubility of Pb, Cu, Zn or Cd can be carried out as follows. The representative soil sample is added to a volume of buffer, dispersed and allowed to react for about 10 minutes. Any soluble metals will quickly go into solution. The sample is quickly filtered and analysed using the ASV analyser. If the trace shows peaks in the Zn, Pb, Cd or Cu windows, the sample will probably fail the WAC test. The absence of peaks shows the metals are not soluble at neutral pH and the sample should pass the WAC test for these metals.
ASV can easily be set up to detect As, Cd, Cr, Cu, Hg, Pb and Zn. ASV interestingly only detects the most toxic stable oxidation states of a metal, so for example only detects Cr(VI) and As(III). Simple sample preparation converts any metals in solution into the correct oxidation state.
