Techniques involved in the measurement of arsenic.

All analytical methods consist of a number of stages, which typically include: sampling, sample pretreatment, instrumental measurement, calibration, interpretation, evaluation and decision. Each research group investigated an aspect of a method for the determination of arsenic in environmeal materials. Arsenic has the ability to form volatile hydrides. For example arsenite in solution can be transformed to arsine (AsH_3_) gas by a variety of reagents. This is a very useful property that is exploited in a number of the emthods of analysis. Phosphorus does not form a volatile hydride under the same conditions as so this reaction is a way of separating arsenic compounds from phosphorus compounds. It is useful to be able to do this, as phosphorous compounds can cause positive interferences in some procedures for the measurement of arsenic.

1. Measurement by of the ions, produced by an inductively coupled plasma (ICP) source from a solution of arsenic sprayed into the plasma, by a mass spectrometer (MS). The technique is ICP-mass spectrometry (ICP-MS)

2. Measurement of the light emittted by hot atoms (or ions) produced by inductively coupled plasma (ICP) source. The technique is ICP-optical emission spectrometry (ICP-OES).

3. Measurement of the intensity and energy of X-rays emitted following ionization (also by X-rays). The technique is X-ray fluorescence (XRF) spectrometry. There are several devices that can be taken out of the lab to make measurements on site.

4. Measurement by the absorption of light by molecules in solution (solution spectrophotometry).

5. Measurement by the light emitted following absorption of light by atoms in the gas phase produced by the decomposition of the volatile hydride, arsine (hydride generation atomic fluorescence spectrometry, HG-AFS). Arsine is produced by reation of arsenite with borohydride, BH_4_^-^.

6. Measurement by the absorption of light by atoms in the gas phase produced by the decomposition of the volatile hydride, arsine (hydride generation atomic absorption spectrometry, HG-AAS). Arsine is produced by reation of arsenite with borohydride, BH_4_^-^.

7. Measurement by means of the intensity of the color stain produced on a paper impreganted with mercuric bromide by reaction with arsine generated by the reaction of arsenite with the hydrogen produced by the dissolution of zinc in acid solution. The reaction is known as the Gutzeit reaction.

8. Measurement by the absorption of light by atoms in the gas phase produced by the decomposition of the salts produced on drying a few µL of soluiton oin the interior or a graphite furnace, arsine (electrothermal atomization atomic absorption spectrometry, ET-AAS).

9.Measurement of the components of a mixture after separation based on the relative affinities of each component for two phases one of which moves (chromatography). When performed with small (5 µm) particles and a liquid mobile phase, good separations in short times are achieved. The overall technique is high performance liquid chromatography (HPLC). A variety of detectors can be used, including the atomic spectrometry techniques mentioned earlier.

10. Most measurement techniques need the sample components to be in solution. Many samples are solids and thus have to be dissolved, or at least the components that are to be measured must be dissolved. If the analytes are dissolved but not changed and some of the sample is left as a solid, the procedure is known as leaching or extraction. If the analytes are chemically changed, the process is called digestion. Sometimes agressive conditions need to be used to get materials into solution such as heating under pressure with concentrated acids (nitric) and oxidizing agents (peroxide). Sealed vessels in microwave ovens are often used.

11. Measurement of the components of a mixture after separation based on the relative affinities of each component for two phases one of which moves (chromatography). When performed with a gaseous mobile phase, the technique is high gas chromatography (GC). Usually the stationary phase in chemically bonded to the inside walls of a long (30 m) capillary tube. A variety of detectors can be used, including the atomic spectrometry techniques mentioned earlier. There are also a number of commonly used detectors that are specific for GC, such as the flame ionization detector and electron capture detector.