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Copyright © 2003 John Wiley & Sons, Ltd.
ISBNs: 0-470-84421-3 (HB); 0-470-84422-1 (PB)
• To appreciate the different approaches available for the preparation of solid samples for elemental analysis.
• To be able to carry out acid digestion (hot-plate and microwave) in a safe and controlled manner.
• To be aware of other decomposition methods (e.g. fusion and dry ashing).
• To understand the importance of chemical speciation studies.
• To appreciate the importance of chemical species identification with respect to mercury, tin, arsenic and chromium.
• To be able to carry out methylmercury extraction in a safe and controlled manner.
• To be able to carry out organotin extraction in a safe and controlled manner.
• To be able to carry out organoarsenical extraction in a safe and controlled manner.
• To be able to carry out hexavalent chromium extraction and analysis in a safe and controlled manner.
• To understand the relevance of selective extraction methods for soil studies.
• To be able to carry out single extraction procedures using EDTA, acetic acid and DTPA in a safe and controlled manner.
• To be able to carry out a sequential extraction procedure on a soil sample in a safe and controlled manner.
• To be able to carry out a simulated gastro-intestinal extraction of foodstuffs in a safe and controlled manner.
• To be able to carry out a physiologically based extraction test on soil in a safe and controlled manner.
Methods for Environmental Trace Analysis
Analysis for metals in solids can be carried out by two different approaches, namely direct analysis of the solid, or after decomposition of the matrix to liberate the metal. Samples can be analysed directly for metals by using, for example, X-ray fluorescence (XRF) spectroscopy (see Chapter 11). This present chapter principally focuses on methods of decomposition of the matrix to liberate its metal content. In addition, selective methods of metal extraction are considered, together with appropriate methods of analysis.
5.2 Decomposition Techniques
Decomposition involves the liberation of the analyte (metal) of interest from an interfering matrix by using a reagent (mineral/oxidizing acids or fusion flux) and/or heat. The utilization of reagents (acids) and external heat sources can in itself cause problems. In elemental analysis, these problems are particularly focused on the risk of contamination and loss of analytes. It should be borne in mind that complete digestion may not always be required as atomic spectroscopy frequently uses a hot source, e.g. flame or inductively coupled plasma, which provides a secondary method of sample destruction. Therefore, methods that allow sample dissolution may equally be as useful.
Consider the difference between the terms ‘digestion’ and ‘dissolution’.
Digestion infers the complete destruction of the sample matrix whereas dissolution considers the liberation from the matrix of the analyte of interest (without the requirement for complete destruction of the matrix).
In the latter case, it may still be possible to identify ‘organic’ parts of the matrix by using appropriate techniques.
5.3 Dry Ashing
Probably the simplest of all decomposition systems involves the heating of the sample in a silica or porcelain crucible in a muffle furnace in the presence of air at 400-800oC. After decomposition, the residue is dissolved in acid and transferred to a volumetric flask prior to analysis. This allows organic matter to be destroyed. However, the method may also lead to the loss of volatile elements, e.g. Hg, Pb, Cd, Ca, As, Sb, Cr and Cu. Thus, while compounds can be added to retard the loss of volatiles, its use is limited. Due to the disadvantages of this method, namely:
• losses due to volatilization
• resistance to ashing by some materials
• difficult dissolution of ashed materials
• high risk of contamination
it has largely been replaced by wet ashing.
5.4 Acid Digestion (including the Use of Microwaves)
Acid digestion involves the use of mineral or oxidizing acids and an external heat source to decompose the sample matrix. The choice of an individual acid or combination of acids is dependent upon the nature of the matrix to be decomposed. The most obvious example of this relates to the digestion of a matrix containing silica (SiO2), e.g. in a geological sample. In this situation, the only appropriate acid to digest the silica is hydrofluoric acid (HF). No other acid or combination of acids will liberate the metal of interest from the silica matrix.
Why should HF be so effective for the digestion of silica?
A summary of the most common acids types used and their applications is shown in Table 5.1.
Table 5.1 Some examples of common acids used for wet decomposition“
Acid Boiling point (° C) Comments
Hydrochloric (HCl) 110 Useful for salts of carbonates, phosphates, some oxides and some sulfides. A weak reducing agent; not generally used to dissolve organic matter