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13 SAMPLE DISSOLUTION
13.1 Introduction
The overall success of any analytical procedure depends upon many factors, including proper sample preparation, appropriate sample dissolution, and adequate separation and isolation of the target analytes. This chapter describes sample dissolution techniques and strategies. Some of the principles of dissolution are common to those of radiochemicalseparation that are described in the next chapter, but their importance to dissolution is reviewed in this chapter. Sample dissolution can be one of the biggest challenges facing the analytical chemist, because most samples consist mainly of unknown compounds with unknown chemistries. There are many factors for the analyst to consider: What are the data quality objective requirements for bias andprecision to meet the data quality objectives of the program? What is the nature of the sample; is it refractory or is there only surface contamination? How effective is the dissolution technique? Will any analyte be lost? Will the vessel be attacked? Will any of the reagents interfere in the subsequent analysis or can any excess reagent be removed? What are the safety issues involved? What are thelabor and material costs? How much and what type of wastes are generated? The challenge for the analyst is to balance these factors and to choose the method that is most applicable to the material to be analyzed. The objective of sample dissolution is to mix a solid or nonaqueous liquid sample quantitatively with water to produce an aqueous solution (homogeneous mixture),so that subsequentseparation and analyses may be performed. Because very few natural or organic materials are water-soluble, these materials routinely require the use of acids or fusion salts to bring them into solution. These reagents typically achieve dissolution through an oxidation-reduction process that leaves the constituent elements in a more soluble form. Moreover, because radiochemists routinely add carriers oruse the technique of isotope dilution to determine certain radioisotopes, dissolution helps to ensure exchange between the carrier or isotopic tracer and the element or radioisotope to be determined, although additional chemical treatment might be required to ensure exchange. There are three main techniques for sample decomposition discussed in this chapter: • Fusion; • Wet ashing, acid leaching,or acid dissolution; and • Microwave digestion. Fusion and wet ashing techniques are used singly or in combination to decompose most samples analyzed in radioanalytical laboratories. Generally, fusion techniques are used when a total
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dissolution of a difficult sample matrix is required. Leaching techniques are used to determine the soluble fraction of the radionuclide of interest under specific conditions. Because recent advances in microwave vessel design have allowed for the use of larger samples, microwavedissolution is becoming an important tool in the radiochemistry laboratory. Because of the potential for injury and explosions, it is essential that proper laboratory safety procedures be in place, the appropriate safety equipment be available, a safe work space be provided, and that the laboratory personnel undergo the necessary training to ensure a safe working environment before any of thesemethods are used. Aspects of proper sample preparation, such as moisture removal, oxidation of organic matter, and homogenization, were discussed in Chapter 12, Laboratory Sample Preparation. Fundamental separation principles and techniques, such as complexation, solvent extraction, ion exchange, and co-precipitation, are reviewed in Chapter 14, Separation Techniques. There are many excellent...