Preconcentration in Inorganic Trace Analysis

Preconcentration in trace analysis lowers the detection limit, simplifies the treatment of a representatIve sample, facilitates calibration, reduces the need for various standard reference samples and frequently involves separation of the matrix in analysis of toxic, radioactive and expensive materials. In this paper, the types of preconcentration are considered and quantitative characteristics which describe it are noted. The main methods of preconcentration are discussed: solvent extraction, sorption, precipitation and co—precipitation, evaporation, ashing, directed crystallization, electrochemical methods, fire—assay, partial dissolution of the matrix and so on. A comparison of preconcentration methods is made. Rational combinations of concentration methods and subsequent methods of determination are of great importance. In these cases, the number of elements to be determined, the nature of the concentrate, and the overall error connected with the concentration and determination stages should be taken into consideration. Examples of successful combinations are given: solvent extraction—spectrochemical analysis, solvent extraction —atomic absorption, and others. Peculiarities of preconcentration in analysis of high—purity substances and natural waters are also considered. Preconcentration is used if the detection limit of the analytical technique is higher than the concentration of trace elements in the sample. Concentration often involves separation of the matrix or the bulk of it, and sometimes a number of interfering minor constituents as well. In a prepared concentrate, the relative concentration of trace elements is usually higher than in the initial sample. Moreover, the possibility of increasing the amount of sample analysed means that the absolute amounts of elements to be determined can also be increased. As a result, it is possible to reduce the detection limit of trace elements (sometimes very significantly; by a factor of 100 or 1000). This is the main but not the only reason for the widespread use of preconcentration. Preconcentration is almost essential if trace elements are non—homogeneously distributed in the material. In this case, a representative sample must be quite large; it is difficult to analyse it directly, especially if the method of determination needs a small sample as, for instance, spark—source mass—spectrometry or spectrochemical analysis. To take a classical example: the fire—assay technique concentrates gold and other noble metals, which are non—homogeneously distributed in ores, rocks, etc., from very large samples. In many other cases, preconcentration with preliminary dissolution and production of a small volume of concentrate facilitates the preparation of a representative sample. Samples can also be homogenized during other operations, of course. Concentration facilitates calibration, especially if there is a lack of standard reference materials. It makes it possible to obtain concentrates with identical matrices in analysis of quite different materials, for example, concentrates on carbon powder in spectrochemical analysis. Reference samples are prepared as concentrates of the same type. There is then no necessity to have standard reference materials for all substances analysed. Preconcentration with exhaustive removal of the matrix is desirable in the analysis of toxic, radioactive or,if the matrix can be recovered,very expensive materials. Moreover, it is convenient to add elements as internal standards, if necessary, during decomposition of the sample and concentration. Sometimes, preconcentration allows an increase in the number of trace elements which can be determined by a selected technique or makes it possible for the determination technique to be used at all. These advantages of preconcentration make it an important part of trace analysis. In spite of the progress in sensitive instrumental methods of direct analysis, the significance of