Sample dissolutiondestruction of organic matter

The sample dissolution is a very important step of the analytical procedure in order to obtain a reliable final concentration datum. The sample dissolution is then strictly linked to the kind of analyte to determine. Generally, in the case of food matrices, inorganic contents, and metals in particular are the minor constituents; so it is absolutely necessary to remove the organic matter, the major constituent, before the final instrumental measurement to obtain the element concentration, since several interference problems may arise. Really, the direct determination of metals in food matrices without a sample pre-treatment has seldom been followed [35].

On the contrary, most procedures proposed for the determination of metals in food matrices require a preliminary destruction of organic matter, since this predominant component certainly interferes in the analytical procedure [36,37], altering significantly in positive or negative ways the metal concentration value.

Several methods, aimed to solve the problem relevant to the destruction of the organic matter, have been proposed in the literature [24,38,39]. However, it is not possible to recommend a single particular procedure since, generally, all the proposed methods are strictly linked to personal surveys, convictions or preferences.

In the 1970s, Gorsuch [39] published a very interesting and still now actual treatise, showing that, in all cases, the destruction of organic matter is the more suitable procedure for obtaining a correct metal concentration datum.

The two methods usually employed are: wet and dry ashing, and a third procedure, the fusion, is rarely followed [38]. Obviously, the choice of a particular procedure always represents a compromise in order to obtain the better concentration datum (good precision, accuracy, limits of detection and/or quantification, sensitivity of the analytical procedure, and so on). Both wet and dry ashing present advantages and disadvantages.

Wet ashing involves reasonable times, small amounts of sample, but relatively large volume of reagents, and then contamination due to the reagent impurities may be a serious problem, especially in the case of very low element concentrations.

Dry ashing also requires reasonable times, but a relatively large amount of sample is necessary. Such a procedure presents, however, also contamination problems. In dry ashing, because containers must be left open to the atmosphere, contamination caused by airborne particulate and/or by components of the ashing apparatus may be a problem. Besides, analyte losses may occur in the case of an element with very high volatility as Hg (see Section 3.1) or of hydrides of particular metals, such as As(III), Sb(III), Sn(IV), Te(II), Se(II), which have boiling points lower than 0°C.

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