Reagent volumes and their handling are reduced compared to wet digestion methods. With high-pressure bombs, the residual carbon content may be lower but it is never quantitatively eliminated. This is systematically observed not only for both conventionally heated and open microwave digestions, but also for closed microwave-assisted digestions. The resulting solutions are of very acceptable aspect (clear, colorless, and odorless), rarely the case when wet digestion methods are used and residual carbon content often attains elevated values. This is a prerequisite for ensuring accuracy with some analytical techniques (e.g., ICP-MS or electrochemical methods) wherein analyte response may be influenced by the presence of residual carbon or some undigested organic molecules. The resulting ash is completely free of organic matter. The possibility of processing larger masses of sample, which, upon mineralization, provides a homogeneous solution, helps to minimize subsampling errors. Additionally, heterogeneity is a typical property of many biological materials. Such an advantage is not realizable with wet digestion methods. This permits preconcentration of trace elements in the final solution, which is useful when very low analyte concentrations are to be determined. Possibility of treating large sample amounts and dissolving the resulting ash in a small volume of acid. This is not their unique advantage – compared with wet digestions, dry ashing procedures present several other interesting characteristics: 1. The application of dry ashing methods is simple and large series of samples may be treated at the same time. Depending on the initial sample condition, results are expressed based on a fresh-or dry-weight basis. The solution is diluted to a known volume and analyzed. The resulting inorganic residue (ash) is dissolved using an appropriate acid. The temperature is then progressively elevated, following a convenient heating program, to attain 450☌, and then maintained for several hours. In the usual high-temperature ashing, fresh or dried (generally 103–105☌) samples are weighed into suitable ashing vessels (vitreous silica, porcelain, platinum) and placed in the furnace. In addition, LTA is a particularly time-consuming procedure. The instrumentation is, unfortunately, very expensive and not readily available at present. Additionally, a low-temperature ashing (LTA) procedure in electronically excited oxygen plasma exists, very desirable for sample preparation when volatile elements are to be determined. However, this property is not directly interesting for usual dry ashing procedures, where precisely slow heating ramps are needed. The unique advantage of the latter is the capacity to ensure application of very fast heating ramps. In addition to conventionally heated muffle furnaces generally employed for dry ashing purposes, the market now also provides microwave furnaces especially adapted to attain elevated temperatures. The commonly utilized temperature for this step is ∼450☌. Usually, these procedures are performed by calcination at atmospheric pressure in programmable muffle furnaces. Our own extensive experience in the field of sample preparation has shown that, better than the other known mineralization procedures, dry ashing methods ensure the quantitative decomposition and elimination of organic matter. In contrast, many respected institutions as well as numerous laboratories carry on the use of classical dry ashing in practical analyses of a number of materials of biological origin. Hoenig, in Encyclopedia of Analytical Science (Second Edition), 2005 General PrinciplesĪfter the appearance, at the end of the 1970s, of commercial advertisings praising the universality and absolute necessity of wet digestion microwave heating devices for trace element analysis, several scientific papers have radically condemned dry ashing procedures, despite their long record of usefulness.
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