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The efficiency of oxidation in wet digestion procedures for organic materials can be of great importance for the quality of the analytical data of various measurement techniques. Four items of commercial equipment for pressurised sample digestion were compared and it could be shown that the residual carbon content achieved only depends on the temperature of the digestion solution and not on the equipment, if the digestion time is long enough to reach the steady state. The consumption of nitric acid during the digestion procedure was investigated. This is important to consider, because the signal response of instrumental element determination can be influenced by the acid concentration of the sample solution. The relation between sample amount, maximum digestion temperature and residual carbon content in a closed vessel digestion system was depicted and the influence of incomplete oxidation of organic sample matrix on the determination of As and Se by means of axial ICP-OES was investigated.

The High Pressure Asher (HPA-S) was adapted with a Teflon liner for pressurized wet digestion in open vessels. The autoclave was partly filled with water containing 5% (vol/vol) hydrogen peroxide. The digestion vessels dipped partly into the water or were arranged on top of the water by means of a special rack made of titanium or PTFE-coated stainless steel. The HPA-S was closed and pressurized with nitrogen up to 100 bars. The maximum digestion temperature was 250 degrees C for PFA vessels and 270 degrees C for quartz vessels. Digestion vessels made of quartz or PFA-Teflon with volumes between 1.5 mL (auto sampler cups) and 50 mL were tested. The maximum sample amount for quartz vessels was 0.5-1.5 g and for PFA vessels 0.2-0.5 g, depending on the material. Higher sample intake may lead to fast reactions with losses of digestion solution. The samples were digested with 5 mL HNO(3) or with 2 mL HNO(3)+6 mL H(2)O+2 mL H(2)O(2). The total digestion time was 90-120 min and 30 min for cooling down to room temperature. Auto sampler cups made of PFA were used as digestion vessels for GFAAS. Sample material (50 mg) was digested with 0.2 mL HNO(3)+0.5 mL H(2)O+0.2 mL H(2)O(2). The analytical data of nine certified reference materials are also within the confidential intervals for volatile elements like mercury, selenium and arsenic. No cross contamination between the digestion vessels could be observed. Due to the high gas pressure, the diffusion rate of volatile species is low and losses of elements by volatilisation could be observed only with diluted nitric acid and vessels with large cross section. In addition, cocoa, walnuts, nicotinic acid, pumpkin seeds, lubrication oil, straw, polyethylene and coal were digested and the TOC values measured. The residual carbon content came to 0.2-10% depending on the sample matrix and amount.

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