Vapour phase and pressure digestion

The exploration of gas-phase reactions to dissolve biological matrices has rarely been considered in spite of its well established success for silicates and other refractory inorganic materials. A simple apparatus for the vapour-phase oxidation of blood and other tissues with HN03, has been described by Roos [38]. The sample containers are placed inside a reaction vessel containing cone. HN03 and fitted with a reflux condenser. The sample attack, which takes 3—4h, is from the vapour phase only, ensuring that no acid accumulates in the sample and that contamination is minimised. 

A simplified pressurised digestion technique was used by Lutyen et al. [30]. Tissue samples (250 mg) were placed in Nalgene bottles, 4 ml of (1 + 1) HN03/HC104 was added and the bottles were sealed and kept at 100°C for 2h. After cooling, the bottles were opened and the solutions diluted with water. This procedure did not work well with brain tissue because of the high lipid content. 

Decomposition of biological tissues in a low pressure ( 1 torr) stream of radio-frequency excited 02 gas takes place at relatively low temperatures (70°C) with no volatilisation losses and 99—102% recovery of Sb, As, Cs, Co, Cr, Fe, Pb, Mn, Mo, Se, Na and Zn [43]. Appreciable losses of Hg, I, Ag, Au and Pt do occur, the latter three probably as a result of catalytic reaction with the excited 02 [44], The main disadvantage of this technique is the very long ashing time, which can be up to 32 h for 1 g samples. The use of more reactive gas mixtures can reduce the ashing time. Lopez-Escobar and Hume [45] described a mixed-gas technique in which 1.4 nmol min-1 of 03 in 02 effected the release of 98.5% of Hg from organic matrices in only 

Carter and Yeoman [46] used an equimolar mixture of CF4 and 02 to oxidise 10 mg weights of whole-blood for Cd analysis in 12.5 min compared with 60 min required with 02 alone. Pretreatment of blood samples with HjOj, can further reduce the sample time for complete oxidation with CF4/02 to 6 min for 20 mg (20 pi) samples [47]. The application of mixed reactive gases for the rapid oxidation of biological matrices merits further study. 

All of the sample preparation techniques that have been discussed may be used with solvent extraction and other separation procedures to concentrate the analyte and separate it from the matrix. Where such procedures are applicable they will be discussed together with the methods for determining the individual elements. 

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