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Chemical studies of the degree of decomposition and dissolution in microwave digests

By Helen J. Reid

Abstract

A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.Most methods for the determination of trace elements in organic matter require\ud the sample to be brought into solution, which may be achieved by heating with\ud strong oxidising agents such as concentrated mineral acids, to decompose the\ud organic matrix sufficiently to render the sample soluble, whilst not necessarily\ud achieving complete decomposition. The use of microwave heating for this\ud purpose, using sealed vessels under pressure, was investigated for a variety of\ud food samples.\ud One drawback of this otherwise rapid method is the time taken to cool the vessels\ud before opening. The use of liquid nitrogen to cool the vessel walls - before, during\ud and after the heating cycle - was investigated, and its effect on the progress of the\ud digestion was assessed. Another problem is the control of internal pressure,\ud which can rise very rapidly when digesting samples of high organic content, and\ud ways of controlling excess pressure when digesting larger samples were devised.\ud These included the use of an open pre-digestion under reflux using a microwavetransparent\ud coolant liquid, and the use of vessels capable of withstanding higher\ud pressures. The latter proved more convenient as they enabled complete\ud dissolution to be achieved in a single stage without pre-digestion, although\ud cooling of these vessels was much slower than for the low pressure vessels, so\ud overall sample preparation time was similar.\ud Various methods were developed for the complete dissolution of the samples\ud using nitric acid and hydrogen peroxide. The degree of decomposition achieved\ud for the different methods and sample types was monitored by measurement of\ud residual carbon in the digests. A few samples were virtually completely\ud decomposed with nitric acid alone, while the use of hydrogen peroxide, following\ud a nitric acid digestion, was found to reduce residual carbon substantially in the\ud others. The undecomposed material was analysed by various methods including\ud ultraviolet/visible spectrometry, infrared spectrometry and thin layer\ud chromatography. Decomposition products were found to include carboxylic\ud acids, oxalates, phosphates, nitrates and nitro-aromatics.\ud Complete decomposition of milk powder, which proved most resistant to\ud oxidation, was achieved by further treatment with perchloric acid. This was\ud found necessary for anodic stripping voltammetric analysis, which required the\ud destruction of interfering organic species for accurate determination of trace\ud elements, unlike other techniques such as inductively coupled plasma-mass\ud spectrometry, where complete decomposition was not required

Publisher: © Helen Julia Reid
Year: 1994
OAI identifier: oai:dspace.lboro.ac.uk:2134/15385

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