Chemical studies of the degree of decomposition and dissolution in microwave digests
1994
Most methods for the determination of trace elements in organic matter require
the sample to be brought into solution, which may be achieved by heating with
strong oxidising agents such as concentrated mineral acids, to decompose the
organic matrix sufficiently to render the sample soluble, whilst not necessarily
achieving complete decomposition. The use of microwave heating for this
purpose, using sealed vessels under pressure, was investigated for a variety of
food samples.
One drawback of this otherwise rapid method is the time taken to cool the vessels
before opening. The use of liquid nitrogen to cool the vessel walls - before, during
and after the heating cycle - was investigated, and its effect on the progress of the
digestion was assessed. Another problem is the control of internal pressure,
which can rise very rapidly when digesting samples of high organic content, and
ways of controlling excess pressure when digesting larger samples were devised.
These included the use of an open pre-digestion under reflux using a microwavetransparent
coolant liquid, and the use of vessels capable of withstanding higher
pressures. The latter proved more convenient as they enabled complete
dissolution to be achieved in a single stage without pre-digestion, although
cooling of these vessels was much slower than for the low pressure vessels, so
overall sample preparation time was similar.
Various methods were developed for the complete dissolution of the samples
using nitric acid and hydrogen peroxide. The degree of decomposition achieved
for the different methods and sample types was monitored by measurement of
residual carbon in the digests. A few samples were virtually completely
decomposed with nitric acid alone, while the use of hydrogen peroxide, following
a nitric acid digestion, was found to reduce residual carbon substantially in the
others. The undecomposed material was analysed by various methods including
ultraviolet/visible spectrometry, infrared spectrometry and thin layer
chromatography. Decomposition products were found to include carboxylic
acids, oxalates, phosphates, nitrates and nitro-aromatics.
Complete decomposition of milk powder, which proved most resistant to
oxidation, was achieved by further treatment with perchloric acid. This was
found necessary for anodic stripping voltammetric analysis, which required the
destruction of interfering organic species for accurate determination of trace
elements, unlike other techniques such as inductively coupled plasma-mass
spectrometry, where complete decomposition was not required.
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