We used a closed acid digestion method to release cations, since
these compose the majority of essential elements. The purpose of
digestion was to decompose the sample matrix and solubilise the
analytes of interest for nebulisation in the ICP instrument. The
use of microwave radiation and high pressure for closed-vessel
digestion has the advantages of accelerating sample decomposition
and reducing reagent consumption, while minimising the risks of
contamination and loss of volatile analytes (Borkowska-Burnecka,
2000). Throughput is limited by the capacity of the microwave system,
but overall efficiency is improved because the run time is short
(usually 1 h or less) and digestion does not require supervision
Various combinations of strong acids and oxidants have been
used for digestion of plant materials, with nitric acid being the
most commonly used acid. Previous research has validated the
use of closed microwave digestion in nitric acid at 200 C
(600 psi) to prepare several CRM and foodstuffs for ICP–OES analysis
of a range of dietary nutrients and contaminants, including
the essential minerals of interest to our study (Dolan & Capar,
2002). We opted for a similar nitric acid-based method, with addition
of hydrogen peroxide to boost the oxidising strength of the
bomb. Digestion was judged to be complete if a clear yellowish
solution was formed, without visible particulate matter. The first
microwave program was initially found to be adequate for peanut
samples, but as the experiment proceeded, dissolution was occasionally
incomplete, resulting in unacceptably high variability. This
was attributed to gradual deterioration in the elasticity of the PTFE
vessels due to frequent use, which intermittently affected pressure
control. In view of this, we increased the duration of the holding
period (190–200 C) to ensure complete digestion.