Peanut samples were pulverised using a mortar and pestle after
manual removal of the testa and addition of liquid nitrogen to
encourage shattering. A portion of each sample was set aside for
gravimetric determination of moisture content. Mortars and pestles
were cleaned by abrasion with acid-washed sand, rinsed with
ultrapure water, and oven-dried between samples. The samples
were stored in plastic containers at room temperature until the
time of digestion.
The reaction mixture comprised 0.40 g ground sample or CRM,
5.0 mL nitric acid, 2.0 mL hydrogen peroxide, and 3.0 mL water in
a PTFE vessel. Digestion bombs were allowed to pre-digest for at
least 15 min before loading into the microwave carousel according
to the manufacturer’s instructions. A stepped microwave program
of 100 C and 190 C for 10 min each was initially used. This was
later adjusted to 200 C for 20 min, with adjustments in ramp
times and hold temperatures according to microwave capabilities,
to improve the consistency of digestion. Digestates were unloaded
after internal temperatures had declined to less than 50 C, diluted
to 30 mL with water, and stored at room temperature until the
time of analysis.