To begin, use a clean ceramic knife to manually cut food samples into smaller pieces. Transfer the samples to 250-milliliter glass beakers. Then place the samples in a dryer at 105 degrees Celsius until there is no change in weight.
Next, transfer the dried sample into a mixer beaker. Grind the sample until it turns into a fine powder or a homogenous paste. With a clean plastic spatula, transfer the homogenized sample into a 50-milliliter glass beaker.
Next, weigh 250 milligrams of the sample into an open reaction vessel on an analytical balance. Once the weighing is complete, cover the reaction vessel with its lid. Then transfer it to a fume hood.
Now open the cover lids of the reaction vessels. Then pipette five milliliters of 68%nitric acid and one milliliter of 30%hydrogen peroxide into each reaction vessel. With a 200-microliter pipette, add 37.5 microliters of ICP multi-element standard solution into reaction vessels for a spike recovery test.
Ensure that each unspiked and spiked sample is prepared in triplicate. Place the cover lids on the reaction vessels and screw the thread cover on the reaction vessels to tighten the cover lids. Allow the samples to react with the acids for two to three minutes.
To perform microwave-assisted wet acid digestion, distribute the closed reaction vessels symmetrically in the microwave rack. Insert the rack into the microwave chamber and mount it on a holder. Then close the microwave door.
Digest the samples using the selected digestion program, while monitoring the change in reaction conditions on the screen. Once the digestion is complete, remove the rack from the oven chamber. Close the door of the instrument and switch it off.
Now slowly open the lids of the reaction vessels to release any gases formed during digestion. Turn the reaction vessels towards the fume hood. With a glass funnel, quantitatively transfer the digested sample into a clean 25-milliliter glass volumetric flask.
Dilute the sample with ultrapure water up to the mark on the flask. Then close the mouth of the flask with a stopper and mix the contents well. Next, connect a 20-milliliter plastic syringe to a polyamide syringe filter.
Fill the syringe with the diluted sample. Then filter its contents into a 50-milliliter plastic centrifuge tube. Homogenization resulted in fine sample particulates that were uniform in size and evenly distributed.
Acid digestion caused a steady increase in temperature with time.
