1. Weigh a 24 × 150 mm Pyrex, numbered, digestion tube.
2. Place 1–5 g of homogenised fish muscle into the tube (amount used depends on anticipated mercury concentration).
3. Add 10 mL of concentrated nitric acid. Allow to stand overnight.
4. Transfer the digestion tube to an aluminium block placed on a hot-plate. Adequate temperature control was achieved by using a standard aluminium topped laboratory hotplate, shielded from the fume-cupboard updraft by aluminium foil. Adequate refluxing, with negligible loss of
mercury, occurs because the top of the test tube acts as an air-cooled condenser. Seventy-two tubes can be heated on each hotplate. Raise the temperature to 125 °C over a period of three hours. Allow to gently reflux for a further four hours, then cool.