Cleaning of glassware. To ensure that the glassware
used in the preparation of samples is free of contaminants,
the dishes should be cleaned before using them. To
perform this, 25 ml of 20% (200 g l- ‘) HNO, is added
to each dish, covered with a watch glass and heated on
a hot plate until the solution boils. The heating is
maintained to allow the acid solution reflux on the walls
of the dish during 15 min. The cover is removed and the
acid solution rejected. The dish is washed twice with
deionized water. This procedure is repeated with fresh
aliquots of the acid solution. After the acid solution
treatment, dishes are entirely washed with hot deionized
water and afterwards covered with a clean watch glass
and dried in an oven.