The alkaline persulfate oxidation, converting all N into nitrate, is an eas
y and rapid method for determining total dissolved nitrogen. We found indic
ations that atmospheric N is trapped during the oxidation step, which can b
e impeded by: (1) using always the same type of autoclave bottle and a cons
tant sample volume; (2) minimising the remaining headspace in the bottle; (
3) closing the bottle caps tightly for autoclavation; and (4) determining b
lanks for each test series.