Why does the preservation of a sample with acid change the results to total nitrate + nitrite?
Preservation of samples for nitrate analysis
In a sample that is preserved, any nitrite that was present has been converted to nitrate. Even after neutralizing the sample, the nitrite that was originally in the sample remains as nitrate. On preserved samples, it is not possible to differentiate nitrite from nitrate, so it is not possible to perform nitrite analysis on preserved samples.