A 50 ml volume of a unfiltered sample is pipette into a 100 ml calibrated flask and filled up with buffer solution. If nitrate concentrations in excess of 15 µmol/l are expected, the samples should be dilute 1:2 or 1:5 by correspondingly increased buffer volumes; i.e., only 25 ml or 10 ml sample (instead of 50 ml ) should be filled up to 100 ml with buffer solution