The solid mass is dissolved out from the column with 5 ml of dimethylformamide (DMF) and the absorbance is measured by a spectrophotometer at 543 nm for SAM–NS, 537 nm for SM–NS and 530 nm for SD–NS. The calibration graph was linear over 30–600 ng NO2-N and 22–450 ng NO3-N in 15 ml of final aqueous solution (i.e. 2–40 ng NO2-N ml−1 and 1.5–30 ng NO3-N ml−1 in aqueous sample) for three systems.