3.6. Validation of the present approach In order to test the applicability of the present HS-dynamic in- syringe LPME with in situ derivatization-HPLC–FLD method for the quantitative determination of ammonium ion in aqueous sample, calibration plot was built-up over the concentration from 0.625 to 10 M. The peak area of signal response had a good linear relationship with the quantity of ammonium ion in the equa- tion of y = 1556x + 8368 with a correlation coefficient of 0.9967. The precisions of the developed method (as relative standard deviations, RSD) were between 3.2% and 8.7% for three determina- tions of ammonium ion in the concentration range for calibration. Detection limit was calculated based on three times the standard deviation (n = 3) of the lower concentration in calibration plot (0.625 M) divided by the detection sensitivity (slope of calibration plot), which was 0.33 M (5.6 ng mL−1 ).