The inductively coupled plasma atomic emission spectrometer with discrete nebulisation gives a spike-like signal. The peak height of the signal is proportional to the metal concentration in a sample solution at a constant injection volume. The peak area, on the other hand, corresponds to the absolute amount of metal in the injection volume. Linear plots of peak area versus absolute amount, i.e., calibration graphs, are obtained by injecting (1) a constant volume of standard solutions of different concentrations and (2) different volumes of a standard solution of a constant concentration. The calibration graphs thus obtained are identical with each other. With these calibration methods, manganese was successfully determined in the standard biological material NIES-CRM No. 6 Mussel.