different volume of digested sample solutions was compared with
the slope of an external calibration equation. The
fluorescence
signal was obtained with a gain of 1000 due to the high dilution of
sample solutions used to avoid the interference of nitrous acid and
nitrous oxides generated during the digestion. The external
calibration and standard addition method were both measured
under the same conditions. Table 7 summarises the values of the
slopes obtained for the external calibration and the ratio for the
standard addition curves with different digestion mixtures