The validation guideline suggests applying the Horwitz Equation to calculate the minimum intermediate precision. For most of the spiking concentrations used in this method (<0.1 ng mL -1 ), employing this equation yields tolerated deviations that were comparably high, i.e., 45% (1 ng mL -1 ), 64% (0.1 ng mL - 1 ) and 91% (0.01 ng mL -1 ). Therefore, the decision was made to apply stricter limits. The highest acceptable deviation for the intermediate precision was thus defined as 32% for concentrations <10 ng mL -1 , 27% for concentrations between 10-100 ng mL -1 , 23% for concentrations between 100-1000 ng mL -1 , and 16% for concentrations >1,000 ng mL -1 . The same limits were used for the repeatability. A recovery range (defined as the proportion of the detected concentration to the spiked amount that had been added to the blank matrix prior to sample preparation) of 50-120% was accepted for all analytes independent of the spiked concentrations because the quantitative results of the unknown samples were corrected by the respective average recovery.