The precision data were calculated as relative standard deviation (RSD) of replicate measurements. The RSD value under repeatability conditions (RSDr) should not be higher than the half or two thirds of the value calculated by Horwitz equation (Eq. 5). For mass fractions of 150 μg/kg, this means that the RSDr should be lower than 14%. However, using this equation, mass fractions lower than 100 μg/kg will result in unacceptable high values.