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. Therefore, in this case, the RSDr should be as low as possible.
With regard to the RSD under within-laboratory reproducibility conditions (RSDR), the observed values should not be greater than those calculated using the Horwitz equation. For mass fractions of 150 μg/kg, this means that the RSDR should be lower than 21%. However, similarly to the RSDr, the RSDR for mass fractions lower than 100 μg/kg should be as low as possible. Data presented in indicate the good precision of the method.