Self test 9.3

The goal of the analysis method is to estimate whether the metabolites that indicate the use of an illegal doping are present in the urine of a professional athlete. No LoD value is needed. 

The analysis method measures whether any mercury is present in food by using cold vapor atomic fluorescence spectroscopy. LoD must be estimated for the method as the mercury level in the sample can often be close to the LoD of the analysis method. Therefore, LoD must be known to interpret the results. 

The analysis method measures Na⁺ and K⁺ content in drinking water with atomic absorption spectroscopy. These ions are always present in high enough quantities so the LoD value is always lower than the concentration of the analytes in the sample. Therefore determination of LoD is not critical.

The analysis method measures protein content in milk by the Kjeldahl method. In the case of this analysis the LoD value is very important.

Data in Plot A are homoscedastic 

Data in Plot B are heteroscedastic

If CC_β is estimated by using the standard deviation of blank samples, but the analytical method gives strongly heteroscedastic data, then the LoD estimate is overestimated. 

If LoD is estimated from the standard deviation of residuals, but the data used are not linear, then the LoD value will be overestimated (assuming that in this case the slope of the calibration function does not influence the LoD result significanlty).

S/N can be used to estimate LoD for chromatographic methods by performing a single measurement of a sample at different concentrations.