The uncertainty of the present method can be evaluated by two types of false identifications: false negatives and false positives. Afalse negative is defined as a TP which is formed in the incubationsystem but discarded due to a failure in any of the screening steps.The three main causes are: (1) removal of a monoisotopic molecularpeak as noise; (2) absence of a 13C isotopic peak; or (3) absence of areasonable time-trend during the incubation period. The first two reasons directly depend on the sensitivity of the instrument and the concentration of the parent compound as peaks with too lowintensity are discarded during the noise removal step.