The stability of the analytes in the QC urine samples wasassessed during storage at −24◦C over a period of up to 40 days.For this, a urine sample with a mixture of the 10 organic acids in aconcentration corresponding to the mean normal concentration foreach acid was used. In addition, the stability of the trimethylsilyl(TMS) derivatives during their stay on the autosampler at 5◦C wasinvestigated. Stability of the TMS derivatives of the organic acidswas determined by five determinations over a period of 40 h.