[en] In an attempt to develop quantitative mass spectrometry toward a definitive method for organic constituents, each step of the GC-MS procedures have been subjected to critical evaluation of their effects on the ratio measurement between corresponding fragments, using isotope dilution technique and glucose and glycerol labelled with ²H and ¹³C. The results show that in particular ion source settings strongly influence the fragment ratio measurements. The data suggest that in high precision quantitative analyses the use of relative measurements (standard curves) should be used. By optimizing all parameters a coefficient of variation between parallels close to 0.3% and with no known inaccuracy was obtained as shown for glucose. (Auth.)