To reduce the demand for fossil resources, biotechnological processes are increasingly applied. Hence, the used biological hosts have to be optimized in order to achieve the highest possible yield. Prior to this optimization, which is usually carried out by means of metabolic engineering strategies, the intracellular processes need to be investigated by methods such as 13C-MFA. The present thesis focussed on improving the labelling determination analysis for 13C-MFA based on GC-MS and GC-MS/MS.