Monitoring of the fermentation media of citric acid by the trimethylsilyl derivatives of the organic acids formed

In this approach, a derivatization method is described for monitoring of organic acids in fermentation media without any separation step. The aqueous phase of fermentation media was evaporated and heated in a silylation reagent to form trimethylsilyl (TMS) derivatives. The silylated compounds are analyzed by Si-29 nuclear magnetic resonance (Si-29 NMR) and gas chromatography-mass spectrometry (GC-MS). Si-29 NMR can qualitatively monitor the components produced in the Krebs cycle. Quantification of these compounds is investigated by using selected ion monitoring mode of mass spectrometry. In this mode, mass to charge (m/z) values of their [M - 15](+) ions, which are 465, 275, 247, 221, 335, 251, and 313 of TMS derivatives of citric, alpha-ketoglutaric, succinic, fumaric, L-malic, oxaloacetic, and palmitic (as an internal standard), acids, respectively, are used. The limit of detection and the linear working range for derivatized citric acid were found to be 0.1 mg L-1 and 10-3 x 10(4) mg L-1. The relative standard deviation of the method for five replicates was 2.1%. The average recovery efficiency for citric acid added to culture media was similar to97.2%. Quantitative results of GC-MS are compared with those obtained by an ultraviolet-visible method.