Magnesium interference and different efficiencies of diastereoisomeric cluster formation in phenylalanine enantiomeric discrimination by the kinetic method

The kinetic method has been applied for determination of d-Phe/l-Phe enantiomeric ratio. Discrimination of enantiomers was inferred from product ion mass spectra of trimeric cluster ions containing the analyte (l,d-Phe), Cu2+ as a central metal and l-Trp as a chiral reference ligand. Unsatisfactory quantitative results achieved on an ion trap were rationalized by high-resolution mass spectrometry. The formation of Mg2+-containing cluster isobaric to trimeric cluster [Cu(l-Trp)2Phe]+ was observed. Interference like this was identified as a possible reason for deterioration of quantitative low-resolution mass spectrometric analyses of real-world samples based on the kinetic method. Cation-exchanger was used for easy removal of magnesium from a sample and improvement of quantitation.Chiral dependence of formation of the Cu2+-containing trimeric cluster was also observed. Heterochiral diastereoisomeric ions were created less effectively.