SELECTION OF BUFFERS AND OF AN ION-PAIRING AGENT FOR THERMOSPRAY LIQUID-CHROMATOGRAPHIC MASS-SPECTROMETRIC ANALYSIS OF IONIC COMPOUNDS

The applicability of ammonium formate, ammonium acetate and ammonium bicarbonate as volatile electrolytes for thermospray liquid chromatographic-mass spectrometric analysis of three decomposition products of alpha-aspartame is demonstrated. With these buffers, the pH range of the mobile phases for silica-based reversed-phase columns can be covered. With respect to the buffer capacity and ultraviolet transparency, a buffer salt concentration of 50 mM was used in the mobile phase. The retention times of the compounds studied in the volatile buffers were compared with those obtained in sodium acetate and sodium phosphate buffers and were found to match quite well. For the reversed-phase ion-pair separation of alpha-aspartame and beta-aspartame in combination with thermospray mass spectrometry, a method is described using trifluoroacetate as pairing ion. Liquid chromatographic-mass spectrometric analysis of L-aspartyl-L-phenylalanine shows that the highest signal-to-noise ratio is obtained in ammonium formate. For the compounds studied, no correlation could be found between the signal-to-noise ratio and source temperature, or between signal-to-noise ratio and vaporizer temperature.