Strategy for selecting separation solutions in capillary electrophoresis mass spectrometry

A simultaneous determination of cationic, neutral and anionic analytes was performed by capillary zone electrophoresis-mass spectrometry (CZE-MS). An ammonium acetate buffer (pH 6.0) was used as a weakly acidic running solution, an ammonium phosphate buffer (pH 7.0) was used as a neutral one, and an ammonium carbonate buffer (pH 8.5) was used as an alkaline one. The best CZE separation was achieved using the alkaline running solution because several analytes having acidic hydroxyl groups were deprotonated near their pK(a) values. The ammonium phosphate buffer could also be used as the running solution in CZE-MS without a deterioration of the detection sensitivity. As for a sheath liquid, a mixture of the alkaline buffer (pH 8.5)-methanol (1:1) was suitable because a stable negative electrospray was obtained with an alkaline solution for the detection of anionic analytes by negative ion mode. Electrokinetic chromatography-MS (EKC-MS) was performed for the separation of neutral analytes. Nor only sodium dodecyl sulfate (SDS) but also sulfobutyl ether beta-cyclodextrin [beta-CD-SBE(IV)] and 4-sulfonated calix[6]arene (Cx-SO3) were employed as pseudo-stationary phases. The interference of the MS detection by SDS was investigated using the alkaline running solution (pH 8.5), but the deterioration of the detection sensitivity of analytes was not observed. ll significantly different selectivity was achieved when beta-CD-SBE(IV) and Cx-SO3 were used as pseudo-stationary phases in comparison with SDS micelles. Furthermore, no background signal on the MS detection caused by the elution of the pseudo-stationary phases was observed. The charged CD and the charged calixarene were also useful for the separation of neutral analytes by EKC-MS. (C) 1998 Elsevier Science B.V. All rights reserved.