OPTIMIZATION OF CAPILLARY ZONE ELECTROPHORESIS ELECTROSPRAY MASS-SPECTROMETRY FOR CATIONIC AND ANIONIC LASER-DYE ANALYSIS EMPLOYING OPPOSITE POLARITIES AT THE INJECTOR AND INTERFACE

On-line capillary zone electrophoresis-electrospray mass spectrometry (CZE-ES-MS) has been used to investigate the purity of laser dyes. A CZE-ES-MS interface linking a Dionex CE System I and a Vestec 201 electrospray mass spectrometer was constructed and employed for this purpose. Interface optimization studies revealed that maximum sensitivity conditions are significantly different from maximum stability conditions. Use of a cationic surfactant, cetyltrimethylammonium chloride, proved effective at reducing the interaction between the cationic laser dye, Rhodamine 6G, and the wall of the silica capillary, a problem which can otherwise persist even at low pH. To direct electroosmotic flow towards the detector, negative voltages were applied to the CZE source vial. This enabled efficient separation of Rhodamine 6G from low level isomeric and analogue impurities which were detected using positive voltages at the mass spectrometer ion source. Despite intrinsic difficulties associated with negative-ion ES-MS, negative-ion detection was used in combination with positive CZE potentials to separate and characterize analogue impurities found in the anionic laser dye Eosin Y. Impurities present in such laser dyes can have profound effects on the operating efficiencies of dye lasers and the usable lifetime of the dye.