Reversed phase continuous full filling CEC-ESI-MS

Nanoparticles were used as a pseudostationary phase in capillary electrochromatography (CEC) electrospray ionisation-mass spectrometry (ESI-MS) for separation of both neutral analytes by a reversed phase mechanism, as well as for cationic analytes by a cation exchange mechanism. Nanoparticles suspended in electrolyte, were injected as a plug prior to the sample using a partial filling technique (PF), or used as electrolyte in a continuous full filling (CFF) technique. An orthogonal ESI probe was used to hinder the nanoparticles from entering the mass spectrometer and to allow detection of analytes co-eluting with concentrated nanoparticle slurries. Two types of nanoparticles were synthesised and used, both of them having a hydrophobic core and a hydrophilic surface. The hydrophobic core gave the nanoparticles reversed phase properties and the hydrophilic surface promoted the formation of stable slurries of nanoparticles in electrolytes with a low concentration of organic modifier. The surface of one of the nanoparticle types was covered with sulphate groups that, besides from enhancing slurry stability and thus enabling reversed phase CEC, also enabled ion exchange CEC. Both nanoparticle types showed reproducible and interpretable retention properties.