Mechanism of Signal Suppression by Anionic Surfactants in Capillary Electrophoresis−Electrospray Ionization Mass Spectrometry

Micellar-mediated capillary electrophoresis (CE) is used for a wide variety of applications, including the separation of pharmaceuticals, environmental contaminants, illicit drugs, DNA fragments, and many other biological samples. The electrospray ionization interface is one of the most common CE-MS interfaces. Coupling micellar-mediated CE separations with MS detection combines two very powerful, widely applicable analytical techniques. Some types of surfactants strongly interfere with electrospray ionization mass spectrometric (ESI-MS) detection of analytes, and in many cases the ESI-MS analyte signals are completely quenched. Only a few reports have appeared that describe the ESI-MS detection of analytes in the presence of surfactants; however, the exact mechanism of ionization suppression has not yet been addressed. In this work, a modified aerosol ionic redistribution (AIR) model is presented that qualitatively explains the results of previous studies, including those using “polymeric surfactants”. Analyte ionization suppression by surfactants appears to be caused by Coulombic interaction between oppositely charged solute and surfactant ions in the ESI-produced offspring droplets. It appears that the ability of surfactants to quench electrospray ionization is directly related to the surface activity and the charge of the surfactant. Also, highly surface active components tend to be enriched in ESI-produced offspring droplets. Analyte ion signals can be detected under conditions that lower the surface concentration of oppositely charged surfactant ions in aerosol droplets. The mechanistic information outlined here may be used to design micellar-mediated CE separations that allow detection of analyte ions by ESI-MS.