Capillary electrophoresis coupled to negative mode electrospray ionization-mass spectrometry using an electrokinetically-pumped nanospray interface with primary amines grafted to the interior of a glass emitter.

We demonstrate an electrokinetically pumped sheath flow nanospray interface for capillary electrophoresis coupled to negative mode electrospray mass spectrometry. In this interface, application of an electric field generates electro-osmotic flow at the interior of a glass emitter that is pulled to a 10-20µm inner diameter orifice. Electro-osmotic flow pumps liquid around the distal tip of the separation capillary, ensheathing analyte into the electrospray electrolyte. In negative ion mode, negative potential applied to an untreated glass emitter drives sheath flow away from the emitter orifice, decreasing the stability and efficiency of the spray. In this manuscript, we treat a portion of the interior of the electrospray emitter with 3-aminopropyltrimethoxysilane, which grafts primary amines to the interior. The amines take on a positive charge, which reverses electro-osmosis and generates stable sheath flow to the emitter orifice under negative potential. Negative mode operation is demonstrated by analyzing a metabolite extract from stage 1 Xenopus laevis embryos. Production of the treated emitters was quite reproducible. We evaluated the performance of three emitters using a set of amino acids; the relative standard deviation in peak intensity was 7% for the most intense component.