Surface roughening of a non-tapered open tubular emitter for improved electrospray ionization mass spectrometry performance at low flow rates

A non-tapered open tubular emitter with 75 microm internal diameter (i.d.) and 360 microm external diameter (o.d.) was developed by simply grinding the exit aperture of a fused-silica capillary. The roughened emitter, with a relatively large aperture, generates stable electrospray signals (generally <5% relative standard deviation (RSD) for most conditions studied) at less than 500 nL/min flow rates, and was characterized with atomic force microscopy. The surface treatment greatly extends the operational range of an open tubular emitter to lower flow rates, compared to that of a cleaved capillary with similar dimensions. The stabilized nanoelectrospray is attributed to the increased surface roughness and modified wetting characteristics of the emitter exit resulting from grinding. Electrospray performance was evaluated, and as a result of the enhanced sensitivity from a roughened emitter, five femtomoles of leucine enkephalin were detected at a 50 nL/min flow rate with a signal to noise (S/N) ratio of 48. Furthermore, trypsin-digested bovine serum albumin (BSA) was used to demonstrate the application of the emitter in protein identification, giving a sequence coverage of 60%. These emitters are robust, and may become a facile alternative to tapered emitters at moderate nano flow rates (e.g. 50 to 500 nL/min).