Long-lived metallized tips for nanoliter electrospray mass spectrometry

Sheathless electrospray at nL/min flow rates combined with Fourier-transform mass spectrometry has made possible high resolving power (gr 50,000) mass spectra of subattomole samples of gr 8 kDa proteins separated by capillary electrophoresis (Valaskovic, G. A.; Kelleher, N. L.; McLafferty, F. W. Science, 1996, 273, 1199-1202). However, for this new method the mechanical stability of the thin (35 to 100 nm) gold film electrodes has limited tip lifetime to 15 to 30 min. A technique for SiO_x coating of the gold is described that provides a steady ion current (± 10 pA) for 1 to 2 h, even with arcs or interruptions of the electrospray voltage.