USE OF SMALL-DIAMETER CAPILLARIES FOR INCREASING PEPTIDE AND PROTEIN-DETECTION SENSITIVITY IN CAPILLARY-ELECTROPHORESIS MASS-SPECTROMETRY

The use of small ID capillaries is shown to provide a substantial incr ease in sensitivity for capillary electrophoresis-electrospray ionizat ion/mass spectrometry (CE-ESI/MS). In a comparison using capillaries r anging from either 100 to 10 mum or 50 to 5 mum ID and chemically modi fied with aminopropylsilane, a 25- to 50-fold increase in sensitivity was observed for both peptide and protein mixtures. This enhanced solu te sensitivity allowed the detection of approximately 150 attomoles of melittin (2845 Da) with selected ion monitoring and 600 attomoles of carbonic anhydrase (29 157 Da) while scanning forCE-MS with a quadrupo le mass spectrometer. For the protein mixture, mass spectra of suffici ent quality for precise molecular weight determination (less-than-or-e qual-to 0.05 %) were obtained for 600 attomole injections using a 5 mu m ID capillary. The increase in sensitivity with small capillary diame ters can be primarily attributed to a reduced mass flow rate of buffer and other background constituents into the electrospray source, which allows for greater sample ionization efficiency. A model that qualita tively accounts for the results is presented, but quantitative agreeme nt is precluded due to difficulties in accounting for contributions du e to a liquid sheath flow used with the electrospray source. The model accounts for the observation that the ESI/MS appears to function as a concentration-sensitive detector under many conditions using large-di ameter capillaries. A transition occurs, however, to a regime where th e ESI/MS functions as a mass flow-sensitive detector for small-diamete r capillaries, where the ESI current is limited by the rate of deliver y to the ESI source of charge carrying species in solution. These resu lts suggest peptide and protein analysis at low attomole and subattomo le levels should be obtainable with alternative types of mass spectrom eters.