Use of small-diameter capillaries for increasing peptide and protein detection sensitivity in capillary...

The use of small ID capillaries is shown to provide a substantial increase in sensitivity for capillary electrophoresis-electrospray ionization/mass spectrometry (CE-ESI/MS). In a comparison using capillaries ranging from either 100 to 10 microns or 50 to 5 microns ID and chemically modified with aminopropylsilane, a 25- to 50-fold increase in sensitivity was observed for both peptide and protein mixtures. This enhanced solute 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 for CE-MS with a quadrupole mass spectrometer. For the protein mixture, mass spectra of sufficient quality for precise molecular weight determination (< or = 0.05%) were obtained for 600 attomole injections using a 5 microns ID capillary. The increase in sensitivity with small capillary diameters 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 qualitatively accounts for the results is presented, but quantitative agreement is precluded due to difficulties in accounting for contributions due 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-diameter capillaries. A transition occurs, however, to a regime where the ESI/MS functions as a mass flow-sensitive detector for small-diameter capillaries, where the ESI current is limited by the rate of delivery to the ESI source of charge carrying species in solution. These results suggest peptide and protein analysis at low attomole and subattomole levels should be obtainable with alternative types of mass spectrometers.