There is a strong interest in the development of analytical techniques with enhanced sensitivity for biochemical applications, because in many cases sample size is limited or obtained at great effort and expense. This enhanced sensitivity is beneficial for all applications, but for large biopolymers (proteins, DNA segments, and larger carbohydrates), the difficulties and analytical demands increase significantly. Some of the most exciting and demanding applications require methods capable of analyses that are on the single-cell level,' where the maximum amounts of components would typically be in the attomole range, and that extend down to zeptomoles (10-21 mol) and below. In general, analytical methods being developed for such applications are more useful when they combine a separation method with a sensitive, selective, and broadly applicable detection method.In this regime of ultrasensitive analysis and, in particular, for biopolymer characterization, mass spectrometry (MS) has not generally been considered sufficiently sensitive when compared with techniques such as laser-induced fluoregcence2-' and electrochemical detection.& Indeed, Dovichi? Zare,3 Gman,' and their respective co-workers, among others, have successfully demonstrated capillary electrophoresis (CE) of fluorescently labeled compounds at the low-zeptomole level. These detection methods have provided the most impressive sensitivities yet demonstrated using CE, but application is restricted and information for compound identification is generally limited due to a reliance on electrophoretic mobilities. The attraction of MS detection is that accurate molecular weight information and component identification can be performed, even for proteins,B and techniques for obtaining structural information based upon tandem methods are being currently extended to larger molecules.6-g With the recent dramatic developments in ionization techniques, the primary limitation for the use of mass spectrometry is due to sensitivity constraints. For electrospray ionization (ESI) sensitivity generally decreases as molecular weight increase^,^ an observation largely attributable to the increased extent of charging and broader charge state distribution for large molecules. Previous work by Thibault and co-workers,lo Moseley et al.," and this labo-(l)Kennedy, R. T.; Oates, M. D.; Cooper, B. R.; Nicherson, B.; Jorgenson, (7) Smith, R. D.; Fields, S. M.; Loo, J. A.; Barinaga, C. J.; Udseth, H. (8) Smith, R. D.; Loo, J. A.; Barinaga, C. J.; Edmonds, C. G.; Udseth, (9) Smith, R. D.; Loo, J. A.; Edmonds, C. G.; Barinaga, C. J.; Udseth, (10) Thibault, P.; Paris, C.; Pleasance, S. Rapid Commun. Mass (11) Moseley, M. A.; Jorgenson, J. W.; Shabanowitz, J.; Hunt, D. F.; 0003-2700/92/0384-3194$03.00/0ratory12 have generally used sample sizes on the order of 100 fmol and larger to obtain mass spectra of proteins with sufficient quality for precise molecular weight measurements.In this correspondence we report the use of chemically modified 5-rm4.d. capillaries for C S M S of proteins....