Electrospray ionization with an ultralow flow rate (less than or equal to 4 nanoliters per minute) was used to directly couple capillary electrophoresis with tandem mass spectrometry for the analysis and identification of biomolecules in mixtures. A Fourier transform mass spectrometer provided full spectra (>30 kilodaltons) at a resolving power of approximate to 60,000 for injections of 0.7 x 10(-18) to 3 x 10(-18) mole of 8- to 29-kilodalton proteins with errors of <1 dalton in molecular mass. Using a crude isolate from human blood, a value of 28,780.6 daltons (calculated, 28,780.4 daltons) was measured for carbonic anhydrase, representing 1 percent by weight of the protein in a single red blood cell. Dissociation of molecular ions from 9 x 10(-18) mole of carbonic anhydrase gave nine sequence-specific fragment ions, more data than required for unique retrieval of this enzyme from the protein database.