This article describes developments in fundamental and applied aspects of separations, electrospray ionization phenomena, and mass spectrometric instrumentation that are interrelated and important for making more effective and quantitative measurements, particularly for proteomics applications. The basis for better quantitation and ultrahigh sensitivity is highlighted for high-resolution capillary liquid chromatography separations that provide low nanoliter per minute flow rates to an electrospray ionization interface. The increased dynamic range of measurements and low zeptomole regime detection limits obtainable open new avenues for biological research.