The advent of new analyzer systems based on Fourier-transform ion cyclotron resonance (FT-ICR) and quadropole time-of-flight (Q-TOF) technologies allows protein digests to be analyzed under high-resolution conditions, resulting in mass spectometry (MS) and tandem MS (MS/MS) peptide mass spectra characterized by a resolution of 10,000 [full width half-maximum (FWHM) definition] or better. From mass spectra recorded under these conditions, the m/z values of peptides can be measured with mass errors between somewhat below 1 ppm and 50 ppm, depending mainly on the type of instrument used, the sample purity, the signal intensity, and the mode of calibration employed.