Cox J, Mann M. J Am Soc Mass Spectrom. 2009 Aug;20(8):1477-85. Precision proteomics requires high-resolution and high mass accuracy peptide measurements. The Orbitrap instrument achieves excellent resolution on a chromatographic time scale and its design is favorable for very high mass accuracy. Here we describe how mass precision for each peptide increases successively by considering all associated measurements, starting from the MS peak and proceeding to its chromatographic elution profile, isotope envelope, and stable isotope pair in SILAC measurements. We extract peptide charge pairs to perform nonlinear recalibration of the Orbitrap mass scale through spline interpolation. The deviation of mass values determined from charge pairs is used to convert mass precision to mass accuracy for subsequent database search. The corrected mass precision is consistent with the mass accuracy independently determined by database identification. Individual mass deviations range from below 100 ppb for peptides with many associated mass measurements and good signal intensities to low ppm for peptides with few mass measurements and signals close to the noise level. This extremely high and individualized mass accuracy is equivalent to a substantial increase in database identification score.