Many software utilize retention time calibration to improve database searching and identification, and these retention time standards can also be used for QC . Retention time calibration standards are typically added to each individual sample in liquid chromatographytandem mass spectrometry (LC-MS/MS) in order to align chromatograms or normalize retention times . These standards can also function as QC1 mixtures and be used for some QC tasks. Mass spectrometry calibration standards are less suited however, as they are typically injected directly into the mass spectrometer. Conversely, both retention times and mass spectra 9 can be aligned (retention time) or calibrated (m/z) without standards, using identified peptides that reveal drifts in either retention time or mass measurement. Typically, LC-MS/MS datasets are aligned in order to assist peak integration across runs and transfer identifications between the runs , a practice also known as "match between runs" . Normalizing or calibrating retention times takes this one step further, and express retention times as a normalized elution time relative to the elution profiles of tryptic peptides, or estimate the fraction of organic solvent (e.g., acetonitrile) at elution, as recently demonstrated by Bouwmeester et al. . Knowledge of downstream analysis capabilities and whether a specific QC1 is required is essential to taking advantage of these techniques in the core.