How QC materials are used in a mass spectrometry proteomics core largely depends on the needs and experimental designs. In a very basic sense, system suitability testing (SST) should be universal. All mass spectrometers need to be periodically calibrated as mass measurements are affected by changes in the environment, notably temperature, in electronic components, in detectors and in ion optics, especially as contamination builds up on internal components. System suitability testing is performed prior to the analysis of samples to confirm that the instrument is performing within instrument and experiment-specific operational margins, a concept formally known as statistical process control 2 . This concept relies on longitudinal QC measurements to establish the acceptable (inherent) variation of a system in order to detect unexpected deviations from this baseline that must be addressed before continuing. Normally, SSTs should encompass as much of the data acquisition path as possible. Since most mass spectrometry-based proteomics methods utilize liquid chromatography (LC), the SST should include the LC and not simply rely on direct infusion of a QC material onto the instrument.