Two or more well-defined materials for QC1 or QC2 can be combined to make bespoke but still well-defined materials for QC3 or QC4. Protein or proteome digests can be combined in known ratios for quantitative quality control . Spiking peptides into a lysate or digest provides additional utility, e.g., for simple chromatography and mass measurement quality control. The drawback of custom mixed standards versus commercially available pre-mixed standards, is that since they are only in their respective components, users must mix their own, meaning that data from these mixes varies across labs. However, some vendors now sell pre-mixed QC1/QC2 materials, such as HeLa digests with PRTC Standard. Lastly, though we have described QC4 materials above (i.e., a suite of QC2 materials), there are currently no commercially available suites of materials for proteomics that fit this definition (aside from the sPRG suite currently in development). Arguably the Pierce TMT11plex Yeast Digest provides similar utility as a true QC4, however as it is pre-labeled it cannot be used to assess quantification across the complete experimental workflow. That said, there are many suites of biological materials available from NIST that could fit this need, albeit they are not explicitly listed as fit for purpose for proteomics.