These definitions reflect the types of material and their use-cases, determining what QC information they can provide. A QC1 sample can be run on a short gradient independently of an experiment, can be designed to be shelf stable, and is relatively inexpensive. A QC2 sample would typically be analyzed with the same gradient length as the experimental samples, and therefore be slightly more expensive in terms of instrument time but is excellent at checking experimental and instrument performance. Likewise, a QC3 sample can provide the same information as QC2 samples with the addition of quantitative detection limits of the method and/or retention time calibration, and in most labs will be used nearly as frequently as QC2. In contrast, QC4 requires multiple injections and therefore more instrument time, and may not be run as regularly as QC1, QC2, or QC3 in most cores, despite potentially providing valuable insight into method accuracy and precision. The frequency of using a type of QC will vary between cores depending on the types and scale of experiments that are analyzed. For instance, a core providing quantitative proteomics measurements requires higher levels of QC than a core that only provides protein identifications. While a core performing metaproteomics analyses has more need for mixed proteome materials than a core that only analyzes human samples.