As an alternative to peptide mixtures, but still relatively simple, purified undigested proteins can also be used for QC (Table ). When undigested, these fall under the QC2 category, and unlike QC1 level peptides, they can be used to provide information on efficiency of reduction, alkylation and digestion. A good example is the QconCAT 29 method, where a protein is made from a genetic construct, concatenating peptides into a protein that is added to samples to control for variability in reduction, alkylation and digestion. The resulting peptides can be chosen from proteins targeted for quantitation by MRM and the entire protein inexpensively isotopically labeled for the resulting peptides to be used as internal standards. If the concatenated peptides span the chromatographic separation, they can also be used for retention time calibration and LC monitoring . Open-source software can be used to design these concatenated constructs .