Complex biological samples consist of multiple proteins. Understanding the presence and relative abundance of different proteins in a sample can provide important information about the biological processes underlying human health and disease. Some protein detection techniques identify proteins using antibodies or other affinity-based reagents with specificity for the proteins of interest. These methods require the generation of protein-specific antibodies or the use of commercially available or custom targeted antibody panels. The need to source or develop high-quality antibodies with the correct specificity for target proteins and panels limits the proteins that can be identified and typically places key information about proteins with variant peptide sequences out of reach.