Systems and methods for biomolecule quantitation

An array composition, comprising:

• (a) a solid support comprising a plurality of sites, wherein each site is optically resolvable at single-analyte resolution from any other site of the plurality of sites, wherein a plurality of polypeptides is attached to sites of the plurality of sites, wherein each of the sites of the plurality of sites is attached to a single polypeptide of the plurality of polypeptides;
• and
• (b) affinity reagents of a plurality of affinity reagents bound to polypeptides of the plurality of polypeptides, wherein each affinity reagent of the plurality of affinity reagents recognizes a characteristic of a proteoform of a species of polypeptide,
• wherein polypeptides of the plurality of polypeptides are of the species of polypeptide, and wherein the plurality of polypeptides comprises two or more proteoforms of the species of polypeptide.

A system for detecting the presence of a polypeptide proteoform, comprising:

• (a) a solid support comprising a plurality of sites, wherein each site is optically resolvable from any other site of the plurality of sites, wherein a plurality of polypeptides is attached to sites of the plurality of sites, wherein each of the sites of the plurality of sites is attached to a single polypeptide of the plurality of polypeptides;
• (b) a plurality of detectable affinity reagents, wherein each detectable affinity reagent of the plurality of detectable affinity reagents recognizes a proteoform of a species of polypeptide;
• and
• (c) a detector that is configured to detect presence or absence of an optical signal from a detectable affinity reagent at each site of the plurality of sites,
• wherein polypeptides of the plurality of polypeptides are of the species of polypeptide, and wherein the plurality of polypeptides comprises two or more proteoforms of the species of polypeptide.

A method for detecting a polypeptide proteoform, comprising:

• (a) providing a solid support comprising a plurality of sites, wherein each site is optically resolvable from any other site of the plurality of sites, wherein a plurality of polypeptides is attached to sites of the plurality of sites, wherein each of the sites of the plurality of sites is attached to a single polypeptide of the plurality of polypeptides;
• (b) contacting to the solid support a plurality of detectable affinity reagents, wherein each detectable affinity reagent of the plurality of detectable affinity reagents recognizes a proteoform of a species of polypeptide;
• and
• (c) detecting at single-analyte resolution for each site of the plurality of sites a presence or absence of an optical signal from a detectable affinity reagent at each site of the plurality of sites, thereby detecting a presence or absence of the proteoform of the species of polypeptide at each site of the plurality of sites.