Methods for detecting biopolymers such as proteins, peptides, and nucleic acids in a matrix are provided, in which the methods include contacting the matrix with a sensitizing reagent that includes a substituted heteroaromatic compound; contacting the matrix with a reduceable metal salt to stain the biopolymer(s); and detecting the stained biopolymer(s). Compositions for performing the methods can include sensitizing agents, staining reagents, reducing agents, enhancing agents, and destaining agents. The compositions presented are compatible with mass spectrometry, and methods are provided for staining a biopolymer using the compositions of the invention and performing mass spectrometry on one or more biopolymers identified by the staining methods. Kits for staining biopolymers in matrixes can include a sensitizing reagent and a reduceable metal salt, and optionally, one or more enhancing agents, reducing agents, destaining agents, fixing reagents, chelating agents, biopolymers, matrices, cleavage reagents, buffers, or pH indicators.