Example of HM2 high-mass MALDI-TOF spectra obtained by Participant 31 for mixtures of (A) protein-A plus protein-B, (B) protein-A plus protein-C, (C) protein-A plus protein-C incubated for 1 h, followed by addition of protein-B, and (D) protein-A plus protein-B incubated for 1 h, followed by addition of protein-C. In each panel, the blue spectra were obtained in the absence of chemical cross-linking, whereas the red spectra were obtained following chemical cross-linking.