In situ protein identification, mapping and depth profiling using 3D OrbiSIMS

Label-free protein characterisation at surfaces requires digestion or matrix application prior to mass spectrometry. We report the assignment of undigested proteins at surfaces using secondary ion mass spectrometry (SIMS). Ballistic fragmentation induced by a gas cluster ion beam (GCIB) produces peptide fragments for subsequent OrbitrapTM analysis. We annotate 16 example proteins (up to 272 kDa) by de novo sequencing and apply the methodology to characterising a protein monolayer biochip and in situ depth profiling of proteins through human skin.