Schematic view of the established technique for modification and enrichment of citrullinated peptides (top). Enrichment of the synthetic peptide RPSQ-Cit-HGSK (0.5 pmol) from a complex sample (8.2 nmol) (bottom). After enrichment an amount corresponding to 50 fmol of the spiked-in peptide was analysed and is observed as a prominent signal (m/z 1569.85)