Extracted ion chromatograms for m/z 258.1090 acquired for (a) m4C standard, (b) a 1:1 mixture of m4C and m5C, and (c) m5C standard. Similar chromatographic behavior is observed if LC methods 1 or 3 are used. CID MS/MS of (d) m4C standard, (e) 1:1 m4C/m5C mixture, and (f) m5C standard. HCD-based fingerprint of (g) m4C standard, (h) 1:1 m4C/m5C mixture, and (i) m5C standard.