Table 1.

Mass transitions of 7‐methylguanosine isotopomers in methylation discrimination NAIL‐MS.

Nucleoside	Labeling[a]	Precursor[b]	Product[c]	t R [min]
guanosine	unlabeled	284	152	3.7
	SIL‐IS[d]	299	162	3.7
7‐methylguanosine	unlabeled[e]	298	166	2.0
	CD3‐label[f]	301	169	2.0
	SIL‐IS	314	177	2.0

[a] Used labeling technique. [b] Precursor ion m/z. [c] Product ion m/z. [d] SIL‐IS: stable isotope labeled internal standard prepared from a complete ¹³C/¹⁵N‐labeled E. coli culture. [e] From MMS damage. [f] Labeling with CD₃‐methionine to detect enzymatic methylation.