Fig. 1

LC–MS/MS chromatogram of reference compounds using MRM (multiple reaction monitor) mode. A 10 ng/mL 5mdC and 20 ng/mL ¹⁵N₅-dG were dissolved in 5% MeOH + 0.1% FA. B sample spike with 20 ng/mL ¹⁵N₅-dG was dissolved in 5% MeOH + 0.1% FA