Table 1. Selected transformation products of N⁴-hydroxycytosine, cytosine, uracil, pyrimidin-2-one, molnupiravir, and EIDD-1931 with [M + H]⁺, retention time (R_t in min, in ascending order), and representative MS/MS fragments, reported in descending order of fragment ion intensity. Color coding indicates TPs that are identical to authentic standards: N⁴-hydroxycytosine (grey), cytosine (yellow), uracil (red), and pyrimidine-2-one (green). TP [M + H]⁺ = 111.01 (blue) was formed during ozonation of N⁴-hydroxycytosine, pyrimidine-2-one, and uracil. For structure proposals see SI Tables A.2–A.7.