Schematic procedure for the SRM method development. (a) Full-scan mode analysis of nucleobase standards. (b) Comparison of the sample mass spectrum at 14.32 minutes with the NIST standard database spectrum, leading to the selection of m/z 193 and 307 as parent ions for hypoxanthine. (c) Daughter ion spectra derived from the parent ion m/z 193 at collision energies of 10, 20, and 30 eV (signals normalized to the most intense peak, m/z 84 at 30 eV). Ions at m/z 84 and 166 were selected for further analysis. (d) Collision energy optimization for the two selected ion pairs.