Fig. 2

Mass spectra of l-Leu irradiated under vacuum condition in positive ion mode at low mass range (m/z 15–200) (a) and normal mass range (m/z 50–800) (b). The irradiated l-Leu was dissolved into a solution 1:1 of MeOH-AcNH₄ 45 mM to obtain a concentration of 7.8 mM. The mass spectra for l-Leu standard are reported in the Additional file 1