Metronidazole (MW 171.15) has a broad-spectrum antibacterial and antiprotozoal effect, and is mainly used for the prevention and treatment of infections caused by anaerobic bacteria. The mass spectra data obtained by MPTDI-TOF MS is shown in Figure 5c. It can be seen that there are two peaks at m/z 172 and 128. After the analysis of MSn data obtained by MPT-LTQ MS (Supplementary Figure S-1c), it can be confirmed that the peaks at m/z 128 and 82 are the fragment ions of protonated metronidazole, and m/z 172 is the protonated molecular ion. The fragment ion at m/z 128 is likely formed by losing one molecule of vinyl alcohol from protonated metronidazole (Supplementary Figure S-2c). However, ion at m/z 82 is not found in the full-scan mass spectrum of MPTDI-TOF MS (Figure 5c), possibly because the bond cleavage to lose NO2 requires more energy.