Similar with ribavirin, acyclovir (MW 225.21) is a kind of anti-viral drug, which is mainly used for herpes virus infection onset and recurrence. Both the protonated molecular ion m/z 226 and fragment ion m/z 152 of acyclovir can be observed in the full-scan mass spectrum obtained by the present simple TOF mass spectrometer, as shown in Figure 5f. Meanwhile, information of the fragment ion can be proven by the data from literatures [35, 36] and MSn spectrum by MPT-LTQ MS (Supplementary Figure S-1f). The fragment pathway (Supplementary Figure S-2f) can be speculated that protonated acyclovir loses a methanal to form the ion at m/z 196. The ion at m/z 196 goes on to lose of oxirane (ethylene oxide) to produce the ion at m/z 152. According to the above results, the main fragment ion of acyclovir can be obtained without tandem mass spectrometry, and accurate qualitative analysis of acyclovir can be completed using only the data of full-scan mass spectrum by MPT ionization source coupled with a simple mass spectrometer.