Isoniazid (MW 137.14) is one of the widely used anti-tuberculosis drugs. Figure 5d shows the full-scan mass spectra of isoniazid obtained from MPTDI-TOF MS, protonated molecular ion at m/z 138, and main fragment ions at m/z 121, 93 can be found in the mass spectrum. This result is consistent with the MSn result obtained by LTQ MS (Supplementary Figure S-1d). The possible fragment pathway was that protonated isoniazid occurred α-cleavage. The ion at m/z 121 corresponds to a loss of ammonia, ion at m/z 93 corresponds to a continuing loss of a carbon monoxide molecule. It can be proven that MEISD has both soft and hard ionization characteristics based on the analysis of the data obtained by MPTDI-TOF MS and MPT-LTQ MS.