In order to increase the ionization efficiency of MPTDI source, the relationship between desorption time and abundance of major fragment ions was studied; the curves of desorption time versus abundance for three typical pharmaceuticals are shown in Figure 3. This method could obtain the response signals of protonated molecular ions and fragment ions in less than 5 s. The results showed that efficiency of desorption and ionization increased with increasing desorption time, and significant changes of the signal intensities were observed in the range of 20–30 s desorption time, as depicted in Figure 3. Desorption times longer than 30 s did not lead to substantial increases in observed signals. With prolonged desorption time, not too much signal increase was observed. So the desorption time of 30 s was chosen as the optimal condition.Figure 3Desorption time (t) resulting the signal intensity variation of major fragment ion of metronidazole (m/z 128), acyclovir (m/z 152), and azithromycin (m/z 592)