Influence of the microwave power of MPT ion source on signal intensities obtained for theophylline (a) and metronidazole, (b) in positive mode. *b1 is the signal intensity versus microwave curves for ions of metronidazole at m/z 172, 128, 98; b2 is the enlarged figure for ions at m/z 99 and 128. Instrument: MPT-TOF mass spectrometer