AbstractA linear quadrupole mass spectrometer operating in the third stability region [(a, q) ≈ (3, 3)] has been developed and utilized to obtain the mass spectra of multiply charged protein ions generated by electro‐spray ionization. The spectra were acquired by scanning the frequency of the quadrupole with a home‐built wide‐band power amplifier from 400 to 100 kHz at constant voltage amplitudes in the range of 100 V. A mass resolution approachingR1/2= 200 was achieved for doubly charged gramicidinSions atm/z570.5. Further improvement of the resolution is anticipated when a higher‐voltage power amplifier is