Linear time-of-flight mass spectrometers: Postsource pulse focusing conditions and mass scale

The condition to cancel aberrations that result from initial ion velocities and length of ionization process by postsource pulse focusing in linear time-of-flight mass spectrometers is derived. The ion arrival time distribution as a function of ion mass is given, which permits mass scale calibration. Simple formulas allow calculation of the voltage needed for postsource pulse focusing and the limits of the mass range where the focusing action is felt. The theory is illustrated for a specific instrument case.