Computational evaluation of a new digital tandem quadrupole mass filter.

A tandem mass filter consists of two low-resolution mass filters arranged in series that operate with a small offset between their mass windows. In principle, the overlap of the two individual mass windows defines the tandem window. Tandem operation provides improved resolution and transmission compared to a single mass filter operated with the same mass window. The improvement in transmission is owed to the larger acceptance of the low-resolution quadrupoles. The tandem filter resolution and transmission are adjusted by changing the amount of offset separating the mass windows of the individual filters. Sine wave systems create this offset through voltage changes. Digital tandem mass filters depart from convention because they do not change voltage. The tandem mass window is created when the individual filters are operated with two slightly different duty cycles. Both quadrupoles operate at the same frequency, phase, and voltage. When the frequency, phase, and voltage of each quadrupole are identical, there theoretically are no changes to the Mathieu parameters to cause ion excitation and loss during transition between the quadrupole pair. The work presented here shows that a fixed AC voltage digital tandem mass filter can only operate in higher stability zones. However, unlike sine mass filters, the mass range of a digital system is not limited. This makes the digital tandem mass filter feasible as a commercial product. For the tandem digital mode to be successful, the duty cycles of each quadrupole must be precisely controlled because the duty cycle differences required to shift the mass windows are small. The creation of these mass window offsets requires waveform generation that can obtain high duty cycle resolution. Our method of generating waveforms can meet this demand; however, modifications to our current printed circuit board must be made. These modifications are minor and will be discussed.