In a quadrupole rod assembly, because rods opposite each other in the array connect electrically, the four rods are considered two pairs of two rods each. The MS applies RF and DC voltages to the rods. In the following figure, although the RF voltages applied to the four rods are the same, the two pairs are 180 degrees out of phase (that is, one pair has a positive voltage and the other a negative voltage). The same is true for the applied DC voltages.
The quadrupole coil module and the rod driver module provide the voltages for operating the quadrupole. The RF voltage applied to the quadrupole rods is of constant frequency.
Because the frequency of this AC voltage is in the radio frequency range, it is referred to as RF voltage. The ratio of RF voltage to DC voltage and their amplitudes determine the ability of the MS to separate ions of different m/z values.
The first and third quadrupole rod assemblies (Q1 and Q3) can act as mass filters or as ion transmission devices. When the MS applies both RF and DC voltages, quadrupoles Q1 and Q3 act as mass filters; when the MS applies only RF voltage, Q1 and Q3 act as ion transmission devices. In the ion transmission mode, the quadrupoles allow ions to pass in a wide window of m/z values.
The quadrupole rod assembly (Q2) operates in the ion transmission mode only. Surrounding the Q2 quadrupole housing is a collision cell where CID takes place if the argon (or nitrogen) collision gas is present in the cell.