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. 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 has a negative voltage). The same is true for the applied DC voltages. The following figure depicts the two pairs and their voltages.
The quadrupole coil module and the rod driver module provide the voltages needed to operate the quadrupole. The RF voltage applied to the quadrupole rods is of a 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 quadrupole mass analyzer can act as mass filter or as an ion transmission device. When the MS applies both RF and DC voltages, the quadrupole mass analyzer acts as a mass filter. When it applies only the RF voltage, the quadrupole mass analyzer acts as an ion transmission device. In the ion transmission device mode, the quadrupole mass analyzer allows ions to pass in a wide window of m/z values.
NOTE
The Ion Concentrating Routing Multipole (ICRM) assembly operates in trapping mode only. The ICRM is sealed and operated at a nominal pressure of 8 mTorr of nitrogen gas, such that higher-energy collision dissociation (HCD) takes place if the ions are accelerated to high enough energies when entering the collision cell.