2
Functional Description
MS Detector
26
Surveyor MSQ Plus Hardware Manual
Thermo Scientific
RF/dc Prefilter
The RF/dc prefilter focuses the ions produced in the API source and transmits them to the
mass analyzer. The RF/dc prefilter is a square array of square-profile rods that acts as an ion
transmission device and as a wide band-pass mass filter. See
During ion transmission, the offset voltage is positive for the positive ion polarity mode and
negative for the negative ion polarity mode. Increasing the offset voltage increases the kinetic
energy of the ions along the axis of the quadrupole through the differential aperture.
Allowable values for the RF lens bias are –10 V to +10 V. In the default Tune file
(
default.tune
), the RF lens bias is set to 1.0 V. The default RF lens bias for the Autotune
procedure is 0.5 V.
Figure 17.
RF/dc prefilter, square array of square-profile rods
Mass Analyzer
The mass analyzer separates ions according to their mass-to-charge ratio and then passes them
to the ion detection system. In the Surveyor MSQ Plus MS detector, the mass analyzer is a
single quadrupole rod assembly.
The mass analyzer is described in detail in the following topics:
•
RF and DC Fields Applied to the Quadrupoles
•
RF and DC Fields Applied to the Quadrupoles
In a quadrupole rod assembly, rods diagonally opposite each other in the array are connected
electrically. Thus, the four rods can be considered to be two pairs of two rods each. Ac and
dc voltages are applied to the rods and these voltages are ramped during the scan. Voltages of
the same magnitude and sign are applied within the rods of each pair. Voltages equal in
magnitude but opposite in sign (dc) and phase (RF) are applied to the different rod pairs. See
The ac 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 determines the resolving power of the quadrupole and the ability of
the MS detector to distinguish between ions of different mass-to-charge ratios.
