Thermo Scientific DSQ II, Hardware Manual

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Thermo Scientific DSQ II Hardware Manual 183
6
Prefilter and Quadrupole
This chapter describes the theory of operations for the prefilter and quadrupole mass filter,
which are located inside the vacuum manifold.
The quadrupole consists of the following components:
• Prefilter
• Entrance Lens
• Quadrupole Mass Filter
• Exit Lens
After ions leave the ion source lenses, they pass through the prefilter. The prefilter is mounted
to the back side of the baffle wall and consists of four metal rods held in a square array. The
prefilter is curved to prevent neutral species (like helium) from reaching the ion detector. Ions
next pass through the entrance lens and into the quadrupole mass filter. The quadrupole mass
filter, like the prefilter, is composed of four metal rods held in a square array. However, these
rods are precision-aligned so the quadrupole should be handled with care. Finally, ions pass
through the exit lens and to the ion detector.
The four quadrupole mass filter rods are charged with a combination of radio frequency (RF)
and direct current (DC) voltages. The magnitudes of these voltages give stable oscillations to
ions with a specific
m/z ratio and unstable oscillations to all others. The unstable ions strike
one of the rods, become neutralized, and are pumped away. The quadrupole mass filter
behaves as a filter allowing only ions with a certain
m/z ratio to reach the ion detector. As the
RF and DC voltages are ramped, ions of different m/z ratios are transmitted through the exit
lens to the ion detector. This produces the mass spectrum.