Functional Description
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_______________________ Finnigan LTQ Hardware Manual _____________________
2-17
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DC Axial Trapping Voltages
The LTQ uses three dc axial trapping voltages, one for each rod section,
which establish axial (Z-axis) trapping by lowering the potential of the center
section below the potential of the front and back sections. For positively
charged ions, LTQ applies a potential of -9 V to the front rod section during
ion storage and a potential of +20 V during mass analysis. LTQ applies a
potential of -12 V to the rear rod section during ion storage and a potential of
+20 V during mass analysis. LTQ applies a potential of -14 V to the center rod
section during both ion storage and mass analysis. For negatively charged
ions the polarity of the dc axial trapping voltages are reversed.
Main RF Voltage
In each quadrupole rod section, rods opposite each other in the array are
connected electrically. Ac voltages are applied to the rods and these voltages
are ramped during the scan. Voltages of the same amplitude and sign are
applied to the rods of each pair. However, the voltages applied to the different
rod pairs are equal in amplitude but opposite in sign.
The ac voltage applied to the quadrupole rods is of constant frequency
(1.2 MHz) and of variable amplitude (0 to 10,000 V peak-to-peak). Because
the frequency of this ac voltage is in the radio frequency (RF) range, it is
referred to as the main RF voltage. The application of the main RF voltage to
Figure 2-10. Linear ion trap quadrupome rod assembly
