Ion Detection System; Conversion Dynode - Thermo Scientific TSQ Series Hardware Manual

• To retain the collision gas. Lenses L23 and L3 form two of the walls of the collision cell, so
they tend to hold the collision gas in the collision cell. The collision gas escapes, however,
through the same lens holes through which the ion beam passes.
• To prevent gas from entering the mass analyzers. Lenses L22 and L21 on one side of Q2 and
lenses L32 and L33 on the other side of Q2 act as baffles to help prevent the gas that escapes
from the collision cell from entering the mass analyzers.
• To shield Q1 from the rf voltage applied to Q2 and vice versa (L2x lens set) and to shield Q3
from the rf voltage applied to Q2 and vice versa (L3x lens set).
• To focus the ion beam. The three lenses between Q1 and Q2 (and those between Q2 and Q3)
together form a three-element aperture lens. The first and third lenses are generally set to
similar or identical values and the central lens is set to a value different (either higher or lower)
from the other two.
The voltage applied to each of the lenses can vary from about -300 to +300 V. Typically, however,
the voltage applied to the first and third elements of the L2x lens set is somewhat greater than the
quadrupole offset voltage applied to Q1. Because the Q1 quadrupole offset voltage is generally set
to about ±5 V (depending on the charge of the ions of interest), the voltage applied to lenses L21
and L23 is typically about -10 V for positive ions and +10 V for negative ions. The voltage applied
to the central lens of the L2x lens set is typically about ±225 V.
In the Q3MS scan mode, the voltage applied to the lenses of the L3x lens set is about the same as
that applied to the corresponding lens in the L2x lens set. Note, however, that in the MS/MS scan
modes, the voltage applied to the L3x lens set automatically varies with the quadrupole offset
voltage applied to Q3. As the Q3 quadrupole offset voltage ramps, the voltages applied to the
lenses ramp correspondingly.
Lens L4 is located between Q3 and the ion detection system. L4 is held at ground potential. Its
purpose is to shield Q3 from the high voltage applied to the ion detection system and to shield the
ion detection system from the high rf voltages applied to Q3.

Ion Detection System

The TSQ mass spectrometer is equipped with a high-sensitivity, off-axis ion detection system. This
system produces a high signal-to-noise ratio and allows for voltage polarity switching between
positive ion and negative ion modes of operation. The ion detection system includes a 15 kV
conversion dynode and a channel electron multiplier. See
located at the rear of the vacuum manifold behind the mass analyzer.
The conversion dynode is a concave metal surface located at a right angle to the ion beam. The
TSQ applies to the conversion dynode a potential of +15 kV for negative ion detection or -15 kV
for positive ion detection. When an ion strikes the surface of the conversion dynode, one or more
secondary particles are produced. These secondary particles can include positive ions, negative
ions, electrons, and neutrals. When positive ions strike a negatively charged conversion dynode,
the secondary particles of interest are negative ions and electrons. When negative ions strike a
positively charged conversion dynode, the secondary particles of interest are positive ions. The
curved surface of the conversion dynode focuses these secondary particles and a voltage gradient
accelerates the particles into the electron multiplier.
Thermo Scientific
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Figure 29. The ion detection system is
TSQ Series Hardware Manual
Functional Description
Mass Spectrometer
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