Functional Description
'JOOJHBO-52
___________________________________________________________________ MS Detector
_______________________ Finnigan LTQ Hardware Manual _____________________
2-19
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However, ion motion in the radial direction is enhanced and the ion gains
kinetic energy. After many collisions with the helium damping gas, which is
present in the mass analyzer, the ion gains enough internal energy to cause it
to dissociate into product ions. The product ions are then mass analyzed.
During ion scan out, the resonance ejection RF voltage facilitates the ejection
of ions from the mass analyzer and thus improves mass resolution. The
resonance ejection RF voltage is applied at a fixed frequency and increasing
amplitude during the ramp of the main RF voltage. Only when an ion is about
to be ejected from the mass analyzer cavity by the main RF voltage is it in
resonance with the resonance ejection RF voltage. When an ion approaches
resonance, it moves farther away from the center of the mass analyzer, where
the field generated by the main RF voltage is zero (and space-charge effects
are strong), into a region where the field produced by the main RF voltage is
strong (and space-charge effects are small). As a result, the ejection of the ion
is facilitated, and mass resolution is significantly improved.
Helium Damping Gas in the Mass Analyzer Cavity
The mass analyzer cavity contains helium that is used as a damping gas and a
collision activation partner. The helium damping gas enters the mass analyzer
cavity through a gap between the quadrupole rods. The flow of gas
(~1 mL/min) into the mass analyzer cavity is regulated by a pressure regulator
and a capillary restrictor. The flow of gas out of the mass analyzer cavity (and
into the turbomolecular pump) is restricted by the openings in the mass
analyzer. The flows into and out of the cavity are matched so that the partial
pressure of helium in the mass analyzer cavity is maintained at approximately
0.1 Pa (10
-3
Torr).
The collisions of the ions entering the mass analyzer with the helium slow the
ions so that they can be trapped by the RF field in the mass analyzer.
The presence of helium in the mass analyzer cavity significantly enhances
sensitivity and mass spectral resolution. Before their ejection from the mass
analyzer cavity, sample ions collide with helium atoms. These collisions
reduce the kinetic energy of the ions, thereby damping the amplitude of their
oscillations. As a result, the ions are focused to the axis of the cavity rather
than being allowed to spread throughout the cavity.
Helium in the mass analyzer cavity also serves as a collision activation
partner. During the collision induced dissociation step of an SRM, CRM, or
MS
n
(n > 1) full scan analysis, the resonance excitation RF voltage applied to
the endcap electrodes drives parent ions into the helium atoms. After gaining
sufficient internal energy from the resulting collisions, the parent ion
dissociates into one or more product ions.
