Functional Description
MS Detector
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______________________ Finnigan LTQ Hardware Manual _______________________
The lens L1 is a metal disk with a circular hole in the center through which
the ion beam can pass. An electrical potential can be applied to the lens to
accelerate (or decelerate) ions as they approach the lens and to focus the ion
beam as it passes through the lens. The value ranges between 0 and ±300 V.
Lens L1 also acts as a vacuum baffle between the Q0 ion guide chamber and
the mass analyzer chamber. The Q0 ion guide chamber is evacuated to a
pressure of about 1 mTorr by the interstage inlet of the turbomolecular pump.
Q1 Ion Guide
The Q1 ion guide transmits ions from the Q0 ion guide to the mass analyzer.
The Q1 ion guide includes the Q1 octapole and the gate lens. See Figure 2-9.
The Q1 octapole is an octagonal array of round-profile rods that acts as an ion
transmission device similar to Q00 and Q0. An RF voltage that is applied to
the rods gives rise to an electric field that guides the ions along the axis of the
octapole. The Q1 offset voltage increases the translational kinetic energy of
ions emerging from Q0.
The gate lens is used to start and stop the injection of ions into the mass
analyzer.
Mass Analyzer
The mass analyzer is the site of mass analysis (that is, ion storage, ion
isolation, collision induced dissociation, and ion scan out). This section
describes the components of the mass analyzer, the voltages applied to the
mass analyzer electrodes, the presence of helium damping gas in the mass
analyzer cavity, and the operation of the mass analyzer during mass analysis.
Components of the Mass Analyzer
The mass analyzer consists of a front lens, linear ion trap, and back lens
(Figure 2-9). The front and back lenses are metal disks with a circular hole in
the center through which the ion beam can pass. An electrical potential of
-21 V is applied to the front lens, and an electrical potential of +22 V is
applied to the back lens. The purpose of the front and back lenses is to provide
conductance limits. To clear ions from the trap, the ions are ejected through
the back lens.
The basic design of the linear ion trap is shown in Figure 2-10. The linear ion
trap is a square array of precision-machined and precision-aligned hyperbolic
rods. Each rod is cut into three sections of 12, 37, and 12 mm length. Two of
the center section rods, called the exit rods, have a 0.25 x 30 mm slot through
which the ions are ejected during scan out. Quartz spacers act as electrical
insulators between adjacent rods. In each quadrupole rod section, rods
opposite each other in the array are connected electrically. Thus, the four rods
of each section can be considered to be two pairs of two rods each.
