Functional Description
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___________________________________________________________________ MS Detector
_______________________ Finnigan LTQ Hardware Manual _____________________
2-15
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Ion Optics
The ion optics focus the ions produced in the API source and transmit them to
the mass analyzer. The ion optics consist of three ion guides:
•
Q00 ion guide
•
Q0 ion guide
•
Q1 ion guide
Q00 Ion Guide
The Q00 ion guide is the ion guide that is located closest to the API source.
The Q00 ion guide includes the Q00 quadrupole and lens L0. See Figure 2-8
and Figure 2-9.
The Q00 quadrupole is a square array of square-profile rods that acts as an
ion transmission device. An RF voltage that is applied to the rods gives rise to
an electric field that guides the ions along the axis of the quadrupole. A dc
voltage offset from ground applied to Q00—called the Q00 offset
voltage—increases the translational kinetic energy of ions emerging from the
skimmer. During ion transmission, the offset voltage is negative for positive
ions and positive for negative ions. Increasing the offset voltage will increase
the translational kinetic energy of the ions. Typical values of the Q00 offset
voltage are –4 V to +4 V.
The lens L0 is a metal cylinder with a small hole in one end through which the
ion beam can pass. A potential of between 0 and
±
5 V (negative for positive
ions and positive for negative ions) is applied to lens L0 to aid in ion
transmission. Lens L0 also acts as a vacuum baffle between the Q00 and Q0
ion gauge chambers. The Q00 ion guide chamber of the vacuum manifold is
evacuated to a pressure of 50 mTorr by the third inlet in the molecular drag
section of the turbomolecular pump.
Q0 Ion Guide
The Q0 ion guide transmits ions from the Q00 ion guide to the Q1 ion guide.
The Q0 ion guide includes the Q0 quadrupole and lens L1. See Figure 2-9.
The Q0 quadrupole is a square array of square-profile rods that acts as an ion
transmission device similar to Q00. An RF voltage that is applied to the rods
gives rise to an electric field that guides the ions along the axis of the
quadrupole. The Q0 offset voltage increases the translational kinetic energy of
ions emerging from Q00. Q0 must be at a lower potential than Q00. Because
the gas pressure in Q00 is so high, ions typically have zero kinetic energy
when they emerge from Q00. There must be a downhill potential gradient for
them to enter Q0.
