Linear quadrupole analyzer principle

Figure 2.17. Schematic of a linear quadrupole ion trap (QIT). This type of analyzer consists in principle of a quadrupole analyzer with electrodes at the ends to block ion passage in the z-direction.

Quadrupole ion trap mass analyzers merge the trapping characteristics of the ICR with the physical principles of the linear quadrupole mass analyzer. Quadrupole ion traps produce time-dependent spectra with excellent sensitivity and tandem mass spectrometry capabilities, but unlike the ICR they provide these ion trapping characteristics with physically smaller and considerably less expensive instrumentation, giving them a reputation as a powerful and accessible tool for both qualitative and quantitative mass spectrometry [43-47]. The capability of quadrupole ion traps to be configured with either internal and external ionization sources has expanded their utility for modem analytical applications [48—52]. 

Various types of electric field are used in different mass analyzers. From this perspective the simplest example is provided by the constant field (DC) strengths used to accelerate ions, e.g. out of an ion source into an analyzer, or from an intermediate component into the flight tube of a time of flight analyzer etc. More elaborate fields whose strength is varied in time at frequencies in the range of a few MHz (within the radiofrequency (RF) range) are used in linear quadrupoles and ion traps. However, the same principles of physics are exploited in aU cases, and the following section is an introduction to these principles. 

A three-dimensional ion trap (e.g., Q ion trap) bears the same physical principles as the quadrupole mass analyzer, but the ions are trapped and sequentially ejected mainly by using an RF field, within a space defined by a ring electrode between two end-cap electrodes (Figure 2.6b). A linear quadrupole ion trap is similar to a three-dimensional ion trap, but it traps ions in a two-dimensional, instead of a three-dimensional, quadrupole field. Some of the figures of merit of the ion-trap instrument are as follows ... 

The quadrupole ion trap mass analyzer operates by similar principles as the linear quadrupole described above and is a common mass analyzer found in GC-MS instruments. The ion trap consists of two hyperbolic endcap electrodes and a doughnut-shaped ring electrode (the endcap... 

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