Linear quadrupole mass analyzers

A linear quadrupole mass analyzer consists of four hyperbolically or cyclindrically shaped rod electrodes extending in the z-direction and mounted in a square configuration (xy-plane, Figs. 4.31, 4.32). The pairs of opposite rods are each held at the same potential which is composed of a DC and an AC component. 

Fig. 4. 32. Schematic (a) and photograph (b) of a linear quadrupole mass analyzer. By courtesy of JEOL, Tokyo (a) and Waters Corp., MS Technologies, Manchester, UK. (b).

Figure 3.8 Stability diagram for a linear quadrupole mass analyzer, the first stability region showing a line scan. If the rf and dc voltages applied to the quadrupole are adjusted so that an ion mass m3 is inside the tip of the stability region, then heavier ions of mass m2 and m, and lighter ions m4 are outside the stability region and are filtered out.

Linear quadrupole mass analyzers consist of four parallel metal rods aligned along the instrument s z-axis (Fig. 6). 

The quadrupole ion trap is a three dimensional analogue of the linear quadrupole mass analyzer [71,72]. It consists of a cylindrical ring electrode and two end-cap electrodes. Both end-cap electrodes contain a whole for injecting and ejecting ions into and out of the ion trap (Fig. 8.11). A relatively high pressure of helium damping gas (about 0.1-0.4 Pa) is present in the ion trap in order to kinetically cool the trapped... 

An impressive diversity of mass analyzers are utilized in modem analytical instrumentation. An overview of the common mass spectrometer analyzers follows, with particular emphasis on linear quadrupole mass analyzers, quadrupole ion traps, and time-of-flight mass analyzers, as they arguably constitute the quantitative MS workhorses of the pharmaceutical industry. The description of alternate analyzer systems should provide a framework in which the utility of these three particular systems provides the most cost-effective analytical mass spectrometer systems for pharmaceutical analysis. 

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]. 

The methane PICI spectrum of TATP obtained with a linear quadrupole mass analyzer has been reported to contain ions of m/z 103,117,133, and 223, corresponding to the pseudomolecular ion [TATP -i- H]" [35]. A full-scan methane PICI spectrum of TATP has also been reported to contain ions of m/z 43(100%), 59,74,75,91, and 223(<5%), as shown in Figure 16.4B [30].The LOD for methane PICI analysis of TATP with linear qudaru-pole and ion trap mass analyzers were reported in the same work as 1 and 2 ng, respectively. Quantitation was based on extracted ions m/z 43, 59, 75, and 91 in both cases. When the m/z 91 ion was isolated in an ion trap mass analyzer and fragmented under CID, ions of m/z 43 and 74 were observed [30]. The third-order tandem mass spectrometry (MS ) experiment involving isolation of m/z 91 with subsequent CID and isolation of the m/z 74 product ion leads to the formation of a m/z 43 ion upon further CID. These results provide a scheme for analysis of TATP by selected reaction monitoring (SRM). 

The linear quadrupole mass analyzer is a mass filter. It consists of four hyperbolic rods that are placed parallel in a radial array. Opposite rods are charged by a positive or negative DC potential Uat which an oscillating radiofrequency voltage Vo cos cor is superimposed. The latter successively reinforces and overwhelms the DC field. Ions are introduced into the quadrupole field by means of a low accelerating potential, typically only a few volts. The ions start to oscillate in a plane perpendicular to the rod length as they traverse through the quadrupole filter. The trajectories of the ions of one particular m/z are stable. These ions are transmitted toward the detector. Ions with other m/z have unstable trajectories and do not pass the mass filter because the amplitude of their oscillations becomes infinite. Ions of different m/z can consecutively be transmitted by the linear quadrupole filter toward the detector when the DC and AC potentials are swept, while their ratios and oscillation frequencies are kept constant. 

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