Linear quadrupole analyzer

With the advent of linear quadrupole analyzers the full width at half maximum (FWHM) definition of resolution became widespread especially among instruments manufacturers. It is also commonly used for time-of-flight and quadrupole ion trap mass analyzers. With Gaussian peak shapes, the ratio of / fwhm to Rio% is 1.8. The practical consequences of resolution for a pair of peaks at different m/z are illustrated below (Fig. 3.17). 

Fig. 4.33. Stability diagram for a linear quadrupole analyzer showing four stability regions (I-IV) for X- and y-motion. Reproduced from Ref. [109] with permission. John Wiley Sons Inc., 1986.

Fig. 4.34. Detail of the upper half of region I of the stability diagram for a linear quadrupole analyzer. Reproduced from Ref. [104] with permission. World Scientific Publishing, 1993.

Tandem methods for chemical measurements are now widely accepted and employed even when tandem is defined in the narrowest sense, such as two-dimensional gas chromatography or tandem mass spectrometry. Tandem or triple quadrupole MSs became commercially available in the early 1980s, and today these instruments are routine concepts as linear quadrupole analyzers or as ion traps. Tandem or sequential methods result in loss of signal current, yet noise is lost more so thus, S/N is increased. 

In GC-MS systems, sample introduction is performed from the open capillary chromatographic column, either directly or via an open split coupling (see section 4.1). The ionization of the analytes is generally performed by either electron ionization or chemical ionization (see section 3.2). After the production of ions, these are separated according to their m/z ratio in the mass analyzer. Although linear quadrupole analyzers are most widely applied, other analyzer types, i.e, (magnetic) sector, quadrupole ion trap, and time-of-flight, are applied as well (see section 3.3). The detection of ions is mostly performed by means of an electron multiplier. 

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.

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).

The operation of magnetic sector (Chap. 4.3), linear quadrupole (Chap. 4.4), or quadrupole ion trap (Chap. 4.5) mass spectrometers in the repetitive scanning mode is useful for the identification of the components of a mixture. If quantitation is a major issue (below), selected ion monitoring (SIM) is preferably employed the term multiple ion detection (MID) and some others are also in use. [33] In the SIM mode, the mass analyzer is operated in a way that it alternately acquires only the ionic masses of interest, i.e. it jumps from one m/z value to the next. [34-39] The information obtained from a SIM trace is equivalent to that from a RIC, but no mass spectra are recorded. Thus, the scan time spent on a diagnostically useless m/z range is almost reduced to zero, whereas the detector time for the ions of interest is increased by a factor of 10-100. [40] An analogous improvement in sensitivity (Chap. 5.2.3) is also observed. 

Dynamic mass separation systems use the fact that ions with different masses (accelerated with the same voltage) possess several velocities and consequently their flight times are different. There are about 50 dynamic separation systems known2 using several types of ion movements (linear straight ahead, linear periodic or circular periodic as a function of the electric or magnetic sector field applied). The simplest dynamic mass separation system is a linear time-of-flight (ToF) mass analyzer, and a widely applied mass separation system is the quadrupole analyzer. 

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... 

Mass analyzer/ Feature Linear Quadrupole Quadrupole ion trap Linear time-of-flight Reflectron time-of-flight... 

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