Linear quadrupole mass filters,

Essentially, the ion storage trap is a spherical configuration of the linear quadrupole mass filter. The operations, however, differ in that the linear filter passes the sorted ions directly through to the detector, whereas the ion trap retains the unsorted ions temporarily within the trap. They are then released to the detector sequentially by scanning the electric field. These instruments are compact (benchtop), relatively inexpensive, convenient to use, and very sensitive. They also provide an inexpensive method to carry out GC/MS/MS experiments (Section 2.2.7) (GC is gas chromatography). 

The first idea was to build a trap from the linear quadrupole mass filter structure but, rapidly, the properties of multi-pole potentials were exploited also. For a quadrupole linear trap, the RF electric field [19] is transverse to the z-axis of the ion trap near this axis, the time potential, ( ), in the x- and y-directions can be expressed by... 

Figure 6.11 Computed trajectories through a linear quadrupole mass filter for ions of mIz (a) 100, (b) 99 and (c) 101. In all cases the top trajectory is for the xz plane and the other for the yz plane. All trajectories were calculated for the same initial conditions (ion energy and point and angle of entry) under a, q) conditions chosen such that ions of miz 100 are close to the apex value (0.701,0.2353) within the stability range (Figure 6.15(b)) and are thus transmitted as shown in (a). However, the scan-line slope f scan = 2U/Vo was Set so that the corresponding a, q) values for miz 99 and 101 lie outside this intersected stability region at the instantaneous values of U and Vq that led to stable trajectories for the m z 100 ions, and are thus not transmitted but are lost on the quadrupole rod as shown in (b) and (c). (Recall that the z-direction is the main axis of the quadrupole, i.e., the horizontal direction in this sketch). Reproduced from Campana, Int. J. Mass Spectrom. Ion Phys. (1980) 33, 101, copyright (1980), with permission of Elsevier.

In 1989 the Nobel Prize in physics was shared by Wolfgang Paul (for development of the three-dimensional quadrupole ion trap as an extension of the linear quadrupole mass filter) and Hans Dehmelt (for spectroscopic studies of ions suspended in ion traps of various kinds, including the Paul trap) the Nobel award lectures (Paul 1990 Dehmelt 1990) incidentally also provide accounts of their work that are interesting historically and also lucid and accessible to nonexperts. Other early work on development of the same general principles for ion trapping (Good 1953 Wuerker 1959) should also be... 

U DC potential applied between the two pairs of opposing rods in a linear quadrupole mass filter, v when used in bold font this represents the vector quantity velocity (magnitude and direction), but in normal font is often used to represent the scalar magnitude of the velocity (the speed). 

Figure 3.16 Ion trajectory stability diagram for a linear quadrupole mass filter. The region inside the solid lines represents the possible values of U and V where stable trajectories are obtained for a given m/z. If the U/V ratio is varied such that it follows the scan line, shown by the dashed line in the figure, then at each time in the scanning period the filter will be optimized to transmit ions of a particular mass. This scanning process allows the quadrupole filter to act as a mass spectrometer by adding an ion detector at the downstream end of the filter.

The first description of an ion trap in a PTR-MS instrument was published in 2003. To date there are three distinct research groups that have constructed PTR ion trap mass spectrometers (PTR-IT-MS) [51-53]. Some of the driving forces for replacing a linear quadrupole mass filter with a 3D quadrupole trap have been described above, including the convenience for ion-selective CID and the small size of the trap. Another motivation... 

In mass spectrometers, ions are analysed according to the ml7. (mass-to-charge) value and not to the mass. While there are many possible combinations of technologies associated with a mass-spectrometry experiment, relatively few forms of mass analysis predominate. They include linear multipoles, such as the quadrupole mass filter, time-of-flight mass spectrometry, ion trapping forms of mass spectrometry, including the quadrupole ion trap and Fourier-transform ion-cyclotron resonance, and sector mass spectrometry. Hybrid instruments intend to combine the strengths of the component analysers. 

Principle. The cylindrical quadrupole ion trap is based on the same principle as the quadrupole mass filter, but the geometry is different (Fig. 2.16). The cylindrical QIT, or Paul trap, was developed almost simultaneously with the quadrupole mass filter [232, 233]. Recently, a variant of the theme has emerged, the linear quadrupole ion trap [236], which is a device built like a quadrupole mass filter with extra trapping end electrodes for the axial direction. Under stable conditions, ions moving around inside such traps will ideally continue to do that forever. 

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.

Quadrupole mass filters are one of the most common and cost effective mass analyzers. Although they have limited mass resolution (Table 10.2) and are less sensitive than other mass analyzers, they are durable and suitable for high-throughput analyses. To perform a tandem mass analysis (MS/MS), a triple quadrupole MS is used in which three quadrupoles are placed in series to select, fragment, and analyze ions of interest. Quadrupole ion-trapping devices such as linear 2D ion traps are... 

A quadrupole mass filter consists of four hyperbolic rods on which a potential U F0cos cot is applied (see Fig. 2.6). The hyperbolic shape lead to the production of a quadrupolar field, where the field intensity is linearly dependent on space. However, these conditions can be validly approximated by the use of cylindrical rods (see Fig. 2.7). The ions are injected in the z direction and experience a field due to the direct current (dc) and radio frequency (rf) potentials applied on... 

Linear ITs have been developed to obtain a high ion storing efficiency. They are based on the use of a quadrupolar field, but their operative conditions are deeply different from those of quadrupole mass filters and quadrupole ion traps. As shown in Fig. 2.21, the first proposed configuration of this device is based on the use of a quadrupole mass... 

A triple-quadrupole linear ion trap (QqLIT), which is the most widely used hybrid linear ion trap, is based on the ion path of a triple-quadrupole mass spectrometer with Q3 operated as either a conventional RF/DC quadrupole mass filter or a linear ion trap mass spectrometer. " A QqLIT combines the advantages of a QqQ and a QIT within the same platform without compromising the performance of either mass spectrometer. It retains classical QqQ functions such as MRM, product ion scan, precursor ion scan, and constant neutral loss scan for quantitative and qualitative analysis, and possesses MS" ion accumulation... 

Quadrupole Mass Filters Quadrupole mass analyzers consist of four electrodes, ideally of hyperbolic rods, that are accurately positioned in a radial array. For practical as well as economic reasons, most quadrupole mass filters have employed electrodes of circular cross section. A potential is applied to one pair of diagonally opposite rods consisting of a DC voltage and an rf voltage. To the other pair of rods, a DC voltage of opposite polarity and an rf voltage with a 180° phase shift are applied. The ion motion under the influence of this two-dimensional (2D) field can be described mathematically by the solutions to the second-order linear differential equation, known as Mathieu equation, from which the Mathieu parameters, and can be derived as... 

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