Quadrupole mass filter principle

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

Figure 2.15. Schematic of a quadrupole analyzer, (a) A hyperbolic cross-section (b) cross-section of cylindrical rods (c) the operating principle of a quadrupole mass filter. The x-direction pair of rods acts like a high pass filter so ion C (with low m/z) is not allowed through, and the y-direction pair of rods acts like a low pass filter and takes care of ion A (with high m/z). Only ion B having an m/z in the stable range is allowed through the quadrupole mass filter for subsequent detection. Reprinted from A. Westman-Brinkmalm and G. Brinkmalm (2002). In Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, J. Silberring and R. Ekman (eds.) New York John Wiley Sons, 47-105. With permission of John Wiley Sons, Inc.

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. 

The basic principles of the quadrupole mass filter were published in the early 1950s by Paul and Steinwedel. " It has now become one of the most widely used types of... 

Two types of mass analyzers have been used extensively in atmospheric applications quadrupole mass filters and time-of-flight (TOF) instruments. The use of ion traps is also being increasingly explored for this application. For the fundamental principles of mass... 

The principles behind an ion trap mass spectrometer are similar to those of the quadrupole mass filter, except that the quadrupole field is generated within a three-dimensional cell using a ring electrode and no filtering of the ions occurs. All of the steps involved in the generation and analysis of the ions take place within the cell, and in order to detect the ions they must be destabilized from their orbits, by altering the electric fields, so... 

The quadrupole mass filter (QMF) is a mass analyzer on whose operation use of an MEM is not necessarily dependent. The ion currents produced are of sufficient magnitude to be measured by means of a Faraday cage and a suitable amplifier such as a vibrating-reed electrometer. The QMF is a true M/z filter which requires no magnetic fields. Since first being proposed by Paul and Steinwedel (30), the QMF has been investigated extensively, and the principles and methods of operation are well known (see, for example, ref. 31). 

It should also be noted that the RF-only quadrupoles (as well as the related RF-only hexapoles and octapoles) can be operated as ion lenses, often referred to as RF-only ion guides or simply ion guides. RF-only quadrupoles (denoted as lower case q , nonitalicized, to distinguish them from the closely related quadrupole mass filters Q) also act as efficient collision cells for collision induced dissociation in triple quadrupole analyzers (QqQ, Section 6.4.3) and hybrid tandem quadrupole-time of flight instruments (QqTOF, Section 6.4.7). Discussion of these devices must be postponed imtil the principles of all hnear quadrupoles have been considered (Section 6.4.2). 

Radiofrequency (RE) electric quadrupole mass (really miz) filters represent a considerable majority of analyzers in current use, particularly in trace quantitative analysis for this reason the operating principles of these devices will be discussed in some detail to emphasize their advantages and limitations. (The RF range corresponds to a few MHz.) These analyzers are used as stand-alone (non-tandem) MS detectors, as the components of the workhorse QqQ tandem instruments and as the first analyzer in the QqTOF hybrid tandem instrument. Quadrupole mass filters (Q) are essentially the same device as the RF-only collision cells and ion guides (q) discussed later in this section and are intimately related to the RF ion traps described in Section 6.4.5. In this regard, it can be mentioned that Q and q devices are not called quadrupoles because they are constructed of four electrodes (rods), but because a quadrupolar electric field (see Equation [6.11]) is formed in the space between the rods indeed the three-dimensional (Paul) trap (Section 6.4.5) creates a quadrupolar field using just three electrodes ... 

Quadrupole ion traps are now available in two geometries, the recently introduced 2D linear ion trap and the (now classical) 3D quadrupole ion trap (Paul trap). The operating principles of 3D ion traps are closely related to those of the quadrupole mass filter but are considerably more complex and thus demand more space for explanation. However, they are discussed here in some detail both because of their popularity and because the basic principles underlying their operation are shared by... 

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... 

Figure 13.10 Principle of quadrupole mass filter, acting as a mass band-pass filter depending on their mass, ions react differentlytothemodulationfieldl RF,asshowninthelowerpart).AdaptedfromRouessacandRouessac C/iemjcaMna/>>sjs,2007, with permission of John Wiley Sons Ltd...

The electric quadrupole mass filter, developed by W. Paul and coworkers [250, 251], operates on the principle that ion trajectories in a two-dimensional quadrupole field are stable if the field has an ac component superim-... 

FIGURE 7.3 Schematic representation showing principles of mass separation using a quadrupole mass filter. 

FIGURE 12.1 Principles of mass selection with a quadrupole mass filter... 

The operating principles of quadrupole mass filters are described in Section 3.5.2. 

The quadrupole mass filter was the type of mass analyser used in the earliest PTR-MS instruments and it is still the most popular choice at the time of writing. The popularity of the QMS stems from its relatively compact size and reasonable price. In this section the basic operating principles of QMS systems will be described along with a discussion of some of their performance characteristics. 

Three-dimensional ion traps that operate on the principle of the quadrupole are another type of mass analyser (with or without a DC component). In ion traps, the ions are confined between the electrodes which have a particular shape that resembles the set-up of a quadrupole. Although they are physically simple devices, the fundamental principle of ion traps is very complex and they are more sensitive yet less expensive than quadrupoles. The volume determined by the so-called annular, superior and inferior electrodes is simultaneously the ion source and the mass-filter (see Fig. 16.12). Ion traps are usually coupled to separation techniques (GC/ MS, LC/MS). 

Figure 16.10 Partial spectrum of a hydrocarbon obtained with a gas analyser operating under the quadrupole filter principle. The recording reveals that the instrument has heen used in the mode for which Am is constant for the whole range of masses.

Other mass spectrometers are equipped with three-dimensional ion traps of which the geometry is much different to the quadrupoles previously described. In an ion-trap, the ions are confined between three electrodes (one toroidal and two end-caps), whose particular shape appears to result from a sort of anamorphosis of the four-bar set-up of a classic quadrupole. As in the previous category they operate under the effect of a variable electric field (with or without a superimposed fixed field). Although they are, in appearance, physically simple devices, the fundamental principle of ion trap is complex. These ion trap detectors are sensitive, less costly than quadrupoles and compatible with different ionization techniques. The volume defined by the electrodes, named superior, inferior and annular, is simultaneously the ion source and the mass filter (Figure 16.11). These analysers are almost exclusively linked with a separative technique (GC/MS). 

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