Three-Dimensional Paul Traps

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 

An early account (Dawson 1976) of the bridge between the subsequent developments of the device by physicists (who refer to the device as the Paul Trap ) and chemists (who have used several names, most often simply ion trap but sometimes quadrupole ion storage trap, QUISTOR , 3D trap and, in one commercially available form, ion trap mass spectrometer ), was followed by extensive reviews written by and for chemists (e.g., Todd 1991 March 1992) and a three-volume set (March 1995) covering theory, practicalities and applications. More recently an excellent first introduction for chemists (March 1997) was updated (March 1998) and followed by a comprehensive treatise on the subject (March 2005). An interesting personal perspective by one of the leading contributors to the field (Stafford 2002) describes the additional problems faced in producing a commercial instrument. 

It can be shown (March 2005) that the electrical potential I at any point (x,y,z), within a hyperboloid trap satisfying condition [6.27], is given by  

The ion trajectory within such a potential is described by the Newton equations of motion (Equation [6.2], Appendix 6.1), one for motion in the z-direction and one for each of the x- and y-directions. The component of force exerted on an ion with charge magnitude ze in the x-direction, for example, is ze.E where E, is the electric field component (Equation [6.9])  

Clearly, a = —2 a and q = —2 q, i.e., the stabihty parameters for the z- and r-directions differ by a factor of —2. This differs from the case of the linear quadrupole for which the stability parameters for the X- and y-directions differ by a factor of — 1 (see discussion of Equations [6.13-6.15]). 

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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 ... 

An alternative to three-dimensional Paul traps, linear (two-dimensional) ion traps have demonstrated great utility as mass... 

QQ (TOF) 3DQIT = three-dimensional (Paul) quadrupole ion trap... 

The Paul trap, popularly known as a quadrupole ion trap (QIT), was introduced in 1958 by Paul and colleagues [33]. This contribution was recognized by the award of the 1989 Nobel Prize for Physics to Wolfgang Paul. Because it is a three-dimensional analog of a quadrupole mass filter, it is also called a three-dimensional ion trap to distinguish it from the two-dimensional ion trap described in Section 3.7. The QIT became popular as a mass spectrometer after development of the mass-selective instability mode of mass analysis by Stafford and co-workers [34]. For further reading, several review articles [35-41] and books are cited at the end of the chapter. 

A few selected properties of Coulomb crystals are illustrated in Fig. 6.3. The left part shows a photo from the pioneering experiment performed by Wuerker et al An ensemble of 32 charged aluminum particles having a diameter of a few jim was stored in the effective potential of a three dimensional quadrupole trap operated with an ac voltage of some hundred Hz. Cooling of the translational motion was achieved by collisions with room temperature buffer gas. In this example the ions were confined in the plane of the ring electrode of the Paul trap leading to a radial oscillation with an amplitude that increases with the distance from the center. The other two panels of Fig. 6.3 show results from numerical simulations of a 1000-ion... 

Three-dimensional quadrupole ion trap Quadrupole (RF) ion traps are the newest of the commercially available mass analyzers, despite having been invented at about the same time as the quadrupole mass filter, nearly 50 years ago. The Paul ion trap... 

The ion trap is a device that utilizes ion path stability of ions for separating them by their m/z [53]. The quadrupole ion trap and the related quadrupole mass filter tvere invented by Paul and Steinwedel [57]. A quadrupole ion trap (QITor 3D-IT) mass spectrometer operates with a three-dimensional quadrupole field. The QIT is formed by three electrodes a ring electrode with a donut shape placed symmetrically between two end cap electrodes (Fig. 1.20). 

In a quadrupole device, not as accurate and precise as double-focusing instruments but fast, a quadrupolar electrical field comprising radio-frequency (RF) and direct-current components is used to separate ions. Quadrupole instruments as mass analyzers are used together with ESI as the ion source the configuration employing a three-dimensional quadrupolar RF electric field (Wolfgang Paul, University of Bonn, 1989 Nobel prize for physics) is referred to as an ion trap analyzer (see below). 

The three-dimensional quadupole field ion trap - or Paul trap is a three-electrode device [see Figure 4.5(b)]. Ions are injected into the device and collected in packets from an ESI or MALDI source. The ion trap analyzer is capable of MS, MS" (MS = MS-MS-MS) and high-resolution scans (R = 20,000). The ion packets enter through an entrance-end cap and are analyzed by scanning the RF amplitude of the ring electrode. The ions are resonated sequentially from low to high m/z and are ejected from the ion trap through the exit-end cap electrode to a detector. Unlike the triple quadrupole (QqQ) mass spectrometer discussed previously, the ion trap performs tandem mass spectrometry (MS-MS) scan modes in the same analyzer. 

Figure 6.9 Sketches of the two commonly used types of RF quadrupole devices (a) the linear m/z filter (two-dimensional quadrupole field), and (b) the Paul ion trap (a three-dimensional quadrupole field). Reproduced from Dawson, Mass Spectrom. Revs. 5, 1 (1986), with permission of John Wiley Sons, Ltd. (c) Diagram of the cross-section of a linear quadrupole m/z analyzer at z = 0, showing the potentials applied to the two pairs of electrically connected rods the rod spacing is 2tq at the closest approach, as shown. Reproduced from Farmer, in Mass Spectrometry (CA McDowell, Ed), McGraw-Hill (1963), with permission of John Wiley Sons, Ltd.

Quadrupole mass spectrometry (Q-MS) has wide functionality making it the most popular form of MS. The family of quadrupole mass analyzers includes a variety of devices, such as quadrupole mass filter (QMF), two-dimensional (2D) ion trap (IT) or linear ion trap (LIT), and three-dimensional (3D) IT. The 3D IT device is also called the Paul trap in honor... 

In contrast to the linear two-dimensional quadrupole field of the LIT, the three-dimensional quadrupole ion trap (QIT) developed by Wolfgang Paul creates a three-dimensional RF quadmpole field of rotational symmetry to store ions within defined boundaries. Its invention goes back to 1953 [80-82] however, it took until the mid-1980s to access the full analytical potential of quadrupole ion traps [119,124-128]. 

The mass analyser described in the preceding section is sometimes referred to as a linear quadrupole, since the electrodes consist of parallel rods. Another type of mass spectrometer based on the use of a quadrupolar electric field is the three-dimensional (3D) quadrupole ion trap, which is sometimes also known as a Paul trap. This is a device that has been in existence for several decades, but it is only in the past twenty years or so that it has moved from a small number of research laboratories out into the commercial domain. Ion traps are now popular devices for a whole range of mass spectrometry applications, their popularity being enhanced by their very compact size and the ease with which they can be used in MS experiments (see Section 3.5.3.2). Several PTR-MS instruments have been constructed with a quadrupole ion trap and so a description of the basic operating principles... 

Ion Trap MS The evolution of ion trap mass spectrometry started also in 1953 with the same patent of Paul and Steinwedel that described the quadrupole mass selective detector. The applicability of a three-dimensional quadrupole to trap ions was recognized in the late 1950s, followed by several comparable observations using a circular two-dimensional (linear) ion trap. ... 

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