Paul ion trap

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... 

An alternative to the 3D quadrupole ion trap (Paul trap) is the linear quadrupole ion trap. The linear ion trap is akin to a hybrid of the quadrupole mass filter and the 3D ion trap in that it consists of a four-rod assembly, like the quadrupole filter, but also it has entrance and end electrodes like the 3D ion trap. Confinement of ions along the axial direction is provided by DC potentials applied to the end electrodes. The quadrupole rods produce radial motion of the ions through application of an RF electric field, in a similar manner to that already described for the quadrupole mass filter. To record a mass spectrum axial ion ejection, initiated by RF excitation, can be used in a procedure similar to that used for the 3D ion trap. 

Another approach to mass analysis is based on stable ion trajectories in quadnipole fields. The two most prominent members of this family of mass spectrometers are the quadnipole mass filter and the quadnipole ion trap. Quadnipole mass filters are one of the most connnon mass spectrometers, being extensively used as detectors in analytical instnunents, especially gas clnomatographs. The quadnipole ion trap (which also goes by the name quadnipole ion store, QUISTOR , Paul trap, or just ion trap) is fairly new to the physical chemistry laboratory. Its early development was due to its use as an inexpensive alternative to tandem magnetic sector and quadnipole filter instnunents for analytical analysis. It has, however, staned to be used more in die chemical physics and physical chemistry domains, and so it will be described in some detail in this section. 

H. G. Dehmelt (University of Washington, Seattle) and W. Paul (Bonn) development of the ion trap technique. 

Figure 4.12 Mass spectrum typical of beeswax (wax sculpture of the head of Degas by Paul Valery). Spectrum obtained by Dl El MS on a GCQ Finnigan device equipped with an ion trap...

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. 

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... 

Fig. 11.10. Diagram illustrating the inner surfaces of the primary components of a Paul (3D) quadrupole ion trap. Ions generated by an external source are injected into the trap through an aperture in one of the end caps. Scan functions for isolating ions in the trap, exciting the mass selected ions to induce unimolecular dissociation, and ejecting ions from the trap (for detection) are implemented through the application of DC and RF voltages to the ring electrode.

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

What is one of the significant advantages of a linear (2D) ion trap mass analyzer when compared to a 3D (Paul) ion trap (increased ion capacity, which leads to improved sensitivity). 

Note Paul himself preferred to call the device lonenkdfig (ion cage) rather than the nowadays accepted term quadmpole ion trap because it does not actively act to catch ions from outside. The acronym QUISTOR derived from quadrupole ion store was also widespread in use. 

Quadrupole mass spectrometers [10] or quadrupole ion traps are today the most widely used mass spectrometers. The physical bases were described in the early 1950s by Paul and Steinwedel. For his work Paul received the Nobel Prize in 1989 [11]. Triple quadrupole mass spectrometers have become very popular instruments for qualitative and quantitative analysis. Yost et al. [12] built in 1978 the first instrument and it took four years before this type of instrument was commercialized. The coupling with liquid chromatography or gas chromatography is well established and benchtop ion traps or quadrupoles are nowadays part of the standard equipment of many analytical laboratories. 

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). 

Fig. 2. Ion trap mass analyzer. Reproduced from http //www.chm.bris.ac.uk/ms/images/iontrap-schematic.gif, with permission from Dr Paul Gates, School of Chemistry, University of Bristol, UK.

Elemental mass spectrometry has undergone a major expansion in the past 15-20 years. Many new a, elopments in sample introduction systems, ionization sources, and mass analyzers have been realized. A vast array of hybrid combinations of these has resulted from specific analytical needs such as improved detection limits, precision, accuracy, elemental coverage, ease of use, throughput, and sample size. As can be seen from most of the other chapters in this volume, however, the mass analyzers used to date have primarily been magnetic sector and quadrupole mass spectrometers. Ion trapping devices, be they quadrupole ion (Paul) [1] traps or Fourier transform ion cyclotron resonance (Penning) traps, have been used quite sparingly and most work to date has concentrated on proof of principal experiments rather that actual applications. 

W. PAUL and H.S. STEINWEDEL describe the quadrupole analyser and the ion trap or quistor in a patent [31], W. PAUL, H.P. REINHARD and U. Von ZAHN, of Bonn University, describe the quadrupole spectrometer in Zeitschrift fur Physik in 1958. PAUL and DEHMELT receive the Nobel Prize in 1989 [32],... 

Historically, the first ion traps were 3D ion traps. They were made up of a circular electrode, with two ellipsoid caps on the top and the bottom that creates a 3D quadrupolar field. These traps were also named quadrupole ion traps (QITs). To avoid confusion, this term should not be used but should be replaced preferably with Paul ion trap. The acronym QUISTOR derived from quadrupole ion storage is also largely used but not recommended. 

Conceptually, a Paul ion trap can be imagined as a quadrupole bent in on itself in order to form a closed loop. The inner rod is reduced to a point at the centre of the trap, the outer rod is the circular electrode, and the top and bottom rods make up the caps. 

In using the Mathieu equations to locate areas where ions of given masses have a stable trajectory, the equations are very similar to the ones used for the quadrupole. However, in the quadrupole, ion motion resulting from the potentials applied to the rods occurs in two dimensions, x and y, the z motion resulting from the kinetic energy of the ions when they enter the quadrupole field. In the Paul ion trap the motion of the ions under the influence of the applied potentials occurs in three dimensions, x, y and z. However, due to the cylindrical symmetry x2+y2 = r2, it can also be expressed using z, r coordinates (Figure 2.13). 

Figure 2.33 represents such a linear trap. The two detectors allow the use of all the ions expelled from the trap. Trapping efficiency is in the range 55-70 % while it is only 5 % in the Paul ion trap. Unit resolution is achieved at 16 700 Th s 1 scan rate. At 27 Th s 1, Am = 0.05 is observed at m/z 1520, corresponding to a resolution of 30000 FWHM. The ion capacity is about 20 000, 40 times more than in the Paul ion trap. 

Some mass spectrometers combine several types of analysers. The most common ones include two or more of the following analysers electromagnetic with configurations EB or BE, quadrupoles (Q), ion traps (ITs) with Paul ion traps or linear ion traps (LITs), time-of-flight (TOF), ion cyclotron resonance (ICR) or orbitrap (OT). These are named hybrid instruments. The aim of a hybrid instrument is to combine the strengths of each analyser while avoiding the combination of their weaknesses. Thus, better performances are obtained with a hybrid instrument than with isolated analysers. Hybrids are symbolized by combinations of the abbreviations indicated in the order that the ions travel through the analysers. 

---