Three-dimensional quadrupole field

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). 

A number of mass analyzers in use today have been coupled to these sources. These include two- and three-dimensional quadrupole field, time-of-flight (TOF), quadrupole-TOF hybrids, magnetic sector, and Fourier transform mass spectrometers. Paramount to the mass spectrometer analyzer used in the analysis is proper sample preparation. With proper preparation of proteins and peptides, their molecular weights can be determined with high mass accuracy. Conversely, a poorly prepared sample will lead to poor or no mass spectrometer results. For peptides and proteins, the mass accuracy is typically better than 0.01%. 

With reference to Fig. 4.44, a symmetrical electrical field allows a consideration of only a radial and z displacement. The equations of motion of an ion in the three-dimensional quadrupole field are derived as in the case of the quadrupole mass filter ... 

The Mathieu equation (3.7) of the form discussed in Section 3.4.1 can also be written to describe the motion of ions in a three-dimensional quadrupole field. The corresponding Mathieu coordinates and q are given by... 

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.

A three-dimensional ion trap (e.g., Q ion trap) bears the same physical principles as the quadrupole mass analyzer, but the ions are trapped and sequentially ejected mainly by using an RF field, within a space defined by a ring electrode between two end-cap electrodes (Figure 2.6b). A linear quadrupole ion trap is similar to a three-dimensional ion trap, but it traps ions in a two-dimensional, instead of a three-dimensional, quadrupole field. Some of the figures of merit of the ion-trap instrument are as follows ... 

Figure 5 Several Mathieu stability regions for the three-dimensional quadrupole field. (A) Diagrams for the z direction of space. (B) Diagrams for the rdirection of space.

A three-dimensional quadrupole field is formed by the three electrodes when a suitable RF voltage (VRijCos(27r/Rf t)) is applied to the ring electrode as shown in Figure 9.3a. The... 

D, two-dimensional quadmpole field 3D, three-dimensional quadrupole field APCl, atmospheric pressure chemical ionization APPI, atmospheric-pressure photoionization ESI, electrospray ionization MALDI, matrix-assisted laser desorption ionization CID, collision-induced dissociation ETD, electron transfer dissociation. 

Figure 1 Schematic representation of a quadrupole ion trap showing (a) a cut-away view of the three-dimensional quadrupole ion trap, including two endcap electrodes and the ring electrode, and the (b) positive and (c) negative extremes of the quadrupolar field, alternating as a function of the radio frequency (rf) potential applied to the ring electrode.

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. 

Ion imp analysers operate on a different principle than magnetic sector or quadrupole analysers, both of which manipulate ion trajectories. Ion traps store ions in a three-dimensional electrostatic field that is formed by appropriate application of voltages to electrodes in the trap (Fig. 3.4). Ions of a broad distribution of m/z values can be stored in the trap as they precess at a frequency that is dependent on their m/z ratio. A mass spectrum is obtained by increasing the magnitude of the direct and alternating... 

The RF quadrupole ion trap mass spectrometer (ITMS) is a close relative of the QMF and ideally can be thought of as a three-dimensional quadrupole (see Fig. 17.8). The close relationship of these two devices is evident by the fact that ion motion in the two devices is governed by essentially the same mathematical equations. As with the QMF, the ITMS uses DC and RF electric fields and the operation of the IT is described by solutions to the Mathieu equation. Unlike the QMF, ITMS analyzers trap ions within the mass analyzer. Ions are trapped, ejected to select the mass of interest, and then ejected in a controlled manner for detection. 

Two other analyzers now commonly interfaced with GC are shown in Figure 7.8. These include the ion trap analyzer in which ions can be confined by electric and magnetic fields (38-40). Ion traps are essentially three-dimensional quadrupoles. Ions of a specific miz value circulate in stable orbits within the analyzer. A relatively high pressure of helium (10 Torr) is used as a bath gas... 

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]. 

Interaction with electromagnetic fields three-dimensional quadrupole... 

Interaction with electromagnetic fields three-dimensional quadrupole ion trap (3D QIT) analyzers... 

Ion trap analyzer. A mass-resonance analyzer that produces a three-dimensional rotationally symmetric quadrupole field capable of storing ions at selected mass-to-charge (m/z) ratios. 

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 resulting field within the quadrupole is made up partially of a fixed voltage and partially of a variable voltage. Under these conditions, ions entering the quadrupole at O are subjected to a force that is variable both in intensity and in direction. As a result of the effect of the field, the ions follow complex three-dimensional trajectories that are generally unstable, leading to their contact with one of the electrodes. 

QIT The quadrupole ion trap (QIT) utilizes a cylindrical ring and two end-cap electrodes to create a three-dimensional (3D) quadrupolar field for mass analysis. These instruments are capable of selectively trapping or ejecting ions and are often used for the sequential fragmentation and analysis experiments of product ion MS/MS. Also known as a 3D trap due to the configuration (March, 1997). 

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). 

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