End cap electrode

Fig. 3. Schematic diagram of an ion trap where A and B represent end cap electrodes, C the ring electrode, Tq the internal radius of C, and the internal...

The quadrupole ion-trap, usually referred to simply as the ion-trap, is a three-dimensional quadrupole. This type of analyser is shown schematically in Figure 3.5. It consists of a ring electrode with further electrodes, the end-cap electrodes, above and below this. In contrast to the quadrupole, described above, ions, after introduction into the ion-trap, follow a stable (but complex) trajectory, i.e. are trapped, until an RF voltage is applied to the ring electrode. Ions of a particular m/z then become unstable and are directed toward the detector. By varying the RF voltage in a systematic way, a complete mass spectrum may be obtained. 

The ion trap mass analyzer is similar to the quadrupole but with the important distinction that it can isolate and trap ions in an electrical field. Notably, the ion trap differs significantly from quadrupoles in design and operation in that triple quadrupoles perform tandem mass analysis on ions as they pass through an analyzer ion traps are capable of isolating and retaining specific ions for fragmentation upon collision with an inert gas in the same cell. An ion trap is about the size of a tennis ball and consists of a donut-shaped electrode and two perforated disk-like end-cap electrodes. 

In most commercial cylindrical ion trap instalments the end-cap electrodes are held at ground potential and usually only a RF potential is applied to the ring electrode. When the RF amplitude is set to a low, so-called storage voltage, all ions above a certain m/z are trapped. This voltage is usually chosen so the lowest trapped m/z is greater than those of water, air, and solvent ions (i.e., above 100 to 150 Th), depending on the nature of the measured species. 

The ion trap is a similar analyzer. There are two end cap electrodes that are at ground potential. An electrostatic field is generated hy a donut-shaped hyperbolic electrode within the cap, which maintains ions in a stable trajectory. Changing electrode voltages ejects ions of a particular mass from the trap into the detector (Honour, 2003). 

Ions produced in the source enter the trap through the inlet focusing system and the entrance end-cap electrode. Various voltages are applied to the electrodes to trap and eject ions according to their mass-to-charge ratios. 

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). 

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). 

The ion trap is a 3D analog of the linear quadrupole. Two of the rods form the end-cap electrodes, whilst the other pair of rods is bent round to form the doughnut-shaped ring electrode (Fig. 12). 

The quadrupole ion trap is a three dimensional analogue of the linear quadrupole mass analyzer [71,72]. It consists of a cylindrical ring electrode and two end-cap electrodes. Both end-cap electrodes contain a whole for injecting and ejecting ions into and out of the ion trap (Fig. 8.11). A relatively high pressure of helium damping gas (about 0.1-0.4 Pa) is present in the ion trap in order to kinetically cool the trapped... 

In contrast to triple quadrupole instruments, where MS-MS experiments can be conducted in space in separate regions of the instrument, ion traps enable MS-MS sequentially in the same physical space, and thus, occur tandem in time. After the ions have been formed an trapped, a parent ion is selected by resonance ejection of all ions except those of the selected m/z ratio. This is done by applying a resonance ejection radiofrequency voltage to the end-cap electrodes which stimulates motion of the ions in the axial direction. The next step in the MS-MS sequence is to effect collisionally... 

The ion trap mass analyzer [2] (Fig. 2) consists of two end-cap electrodes, held at ground potential, and an interposed ring electrode, to which DC and RF voltages are applied. The ring electrode is a single surface formed by a hyperboloid of rotation. The end-caps are complementary hyperboloids having the same conical asymptotes z is an axis of... 

Ions produced in the source enter the trap through the inlet focusing system and the entrance end-cap electrode. 

The cylindrical ion trap, machined from stainless steel, has a radius (r0) of 2.5 mm and a center to end-cap distance (z(1) of 2.7 mm. Planar end-cap electrodes allow for ion entrance and exit through 1-mm-radius apertures. The instrument measured 46 x 50 x 38cm, with a weight of 38kg and 210W power consumption while in operation. 

Fig. 2a-g. A diversity of ICR reaction chamber configurations, with excitation, detection, and trapping (end cap) electrodes indicated as E, D and T. Configurations shown are a cubic b cylindrical c the infinity trap with segmented end caps d open-ended e open-ended with ca-pacitative rf coupling between sections f dual g matrix-shimmed . From [12]... 

The quadrupole ion trap is a three-dimensional (3-D) analog of the linear quadrupole [33]. It consists of two end-cap electrodes with hyperbolic cross sections and one ring electrode located between the end caps (Figure 7-5B). The RF voltage is applied to the ring electrode and the ground potential is normally operated on the end caps. A rotationally symmetric electric... 

The trapped ions possess characteristic oscillation frequencies. The stable motion of ions in the trap is assisted by the presence of a helium buffer gas (1 mtorr) to remove kinetic energies from ions by collisions. When a supplementary AC potential, corresponding to the frequency of a certain m/z ion, is applied to the end-cap electrode, ions are resonantly ejected from the trap. This method of resonance ejection is used to effectively extend the mass-to-charge ratio of the ion trap. Some other characteristic features of a 3-D ion trap include high sensitivity, high resolution with slow scan rate, and multiple-stage MS capability (see the section on tandem MS). In addition, it is inexpensive and small in size. As a result, a 3-D ion trap is widely used in LC/MS and LC/MS/MS applications. 

An important development in quadnipole technology is the three-dimensional ion trap [38-39]. A quadmpole ion trap consists of a cylindrical ring electrode to which the quadnipole field is apphed, and two end-cap electrodes (Figure 2.3). One end-cap contains holes for the introduction of electrons or ions into the trap, while the other has holes for ions ejected out of the trap towards the electron multiplier. In LC-MS systems, ions are generated in an external ion source. The ions are introduced to the trap in a pulsed mode and stored there. A helium bath gas (0.1 Pa) is present in the trap to stabilize the ion trajectories. 

At this stage, different MS experiments can be performed. In the full-scan mode, ions of different m/z are consecutively ejected from the trap towards the external detector by ramping the RF voltage at the ring electrode. Resonant ion ejection may be supported by additional waveforms applied to the end-cap electrodes. Ion ejection can be achieved with unit-mass resolution, or at enhanced resolution by slowing down the scan rate [42]. 

The /w/z-selective instabihty mode for mass analysis in the ion trap allows additional experiments to be performed in the ion trap. When it is possible to selectively store ions of a particular m/z and eject other ions, the selection of a precursor ion for product-ion MS-MS is possible as well. The precursor ion is selected by applying two consecutive waveforms which eject all ions with m/z values on either side of the selected m/z. The isolated m/z is then excited by the application of a wz/z-selective excitation waveform to the end-cap electrodes. The... 

Ion trap Storage of ions in space defined by ring and end cap electrodes. Electric field sequentially ejects ions of increasing m/z values. 

Using mass selective instability with resonance ejection, ions are scanned out of the trap through slits in the center of two opposite center section rods and focused onto two separate conversion dynodes. In the case of the QIT, where ions are scanned out of both end cap electrodes, the only place for a detector is behind the end cap opposite the ion entrance, so that only half of the ions scanned out of the trap are detected. Both the QJT and LIT operate at unit mass resolution with similar scan rates and both have the capacity to generate higher resolution spectra at slower scan rates. 

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