Ion storage devices

By applying a fundamental RF potential, the QIT can be described as a small ion storage device where ions are focused toward the center of the trap by collision with helium gas. In the QIT, because of the cylindrical symmetry of the trap, the x and y components of the field are combined to a single radial r component, where + y. The motion of ions in the trap is characterized by one... 

Trapped Ion Cell An electrostatic-magnetic ion storage device. 

The obvious benefit of the quadrupole ion trap is that it is an ion storage device. Therefore, ions can be both accumulated and stored for extended periods. Accumulation can occur over a continuous ionization event or over multiple pulsed ionization periods. When used with pulsed ionization sources, duty cycle, defined in terms of sample utilization, can be as high as 100%. Because a broad range of atomic ions can be stored simultaneously, the quadrupole ion trap is a promising analyzer for transient peak analysis. 

Basically, a tandem mass spectrometer can be conceived in two ways performing tandem mass spectrometry in space by the coupling of two physically distinct instruments, or in time by performing an appropriate sequence of events in an ion storage device. Thus there are two main categories of instruments that allow tandem mass spectrometry experiments tandem mass spectrometers in space or in time. 

Quadrupole ion traps ions are dynamically stored in a three-dimensional quadrupole ion storage device (Fig. 10.6) [37]. The RF and DC potentials can be scanned to eject successive mass-to-charge ratios from the trap into the detector (mass-selective ejection). Ions are formed within the ion trap or injected into an ion trap from an external source. The ions are dynamically trapped by the applied RF potentials (a common trap design also makes use of a bath gas to help contain the ions in the trap). The trapped ions can be manipulated by RF events to perform ion ejection, ion excitation, and mass-selective ejection. This provides MS/MS and MS experiments, which are eminently suited for structure determinations of biopolymers [38] (see Section 10.4). 

In tandem-in-time mass spectrometers all operations take place within a single-ion storage device, but at different times. The most common tandem-in-time instrument is the quadrupole ion trap (QIT). The ongoing development of a higher-sensitivity counterpart, the linear ion trap (LIT), will presumably lead to an eclipsing of the QIT. 

The precursor ion scan mode scheme is scan-disso-ciate-select (o-> in Kondrat symbols, and the opposite of the product ion scan mode). This mode is used in instruments containing two or more analyzers in series, but is unavailable in ion storage devices. The first analyzer scans the ion stream over a particular mass range and ions of increasing miz value are passed to the collision cell and fragmented. The second analyzer is fixed to select product ions of a specific miz ratio. Therefore, ions passing through... 

Linear (2D) ion traps have more recently been introduced for use as ion storage devices, as stand-alone mass spectrometers and as components of hybrid tandem instruments. Although no example of their use in rigorously validated analytical methods has been published to the best of the knowledge of the present authors, there are reasons to believe that this situation might change. For... 

Two types of ion-storage device (ion traps) are in use today as mass spectrometers, as discussed already, with built in MSfMS capabilities the magnetic trap as ion cyclotron resonance spectrometer, which is. in its modem version, the fou-rier transform mass spectrometer [144]- [147] and the electric field version, called ion trap [148]-[152]. Both types of instrument allow all but the one selected ion to be ejected, which then undergoes ion-molecule reactions, collisions, or photodissociation to yield fragments. This process may be repeated, allowing not only MS/MS but also MS/MS/MS and even MS", which allows the detailed study of fragmentation reactions. However, currently only daughter-ion scans are possible. 

In time-of-flight experiments, the charged clusters travel across an electric field region over times that exceed microseconds, being thus macroscopic with respect to the vibrational time scale of the picosecond. Ion storage devices can reach even longer time scales of a fraction of second. However,... 

Perhaps the most suitable type of ion storage device for cooling and spectroscopy of cold molecular ions is the radio frequency (RE) ion trap, which uses an AC electric field at RE frequencies (typically hundreds of kilohertz to a few megahertz) in conjunction with static electric fields. Because of their importance, we describe them here in more detail. 

One particular type of RF ion storage device that can overcome these limitations is the linear ion trap. In a linear ion trap of cylindrical geometry, ions are confined radially by an RF electric field created by a set of In identical linear electrodes (poles), evenly spaced on a circle with inscribed radius Tq. In linear quadmpole ion traps (n = 2), four electrodes are alternatively connected to two sinusoidal RF waveforms of opposite sign (Fig. 2), while two end-caps are typically kept at a few volts DC relative to the electrode bias to stop ions from exiting the trap in the axial direction. 

A German patent was granted for the QUfSTOR (quadrupole ion storage device) together with the quadrupole mass spectrometer. 

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