Principle of Ion Cyclotron Resonance

As we know from the discussion of magnetic sectors, an ion of velocity v entering a uniform magnetic field B perpendicular to its direction will move on a circular path by action of the Lorentz force (Chap. 4.3.2), the radius of which is determined by Eq. 4.13  

Upon substitution with v = va) the angular frequency (Bc becomes  

The disadvantages of this concept are clear i) mass accuracy and resolution are limited to = number of half cycles) ii) the electric signal for ion detection 

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Fig. 35. Operating principles of ion cyclotron resonance mass spectrometry. (Reprinted with permission from Ref. [81].)... 

It is the purpose of this article to explain the basic principle of ion cyclotron resonance, to describe in detail its practical realization in an ICR spectrometer, and to explain the different techniques that have been developed. Because ICR spectrometry differs in some important respects from conventional mass spectrometry, some remarks about the information conveyed by ICR signals are added. The attempt is made to summarize previously reported results both in descriptions and in tables of data on ion-molecule reactions (IMR) as observed with ICR techniques. There is a very complete bibliography of publications dealing with ICR, although it was not possible to review all of the papers hsted there. 

FTICR-MS instruments operate on the principle of ion cyclotron resonance. As ions have resonant frequencies, these frequencies can be used to isolate the ions prior to further fragmentation or manipulation. For example, a resonant frequency pulse on the excite plates (E+/— in Figure 2.8b) will eject the ions at, or near, that frequency. Furthermore, frequency sweeps - carefully defined to not excite the ion of interest - can be used to eject unwanted ions. However, the most elegant method for ion isolation is that of Stored Waveform Inverse Fourier Transform (SWIFT) [86] in which an ion-exdtation pattern of interest is chosen, inverse Fourier-transformed, and the resulting time domain signal stored in memory. This stored signal is then clocked-out, amplified, and sent to the excite plates when needed. The typical isolation waveform in SWIFT uses a simple excitation box with a notch at the frequencies of the ion of interest, a few kHz. 

Magnetic field Fig. 37. Operating principles of ion cyclotron, / resonance mass spectromet. (Reprinted with... 

Fourier transform mass spectrometry (FTMS) offers the highest mass resolution and mass measurement accuracy of all mass analyzers. FTMS, also referred to as Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS), can be adapted to a wide variety of ion sources and ion dissociation methods.The fundamental behavior of ions in FTMS instmments is based on the principle of ion cyclotron resonance (ICR), conceived and developed by E. O. Lawrence in the 1930s to build ion accelerators for nuclear physics experiments. ICR was first implemented in mass spectrometry in an instrument called the omegatron, developed by scientists at the National Bureau of Standards in the 1950s. Advances such as the application of Fourier transform methods to ICR spectrometry and the trapped analyzer cell resulted in the development of a powerful analytical instmment. 

Traditionally, ion traps have been divided into three classes QIT, which rely on RF fields to provide ion trapping a linear ion trap, which is closely related to the QIT in its operating principles and ion cyclotron resonance (ICR) mass spectrometers, which rely on a combination of magnetic fields and electrostatic fields for trapping. 

The result of the Back-to-Basics series is an accumulation of some 50 separate but interrelated expositions of mass spectrometric principles and apparatus. Some areas of mass spectrometry, such as ion cyclotron resonance and ion trap instruments, have not been covered except for passing references. This decision has not been due to any bias by the authors or Micromass but simply reflects the large amount of writing that had to be done and the needs of the greatest proportion of users. 

Principle. The principle of the ion cyclotron resonance was developed in the early 1930s by Lawrence and coworkers [252, 253]. The utilization of the ion cyclotron resonance (ICR) technique for mass spectrometry was introduced around 1950 by Sommer et al. [254, 255], and combination with the Fourier transform (FT) technique was developed by Comisarow and Marshall in 1974 [256], Coupling of external sources to an FTICR analyzer was first done in 1985 [257, 258],... 

Fourier transform ion cyclotron resonance (FTICR) analyzer is excellent for MSn measurements (see Section 2.2.6), perhaps even more so, since the ions remain in the cell after detection. In principle one injection of ions is enough for a whole MSn sequence, including acquisition of a mass spectrum of each step. 

Chemical ionization is, as might be expected from its name, more chemically interesting and is closely allied to ion cyclotron resonance, which will be discussed in the next section. The principle of chemical ionization is simple. The molecule to be studied is injected into the ionizing region of the mass spectrometer in the presence of 0.5-1.5 mm Hg pressure of a gas, usually methane. Electron impact causes ionization of the methane, which is present in relatively large concentration. The ionization products of methane then react with the compound to be analyzed and convert it to ions. The gas mixture then exits into a low-pressure zone (10 4 mm) and the ions are analyzed according to mje in the usual way. 

Figure 7.8 Excitation (a) and detection (b) of the ion cyclotron motion within an FTMS mass analyzer cell. Reprinted from Marshall, A.G. and Flendrickson, C.L., Fourier transform ion cyclotron resonance detection principles and experimental configurations. International Journal of Mass Spectrometry, 215, 59-75. Copyright (2002), with permission from Elsevier.

To benefit general readers, the discussion has been limited to methodologies that are accessible to nonspecialists and that can be carried out on commercially available spectrometers without special modifications. The chapter illustrates the principles of mass spectrometry by demonstrating how various techniques [MALDI, ESI, Fourier transform ion cyclotron resonance (FT-ICR), ion traps, and tandem mass spectrometry (MS-MS)] work. It also provides examples of utilizing mass spectrometry to solve biological and biochemical problems in the field of protein analysis, protein folding, and noncovalent interactions of protein-DNA complexes. 

At the National Institute of Chemieal Physies and Biophysics (NICPB), Laboratory of Chemical Physics the research activities range from studying fundamental aspects to applications. Strong emphasis is given to the deployment of modem physical methods in chemistry and biochemistry, which include principles of mass-spectroscopy and ion-cyclotron resonance environmental analyses and theoretical and experimental particle physics. 

Several techniques have been used to investigate the reactivity of the metal carbide cluster ions formed in a laser vaporization source. The earliest investigations performed by Castleman s group relied on a preliminary mass selection of the desired cluster. The ion beam was then injected into a drift tube where the selected cluster encounters the reactant mixed with helium as a buffer gas. The FTICR (Fourier-transform ion cyclotron resonance) mass spectrometer studies reported by Byun, Freiser and co-workers basically rely on the same principle even though the total pressure of the reaction chamber is 10 torr, compared with 0.7 torr in Castleman s experiments. A new method of forming met-car ligand complexes was then reported by Castleman et al. this involved the direct interaction of the vaporized metal with mixtures of methane and selected reactant gases. ... 

Ion cyclotron resonance spectroscopy is responsible for some of the increase in pubUcations about ion-molecule collision processes during the last decade. Although ion cyclotron resonance has been known in principle for some time, specially designed apparatus for scientific applications has only recently become available. The technique is now well established and the ion cyclotron double-resonance (ICDR) technique, in particular, has furnished the means for rapid surveys of interesting ion-molecule interactions. The flexibility of ICR techniques makes them suitable for many applications. 

Fourier Transform Mass Spectrometer (FTMS) Fourier transformation (FT) of time-dependent image from the detector to m/z intensity is utilized for two types of mass spectrometers ion cyclotron resonance (ICR) and Orbitrap. FTICR mass spectrometers operate based on the ion cyclotron resonance principle ions in a magnetic field (B) move in circular orbits at frequencies (ft>c) characteristic of their m/z values as shown below (Marshall et al., 1998,2002) ... 

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