Ion activation methods

It is convenient to categorize popular excitation methods according to their rate of energy input. Rapid or instantaneous ion activation occurs in roughly s, which is typ- 

Activation of the vibrational energy of ions can also be induced by the absorption of IR radiations. A popular type of IR radiation source is far-IR laser. In fact, many molecules have a broad IR absorption band. The most widely used IR source is a continuous wave (c.w.) CO2 laser, with the wavelength of 10.6 pm. This wavelength corresponds to an energy of 0.3 eV per laser photon. Because decomposition of a chemical bond requires 1 eV, laser excitation has to extended over hundreds of milliseconds to allow ions to absorb multiple IR photons. This method is known as infrared multiphoton dissociation (IRMPD). Another type of similar technique is black-body infrared radiative dissociation 

It is possible to excite ions instantaneously and induce fragmentation before the energy can be redistributed, or when it is only distributed over a small portion of an ion. This option is in contrast to the slow activation processes described above. Such fast heating processes can be achieved by a single high-energy colhsion between an ion and its colhsion partner, irradiation of ions with a UVAds laser [41,42], or by means of charge-neutralization reactions [43,44]. 

Cotter, R. J. (1997) Time of Elight Mass Spectrometry Instrumentation and Applications in Biological Research. American Chemical Society, Washington, DC. 

Guilhaus, M. (1995) Principles and Instrumentation in Time-of-flight Mass-spectrometry - Physical and Instrumental Concepts. J. Mass Spectrom. 30 1519-1532. 

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No tandem MS experiment can be successful if the precursor ions fail to fragment (at the right time and place). The ion activation step is crucial to the experiment and ultimately defines what types of products result. Hence, the ion activation method that is appropriate for a specific application depends on the MS instrument configuration as well as on the analyzed compounds and the structural information that is wanted. Various, more or less complementary, ion activation methods have been developed during the history of tandem MS. Below we give brief descriptions of several of these approaches. A more detailed description of peptide fragmentation mles and nomenclature is provided in Chapter 2. An excellent review of ion activation methods for tandem mass spectrometry is written by Sleno and Volmer, see Reference 12, and for a more detailed review on slow heating methods in tandem MS, see Reference 13. 

L. Sleno and D. A. Volmer. Ion Activation Methods for Tandem Mass Spectrometry. J. Mass Spectrom., 39(2004) 1091-1112. 

As a consequence of these suppositions, CID is an ergodic ion activation method, which allows a redistribution of the energy in the vibrational modes of the ion because the dissociation rate is slower than the rate of energy randomization. In these conditions, the... 

Two other ion activation methods were developed to replace the gas molecules as targets by laser beams (photodissociation or infrared multiphoton dissociation IRMPD) or by electron beams (electron capture dissociation ECD). These two methods can be applied to ions that are trapped during their excitations by photons or electrons, respectively. Thus, they are most often used with ion trap or ICR analysers because the residence time and the interaction time are longer. 

The photodissociation has some advantages over the other ion activation methods. Indeed, this method is selective because only the ions that absorb at the wavelength of the incident radiation can be photodissociated. Furthermore, the amount of energy that is acquired by photodissociation is well defined and the distribution of the internal energy is narrow. 

Table 4.1 Different ion activation methods used in mass spectrometry.

Sleno, L. and Volmer, D.A. (2004) Ion activation methods for tandem mass spectrometry. 

Therefore, ECD is a powerful tool for structural analysis of peptides and proteins that is complementary to the other ion activation methods. However, ECD is not compatible with instruments such as ion traps or QTOF. As a consequence, ECD analysis of peptides and proteins is typically performed on FTICR mass spectrometers. 

Another ion activation method that is well suited for identification and sequence analysis of phosphopep-tides in the positive and negative polarity modes is infrared multiphoton dissociation (IRMPD).105,112 In this technique, phosphopeptides are irradiated with 10.6 pm photons emitted from a C02 laser. The phosphate group behaves like a chromophore for these photons, allowing evenly distributed cleavages in the peptide chain and more sequence coverage than the CID technique.105... 

D Ammann, P Caroni. Preparation and use of microelectrodes and macroelectrodes for measurement of transmembrane potentials and ion activities. Methods Enzymol 172 136-155, 1989. 

Fig. II.9.8 Graphic description of the evaluation of the selectivity coefficient with the help of the fixed interfering ion activity method...

As noted earlier, the ideal mass spectrometer-based activation method would require only a single step. However, as we have learned more about ion activation methods, both their abilities and their limitations, we have arrived at the conclusion that no single ion activation method is best suited for interrogating every possible precursor ion. Hence, the experiments discussed here highlight the utility of mixing multiple activation methods in a single scan as a viable alternative when CID is combined with ETD, ETD-like dissociation is observed wherein rich product ion mass spectra permit identification of precursor ions that have low charge states and m/z-values. 

As an alternative to the methods above described, a further ion activation method was proposed in 1991 [11-14]. In this method, the working point (that is, the point (a, on a QIT stability diagram defined by the magnitudes of the trapping parameters and is moved close to one of the boundaries of the stability diagram this method can be realized with the combined effect of suitable DC and RF potentials applied to the ion trap... 

Comprehensive summaries of ion activation methods can be found in Volume 3 of this series [31,32]. In all of the above methods, precursor ions are mapped at a fixed -value for the duration of the activation period. However, the non-ideal quadrupole field and space charge will cause the ion species secular frequency to deviate away from its theoretical value such that it no longer matches the frequency of the supplemental electric field calculated for the fixed value of q. Due to the lack of a technique... 

Ahmed, F. E. 2009. Utility of mass spectrometry for proteome analysis Part II. Ion-activation methods, statistics, bioinformatics and annotation. Expert Rev. Proteomics 6(2) 171-97. 

In contrast, MFI involves the application of a resonant excitation waveform consisting of several frequency components while the value of is held constant. The frequency components of the waveform bracket the entire anticipated range of the secular frequency so as to compensate for frequency shifts. Other variants of multi-frequency irradiation (March 1998) include random noise, swept frequency and broadband excitation. A rather different ion activation method (Paradisi 1992, 1992a Curcuruto 1992) is accessible with ion trap instruments equipped with a DC power supply so that non-zero values of U and thus a are available. The method involves moving the working point of a given ion species to either the P,. or the p, boundary of the stability diagram... 

In practice the situation is more complex. It is necessary to take into account the average value of Ej , reached by the particular ion activation method used, and also the timescale available for reaction to occur (in general longer for a tandem-in-time (trapping) instrument than for a tandem-in-space (beam) instrument), which in turn determines the range of values of that can produce... 

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