Collision-induced dissociation spectroscop

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Other means of manipulating ions trapped in the FTMS cell include photodissociation (70-74), surface induced dissociation (75) and electron impact excitation ("EIEIO")(76) reactions. These processes can also be used to obtain structural information, such as isomeric differentiation. In some cases, the information obtained from these processes gives insight into structure beyond that obtained from collision induced dissociation reactions (74). These and other processes can be used in conjunction with FTMS to study gas phase properties of ions, such as gas phase acidities and basicities, electron affinities, bond energies, reactivities, and spectroscopic parameters. Recent reviews (4, 77) have covered many examples of the application of FTMS and ICR, in general, to these types of processes. These processes can also be used to obtain structural information, such as isomeric differentiation. 

In addition to UV/visible flash photolysis and TRIR spectroscopy, other techniques have been used for the detection of transition metal-noble gas interactions in the gas phase. The interaction of noble gases with transition metal ions has been studied in detail. A series of cationic dimeric species, ML" " (M = V, Cr, Fe, Co, Ni L = Ar, Kr, or Xe), have been detected by mass-spectroscopic methods (55-58). It should be noted that noble gas cations L+ are isoelectronic with halogen atoms, therefore, this series of complexes is not entirely unexpected. The bond dissociation energies of these unstable complexes (Table IV) were determined either from the observed diabatic dissociation thresholds obtained from their visible photodissociation spectra or from the threshold energy for collision-induced dissociation. The bond energies are found to increase linearly with the polarizability of the noble gas. 

Traditional methods to map posttranslational modification sites, like those of phosphorylation, have been anchored by protein digest and mass spectroscopic (MS) approaches (for a review on the classic evaluation and for MS analyses of O-glycans, see Reference (56)). Unfortunately, like many posttranslational modifications, O-GlcNAcylation occurs routinely on a protein population with substoichiometric frequency, which results in a very small detectable population of a O-GlcNAc-modified product. Also, much like O-phosphate additions, the protein-O-GlcNAc bond is labile and is detached by collision-induced dissociation (CID) during MS analysis. Often, the bond is lost before it can be detected on the peptides analyzed (57, 58). Phosphate modifications, however, can overcome this limitation by emiching the peptide mixtures... 

Dua et al. employed collision-induced dissociation (CID) mass spectra to produce molecular precursors that were selected to yield OSCN , ONCS , and OCNS (23). When combined with the aforementioned theoretical calculations, the CID spectra provided evidence that OSCN is a product of the oxidation of SCN by H2O2, but it was suggested that some of the other ions that were observed by Arlandson et al. were possibly due to ONCS (or other isomers). In addition to conflicting conclusions that arise when comparing the aforementioned theoretical and spectroscopic studies, it is noted that many of the experimental results that have been previously reported appear to be inconsistent with the estimated lifetimes of h5rpothiocyanite under the conditions of the experiments (vide infra). 

Polfer, N.C. Oomens, J. Suhai, S. Paizs, B. Spectroscopic and theoretical evidence for oxazolone ring formation in collision-induced dissociation of peptides. J. Am. Chem. Soc. 2005,127,17154-17155. 

TABLE XVII. Comparison of Thresholds for Collision-Induced Dissociation of Ground-State Ions with Spectroscopic Dissociation Energies... 

This chapter was intended to give some flavors of molecular dynamics-based methods for the calculation of gas phase vibrational spectra and collision induced dissociations of gas phase molecules and complexes. The examples were taken from our own work. We hope we have convincingly shown the essential ingredients entering these simulations, their usefulness in relation to IR-MPD and IR-PD action spectroscopic experiments, their usefulness in relation to CID experiments, and how essential these methods are to produce definitive assignments and microscopic interpretation of experimental features. 

Introduction. - Fundamental Physical Applications of Laser Spectroscopy. - Two and Three Level Atoms/High Resolution Spectroscopy. - Rydbeig States. - Multiphoton Dissociation, Multiphoton Excitation. - Nonlinear Processes, Laser Induced Collisions, Multiphoton Ionization. - Coherent Transients, Time Domain Spectroscopy, Optical Bistability, Superradiance. - Laser Spectroscopic Applications. - Laser Sources. - Postdeadline Papers. - Index of Contributors. 

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