Mass spectrometry primary ionization

SIMS refers to the mass spectrometry of ionized particles (secondary ions) emitted by a beam of primary ions bombarding a surface. SIMS provides a characterization of a target surface by means of mass spectra, depth profiles, and secondary ion images. Surface mass spectra allow the identification and quantification of all constituent elements, isotopes, and molecular species... 

Static Secondary Ion Mass Spectrometry (SIMS) involves the bombardment of a sample with an energetic (typically 1-10 keV) beam of particles, which may be either ions or neutrals. As a result of the interaction of these primary particles with the sample, species are ejected that have become ionized. These ejected species, known as secondary ions, are the analytical signal in SIMS. 

J. A. Loo, C. G. Edmonds, and R. D. Smith. Primary Sequence Information from Intact Proteins by Electrospray Ionization Tandem Mass Spectrometry. Science, 248(1990) 201-204. 

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

K.L.Johnson, T.D. Veenstra,J.M. Londowski, A.J.Tomlinson, R. Kumar, S. Naylor, On-line sample clean-up and chromatography coupled with electrospray ionization mass spectrometry to characterize the primary sequence and disulfide bond content of recombinant calcium binding proteins, Biomedical Chromatography 13 (1999)37-45. 

Fig. 9.4. Simple illustration of an instantaneous collision cascade generated as a result of primary particle impact in desorption/ionization mass spectrometry. Adapted from Ref. [24] by permission. John Wiley Sons, 1995.

More advanced techniques are also available. For instance, mass spectrometry (MS) can be used by converting neutral radicals to ions, via chemical ionization and electron impact these ions are then separated and detected according to their mass-to-charge ratios (m/z). The primary drawback of this method is that it cannot directly... 

This interpretation was proved correct by considering the oxidation of a sample of diphenylmethane that had an isotopic purity of 97.0% a,a-dideuterio and 2.7% a-deuterio by mass spectrometry. The oxidation rate observed after the initial 15-second period (see Figure 2), during which the undeuterated and monodeuterated material were destroyed, yielded a second-order rate constant, ki = 0.0148 mole"1 per second. There is thus an appreciable isotope effect ku/kD of about 6 in the ionization of diphenylmethane by potassium ferf-butoxide in DMSO(80%)-tert-butyl alcohol (20% ) at 25°C. This compares with a value of fcH/ D of 9.5 reported for the ionization of triphenylmethane (16). The observation of primary isotope effects of this magnitude requires that the protonation of the diphenylmethide ion by tert-butyl alcohol in DMSO solution does not proceed at the diffusion rate which would, by the principle of microscopic reversibility, require the absence of an isotope effect in the deprotonation step. 

Over the last decade or so, new ionization techniques such as ES and MALDI have been introduced and have increased still further the use of mass spectrometry in biology. Identification of proteins and characterization of their primary structure is a rapidly growing field in the postgenomic era. ES ionization was the first method to extend the useful mass range of instruments to well over 50 kDa (Mano and Goto... 

Resonance ionization mass spectrometry as a combination of resonance laser ionization with mass spectrometry can be performed on gas atoms only. Therefore, in RIMS of solid samples, before resonance ionization, a neutral gas has to be produced using several methods known from solid state mass spectrometry. During the evaporation of solid material, e.g., by laser evaporation, thermal evaporation or by sputtering with a primary ion beam, the formation of ions should to be avoided. In RIMS, mostly the thermal evaporation of sample from a heated W or Re filament is applied. 

For molecules containing atoms of high electron affinity the photocurrent at the first threshold may be due to a dissociation into ions. This phenomenon was already demonstrated in the early 1930 s by Terenin and Popov28 for TIHal vapors, which split into T1+ + Hal - as a primary photoprocess. Such was the first instance of the application of mass spectrometry to the study of the photoionization of gases. A similar process has been later shown by Morrison et al.8 for Br2 and I2, the first threshold, corresponding to a pre-ionization, accompanied by the dissociation into Hal+ + Hal-. 

Insofar as electron impact mass spectrometry is concerned, the cycloproparenes almost always display a molecular ion. The primary source of fragmentation is by loss of a C(1) substituent radical to provide a cycloproparenyl cation. However, labelling studies have shown that loss of H from 1 and 11 (at least) occurs only after complete scrambling of the carbon atoms216217. The use of appearance energy measurements218 for the loss of Br from ionized 2-bromocyclopropabenzene (57a), when coupled with thermochemical data, have led to the prediction that cation 110 is more stable than the phenyl cation by at least 27.6 kcal mol"1 AHf of 110 is estimated at 311 kcalmol"1. 

Evtuguin, D. V., and Amada, F. M. L., 2003, Application of electrospray ionization mass spectrometry to the elucidation of the primary structure of lignin, Macromol. Biosci. 3 339-343. 

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