Metastable fragmentation

By linking the way in which the first and third quadrupoles are scanned, specific metastable fragments can be detected, which is one form of linked scanning. 

Note The detection of neutrals formed by metastable fragmentation in linear TOP analyzers is one of the few exceptions where neutrals are handled and give rise to a signal in mass spectrometry. 

Isomeric C Hn radical ions fragment not very differently by the different mass spectro-metric methods. The metastable decays are nearly identical, but the collisionally activated spectra of 14 isomeric hexenes, measured by Nishishita and McLafferty240, exhibit some quantitative differences. Bensimon, Rapin and Gaumann251 compared the metastable decay and the photoinduced fragmentation by infrared photons of long-lived parent ions of six hexene isomers and cyclohexane. If the linear isomers are practically identical, some notable differences are observed for branched isomers. Cyclohexane behaves similar to n-hexenes. The metastable fragmentation of H/D-labeled 4-Me-2-pentene, 2-Me-2-pentene... 

A common drawback of these two instruments is the impossibility to observe any metastable fragmentation which occurs in the first mass analyser between the source and the first reflectron. Because these fragment ions penetrate in the first reflectron, they do not arrive at the deflection gate and collision cell at the same time as their precursors. Thus, they do not contribute to the detected fragment ions and are not recorded in the fragmentation spectrum of the selected ion. Since such metastable fragmentations are generally not minor processes, this reduces the efficiency of these mass spectrometers. 

The ion issuing from the metastable fragmentation shows up at the apparent mass rrf, linked to mp and mf through the relation m = ml/mp. The kinetic energy released during the fragmentation brings up a dispersion in velocity, thus an alteration in the resolution. 

Figure 2.55 shows as an example the spectrum of theobromine, the metastable fragmentation of which is shown in the scheme. 

Metastable fragmentations have the advantage that the relation between parent and fragment ion are defined and that—to a certain limit—the kinetic energy released can be determined. Rosenstock and coworkers, after an unsuccessful attempt could only find a weak metastable reaction in (equation 32). 

The Bu ion loses methane and ethylene in its metastable fragmentation. Because the first measurements by Johnson and Langer already showed a positional scrambling of the thermochemical stability of the isomers of 4119, and the kinetics of their formation, interconversion and fragmentation has raised much interest. However, three different categories of problems should be clearly distinguished, since they are sometimes confused ... 

It has been shown above that Bu ions, at least on a long time scale, scramble the carbon and hydrogen atoms randomly, whereas heptyl ions seem to follow some unknown distribution law directed by the way the lowest-energy structure(s) is attained. Practically only olefins are eliminated. The metastable fragmentation of the dodecyl ion, formed in the fragmentation of n-tetradecane (6% C7H14, 17% 30% CgHio 30%... 

One might envision two limiting cases 1) absorption of n-photons by the parent followed by a concerted or a sequential series of dissociations into metastable fragments, or 2) a series of sequential photodissociation steps wherein the daughter species subsequently absorb at the same frequency as the parent to form smaller and smaller fragments. 

MALDI Post-Source Decay The fast-metastable fragmentations have a decaytime constant on the order of the acceleration event and are considered a nuisance for the MALDI-TOF-MS analysis of large oligonucleotides. In contrast, the... 

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