Intact molecular ion formation

Intact Molecular Ion Formation of Some Organic Molecules by Femtosecond Lasers... 

The mechanisms of intact molecular ion formation or fragmentation in intense femtosecond fields are interesting and an important subject to be clari-... 

Cgo has been ionized by pulses from 5 ps to 25 fs the shortest pulse of 25 fs produced Cqq (z = 1,2) without heavy fragmentation [20] at 0.79 pm. Very recently, Hertel and coworkers detected a single peak of the singly charged ions [21], where they irradiated C60 with a 9-fs pulse at 0.8 pm. It should be noted that this wavelength is resonant with the cation electronic level. From the above results, it may be considered that with sub ten fs pulse, intact molecular ion formation is possible for any molecule, even if the excitation wavelength is resonant with the cation absorption levels. 

Intact molecular ion formation has a great advantage for application to femtosecond laser mass spectrometry (FLMS). The Ledingham group has used... 

Intact molecular ion formation of organic compounds is a distinct feature of femtosecond laser excitation, although some molecules show heavy fragmen-... 

Electrospray analysis can be performed in positive and negative ionization modes. The polarity of the ions to be analyzed is selected by the capillary voltage bias. A novel feature of the ESI mass spectrum is the formation of intact molecular ions of the analyte. Fragmentation, if desired, can be induced in the ion-transport region of the ESI source by increasing the sampling cone voltage. This process is known as in-source collision-induced dissociation (CID) or nozzle-skimmer (NS) dissociation. 

Fig. 11.12. Typical appearances of signals representing the intact molecular mass in case of (a) molecular ion formation, (b) protonation, (c) silver cationization, d) molecular ion and protonation, (e) protonation plus alkali cationization, and (f) protonation, ammonium plus alkali adduct formation. The relative abundances of the respective contributions are subject to wide variations. The abscissa gives the corresponding M-hX nominal mass value artificial isotopic patterns are added for more realistic appearance.

Mass analysis determines the mass-to-charge ratio (tn/z) of ions derived from the analyte. For peptide ions, two characteristics can be obtained. The first characteristic is the molecular weight of the peptide, which can be calculated from the measured m/z of the source-generated intact peptide ion (the so-called molecular ion). The second characteristic is structural information that is obtained via an MS/MS analysis. An MS/MS experiment measures gas-phase dissociations of an activated molecular ion of the peptide to yield product-ion data that are diagnostic of the peptide sequence. The basic sequence of events in MS/MS includes 1. mass selection of the peptide ion of interest (that is population of ions of a single m/z) as a so-called precursor ion 2. activation of the precursor ion, most commonly through collisions with an inert gas, followed by dissociation of the activated precursor and formation of product ions 3. mass... 

Laser desorption (LD) is defined as the use of short, intense pulses of laser light to induce formation of intact gaseous molecular ions or neutral molecules by direct removal of very small amounts of material (estimated at some tens of ng) from... 

The ionization typically proceeds in two steps. In the first step (primary ion formation), the matrix absorbs the laser energy. Together with intact macromolecules, the formed matrix ions desorb into the gas phase. This process is very fast and happens in a few nanoseconds. A dense plume is formed in which the second step, the charge transfer from the matrix ions to the maaomolecules, occurs. This is mostly done by a gas phase cation (H, Na, K ) transfer. A quantitative two-step rate equation model of the ionization process was developed by Knochenmuss. This approach was extended by introducing a quantitative molecular dynamics model. According to Karas et al.. ..single charged ions are the lucky survivors.... These ions are accelerated in an electric field of several kilovolts and introduced into the mass analyzer. 

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