EJMS RSS Feed

Electrospray tandem mass spectrometry of β-nitroalkenyl meso- tetraphenylporphyrins

β-Nitroalkenyl meso- tetraphenylporphyrins [β-TPPCHC(NO₂)R)], as free-bases and Zn(II) complexes, were studied by electrospray mass spectrometry (ESI-MS). Under this ionisation condition the [M + H]⁺ ions are formed. The fragmentation pattern of the resulting [M + H]⁺ ions were studied by electrospray tandem mass spectrometry (ESI-MS/MS). The ESI-MS/MS of β-nitroalkenylporphyrins, either as free-bases or as Zn(II) complexes, show several interesting features, distinct from the typical behaviour of nitro compounds. For the studied compounds, common main fragmentation patterns are observed, namely characteristic losses of NO₂^•, HNO₂, 2OH^•, RNO₂, RCNO, RCNO₂, RCH₂NO₂, C₆H₅^• plus NO₂^• and the formation of the protonated macrocycle, [TPP + H]⁺ or [ZnTPP + H]⁺. However, depending on the presence or absence of the metal and the nature of the R substituent, important differences are observed on the relative abundances of the ions formed by the same fragmentation pathway. The presence of bromine in the alkenyl group leads to a peculiar behaviour, since the main fragmentation pattern corresponds to the combined elimination of the bromine atom with the typical nitro group fragments. When R = Br, the loss of the nitro group occurs in low relative abundance (11–16%). However, when R = CH₃, the relative abundance of the ion due to the loss of HNO₂ changes drastically from 100%, observed for the free-base porphyrin, to 29% in the case of the Zn(II) complex. These variations of the relative abundance of the fragment corresponding to the loss of the nitro moiety (typically considered as a diagnostic fragment) can induce to an erroneous interpretation of their MS/MS spectra. Some fragmentations are observed only for the free-base porphyrins, namely the loss of ^•CH(NO₂)R and HNO₂ plus C₂H₂, while the loss of OH^•, H₂O, OH^• plus H₂O and RCCH plus H₂O is observed only for the complexes. Unusual and unexpected fragmentations are also observed, namely the losses of RCNO, RCNO₂ and HNO₂ plus C₂H₂. This work demonstrates that valuable structural information about theβ-nitroalkenyl substituents linked to meso-tetraarylporphyrins can be achieved using MS/MS. These results can also be useful for the interpretation of the mass spectra of other nitroalkenyl substituted compounds.

Structural characterization and differentiation of isomeric ω- bromoalkoxy ...

Mass spectrometry with electron ionization and electrospray ionization have been applied to characterize and differentiate the isomeric ortho-, meta- or para-(E)-bromoalkyloxychalcones 1–15. The difference in the values of µ1–µ5 (i.e. the ratio of abundances of the selected fragment ions to those of the molecular ions) in the series of isomeric chalcones studied and the so-called “in-source” fragmentation induced by increasing cone voltage have been found to be important and reliable indicators differentiating the isomers studied.

Differentiation of 2-alkylthioorotic acids, methyl and ethyl 2-alkylthioorotates ...

Electron ionization mass spectra of 12 derivatives of 2-thioorotic acid have been discussed and general fragmentation routes of their molecular ions have been proposed. The compounds under discussion were three groups of four chemical species (2-alkylthioorotic acid, methyl 2-alkylthioorotate, ethyl 2-alkylthioorotate and 2-alkylthioorotic hydrazide) each with the same relative molecular mass. The comparison of selected ions relative abundances and their correlation with the abundance of molecular ions enable differentiation between isomeric or isobaric species in this class of compounds.

Electron-transfer-induced decomposition of 1,2-dioxetanes in negative-mode matrix-assisted ...

1,2-Dioxetanes bearing an aromatic electron donor undergo intramolecular charge-transfer-induced chemiluminescence (CTICL). Although there has been some controversy regarding the mechanisms involved, there is little experimental evidence to strongly support any of the proposed mechanisms. In the course of our investigations, to clarify these mechanisms, we tried to effectively ionize dioxetanes bearing a phenolic group and found that poly(3-octylthiophene-2,5-diyl) was a promising matrix for negative-mode matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS). Electron-transfer ionization was found to take place for dioxetanes bearing a hydroxyphenyl moiety that had been further substituted with an aromatic group, which acted as an antenna to catch an electron from the matrix. Furthermore, the characteristic fragmentation of dioxetanes 3c–3d was thought to occur by the elimination of 2-methyl-1-propene (56 u) and pivalaldehyde (86 u) from deprotonated ion [M – H]^– of dioxetanes, based on the results of muliple mass spectrometry measurements of dioxetanes using MALDI quadrupole ion trap ToF-MS. Based on a comparison of fragmentation in dioxetanes and the corresponding keto esters, dioxetanes were presumed to initially generate excited keto esters from which fragmentation took place.

Ion optical evaluation of a miniature double-focusing mass spectrograph

A new miniature double-focusing mass spectrograph has been designed and constructed. The ion optical system was designed based on Mattauch–Herzog geometry. The mass spectrograph employs a focal plane detector consisting of a microchannel plate, a phosphor layer, a fiber-optic plate and a charge-coupled device. For the evaluation of the ion optics of the instrument, the energy and angular focal planes were investigated both experimentally and by simulation. Double focusing was satisfactorily achieved along a straight line over a wide mass range, and the experimental and simulated results were mutually consistent. A second-order element of the transfer matrix was also measured experimentally and proved to be in good agreement with the simulated result.

Misphasing of ion motion in quadratic potential induced by space-periodic disturbance

An ideally quadratic potential used in different types of ion mass analyzer such as Paul trap, Kingdon trap and quadratic field reflectron may be space-periodically disturbed due to inaccuracy of fabrication and design features. If ion motion in such devices is computer-simulated, disturbances of potential may be caused by the peculiarity of the computation method. The problem investigated in this work is the effect that weak space-periodic disturbance of a quadratic potential takes on the ion motion in such a potential. The effect of the disturbance we considered is the misphasing of an ion cloud oscillating in a disturbed quadratic potential. A method to evaluate the characteristic misphasing time is presented. For the case of disturbance amplitude being constant along ion trajectories, the designated problem may be considered analytically. If the disturbance amplitude depends on oscillation co-ordinate, the result can be obtained by use of numerical integration. An example of numerical calculation is presented.