Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal desorption source

Wavelength-dependent measurements of the RNA base uracil, undertaken with nanosecond ultraviolet laser pulses, have previously identified a fragment at m/z = 84 (corresponding to the C3H4N2O+ ion) at excitation wavelengths ≤232 nm. This has been interpreted as a possible signature of a theoretically predicted ultrafast ring-opening occurring on a neutral excited state potential energy surface. To further investigate the dynamics of this mechanism, and also the non-adiabatic dynamics operating more generally in uracil, we have used a newly built ultra-high vacuum spectrometer incorporating a laser-based thermal desorption source to perform time-resolved ion-yield measurements at pump wavelengths of 267 nm, 220 nm, and 200 nm. We also report complementary data obtained for the related species 2-thiouracil following 267 nm excitation. Where direct comparisons can be made (267 nm), our findings are in good agreement with the previously reported measurements conducted on these systems using cold molecular beams, demonstrating that the role of initial internal energy on the excited state dynamics is negligible. Our 220 nm and 200 nm data also represent the first reported ultrafast study of uracil at pump wavelengths 3(1ππ*) state. These measurements do not, however, provide any evidence for the appearance of the m/z = 84 fragment within the first few hundred picoseconds following excitation. This key finding indicates that the detection of this specific species in previous nanosecond work is not directly related to an ultrafast ring-opening process. An alternative excited state process, operating on a more extended time scale, remains an open possibility

Fragmentation processes of ionized 5-fluorouracil in the gas phase and within clusters

We have measured mass spectra for positive ions produced from neutral 5-fluorouracil by electron impact at energies from 0 to 100 eV. Fragment ion appearance energies of this (radio-)chemotherapy agent have been determined for the first time and we have identified several new fragment ions of low abundance. The main fragmentations are similar to uracil, involving HNCO loss and subsequent HCN loss, CO loss, or FCCO loss. The features adjacent to these prominent peaks in the mass spectra are attributed to tautomerization preceding the fragmentation and/or the loss of one or two additional hydrogen atoms. A few fragmentions are distinct for 5-fluorouracil compared to uracil, most notably the production of the reactive moiety CF+. Finally, multiphoton ionization mass spectra are compared for 5-fluorouracil from a laser thermal desorption source and from a supersonic expansion source. The detection of a new fragment ion at 114 u in the supersonic expansion experiments provides the first evidence for a clustering effect on the radiation response of 5-fluorouracil. By analogy with previous experiments and calculations on protonated uracil, this is assigned to NH3 loss from protonated 5-fluorouracil

Les innombrables applications de la chromatographie bidimensionnelle en phase gazeuse

National audienceLa chromatographie gazeuse monodimensionnelle (GC)est la technique de choix dans de nombreux laboratoires à travers le monde pour l’analyse de composés volatils ou semi-volatils.Toutefois, la complexité d’un grand nombre d’échantillons empêche fréquemment l’identification de tous les composés.Afin de remédier à ce manque de résolution,plusieurs colonnes capillaires de sélectivité différente ont été couplées pour donner naissance à la GC multidimensionnelle(MDGC),et à une approche plus récente,la GC bidimensionnelle intégrale (GC×GC).La GC×GC permet d’améliorer considérablement le pouvoir de séparation en passant d’une séparation linéaire à une séparation spatiale, ce qui constitue un avantage significatif pour l’analyse qualitative et quantitative de composés ciblés ou non ciblés.Dans cet article sont abordées:les évolutions technologiques clés de la GC×GC,sa capacité de séparation structurée comprenant l’invention de nouvelles colonnes GC,l’analyse énantiosélective,la séparation multidimensionnelle ainsi que les nouvelles tendances pour l'identification de composés inconnus dans des mélanges complexes

Analysis of Outcomes in Ischemic vs Nonischemic Cardiomyopathy in Patients With Atrial Fibrillation A Report From the GARFIELD-AF Registry

IMPORTANCE Congestive heart failure (CHF) is commonly associated with nonvalvular atrial fibrillation (AF), and their combination may affect treatment strategies and outcomes