Absolute partial and total electron ionization cross sections of uracil

Communication: Electron ionization of DNA bases

“…The measured cross sections fail to conform to this thumb rule as they range from 8 A 2 for cytosine (mass = 111 amu) to 32 A 2 for guanine (mass = 151 amu). Our measurements on uracil 29 have also stressed the need for measurements on the DNA bases, as they gave cross sections about a factor of two larger than previously reported. 26 The limited absolute partial cross sections [23][24][25] that have been reported are also susceptible to similar uncertainties as in the total ion cross sections.…”

“…29,33 Briefly, it consists of a magnetically collimated and pulsed electron gun, an effusive molecular beam, a Time of Flight Mass Spectrometer (ToFMS), a pair of micro-channel plates (MCPs) in chevron configuration, a Faraday cup, and the associated electronics.…”

“…Details of the measurement procedure used to obtain the ion yield curves as well as absolute cross sections and the estimation of errors in the technique have been discussed for the case of uracil. 29 The estimated maximum uncertainty in the total ion cross sections is 12%. The main sources of this uncertainty are from the vapour pressure data (4%), uncertainty in the detection efficiency for ions over the range of interest (10%), and the cross section for Ar used for normalization to absolute scale (5%).…”

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

“…A noticeable difference with uracil is in the ion yield ratio for these ions. For fluorouracil 86 u / 87 u slowly rises from 0.052 at 40 eV to 0.058 at 100 eV, whereas for uracil 68 u / 69 u slowly rises from 0.39 at 40 eV to 0.42 at 100 eV [33].…”

“…Comparison with the mass spectrum of uracil [12,13,[31][32][33] provides clear indications regarding the main fragmentation pathways of 5-fluorouracil. Comparisons with the mass spectra of 5-bromouracil, 5-chlorouracil and 6-chlorouracil can also be made [11][12][13].…”