International Journal of Mass Spectrometry 232 (2004) 99–105 Electron impact ionization of 5- and 6-chlorouracil: appearance energies S. Denifl, S. Ptasi ´ nska, B. Gstir, P. Scheier, T.D. Märk ∗ Institut für Ionenphysik, Leopold-Franzens Universität, Technikerstrasse 25, A-6020 Innsbruck, Austria Received 13 October 2003; accepted 25 November 2003 Abstract Electron impact ionization of the gas phase modified DNA/RNA bases 5- and 6-ClU was studied using a crossed electron/neutral beams technique in combination with a quadrupole mass spectrometer. 5- and 6-ClU belong to the class of halouracils which are used in radiation therapy to increase the effect of ionizing radiation to tumours, when they are incorporated into cancer tissue. Besides determining the mass spectra for both molecules at the electron energy of 70 eV, the ionization efficiency curves for each parent ion and the most abundant fragment ions were measured near the threshold and the corresponding appearance energies (AEs) were derived using an iterative, non-linear least square fitting procedure using the Marquart–Levenberg algorithm based on the Wannier threshold law. The most abundant cations observed in mass spectra have a threshold value of AE ((C 3 H 2 ClNO) + /5-ClU) = 11.12 ± 0.03 eV and AE ((C 3 H 2 NO) + /6-ClU) = 12.06 ± 0.03 eV. The present AE value for the parent ion of 5-ClU AE((5-ClU) + /5-ClU) = 9.38 ± 0.05 eV is in fair agreement with previous calculations at the B3LYP level of theory. The AE((6-ClU) + /6-ClU) = 9.71 ± 0.05 eV is 0.33 eV higher than that for 5-ClU. © 2003 Elsevier B.V. All rights reserved. Keywords: Electron impact ionization; 5- and 6-CIU; Mass spectra; Appearance energies 1. Introduction The lethal effect of radiation damage for cells is some- times unwanted like in the case of radioactivity, but it is also desirable as in the case of radiation therapy. One problem in medicine is that only the cancerous cell material should be destroyed, but healthy tissue should stay unaffected. One way to control this damage is the application of radio sen- sitizers [1,2] that are incorporated into cancer cells. Cancer tissues doped with these sensitizer molecules will be de- stroyed preferentially under radiation exposure, at radiation doses which are low enough that healthy cell material is un- affected. With this method unwanted side effects for patients can be decreased. Generally the genotoxic effect of high energy radiation (, ,  and heavy ions) with living cells is not only caused by the direct impact of the primary high energy projectiles, but primary particles remove secondary electrons from the molecules along the track and due to subsequent charge transfer and energy dissipating processes neutral or ionic radicals are formed. One third of the damage to the genom ∗ Corresponding author. Adjunct professor at Department of Plasma- physics, Comenius University, SK-84248 Bratislava, Slovak Republic. E-mail address: tilmann.maerk@uibk.ac.at (T.D. Märk). due to ionizing radiation is direct, that means energy is de- posited into the DNA and its closely bound water molecules [3]. Two third of the damage is caused indirectly by free radi- cals that are formed close to the DNA in water molecules and other bio molecules. The most important radical for indirect damage is the very reactive hydroxyl radical [4]. The activity of this radical has been also observed in experiments of low energy photons with dry or hydrated DNA [5] resulting in an increase of DNA strand breaks. Secondary electrons that are created due to the primary particles are produced in large amounts (4 × 10 4 electrons per 1 MeV deposited primary quantum energy [6]) with initial kinetic energies up to about 20 eV [7]. In subsequent inelastic collisions with the medium they are thermalized within 10 -12 s before they become in- active because they reach the stage of solvation. As shown recently by Sanche and co-workers [8] such free secondary electrons in a low energy range near and also below the ion- ization limit are able to induce damage in super coiled DNA like single and double strand breaks. Additionally there ex- ists a variety of experiments concerning electron attachment [9,10] and electron ionization [11,12] of isolated gas phase DNA and RNA components to study the intrinsic behaviour of these molecules during interaction with free electrons. These experiments [8–12] were performed with unmod- ified DNA and RNA molecules. We started recently inves- 1387-3806/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ijms.2003.11.010