Coupling of a Rydberg electron capture ion source with a quadrupole mass filter
Résumé
The coupling of a Rydberg electron capture ion source with a Nermag R10-10H quadrupole mass filter is described. Details are given of the addition to this instrument of a creation cell for atoms excited in Rydberg states. Within the Nermag ion source, such atoms allow attachment of electrons of well-defined thermal energy. SF 6 was used for optimization of the main experimental parameters (gas pressures and voltages applied to the electrodes). The procedure by which Rydberg electron attachment was confirmed is described. A polychlorobiphenyl compound was used to illustrate the performance of this ionization technique. Ion formation was observed in the absence of fragmentation. The real-time analysis of a sample from a mass spectrum presents numerous problems when a small quantity of complex molecules in an atmospheric matrix are to be analyzed. In particular, the number of ion fragments produced must be minimized and the presence of the molecules of interest must be ascertained with high specificity while the molecules of the matrix are disregarded. Current ion sources tend to limit fragmentation, for example by proton transfer in positive ion chemical ionization (PICI), 1,2 by thermal electron attachment in electron capture chemical ionization (ECCI), 3 or by means of an electron monochromator (EM). 4 Our Rydberg electron capture (REC) ion source comes within the perspective of a new low-fragmentation negative ion producing source. The ion source has been validated previously on several pollutants 5 using a quadrupole ion trap operated in a non-conventional mode. 6 It was decided to integrate such an ion source with a commercial apparatus. In this way, it was possible to obviate the problems of developing a new mass analysis apparatus 7 and to concentrate on the study of the ion source. In this paper, we present the results obtained after integrating an REC source in a quadrupole mass filter instrument, the Nermag R10-10H. Numerous molecules that are of interest in the environmental field 8,9 can attach an electron by resonant capture provided they have a sufficiently positive electron affinity. In general, many such molecules can attach electrons whose energy can be positive (free electrons) or negative (bound electrons) in a thermal energy range having an absolute value <1 eV. In this work, weakly bound electrons of xenon atoms excited in Rydberg states (designated as Xe**) created by electron bombardment were used. 10 The xenon atoms are excited to different energy levels and the corresponding Rydberg electrons have an energy dispersion of several tens of milli-electron volts. This technique is more straightforward and less costly than that which uses an electron monochromator, for example, for a lower energy dispersion of the electrons. Another great advantage of using Rydberg atoms is the possibility of maintaining at a constant value the energy of the electron even while it is going through zones subjected to weak electric fields; this experimental condition is impossible to achieve with free electrons. The REC process by a molecule MX is expressed by: MX Xe ÃÃ 3 MX ÀÃ Xe 1 The negative molecular ion produced by the reaction is metastable since it is created in an excited state. The lifetime values obtained by Rydberg electron attachment are greater than that which might be expected for attachment of a free electron. The ion is in a less excited state as a result of the negative energy of the Rydberg electron and of the presence of the 'third body' Xe (ion core of Xe**) involved in reaction (1). This Xe ion can stabilize the anion further when the energy level n of the electron is lower than 15. 11,12 Moreover, in the case of polyatomic molecules, the internal energy of the anion is redistributed over a large number of vibrational degrees of freedom. The lifetime is thus enhanced all the more. The electron attachment cross-section a is a function of the electron affinity and gives an indication of the ease with which the anions can be created. Generally, the cross-sections are not well known in the case of low-energy free electrons and Rydberg electrons. Various experimental and theoretical works concerning the Rydberg electron attachment rates on SF 6 molecules, and summarized by Dun-*Correspondence to: