In a time-of-flight mass spectrometer, the ions issued from an ion source are accelerated by an electric field and their mass is determined by measuring the time of flight of the ions until they reach a detector.
In the conventional direct type time-of-flight spectrometer, ions are emitted at one end of the spectrometer and are received, after a direct flight, at the other end. It is possible with these spectrometers to mass-analyze all the ions issued from the source, including molecular ions which decompose in flight, after acceleration, giving rise in some cases to neutral species. But the resolution of direct flight spectrometers can often be inadequate.
It is wellknown to improve mass-resolution by lengthening the trajectory of the ions by reflection, using an ion mirror which receives the ions issued from the source and reflects them towards the detector. The ion mirror is formed by a set of parallel grids, spaced from one another and creating an electrical field capable of decelerating the ions and reflecting them. The ions, before being reflected, penetrate more or less deeply into the mirror, depending on their kinetic energy. It is therefore possible, by adapting a configuration of the mirror, to compensate for the difference in velocities of ions of a same mass, so that these ions reach the detector, at the same time, after reflection. But even though the use of a mirror brings some advantages, it does not permit one to carry out a complete analysis of metastable molecular ions which decompose in flight to give neutral species, the latter being obviously not reflected by the mirror.
It has been proposed to overcome this drawback by using a first detector placed in such a way as to receive the ions reflected by the mirror and a second detector placed behind the mirror in order to receive any neutral species present. Accompanying FIG. 1 shows a configuration such as disclosed in an Article by H. Danigel et al., published in the "International Journal of Mass Spectroscopy and Ion Physics", Vol. 52, Nos. 2/3 September 1983, pages 223-240, Elsevier Science Publishers Amsterdam (NL). The mirror M is tilted at 45.degree. on the trajectory of the ions issued from source S, to reflect the ions towards a detector D1, in a direction perpendicular to the direction of emission, whereas the neutral species and the ions with sufficient kinetic energy to go through the mirror, are received by a detector D2.
This known construction presents a number of drawbacks.
First, it is, in practice, impossible to use the mirror to compensate for the differences in the ion's velocity. Moreover, the study of metastable ions would require a mirror capable of reflecting ions having quite different masses ranging from the mass of the non-decomposed ion to the masses of ionic fragments issued from in-flight decomposition. It would then be necessary to have a mirror of relatively substantial depth and the reflected ion trajectories would be at substantial distances one from the other, depending on the depth of penetration into the mirror. In order to be able to intercept all the reflected ions, it would then be necessary to have a detector D1 of large dimensions, which is difficult, if not impossible, to produce.
The use of a mirror of small depth to reflect ions whose kinetic energy is situated within a fairly wide range means that an intense electrical field is created in the mirror, which causes a sudden reflection. The differences in the dwelling times inside the mirror are then small, even for ions of very different kinetic energy. As a result, for metastable ions, the difference is extremely small between the time of flight of a non-decomposed ion and that of an ionic fragment after decomposition in flight, the complete ion and the ion fraction reaching the mirror with the same velocity. It is then impossible to conduct an accurate study of the metastable ions which implicates that this time-of-flight difference has to be measured.