Image current detector

Ion detectors can be divided into two classes. Some detectors are made to count ions of a single mass at a time and therefore they detect the arrival of all ions sequentially at one point (point ion collectors). Others detectors, such as photographic plates, image current detectors or array detectors, have the ability to count multiple masses and detect the arrival of all ions simultaneously along a plane (array collectors). 

Figure Bl.7.18. (a) Schematic diagram of the trapping cell in an ion cyclotron resonance mass spectrometer excitation plates (E) detector plates (D) trapping plates (T). (b) The magnetron motion due to tire crossing of the magnetic and electric trapping fields is superimposed on the circular cyclotron motion aj taken up by the ions in the magnetic field. Excitation of the cyclotron frequency results in an image current being detected by the detector electrodes which can be Fourier transfonned into a secular frequency related to the m/z ratio of the trapped ion(s).

For FT-ICR-MS, the ICR cell itself must not necessarily differ from one used in scanning ICR-MS (Fig. 4.51). Two of the four side walls (jc-axis) of the ICR cell are connected to the RF power supply during the period of excitation. Then, the image current induced in the detector plates (y-axis) is recorded as transient signal for some period of time (0.5-30 s). The excitation of the ions within the ICR cell has to stop at a level low enough to avoid wall collisions of the lightest ions to be measured. [191,200,201]... 

Note In ICR cells, the ions circulate like separate swarms of birds rather than like matter in the rings of Saturn. If ions of the same m/z non-coherently circulated at the same frequency and radius, but occupied the total orbit rather than a small sector of it, there would be no image current induced upon their passage at the detector plates. 

There are two basic types of detectors used to measure ion signals, current detectors and ion counters. Each type has different implementations. A third type of detector is an imaging detector. In some SIMS instruments, the mass spectrometer is also an ion microscope, which transmits a stigmatic image of the sample to a detector plane. 

Because the number of ions leaving the mass analyser at a particular instant is generally quite small, significant amplification is often necessary to obtain a usable signal. Indeed, 10 incident ions per second at the detector corresponds to an electric current of 1.6 x 10 18 A. In consequence, subsequent amplification by a conventional electronic amplifier is required. Furthermore, with the exception of Faraday cup and image current detection, the other detectors multiply the intensity of the signal by a cascade effect. 

Recently, these studies have led to the production of laboratory diffractometers. Stachs and his collaborators [STA 00], for example, bnilt a device eqnipped with a rotating anode generator and a cylindrical two-dimensional imaging plate detector. The maximum size of these detectors, currently the largest on the market, is close to 30 cm in diameter. Despite this size, the measnrement of diffraction peaks corresponding to relatively small interplanar distances can only be achieved, with this type of device, if the incident beam s wavelength is small. These anthors used the Ka radiation of a silver anode tnbe. However, this choice leads to a significant overlap of the peaks. Other authors performed the same type of measurements with... 

Ion Detection A conventional approach to ion detection is to place a detector just outside the end-cap electrode with more openings. Another means of ion detection is to monitor the image current. Using the principle of resonant excitation, the ions are promoted to higher oscillations with the use of a supplementary fast dc pulse to bring them closer to the detection plates. This nondestructive procedure of detection of ion current is conceptually similar to that used in FT-ICR-MS (Section 3.8.1) [42]. 

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