Focal plane collector

Focal plane collector a detector for spatially disperse ion beams in which all ions simultaneously impinge on the detector plane. 

The array system is discussed in Chapter 29. With array detection, resolution of m/z values depends both on the analyzer and the collector. Historically, the method for recording ions dispersed in space was to use a photographic plate, which was placed in the focal plane such that all ions struck the photographic plate simultaneously but at different positions along the plate, depending on m/z value. This method of detection is now rarely used because of the inconvenience of having to develop a photographic plate. 

Figure 1. Transmission of Zn isotopes as a function of the collector position along the focal plane. This position is parameterized hy mass M. Transmission is the ratio between the number of ions of mass At, arriving at the collector at mass M divided by the number of atoms of mass At, introduced in the mass spectrometer. The parameter u is the linear mass bias coefficient.

Modem isotope ratio mass spectrometers have at least three Faraday collectors, which are positioned along the focal plane of the mass spectrometer. Because the spacing between adjacent peaks changes with mass and because the scale is not linear, each set of isotopes often requires its own set of Faraday cups. 

Special mass spectrometry systems are built for isotope ratio measurements. Most isotope ratio mass spectrometers consist of a single-focusing magnetic sector instrument and a multiple Faraday cup detection system. Because the Faraday cup exhibits a stable response, it is an ideal detector for isotope ratio measurements. Simultaneous collection of relevant ion beams from all isotopes provides high-precision isotopic measurements. A three-Faraday cup detection system is shown in Figure 7.9. The multicup assembly is placed at the focal plane of the mass analyzer and can be used for the simultaneous detection of each isotopic form of the analyte species (e.g., m/z 44, 45, and 46 from CO2). Commercial instruments with up to nine collectors are available. 

Series II includes the specimen plane. The field iris controls the diameter of the area that is viewed. Pointers or scales must be inserted in the primary image plane. Series I is related to the filament. An image of the filament is found at the back focal plane of the objective. The principle behind the uniform illumination of the specimen is the so-called Kohler illumination. Every part of the filament illuminates every part of the specimen. The correct illumination is obtained by adjusting the collector lens and the condenser. Detailed instructions are provided in microscopy manuals. 

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