Ion Detection Systems

When the separated ion beams leave the mass analyzer system the ions are collected and detected using an appropriate ion detection system inserted in the ultrahigh vacuum of the mass spectrometer. Ion currents at the exit of the mass analyzer are in the range of 10 to 10 A. The registration of both high and very small ion currents requires special fast ion detection systems as discussed in this chapter. 

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A mass spectrometer consists of four basic parts a sample inlet system, an ion source, a means of separating ions according to the mass-to-charge ratios, ie, a mass analyzer, and an ion detection system. AdditionaUy, modem instmments are usuaUy suppUed with a data system for instmment control, data acquisition, and data processing. Only a limited number of combinations of these four parts are compatible and thus available commercially (Table 1). 

The ion detection system consists of a high-gain electron multiplier and the signal digitizing system, along with a computer for data acquisition and manipulation. 

The basic components include a Nd YAG pulsed laser system which is coaxial with a He Ne pilot laser and visible light optical system. The latter system enables the analytical area of interest to be located. The TOF-MS has a flight path of 2m in length, with an ion detection system that includes an electron multiplier detector, a multichannel transient recorder, together with a computer acquisition and data processing system. 

This fundamental equation explains that the velocity of heavier ions (iq of ions with mass m,) is lower than of lighter ions (v2 of ions with mass m2, with m, > m2). Equation (10) is used directly in time resolved measurements, for example in time-of-flight mass spectrometers (ToF-MS). The charged ions of the extracted and accelerated ion beam are separated by their mass-to-charge ratio, m/z, in the mass analyzer. Mass-separated ion beams are subsequently recorded by an ion detection system either as a function of time or simultaneously. Mass spectrometers are utilized for the determination of absolute masses of isotopes, atomic weights, relative abundance of isotopes and for quite different applications in survey, trace, ultratrace and surface analysis as discussed in Chapters 8 and 9. 

As an essential part of a mass spectrometer, the ion separation system has the task of separating the fast-flying ions (with different masses m and charges z (with z = n-e) formed in an ion source and extracted from this source using an ion optic system) with respect to their different mass-to-charge (m/z) ratios. The separated ion beams are than supplied to the ion detection system for spatial or time resolved ion detection and registration. The mass spectrum is then the 2D representation of ion intensity as a function of the m/z ratio. 

The exit slit of the magnetic field is located on the imaging curve, e.g., at the focal point A2" (see Figure 3.3). By changing the magnitude of the magnetic force all separated ion beams continuously pass the exit slit and can be detected using a suitable ion detection system (see Chapter 4). 

Different mass spectrometric systems are designed to make use of all the ion sources, ion separation and ion detection systems - as described in the previous chapters. 

The main common parts of an ICP mass spectrometer as discussed above are the sample introduction system, the inductively coupled plasma (ICP) ion source for desolvation, atomization and ion formation of introduced sample material, and the mass spectrometer including the mass analyzer system for separation of extracted ion beams and a fast ion detection system to register separated ion beams as illustrated in Figure 5.1. 

Odanaka et al. [212] have reported that the combination of gas chromatography with multiple ion detection system and a hydride generation heptane cold trap technique is useful for the quantitative determination of arsine, monomethyl-, dimethyl- and trimethylarsenic compounds and this approach is applicable to the analysis of environmental and biological samples. 

In a double-focusing mass spectrometer with an in house built Mattauch - Herzog geometry with ICP or glow discharge ion source, Hieftje et applied an interesting ion detection system... 

A block diagram of the mass spectrometer and the ion detection system is shown in Figme 2. 

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