Mass spectrometers general description

The general description mass spectrometer covers a variety of instruments that vary in size, cost, versatility, performance and operational complexity, but all find specific applications appropriate to their design. The common use of mass spectrometers as chromatographic detectors has resulted in a significant market for low cost instruments with modest performance characteristics that are easy to operate, robust, and require little bench space. Instruments in this category will be the focus of the following sections. 

In general a mass spectrometer consists of an ion source, a mass-selective analyzer, and an ion detector. Since mass spectrometers create and manipulate gas-phase ions, they operate in a high vacuum system. The magnetic-sector, quadrupole, and time-of-flight designs also require extraction and acceleration ion optics to transfer ions from the source region into the mass analyzer. Tables 2 and 3 provide brief descriptions of the most commonly used ionization techniques and the different types of mass spectrometers available, respectively [163,232-235,241,242,244-246]. 

The tandem mass spectrometer is an instrument specifically designed to permit such identifications to be made, and within its limitations, it unquestionably permits the most specific identification of reactant product relationships. However, it generally does not permit truly thermal energy reactions to be observed (kinetic energies between 0.3 and 0.5 eV have been reported), and it is generally limited to the study of primary ion reactions. Hence there are many situations which cannot be examined in a tandem mass spectrometer (see Chapter 11 for a description of the latter). 

In this section the mass analyzers that are important in current practice of trace level quantitation are described in some detail. Before starting the descriptions of specific analyzers, a topic of general applicability (calibration of the m z axis) is discussed. In addition, it should be mentioned here that an extremely important component of any mass spectrometer is the vacuum system this is discussed separately (Section 6.6). Another topic that is important for tandem mass spectrometry is that of colli-sional activation of ions, and discussion is also deferred till later (Section 6.5). 

In VoL 2 of this handbook, the origin of elements has been discussed in detail. Therefore, the present authors will exclude that part, except for some comments on the importance of particular radionucKdes. In this chapter, the principles and instrumentation of accelerator mass spectrometry (AMS), the key player for detection of cosmological radionucKdes in ultra trace scale, will be discussed in detail. Detailed discussion of all the research works carried out to date with cosmogenic radionuclides is out of scope. Only the detection of million-year half-life radionucKdes in ultra trace concentration will be touched, followed by concise description of the required chemistry. Rather than giving a general description, a few of them have been chosen and described in separate sections. Inductively coupled plasma-mass spectrometry (ICP-MS), thermal ionization mass spectrometry (TIMS), secondary ion mass spectrometry (SIMS), or resonant laser ionization mass spectrometer (RIMS), etc. have also been used for detection of cosmogenic radionucKdes. However, these techniques have much lower sensitivity compared to AMS. Brief discussions on these instruments have been appended at the end of this chapter. This chapter ends with a conclusion. 

The mass spectrometers used for isotopic analysis generally comprise three main sections an ion source, a mass analyzer, and an ion collection assembly (Figure 15.5, [1,28]). Barrie and Prosser [2] provide a detailed description of the operation of the IRMS instrument. 

The description of a gas chromatograph modi ed for the snif ng of its ef uent to determine volatile odor activity was rst published by Fuller et al. [63]. In general, GC-olfactometry (GC-0) is carried out on a standard GC that has been equipped with a snif ng port, also denominated olfactometry port or transfer line, in substitution of, or in addition to, the conventional detector. When a FID or a mass spectrometer is also used, the analytical column ef uent is split and transferred to the conventional detector and to the human nose. GC-0 was a breakthrough in analytical aroma research, enabling the differentiation of a multitude of volatiles, previously separated by GC, in odor active and nonodor active, related to their existing concentrations in the matrix under investi gation. Moreover, it is a unique analytical technique that associates the resolution power of capillary GC with the selectivity and sensitivity of the human nose. 

An outline of the process is shown in Fig. 1. Our discussion will include a general description of the hardware to generate the MS raw data (the introduction of the sample at atmospheric pressure into the mass spectrometer, which is under vacuum), the formation of the mass fragments and their separation from one another, focusing, and detection. The raw data thus obtained require some form of evaluation to become intelligible, a process that calls for automatic and/or manual computer work. A few examples of real-life forensic studies will help explain common approaches used in such a pursuit. 

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