Magnetic sector field mass spectrometer

A quite different type of mass spectrometer - the first 180° magnetic sector field mass spectrometer (see Figure 1.7), with directional focusing of ions for isotope analysis, was constructed by Dempster, independently of other instrumental developments in mass spectrometry, in 1918. 

Double-focusing mass spectrometers with high mass resolution are rather bulky and expensive instruments so that the development of single magnetic sector field mass spectrometers was of significance in the following years. 

Figure 1.14 Schematic of a 60° magnetic sector field mass spectrometer (Nier s mass spectrometer, 1940) 5, - entrance slit S2 - exit slit (H.Kienitz (ed.), Massenspektrometrie (1968), Verlag Chemie, Weinheim. Reproduced by permission of Wiley-VCH.)...

In general, ions generated by thermal ionization possess low initial energies (0.1-0.2eV), therefore mostly single magnetic sector field mass spectrometers are used for ion separation. 

The ions formed are extracted by the extraction lens, EL, perpendicular to the electron beam. Due to the low energy spread of ions formed in an electron ionization source (< 1 eV) one single-focusing magnetic sector field mass spectrometer or quadrupole analyzer is sufficient for... 

Fig. 1. Schematic representation of a Knudsen cell-magnetic sector field mass spectrometer system...

Historically, mass spectroscopy was developed to separate atoms and to determine the masses of isotopes accurately [1]. The predecessor of magnetic sector field mass spectrometers was built in 1918 by Dempster [2]. Later, some of the first instruments for the analysis of molecules [3] were used by analytical chemists in refineries to determine hydrocarbons (the first article in the first issue of the journal Analytical Chemistry dealt with mass spectrometry of a mixture of hydrocarbons [4]). Combinations of infrared and mass spectroscopy were reported shortly afterwards [5]. 

Two types of mass spectrometers are mainly used for GC/MS work magnetic sector mass spectrometers, and quadrupole mass filters. There seems to be no clear preference for one or the other t e of instrument. For GC/MS usually low resolution spectra are sufficient. The molecular peak and the fragment peaks form a typical pattern that can either be interpreted by experience or by comparison of the unknown spectrum in the llbraiy. However, If the comparison method falls in identification of the unknown spectrum, a high resolution magnetic sector field mass spectrometer is needed. 

In principle, a helium leak detector detects the partial pressure of a tracer gas in vacuum with a suitable sensor. Small mass spectrometers have become common as a sensor, especially small magnetic sector-field mass spectrometers with permanent magnets. Also the quadrupole mass spectrometer well known from residual gas analysis can be found in some leak detectors. 

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