Sector-type mass spectrometer

A multipoint ion collector (also called the detector) consists of a large number of miniature electron multiplier elements assembled, or constructed, side by side over a plane. A multipoint collector can be an array, which detects a dispersed beam of ions simultaneously over a range of m/z values and is frequently used with a sector-type mass spectrometer. Alternatively, a microchannel plate collector detects all ions of one m/z value. When combined with a TOP analyzer, the microchannel plate affords an almost instantaneous mass spectrum. Because of their construction and operation, microchannel plate detectors are cheaper to fit and maintain. Multipoint detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of any substance are available. For such fleeting availability of ions, only multipoint collectors can measure a whole spectrum or part of a spectrum satisfactorily in the short time available. 

In parallel with the development of the EOID for focal plane mass spectrometers of the Mattauch-Herzog type, similar devices were developed for use with conventional sector-type mass spectrometers (15, 16, 17, 18). A schematic representative of this type detector, versus that implemented on a CEC type 21-490 single focusing mass spectrometer, is shown in Fig. 8. The main differences between these two applications of the EOID are a result of the differences in the ion optics of the two types of mass analyzers, as shown in Fig. 1. First, the detector of a sector type instrument resides outside the magnetic fringe field, thus eliminating the need for angling the primary ion sensors. 

DESCRIPTION OF THE EOID FOR SECTOR TYPE MASS SPECTROMETERS... 

Analytical research applications of mass spectrometers fitted with electro-optical ion detectors are carried out at the present time at JPL and the FOM Institute. These involve" (1) at JPL a 1" EOID on sector type mass spectrometer (CEC type 21-490) in conjunction with an API (armospheric pressure ionization) source for the development of an automated assay procedure for amino acid derivatives produced by a novel Edman type sequenator (23), and (2) at the FOM Institute a 1" EOID on a sector mass spectrometer,fitted with magnetic and electrostatic quadruple "Zoom" lenses, permitting variation of dispersion for focusing a greater number of masses on the 1" detector (24,25). 

Measurement of the evolved He is made by peak height comparison with standard gases on sector-type mass spectrometers such as the MAP 215-50 and VG-3600 (e.g.. Wolf et al. 1996a, Warnock et al. 1997), or by He isotope dilution (ID) on a quadrupole mass spectrometer (QMS). We find that the precision and sensitivity of the ID-QMS technique are superior to those of the sector MS-peak height method. Reproducibility of gas standards suggests that for typical amounts of He evolved from a sample (e.g., of order 1 x 10 cc STP), the ID-QMS technique has a precision of -0.5% (la). The accuracy of this measurement depends on the accuracy of the standard used for calibration, which is probably better than 1% when capacitance manometry is used. 

The use of a smface as collision target was first demonstrated by Cooks et al. in a sector-type mass spectrometer by dissociation of CO-. These authors concluded that SID resulted in a fragmentation spectrum very similar to that of high-energy CID of the same species, and they highUghted experimental convenience as the major advantage of SID. Since this first implementation, SID has been demonstrated in a variety of mass spectrometers, including quadrupole, TOF, hybrid Q-TOF, and FT-lCR. " A crucial aspect of... 

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. 

Figure 1.15 Commercial 60° magnetic sector field mass spectrometer (Type CH4, Atlas MAT) for analysis of gases. (77. Kienitz (ed.), Massenspektrometrie (1968), Verlag Chemie, Weinheim. Reproduced by permission of Wiley-VCH.)...

A special experimental arrangement of double-focusing sector field mass spectrometry - the Mattauch-Herzog type mass spectrometer has double-focusing properties for ions of all masses... 

In commercial instrumentation fairly low-cost quadrupole mass spectrometers and also expensive double-focussing sector field mass spectrometers are usually used (for a survey of mass spectrometers for analytical use, see Ref. [71]) and today new types of mass spectrometers such as time-of-flight mass spectrometers are being find utilized in plasma atomic spectrometry. 

Figure 1 shows a magnetic-type sector field mass spectrometer coupled with a Knudsen cell. The most important part of the instrument is the Knudsen cell. It can be heated up to temperatures above 2500 K. The temperatures are measured with an optical pyrometer or a thermocouple. There would be thermodynamic equilibrium in the Knudsen cell if it were closed. However, real Knudsen cells have an effusion orifice (typical diameter 0.1 to 1 mm) through which a small fraction of the molecules effuse without practically disturbing the equilibrium in the cell. A molecular beam representing the equilibrium vapor in... 

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. 

Approximately 10% of the ICP-MS instruments used worldwide are equipped with a double-focusing sector field mass spectrometer instead of the more traditional quadrupole filter. The use of this type of mass spectrometer requires the ions to be accelerated over a potential difference (e.g., 8000 V) before being introduced into the mass spectrometer [14, 15]. By reducing the width of an entrance and exit slit, the mass resolution can be increased (Figure 2.21). Whereas some instrument types allow continuous variation of the mass resolution, other offer a selection between a limited number of predefined resolution settings only. 

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