Double focusing mass analysers

Figure 7.6 Double focusing mass analyser, Nier-Jordan geometry.

Fig. 3.2. High-resolution double-focusing mass analyser with Nier-Johnson geometry. The electrostatic sector filters ions according to kinetic energy before magnetic sorting according to m/z. The combination of electric and magnet fields produces narrow peaks and high spectral resolution.

The double-focusing mass spectrometer consists of both magnetic sector and electrostatic analysers (ESAs), the latter being a device which focuses ions with the same m jz values but differing energies. The extent to which the beams of ions of closely similar m jz ratios overlap is thus reduced so that in many cases they may be separated. This then allows their mjz ratios to be determined with more accuracy and precision and the atomic composition of the ion to be determined. 

Double-focusing mass spectrometer A mass spectrometer consisting of electrostatic and magnetic sector analysers capable of achieving high-mass spectral resolution. 

Electrostatic analyser (ESA) An energy-focusing device used in a double-focusing mass spectrometer to increase mass specnal resolution. 

Linked scanning A series of techniques in which the electrostatic analyser voltage and magnetic field strength of a double-focusing mass spectrometer are scanned to obtain MS-MS spectra. 

Mass-analysed ion kinetic energy spectrometry (MIKES) A form of MS-MS product-ion scan that may be carried out on a reversed-geometry double-focusing mass spectrometer. 

Reverse-geometry double-focusing mass spectrometer A double-focusing mass spectrometer in which the magnetic analyser precedes the electrostatic analyser. 

Double-focusing mass spectrometer with EB configuration in which a deflection of 90° electrostatic analyser (E) is followed by a 90° magnetic analyser (B) (or reverse geometry - BE). 

Greater resolution can be achieved when a magnetic analyser is coupled with an electrostatic analyser (in a double focusing" mass spectrometer).. Using this combination of analysers, mass accuracies of around 1 part per million (ppm) can be obtained. However, magnetic sector instruments have relatively low sensitivity and are very expensive. 

Figure 16.5—Magnetic analyser mass spectrometer, a) Nier Johnson system, b) directional focusing properties of the magnetic field, c) principle of a double focusing mass spectrometer d) Mattauch-Herzog system.

Low Voltage Mass Spectrometry of Macrocyclic Extractables. Nominal parent mass analyses of the macrocyclic extractables, obtained from typical polymerization products of cyclooctene and 1,5-cyclooctadiene, were performed on a Model MS-9 double focusing mass spectrometer (Associated Electrical Industries, England) at a resolution of 1/1000 and an emission of 7.0 e.v. Samples were introduced directly into the source chamber by the direct-probe technique. The temperature range during the experiment was 125°-200°C., and the source pressure was maintained in the 0.1-3 X 10-6 torr range. In addition, a high resolution measurement was carried out on the mass number 220 to identify the two components present at that mass number. 

Positive ion fast atom bombardment (FAB) mass spectra were obtained with the JEOL HXllO/HXllO tandem double-focusing mass spectrometer with JEOL gun and collisionally induced decomposition (CID) MS/MS. The JEOL instrument was operated at +10 kV with -20 kV postacceleration at the detector. The xenon neutral beam had 6 kV acceleration from the JEOL gun. CID MS/MS was performed with 1 1000 resolution in both MS-1 and MS-2. Helium was used as the collision gas at a pressure sufficient to reduce precursor ion abundance by 75%. Samples were dissolved in methanol-glycerol (1 1, v/v) for all FABMS analyses. 

MS and proton NMR spectroscopy have mainly been used for structure elucidation of isolated compounds. However, there are some reports on mass spectrometric analyses of essential oils. One example has been presented by Griitzmacher (1982). The depicted mass spectrum (Figure 2.9) of an essential oil exhibits some characteristic molecular ions of terpenoids with masses at miz 136, 148, 152, and 154. By the application of a double focusing mass spectrometer and special techniques analyzing the decay products of metastable ions, the components anethole, fenchone, borneol, and cineole could be identi ed, while the assignment of the mass 136 proved to be problematic. 

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