Spectrometer double-focusing mass

The double-focusing mass spectrometer is arguably the finest instrument for obtaining molecular and elemental compositions. 

A double-focusing mass spectrometer can mea.sure mass accurately to several decimal places, thus enabling the determination of molecular formulae and elemental compositions of fragment ions. 

Mattauch-Herzog geometry. An arrangement for a double-focusing mass spectrometer in which a deflection of n/A radians in a radial electrostatic field is followed by a magnetic deflection of nil radians. 

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. 

When the first quadrupole of a triple quadrupole is replaced by a double-focusing mass spectrometer, the instrument is termed a hybrid (i.e. a hybrid of magnetic sector and quadrupole technologies). Figure 3.9 shows the MSi unit as a forward-geometry instrument although there is no reason why this could not be of reversed- or even tri-sector geometry. 

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. 

Forward-geometry double-focusing mass spectrometer A double-focusing... 

Four-sector mass spectrometer A mass spectrometer used for MS-MS studies consisting of two double-focusing mass spectrometers in series. 

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. 

Positive ion FAB mass spectra obtained with a double focusing mass spectrometer produced abundant molecular ions ([M] +) of carotenes and xanthophyUs with minimal fragmentation and no detectable thermal decomposition. Fragmentation of the precursor ion was enhanced by collision-induced dissociation (CID) using helium gas. ... 

Requires expensive double-focusing mass spectrometer... 

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

Double-flash generating plant, 12 531 Double focusing mass spectrometers, 15 664... 

Figure 2.15 A double-focusing mass spectrometer. (After Beynon and Brenton, 1982 Fig. 4-9, by permission of University of Wales Press.)...

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