Double-focusing instruments

Certain regions of a mass spectrometer have no electric or magnetic fields to affect an ion trajectory (field-free regions). Figure 32.3 illustrates three such regions in a conventional double-focusing instrument. 

Peak matching can be done on quadrupole and magnetic-sector mass spectrometers, but only the latter, particularly as double-focusing instruments, have sufficiently high resolution for the technique to be useful at high mass. 

Mass spectrometer configuration. Multianalyzer instruments should be named for the analyzers in the sequence in which they are traversed by the ion beam, where B is a magnetic analyzer, E is an electrostatic analyzer, Q is a quadrupole analyzer, TOP is a time-of-flight analyzer, and ICR is an ion cyclotron resonance analyzer. For example BE mass spectrometer (reversed-geometry double-focusing instrument), BQ mass spectrometer (hybrid sector and quadrupole instrument), EBQ (double-focusing instrument followed by a quadrupole). 

Electrostatic Analyzer In magnetic-sector instruments, an electrostatic sector can be incorporated either before or after the magnet to provide energy resolution and directional focusing of the ion beam. The resolution achievable in these double-focusing instruments is sufficient to separate ions having the same nominal mass (e.g., 28 Daltons) but with different chemical formula (e.g., N2 and CO). 

Accurate mass assignment of highly resolved ion-intensity signals remains problematic. QITMS is a relatively simple and compact apparatus (benchtop) and an order of magnitude less expensive than the multistage (triple quad or tandem double-focusing) instruments traditionally used for structural determination. 

In recent years several new instruments have been developed based on different mass-spectrometer principles. Two different categories of ICP-MS instruments are currently commercially available low-resolution instruments (using either QMS, ITMS or ToF-MS) and focusing high-resolution instruments (DFS, FTMS). Selected specifications for these two categories are shown in Table 8.63. Both the quadrupole-based and the double-focusing instruments allow a sequential multielement measurement, whereas ICP-ToFMS allows... 

Resolving power decreases with increasing mass but spectrometers having a resolution of 1000, i.e. the ability to discriminate between m/z values of 1000 and 1001, or between 100 and 99.9, are adequate for many applications. Double-focusing instruments may be capable of resolutions of 20 000-50 000 or more. 

Figure 9.3 Schematic diagram of a multicollector ICP-MS (MC-ICP-MS). This is a double focusing instrument, with a hexapole (a six-rodded version of the quadropole) and a 90° magnetic selector, and a multiple bank of ion detectors, allowing simultaneous measurement of a number of isotopic ratios.

The separation of ions according to their m/z ratios is achieved using electric and/or magnetic fields in a number of ways. The trajectories of ions moving in such fields are determined by their m/z values and these can be monitored to ascertain their mass. Double-focusing instruments use the combined effects of electric and magnetic fields to effect separation (Figure 3.19). In a typical instrument, after the ions have been accelerated away from the ion source... 

The first instruments used a single magnetic sector (symbol B) to effect separation of the ions. Later, the introduction of double-focusing instruments having an electrostatic sector or electrostatic analyzer (ESA, symbol E) in addition defined a standard which is still valid. 

The combination of an electric sector field with a magnetic sector field to form a double-focusing instrument is advantageous for improving the properties (e.g., the mass resolution) of mass spectrometers. 

In a quadrupole device, not as accurate and precise as double-focusing instruments but fast, a quadrupolar electrical field comprising radio-frequency (RF) and direct-current components is used to separate ions. Quadrupole instruments as mass analyzers are used together with ESI as the ion source the configuration employing a three-dimensional quadrupolar RF electric field (Wolfgang Paul, University of Bonn, 1989 Nobel prize for physics) is referred to as an ion trap analyzer (see below). 

E.G. JOHNSON and A.O. NIER develop double-focusing instruments [30],... 

Besides nonvolatility, the resolving power of the available single-focusing instruments limits the applications of MS to compounds of a molecular weight up to a maximum of about 1,200. Technical improvements and the new double-focusing instruments will probably raise this limit in the near future. 

There are many types of mass spectrometer, from high-resolution double-focusing instruments, which can distinguish molecular and fragment masses to six decimal places, to bench-top machines with a quadrupole mass detector which can resolve masses up to about m/z = 500, but only in whole-number differences. Routinely you are most likely to encounter data from bench-top instruments and therefore only this typie of spectrum will be considered. 

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