Double focusing magnetic sector instrument

Resolution Quadrupole instruments are not capable of achieving the high resolution that is common with double-focusing magnetic-sector instruments. In GC/MS analyses, a compromise is struck between sensitivity (ion transmission) and mass resolution. In the quadrupole instrument, the resolution is set to the lowest possible value commensurate with resolving peaks differing by 1 Dalton (unit resolution). 

Table 6.16 summarises the main characteristics of FI-MS. FT uses high voltages and was once restricted to sensitive double-focusing magnetic sector instruments of relatively high cost. Field ionisation is considered to be the softest ionisation mode. The reproducibility of the non-standard techniques, such as FI-MS and FD-MS, is less well assessed than that of EI-MS. A noticeable drop in FI use occurred after the mid-1980s because of the advent of FAB and other desorption/ionisation methods. FI-MS is only used in a few laboratories worldwide. 

Fig. 4.27. Photograph of a JEOL JMS-700 double focusing magnetic sector instrument.

Classically, high-resolution work is the domain of double-focusing magnetic sector instruments. More recently, TOP and to a certain degree triple quadrupole instruments are also capable of resolutions up to about 20,000. However, the rapid development of FT-ICR instruments has established those as the systems of choice if ultrahigh-resolution (>100,000) and highest mass accuracy (1 ppm) are required (Chap. 4.6). 

During the 1960s, high-resolution, double-focusing magnetic sector instruments became available from multiple manufacturers and were widely used in organic chemistry for exact mass measurements and elemental composition analysis. El was used for generating struc-... 

Similar results can be obtained by operating double-focusing magnetic sector instruments in the B/E - linked scan mode. In this mode, the ratio of B to E is kept constant as B is scanned. The residting spectrum contains fragment ions from a selected precursor ion. Warburton et al. [31] used the B/E - hnked scan mode to show that the peak at m/z 210 in the lemon juice corresponds to citric acid, and the m/z 369 peak in the egg yolk is due to cholesterol. In our laboratories we have used the same technique to elucidate the mechanism of B-carboline formation in food from tryptophan Amadori product and the mechanism of pyrrole formation from lysine Amadori products [32, 33]. 

Figure 16.3 A double focusing magnetic sector instrument showing its BE geometry. Model JMS 700. On this photograph, the characteristic shapes of the electromagnet (magnetic sector) and of the electrostatic sector can be seen. The detector lies at the right the ion source is on the left (reproduced courtesy of Jeol, Japan).

Identifrcation of components in the extracts was conducted by mass spectrometry. The sample was injected onto an HPS890 GC. The chromatographic conditions for the OV-1 column were the same as described for GC analysis. The end of the GC capillary column was inserted directly into the ion source of the mass spectrometer via a heated transfer line maintained at 280°C. The mass spectrometer was a Micromass Prospec high resolution, double-focusing, magnetic sector instrument. The mass spectrometer was operated in the electron ionization mode (El), scanning from ni/z 450 to m/z 33 at 0.3 seconds per decade. 

Accurate mass measurements are often used for the identification or confirmation of the molecular formulas of drug metabolites. The analytical utility of high resolution mass spectrometry began with the introduction of double-focusing magnetic sector instruments, which could routinely achieve... 

The upper mass limit of a double-focusing magnetic-sector instrument is 2500 Da. What would be the upper mass limit when the strength of the magnetic field and the radius of the flight tube are both doubled ... 

The spectrum of benzene (CeHe) was obtained with a double-focusing magnetic-sector instrument that is equipped with a 12.5-KG magnet. Ions exited the source at 6000 V. What would be the radius of trajectory for a ion in the magnetic field ... 

The three most common types of mass analyzers in SIMS systems are (1) double-focusing magnetic sector instruments, (2) TOF mass spectrometers, and (3) quadrupole mass spectrometers. The choice of mass analyzer depends on whether dynamic or static SIMS is needed, on the requirements of mass range and resolution, and on transmission efficiency, among other factors. The mass analyzers have been discussed in Chapter 9 in detail and this chapter should be reviewed as necessary. 

Double-focusing magnetic sector instruments use a magnetic field to separate the ions by mass-to-charge ratio. The mass range covered by this type of analyzer is up to 500 Da with a... 

Double-focusing magnetic sector instruments represent the standard in FAB-MS, because they combine a suitable mass range with the ability to perform high-resolution and accurate mass measurements. Until the advent of ESI and MALDI, FAB-MS/MS on magnetic four-sector instruments was the method of choice for... 

On the downside, there are isobaric interferences such as pCAr]" and [XH]" ions that require R > 5000 to avoid overlap with pure atomic ions [48]. Therefore, double-focusing magnetic sector instruments present the most successful mass analyzers in GD-MS [45]. Another disadvantage of GD-MS as compared to LA-ICP-MS or SIMS (Chaps. 15.4 and 15.5), for example, is the lack of spatial resolution. 

FIGURE 14.4 Separation of As from Ar Cl using high resolving power (5,000) of a double focusing magnetic sector instrument. (From W. Titles, N. Jakubowski, and D. Stuewer, Poster Presentation at Winter Conference on Plasma Spectrochemistry, San Diego, 1994.)... 

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