Double-focusing mode

The mass resolution for a magnetic sector mass selector is high, especially when combined with an electric sector in the so-called double-focusing mode. The disadvantage of this type of mass selector is that the system becomes rather complicated if... 

Figure 9. Schematic diagram showing a second-generation MC-ICPMS instrament (ThermoFinnigan Neptune). This instrument utilizes double-focusing and is equipped with a motorized multiple-Faraday collector block with two channels that can be operated in high-resolution mode. Optional multiple-ion counting channels are also available for the simultaneous measurement of low-intensity ion beams. [Used with permission of Thermo Finnigan.]...

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. 5 Statistical evaluation of LC-MS-based methods for tropane alkaloids referred in this chapter. (a) Relative frequency of ionization methods. +APCI positive atmospheric pressure chemical ionization, +ESI positive electrospray ionization, FAB fast atom bombardment, +TSP positive thermospray, (b) Relative frequency of scan modes used. MS full scan MS, MS/MS tandem mass spectrometry (product ion scan), MRM multiple reaction monitoring, SIM selected ion monitoring, (c) Relative frequency of mass analysers used. EBQtQ2 double focusing sector field mass spectrometer, IT ion trap, QqQ triple quadrupole, SQ single quadrupole. Considered publications were found by PubMed data-based search and references cited in these articles...

Semiquantitative analysis procedures have also been used with laser ablation ICP-MS [71,210-213]. The effect of experimental parameters and potential improvements in accuracy of semiquantitiative analysis using ICP-MS have also been discussed recently [208,214-216], along with the use of semiquantitative analysis with a double-focusing ICP-MS in both low- and high-resolution modes [217]. 

The equipment used for this work consisted of a Hewlett-Packard 5710A GC, with on-coluitm injection, directly coupled to a VG7070E medium resolution double-focusing mass spectrometer. The GC coluitm employed was a 60 m fused silica capillary colurrm coated with a cross-linked methyl silicone stationary phase, DB-1. The initial temperature of the colurrm was 30°C, and after 4 minutes, the temperature was linearly programmed at 8°C/min to 270°C, and held at this final temperature for 15 minutes. The mass spectrometer was operated in the electron impact (El) mode, and the mass range of 20-700u was scanned once a second. The mass spectrometer was linked to an Incos data system which stored the acqnired mass spectra, and allowed these to be compared to the EPA/NIH Mass Spectral Data Base to assist with the identification of compounds detected. 

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]. 

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. 

Fig. 2. Schematic diagram of the high-resolution double-focusing mass spectrometer. The insert shows an enlarged view of the ion extraction optics for the high-extraction-efficiency mode. P, pusher electrode C, collision chamber B, electron beam (z direction) S, collision chamber exit slit S2, penetrating field extraction slit S3, grounded slit S4 and Ss, deflector electrodes.

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