Double-focusing magnetic sector mass analyzers

Walder, A. J., and Freedman, P. A. (1992) Isotopic ratio measurement using a double focusing magnetic sector mass analyzer with an inductively coupled plasma as an ion source. J. Anal. Atomic Spect. 7, 571-575. 

The dwelltime of ions within the ion source is defined by the extraction voltages applied to accelerate and focus them into an ion beam and by the dimensions of that ion source. In standard El ion sources the freshly formed ions dwell about 1 ps before they are forced to leave the ionization volume by action of the accelerating potential. [41] As the ions then travel at speeds of some 10 m s they pass the mass analyzer in the order of 10-50 ps (Fig. 2.9). [9] Even though this illustration has been adapted for a double focusing magnetic sector mass spectrometer, an ion of m/z 100, and an acceleration voltage of 8 kV, the effective time scales for other types of instruments (quadrupole, time-of-flight) are very similar under their typical conditions of operation (Table 2.4). 

The three most common types of mass analyzers in SIMS systems are (1) double focusing magnetic sector instmments, (2) time of flight (TOF) mass spectrometers, and (3) quadru-pole 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... 

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. 

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