Mass spectrometer sector field

Figure Bl.7.4. Schematic diagram of a reverse geometry (BE) magnetic sector mass spectrometer ion source (1) focusing lens (2) magnetic sector (3) field-free region (4) beam resolving slits (5) electrostatic sector (6) electron multiplier detector (7). Second field-free region components collision cells (8) and beam deflection electrodes (9).

Another approach to mass analysis is based on stable ion trajectories in quadnipole fields. The two most prominent members of this family of mass spectrometers are the quadnipole mass filter and the quadnipole ion trap. Quadnipole mass filters are one of the most connnon mass spectrometers, being extensively used as detectors in analytical instnunents, especially gas clnomatographs. The quadnipole ion trap (which also goes by the name quadnipole ion store, QUISTOR , Paul trap, or just ion trap) is fairly new to the physical chemistry laboratory. Its early development was due to its use as an inexpensive alternative to tandem magnetic sector and quadnipole filter instnunents for analytical analysis. It has, however, staned to be used more in die chemical physics and physical chemistry domains, and so it will be described in some detail in this section. 

In a sector instrument, which acts as a combined mass/velocity filter, this difference in forward velocity is used to effect a separation of normal and metastable mj" ions (see Chapter 24, Ion Optics of Magnetic/Electric-Sector Mass Spectrometers ). However, as discussed above, the velocity difference is of no consequence to the quadmpole instrument, which acts only as a mass filter, so the normal and metastable mj ions formed in the first field-free region (Figure 33.1) are not differentiated. 

Reduction of the measurement time for element distributions is possible by simultaneous detection of several masses. This can be achieved only by use of a magnetic sector field spectrometer with Mattauch-Herzog geometry [3.49] (Fig. 3.20) and parallel detection of up to five masses by mechanically adjusted electron multipliers. 

Imaging SIMS. Steeds et al. (1999) included this technique in their study of the distribution of boron introduced into diamond, where it is a well-established dopant that controls the electrical conductivity. SIMS was performed with a field-emission liquid gallium ion source interfaced to a magnetic sector mass spectrometer capable of about 0.1 pm spatial resolution. 

A Thermo Finnigan Element 2 Inductively Coupled Plasma Sector-Field Mass Spectrometer (ICP-SF-MS) with guard electrode was employed for trace element analyses. RSD values derived from internal check standard never exceeded 10%. Accuracy was better than 15% for all elements as determined by analyzing the certified reference standard NWRI TM-RAIN 95 trace metal fortified rainwater, every 5 to 8 samples. 

How can metastable ions be registered with a classic magnetic sector mass spectrometer (See Chapter 2, Section 2.2.2) Let ion mj+ leave the ion source and after acceleration with accelerating voltage V fragment, with formation of ion m2+ and a neutral particle m3° between the source and magnetic analyzer (first field-free region, 1 FFR). 

Nevertheless, a resolution of 80,000 for a sector instrument does not mean that this mass spectrometer allows one to obtain separate peaks of the singly charged ions of masses 79,999 and 80,000. The registered mass is proportional to B2/V. An unlimited increase of magnetic field B is technically unreasonable, while decrease of accelerating voltage V decreases the resolving power. 

The ability to separate ions spatially is called the dispersion of a mass spectrometer. Dispersion is simply the distance between the centers of two ion beams that differ in mass by Am at the collection plate. A simple sector instrument, where the ion beam enters and exist the magnetic field normal to the pole faces and the object and image distances are the same, is known as a symmetrical geometry analyser. Examples are shown in Fig. 8. In this case, the dispersion, D is given by ... 

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