Magnetic sector instrument configuration

Fig. 4.4 Example of a magnetic sector instrument configuration anployed in the Cameca IMS 3-5 series. Reprinted with permission of Cameca Instruments, Inc.

It should be pointed out that FAB, MALDI, and ESI can be used to provide ions for peptide mass maps or for microsequencing and that any kind of ion analyzer can support searches based only on molecular masses. Fragment or sequence ions are provided by instruments that can both select precursor ions and record their fragmentation. Such mass spectrometers include ion traps, Fourier transform ion cyclotron resonance, tandem quadrupole, tandem magnetic sector, several configurations of time-of-flight (TOF) analyzers, and hybrid systems such as quadrupole-TOF and ion trap-TOF analyzers. 

Fig. 10.4 Scheme of a double-focussing magnetic sector instrument with BE configuration. Dependent on the mass-to-charge ratio the ions are distracted by the magnetic field to circular arcs with different radii. 

FIGURE 7.7 The two most popular mass analyzers. The magnetic sector instrument is normally configured in tandem with an electrostatic analyzer that narrows the kinetic energy spread of the ion beam. For this reason, it is called a double-focusing spectrometer. 

Most instruments are configured with a fixed value for the radius of curvature, r, so changing the value of B selectively passes ions of particular values of momentum, mv, tlirough tlie magnetic sector. Thus, it is really the momentum that is selected by a magnetic sector, not mass. We can convert this expression to one involving the accelerating potential. 

Commercial mass analyzers are based almost entirely on quadrupoles, magnetic sectors (with or without an added electric sector for high-resolution work), and time-of-flight (TOE) configurations or a combination of these. There are also ion traps and ion cyclotron resonance instruments. These are discussed as single use and combined (hybrid) use. 

Various tandem MS instrument configurations have been developed, e.g. sector instruments, such as CBCE, CBCECB or CECBCE, and hybrid instruments, e.g. BCECQQ (B = magnetic sector analyser, E = electrostatic analyser, C = collision cell, Q = quadrupole mass spectrometer), all with specific performance. Sector mass spectrometers have been reviewed [168],... 

The performance of a magnetic sector mass spectrometer depends totally on the ability to focus ions from source to detector. To produce ideal focusing a very wide range of factors must be taken into account. Modern computer simulation techniques have now been extensively applied in this instrument and have resulted in an ion optical design closer to the ideal than ever before. This configuration provides for complete image error correction in all planes. 

Instruments with combined magnetic and electric analysers can be assembled according to either of two configurations. The electric sector is located either in front of the magnetic sector, which is the most frequent case, or behind it. The magnetic sector is labelled B and the electric sector is labelledE. The first configuration is called EB (or also Nier-Johnson ). 

Common space instruments have two mass analysers, allowing MS/MS experiments to be performed. A frequently used instrument of this type uses quadrupoles as analysers. The QqQ configuration indicates an instrument with three quadrupoles where the second one, indicated by a lower case q, is the reaction region. It operates in RF-only mode and thus acts like a lens for all the ions. Other instruments combine electric and magnetic sectors (E and B) or E, B and qQ, thus electric and magnetic sectors and quadrupoles. TOF instruments with a reflectron, or a combination of a quadrupole with a TOF instrument, are also used. 

B indicates a magnetic sector, E an electrostatic sector. The most common setups are double-focusing instruments with either a Nier-Johnson geometry (EB) or an inverse Nier-Johnson configuration (BE). Extended setups such as BEBE are available and can be used for up to MS experiments. 

---