The magnetic-sector design

In this assembly, the positive ions generated are first accelerated through a voltage difference U. They take up a velocity v which depends upon their mass m (cf. Section 16.3.1) and are then submitted to a transverse magnetic field B 

The fundamental relationship of dynamics, F = m-a a designates the acceleration), applied to ions of mass m on which is exerted the Lorentz force F = q-vAB leads, after arrangement, to the following relationship  

The orientation of H is such that only the centripetal component of the acceleration vector a gets involved. The ion trajectory lies in a plane perpendicular to B and containing v. Since a= v /R, by substituting a by its form in equation 16.1 (where q = ze is in coulomb, v is in m/s, B is in tesla and m in kg), expression 

2 is attained which shows that separation occurs according to the moment of the ions  

Therefore, the m/z ratio of the ions is only obtained if their speed is known (Equation 16.3)  

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The quadripolar spectrometers whose resolution is limited to about 2000 are of simpler design than the magnetic sectors and are less costly. They are often used in conjunction with gas chromatography (see section 3.3) for purposes of identification. 

The most widely used method for ionization is electron impact (El). In an El source the sample is placed in the path of an electron beam. Although many newer kinds of ion sources have been developed, El is the method commonly used in classical isotope-ratio mass spectrometers (IRMS), i.e. mass spectrometers designed for precise isotopic analysis. In this type of spectrometer the ions, once formed, are electrostatically accelerated, and then ejected through a slit into a magnetic field held perpendicular to the ion trajectory. In the magnetic sector part of the instrument the particles are deflected in an arc described by ... 

Because there is so little mass bias in the mass analyzer, a discussion of ion transfer optics and collectors is not presented. The ion transfer optics of the magnetic sector mass analyzer, and the collectors used for isotope ratio measurements are critical design elements in all isotope ratio mass spectrometers and recent reviews of these items can be found in Habfast... 

Fig. 4.20. Flight tube passing through the gap of the magnetic sector of a JEOL JMS-700 instrument seen from the ESA side. The shapes of the pole pieces of the yoke and the additional blocks around the tube are designed to minimize fringing fields. In addition the pole faces are rotated to increase the mass range.

Fig. 4.28. Schematic design of the above instrument. Horizontal (x) and vertical (y) trajectories are also shown. Quadmpole lenses are used to improve the transmission of the magnetic sector, thus resulting in a qqBqE geometry. Adapted from Ref. [80] with permission. Elsevier Science, 1985.

In general a mass spectrometer consists of an ion source, a mass-selective analyzer, and an ion detector. Since mass spectrometers create and manipulate gas-phase ions, they operate in a high vacuum system. The magnetic-sector, quadrupole, and time-of-flight designs also require extraction and acceleration ion optics to transfer ions from the source region into the mass analyzer. Tables 2 and 3 provide brief descriptions of the most commonly used ionization techniques and the different types of mass spectrometers available, respectively [163,232-235,241,242,244-246]. 

Therefore ions with the same kinetic energy (KE) are, in effect, dispersed with respect to their mass. In addition, the shape of the magnetic sector can be designed to have ion directional focusing properties. A magnetic sector of a particular shape and size will have a particular combination of ion dispersion and directional focusing characteristics. 

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