Magnetic sector analyser:
As the name suggests, these analysers make use of a permanent magnet (or an electromagnet) to separate the fragment ions. It consists of an evacuated curved metallic tube through which the fragment ions pass on their way from the ion source to the detector. A electromagnets are mounted perpendicular to the tube and provide a stable and uniform magnetic field figure.
Figure: Schematic diagram of a magnetic sector mass analyser
As the ions entering the analyser have approximately the same kinetic energy, they have different velocities depending on their masses; heavier ions would be slower. The field of the magnet makes these ions to travel in a circular path generally of 60, 90 or 180 degrees. The mass to charge ratio of the fragment ions is related to the parameters of the instrument as per the equation.
m /z = B2 r 2 e/2V
where,
B = magnetic field strength,
r = radius of curvature of the magnetic sector (metallic tube) ,
V= the accelerating voltage applied on the ion in the ionisation chamber and
e = the electronic charge.
According to this equation the mass spectrum can be obtained by varying any of the three parameters viz., B, V or r. Within most of the modern spectrometers, the separation is achieved by varying the field strength of the electromagnet keeping V and r constant.
At a given magnetic field strength, only the fragments of a certain m/z value would be able to travel through the circular path in the analyser tube; the ions of larger or smaller m/z values would strike the tube and get destroyed. Thus, with the gradual increasing strength of the electromagnet, the fragment ions of increasing m/ z values successfully pass through the analyser and get detected by an ion collector.