Magnetic-Sector Mass Spectrometry

Introduction

Theory

The ion optics in the ion-source chamber of a mass spectrometer extract and accelerate ions to a kinetic energy given by:

K.E. = 0.5 mv² = eV

where m is the mass of the ion, v is it's velocity, e is the charge of the ion and V is the applied voltage of the ion optics.

The ions enter the flight tube between the poles of a magnet and are deflected by the magnetic field, H. Only ions of mass-to-charge ratio that have equal centrifugal and centripetal forces pass through the flight tube:

mv² / r = Hev

centrifugal = centripetal forces.

Where r is the radius of curvature of the ion path, e is the charge, m is the mass, and v is the velocity of the molecule.

r = mv / eH

This equation shows that the m/e of the ions that reach the detector can be varied by changing either H or V.

Instrumentation

• Single Focusing analyzers:
  A circular beam path of 180, 90, or 60 degrees can be used. The various forces influencing the particle separate ions with different mass-to-charge ratios.
• Double Focusing analyzers:
  An electrostatic analyzer is added in this type of instrument to separate particles with difference in kinetic energies.
  

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