Operation - Mass Spectrometry

The principle purpose of a mass spectrometer is to disperse ions according to their mass to charge ratio. To first order, this is accomplished by an ion moving through a magnetic field. Specifically, an ion moving at velocity v through a magnetic field B will experience a force at right angles to both velocity and magnetic field (direction defined by right-hand rule). A constant force acting at right angles to velocity therefore results in the ions moving in a circular motion, with the radius a function of both mass and velocity (for a constant magnetic field).

In isotope-ratio mass spectrometry, the velocity characteristics of the ion beam are immaterial to the measurements. In secondary ion mass spectrometry, a large velocity spread is a characteristic and if not corrected, would cause degradation of peak shape, especially at the high mass resolving power required for SHRIMP analysis. The velocity spread is removed with double-focusing mass spectrometers. These mass spectrometers include an electrostatic sector (Electrostatic Analyzer or ESA) which compensates for the velocity spread produced by the magnet. The ESA consists of parallel plates held at specific potentials so that ions with different energy will travel different pathways, but refocus at the collector slit.

A feature of the SHRIMP mass spectrometers is the Matsuda lens, a quadrupole lens located between the magnet and ESA, that matches the ion optical emittance to the acceptance allowing high transmission and minimal aberrations.