IsoPrime User’s Guide
Version 1.01 Background Information Page 129 of 219
BACKGROUND INFORMATION
This chapter provides some basic information concerning stable isotope mass spectrometers,
and is aimed at those users who are new to isotope ratio mass spectrometers or are unfamiliar
with some of the basic concepts.
The Calculations and Corrections section provides definitions of formulae that may also be of
interest to more experienced users.
BASICS OF MASS SPECTROMETRY
THE MASS SPECTROMETER EQUATION
If an ion of mass M and charge Z is accelerated in a potential V and admitted into a uniform
magnetic field B, then the ion experiences a force and moves in a circular orbit of radius R. The
motion is defined by:
(1)
For singly charged ions, the radius is determined by the nature of the magnetic and electric field.
The combination of fields selects ions of particular mass and forms a mass filter. This principle is
the basis of all magnetic-sector mass spectrometers and equation (1) is frequently termed the
mass spectrometer equation.
MASS SPECTROMETERS
A gas source mass spectrometer comprises a source, flight tube and collector. To analyse a
sample gas, the molecules must be ionised in the source, and the ions are then formed into a
beam and accelerated by an electric field. The ions then pass from the source into the flight
tube, where they are magnetically deflected, and finally they are detected by the collector.
The ionisation is commonly achieved by passing a beam of electrons through the gas sample.
Collision between, or close approach of, an electron and a sample molecule can cause one or
more electrons either to adhere to the molecule and form a negative ion, or to detach from the
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