Isoprime User’s Guide
Version 1.02 Background Information Page 136 of 219
Where peaks are narrow and well separated, values of M will tend to be < 1, and
consequently the resolving power will be greater than the mass. Where M > 1, the resolving
power will be less then the mass.
The mass resolution depends on the following:
The number of ions which have been extracted from the source but which have the
wrong trajectory.
The image size.
Any aberrations in the optics.
In principle, the maximum resolution is directly calculable from the radius and slit width
equation (8). However, the design of the high sensitivity source used in isotope ratio machines
results in a high intensity image that is narrower than the source defining slit. This increases the
actual available resolution of the instrument.
VACUUM SYSTEM
A mass spectrometer must operate in a vacuum for two reasons:
To reduce ion scattering. If the ions collide with any residual gas molecules their
trajectories will be modified, resulting in peak broadening.
To limit contamination. Any residual gases in the ionising chamber would also be
ionised, giving rise to an instrument background
VACUUM MEASUREMENT
Gases are composed of small particles that are in constant motion. As these particles move
around in space they collide with other objects and exert a force. If the number and intensity of
particle impacts can be measured in a unit of area, the result is a pressure measurement.
The pressure P per unit area is:
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