TABLE OF CONTENTS All mass spectrometers consist of three basic components, the:
ion source,
mass analyser,
ion detector.
The role of the ion source is to introduce molecules into the mass spectrometer and convert them to a charged or ionised form. The ion source like the rest of the mass spectrometer is usually, though not always, held at a low pressure. Mass spectrometers are operated under vacuum to prevent the collision of ions with residual gas molecules during their flight from the ion source to the detector. This is because the ions are formed with excess energy and this, together with their charged character, can result in their reaction with other gaseous material present. To avoid this, the levels of contaminants and atmospheric gases such as oxygen within the ion source should be minimised. The ideal operating pressure is that in which the average distance an ion travels before colliding with a gas molecule (its mean free path) is longer than the distance from the source to the detector.
After ions are formed in the source, they are accelerated into the mass analyser where they are separated in vacuo according to their mass and charge through the use of electric and/or magnetic fields. Finally, the ions are passed onto an ion detector producing an electrical current that is amplified and detected.
In most mass spectrometers, these three basic components are physically discrete entities. Thus each of them will be considered separately in order to understand...
