TABLE OF CONTENTS To this point, only the molecular weight of a molecule has been considered to contribute to its appearance in a mass spectrum. However, as recognized by Thomson, the particles detected in mass spectrometry are ions. Thus they have both mass and charge, the latter of which is important to their detection. Mass spectrometers are unable to detect neutral molecules and radicals; a charge must be imparted onto an atom or molecule before it can be studied. The reason for this is that charged molecules can be "handled" or their paths controlled through the use of electric and magnetic fields, while radicals and neutral molecules are unresponsive.
Depending on the nature of the ionisation process, and also the nature of the atoms and molecules themselves, different ion types can be formed. The most common and traditional way in which ions are produced in a mass spectrometer is through the loss of an electron (equation 2.1). This often occurs by the initial collision of a gaseous atom or molecule with an electron in a process known as electron impact or electron ionisation (EI) (Chapter 3).
The product ion formed is known as a radical cation as it is an odd electron species with a positive charge. Since the product, M + , has the same mass as the molecular weight of the compound M from which it was produced, it is known as a molecular ion. In general, the notation
