2. Electron Multiplier
Electron multipliers are probably the most common means of detecting ions, especially when positive and negative ions need to be detected on the same instrument. Their are two types (see figure 2) of electron multiplier, but they both work essentially by extending the principles of the Faraday cup. A Faraday cup uses one dynode and as a result produces one level of signal amplification. One type of electron multiplier (figure 2a) has series of dynodes maintained at increasing potentials resulting in a series of amplifications. The other type (the channel multiplier, figure 2b) has a curved ('horn' shaped) continuous dynode where amplifications occur through repeated collisions with the dynode surface. In both cases, ions pass the conversion dynode (depending on their charge) and strike the initial amplification dynode surface producing an emission of secondary electrons which are then attracted either to the second dynode, or into the continuous dynode where more secondary electrons are generated in a repeating process ultimately resulting in a cascade of electrons. Typical amplification is of the order of one million to one.