5.4.1 Electron Multipliers

After mass analysis, the ions in the beam must be detected. There are several methods for detecting ions, but the method common in commercial instruments utilizes a device called a continuous dynode electron multiplier, or simply electron multiplier for short. An electron multiplier is comprised of a semiconducting medium with a small work function that is placed in the ion beam (after the mass analyzer). Incident ion collisions with the multiplier cause ejection of several electrons per ion collision. The detector is operated in a manner that causes the ejected electrons to likewise collide and eject even more electrons. This process repeats in a 'cascade effect,' in effect multiplying the number of charged particles (the number of electrons leaving the multiplier is much greater than the number of ions that struck it). Gains in current of 10$^{\textrm{7}}$ are possible (and typical), so that single ion events are multiplied to currents sufficient to trigger an electronic amplifier:

1. The charge on a monocation is approximately 10 $^{\textrm{-19}}$coulomb (C)
2. The multiplier produces a current gain of 10$^{\textrm{7}}$C, so the output charge (for a single ion collision event) is 10 $^{\textrm{-12}}$C (1 pC). Charge sensitive amplifiers can be constructed that are sensitive to picocoulomb charges.

Analog current from the amplifier can then be converted to a digital signal (counts) and sent to a multichannel scaler (a scaler is simply a counter). The counts stored in each 'channel' correspond to the signals at each mass; these are read and stored by the data system. Note that in this analog ion current mode, the 'counts' stored by the data system do not correspond to real ion events; the counts are simply the result of an analog to digital conversion of the analog amplified signal.