The MS incorporates a photomultiplier tube (PMT) in its ion detection system. A PMT is an electromagnetic detector that converts photons into an electric signal. The detection system includes two high-energy conversion dynodes, a scintillator, a PMT, an anode, and a preamplifier/digitizer.
| No. | Description | No. | Description |
|---|---|---|---|
1 | Primary dynode | 3 | Secondary dynode |
2 | Photomultiplier tube |
The high-energy conversion dynodes consist of two primary ±12 kV dynodes and a secondary -5 kV dynode. The ion detection system applies a high-positive potential (12 kV) for negative ion detection or a high-negative potential (-12 kV) for positive ion detection to the primary dynode. An ion striking the surface of the primary dynode produces several particles (2–3 on average). Positive ions striking the primary dynode at -12 kV emit electrons, while negative ions striking the 12 kV dynode emit positive particles. The particles emitted from the primary dynode then strike the secondary dynode, which has a fixed negative potential (-5 kV). The purpose of the secondary dynode is to enable negative-ion detection by converting the positive particles emitted from the primary dynode into the electrons for the scintillator.
The electrons produced by the secondary dynode continue and strike a scintillator, which is coupled to the front of the PMT. The scintillator is a material that emits photons when struck by electrons. The emitted photons pass through a window into the sealed PMT.
The PMT contains a photocathode and dynodes. The photocathode converts the photons back into electrons. The electrons are then amplified by the dynodes in a cascade fashion. The dynodes are electrodes in a vacuum tube that serve as an electron multiplier. They are arranged so that the electric fields between them cause the electrons emitted by each dynode to strike the next with an energy of a few hundred electron volts. As a result of secondary emission, the number of electrons increases from dynode to dynode, giving the required multiplication.
Lastly, the anode collects the electrons produced by the electron multiplier and the data system further amplifies, digitizes, and records the current that leaves the electron multiplier through the anode.
NOTE
The main advantage of using a PMT over a regular electron multiplier is that the majority of the electron multiplication is kept sealed in a vacuum tube, preventing contamination, and greatly extending the gain stability and lifetime of the detector.