Electron multipliers are perhaps the most common detectors in modern mass spectrometers due to their exceptionally high gain and low noise. A single ion entering the front of the multiplier can result in upwards of one million electrons exiting the back.
Principle of Operation
The operation of electron multipliers is fundamentally based on the concepts of "dynodes" and "secondary emission". A "dynode" is simply an electrode in vacuum that emits electrons when an ion or electron with sufficient kinetic energy slams into it. This process of emitting electrons is termed "secondary emission". Electron multipliers esentially string together a series of dynodes so that the process of secondary emission happens repeatedly, amplifying the number of electrons exponentially at each step along the way. There are two common geometries for electron multiplier: the "continuous dynode" and the "discrete dynode"...
The "discrete dynode" geometry utilizes a series of dynodes that are chained together by resistors. A relatively large negative voltage is applied to the "front" dynode where the ions enter the detector. Meanwhile, the "back" dynode is held at ground. The voltage difference between the front and back of the dynode chain along with the resistors between each dynode results in a gradual voltage drop down the line. Each dynode is typically 100-200V more positive (less negative) than the preceeding dynode:
The large negative voltage on the front dynode pulls strongly on a positive ion, which slams into the dynode with high kinetic energy. This event causes a handful of secondary electrons to be emitted from the first dynode; for this example, let's just say two electrons are emitted. These electrons now see an electric field between the first dynode and the less negative second dynode. The 100-200V difference between these two dynodes will cause the two negatively charged electrons to accelerate toward the second dynode, slamming into it with 100-200 eV of kinetic energy. When each of these two electrons strikes the second dynode they will emit more secondary electronds. Again, for this example, let's say that each electron generates two secondary electrons. Now we have four electrons being emitted from the second dynode. These electrons see a 100-200V potential drop toward the third dynode, causing them to accelerate into the third dynode with 100-200eV of kinetic energy, emitting even more secondary electrons. This process continues all the way down the dynode chain, with secondary electrons being accelerated into the subsequent dynode. An exponential increase in electron count occurs down the length of the chain, with all of the electrons eventually reaching the back dynode, which is held at ground. These electrons (viz. current) are usually passed into another circuit that further amplifies the signal.
The "continuous dynode" geometry has very similar characteristics to the discrete dynode geometry, with a high negative voltage at the front end and ground at the back end. As with the discrete geometry, an ion is accelerated into the front of the dynode by the high negative potential. The chief difference is that the continuous dynode geometry has one continuous electrode that has sufficient resistance to make the voltage gradually drop from front to back:
As with the discrete geometry, electrons are made when the ion initially strikes the front of the dynode. The secondary electrons are accelerated toward the back of the detector by the potential drop, generating more electrons each time they strike the detector's walls. The increase in electron count is again exponential down the length of the detector. At the end/back of the detector, the electrons are passed into circuitry that further amplifies the response.
Electron multipliers have a few weaknesses as detectors. Among them is their inability to readily detect negative ions and their mass discrimination. Both issues are addressed through the use of a "conversion dynode", which is another electrode that is placed in front of the electron multiplier with an exceptionally high voltage applied to it (e.g. ±15-25kV). Ions are accelerated into the conversion dynode first, causing secondary emission of ions and electrons, which then enter the electron multiplier like normal.
With a standalone electron multiplier whose input is at approximately -1500V, heavy ions (e.g. peptides, proteins, etc.) produce significantly less secondary electrons than light ions when striking the first dynode of the multiplier. This mass discrimination is significantly lessened if the first dynode is held at a much higher voltage, such that ions strike it with much more energy; this is the role of the "conversion dynode". For example, take the figure above, which shows a conversion dynode that is placed in front of the conventional electron multiplier and has -15kV applied to it. Positive ions will strike the conversion dynode with much larger kinetic energies than they would with the multiplier alone. This higher kinetic energy tends to level the playing field across the mass range such that heavy ions produce more secondary particles than they otherwise would. Once the ions strike the dynode, the electrons and negative ions that are emitted from the dynode surface see a steep downhill potential drop between the conversion dynode and front dynode of the multiplier. This causes the secondary particles to accelerate to the front of the electron multiplier, which then operates as explained above.
Negative Ion Detection
In addition to improving the high mass response of the detector, conversion dynodes also provide a means for detecting negative ions, which would be otherwise impossible with a multiplier alone. In order to detect negative ions, the conversion dynode is held at a high positive voltage (15-25kV):
Negative ions are accelerated into the conversion dynode because of its high positive voltage. Upon striking the conversion dynode, positive ions are emitted as secondary particles. These positive particles are then accelerated away toward the front dynode of the electron multiplier. From this point on, the electron multiplier acts as normal.