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Understanding the SEM

Most modern Mass Spectrometers now employ a dual detector system. These detectors are a Faraday cup and a Secondary electron multiplier. Here we explain how they work.


The Faraday Cup

The Faraday cup is a highly polished stainless steel cup that captures ions exiting a mass filter. The ions strike a plate at the bottom of the cup, generating a small current. The greater the number of ions striking the plate, the greater the current generated. For example, if we were to sample air, the current generated by Nitrogen (Mass 28) would be greater than the currents generated by all other components.


The Seconday Electron Multiplier (SEM)

The seconday electron multiplier is a trumpet shaped device which has the inner surface coated with a special semiconducting glass.


Typically ions exiting the mass filter have an energy of +5 to +7 volts.


The mouth of the multiplier has a voltage of around -800V applied to it, This attracts the positively charged ion towards it and the ion strikes the inner surface, releasing electrons from the surface. The shape of the electron multiplier is such that the surfaces are struck many times, creating what is known as a cascade effect. This allows the SEM to amplify the original signal by a factor of up to 10^6, making it a highly sensitive and reliable component of the mass spectrometer.






Channel Plate Multiplier

Cheaper low end instruments use what is known as a channel plate multiplier. These are not used or favoured by ESS as they exhibit dark noise and have a poor detection level.


These work with a beam of electrons are sent towards a thin metal layer called the dynode. When the electrons hit the dynode, they cause the emission of secondary electrons, which are then sent towards the next dynode. This process is repeated several times, resulting in a stronger signal, though it contains lots of noise.



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