Faraday Cup

Faraday cups are perhaps the oldest and most simple detectors used in mass spectrometers.  At a very base level, a Faraday cup is a piece of metal that resides in the mass spectrometer's vacuum chamber and is connected to the instrument's electronics.  Electric fields are utilized to push ions into the piece of metal.  When ions strike the metal, electrons flow through the circuit to meet the ions and neutralize them at the Faraday cup's surface.  This current can be measured and amplified by the instrument's electronics.  The amount of current is proportional to the number of ions hitting the Faraday cup.  Faraday cups have several desirable qualities:

  • Simplicity
  • Quantitative accuracy
  • Robustness
  • Operation at any pressure

Perhaps the biggest disadvantage of Faraday cups relative to more modern detectors is their relatively low sensitivity.  The Faraday itself essentially provides no gain.  For each ion that strikes the plate, an electron comes to meet it.  As such, any amplification must be done in the circuitry that measures the current.  Conversely, detectors like the electron multiplier offer large gains in current (on the order of 106).