Question

Briefly state the principle of a Faraday Cup detector for a mass spectrometer.

Answer 1

A Faraday cup is a metal cup designed to catch charged particles in vaccum. The resulting current can be measured used to determine the number of ions or electrons hitting the cup.

When a beam of ions hits the metal it gains a small net charge while the ions are neutralized. The metal can then be discharged to meausre a small current equivalent to the number of impinging ions. Essentially the Faraday cup is part of a circuit where ions are the charge carriers in vacuum and the Faraday cup is the interface to the solid metal where electrons act as the charge carriers. BY measuring the electric current in the metal part of the circuit the number of charges being carried by the ions in the vacuum part of the circuit be determined. For a continuous beam of ions Faraday cup with an electron - suppressor plate in front

N / t = I / e

where, N is the number of ions observed in a time t in seconds , I is the measured current and e is the elementary charge. Thus a measured current of one nanoamp corresponds to about 6 billio ions striking the Faraday cup each second.

Similarly, a Faraday cup can act as a collector for electrons in a vacuum. In this case electrons simply hit the metal plate and a current is produce. Faraday cups are not as sensitive as electro multiplier detectors, but are highly regarded for accuracy because of the direct relation between the measured current and number of ions, This device is considered a universal charge detector because of its independence from energy, mass, chemistry etc. of the analyte.