Mass spectrometer detector

A mass spectrometer detector turns arriving ions into a recorded electrical signal. It is distinct from the mass analyzer that makes mass-to-charge ratio information distinguishable. The measured response can depend on collection, conversion and amplification as well as on how many ions arrived.

A Faraday cup collects electric charge directly. Ideally, a flux $\Phi$ of ions with known charge $q$ gives $I=q\Phi$. For $10^6$ singly charged positive ions per second, the current is $1.602176634\times 10^{-13}\mathrm{A}$, about $0.1602\mathrm{pA}$. Escaping secondary electrons and background currents must be accounted for when inferring particle delivery.^[ref]

An electron multiplier instead uses secondary emission to amplify the initial conversion signal. The manufacturer's detector description shows discrete dynodes, a continuous-dynode alternative, and a conversion dynode before the multiplier.^[ref] In an illustrative equal-stage model, ten stages with mean yield $\delta =4$ give mean gain $G=4^{10}=1048576$. This is a model calculation, not a measured gain for a named instrument.

Detector gain and detection efficiency answer different questions: the size of an output event and the probability of registering an arriving ion. Increasing gain cannot recover missed ions. At high signals, mass spectrometer detector saturation can also invalidate proportionality between ion delivery and recorded response. Quantitative peak comparisons therefore need a tested response range and appropriate instrument calibration, not merely a large signal.