Inside a PID analyser
Explore a photoionisation detector. Change the gas, UV lamp energy and light intensity to see how photons release electrons and create a measurable signal.
Photons. Charge. Insight.
Inside a PID analyser.
A photoionisation detector turns ultraviolet light into a gas signal.
Explore the energy threshold between a neutral molecule and an ion.
Follow the photoionisation
Light in. A tiny current out.
- UV photon · hν
- Neutral molecule · M
- Electron · e⁻
- Positive ion · M⁺
Explore the detector
About this model
Schematic geometry: in a real detector the electrodes form a thin stack directly on the lamp window, with a gap of a few millimetres, and the UV penetrates only about a millimetre into the gas. Only a tiny fraction of the molecules is ionised (well below 0.1 %), so the sample leaves almost unchanged and the PID is non-destructive. Intensity changes the number of photons, not their energy; use the intensity slider as a stand-in for a fouled window or an ageing lamp. High humidity also lowers the signal (quenching), which is not modelled here.
A photon carries energy.
The UV lamp sends photons through its window into the gas. Lamp energy determines which molecules can be ionised; intensity sets the photon supply.
A molecule loses an electron.
If a photon has sufficient energy, it can form M⁺ and e⁻. Try ethanol at 10.0 and 10.6 eV, or acetylene at 11.7 eV. Methane stays below detection with any standard lamp, which is why a PID is blind to natural gas.
Charge becomes a signal.
The electric field collects the charges. A PID responds to many compounds at once and not equally: it is calibrated on isobutylene, and every other gas reads through its response factor. 100 ppm ethanol reads as about 10 ppm isobutylene equivalent. The total signal does not identify individual gases in a mixture.