Chromatography is a technical field where constituents of a gas sample are separated in order to be individually analysed. Typically, the gas stream outputted by a chromatography column includes one or more impurities or species to be detected carried by a carrier gas, different species being outputted at different moments in time.
Several types of detectors can be used to analyse the composition of the solute outputted by a gas chromatography system. One category of such detectors is Photo Ionisation Detectors (PID). Generally, PIDs are gas detection devices where the gas sample to be analysed is ionized by optical radiation. Photons of energy above the ionisation energy of the constituents of the gas to be analysed are impinged on the sample, breaking the molecules in the sample into free electrons and positively charged ions. This generates an electrical current in the gas sample which can be measured.
The high energy photons used to ionise molecules in a PID can come from various sources. For typical chromatography applications, helium discharge ionisation detectors are often used. FIG. 1 (PRIOR ART) schematically illustrated a basic configuration for a helium discharge ionisation detector as known in the art. Such a device includes a discharge zone where a pure helium gas is inserted, normally supplied from a gas purifier, and an ionisation zone in which the gas to be analysed is received. The discharge zone is submitted to a high voltage electrical field, generating a plasma from which radiation in the UV range is emitted. This radiation is used to ionize the constituents of the gas to be analysed in the ionisation zone.
A helium discharge ionisation detector such as shown in FIG. 1 requires a pure helium supply connected to the discharge zone. The extra helium injected into the detector contributes to dilute the impurities eluting from the chromatography column and inserted at the opposite side of the detector, since this volume of the helium gas is added to the volume of the sample.
There remains a need for improved discharge-based photo ionisation detectors suitable for chromatography applications.