Gas analyser teardown 

The flash tube is a Xenon flash lamp with two internal trigger electrodes, broad spectrum so it required the 4 angled reflective wavelength filters. The continuous wave finger lamp is a UV lamp with a narrower wavelength spread and only has the single band pass filter in front. I have examined a newer instrument and the optical system is still almost the same, CW UV lamp, single filter, heated block, optical feedback and Peltier cooled PMT tube detecting light in the common volume, with the extra band pass filter to block the excitation light and pass the SO2 fluorescence wavelength. It also had a concentric tube-in-tube scrubber that was there for removing hydrocarbon vapours from the sample stream and had an option for an internal span sample source, basically a vial of SO2 with a Teflon seal that diffuses a tiny amount that is calibrated at 75degC I think. The source cannot be left in these machines as it will contaminate everything if the pumping system does not continuously purge the piping to the outside. Ozone does rust stuff. Hollow cathode tubes are generally of a different construction with the optical output emitted from the hollow cathode area. Usually they have heat fins inside the tube too as the cathode runs hot.

Even today 2022 they are surprisingly similar. More computerisation and automated calibration, the sensor in a newer unit I investigated is functionally identical to the second unit Mike disassembled. Look up Teledyne API NOx and SO2 fluorescence analysers, you can find pictures of the internals on the web and they are the same as these old units.

Mark Hodgson He says it at the beginning of almost every video: eBay.

Dan Frederiksen bit late but search for "FTIR"; that detects multiple substances at one go...