for hydrocarbonmethane analysis first valve means 30 is first switched to input 32 so that air containing no hydrocarbons will be input to system 37 and the zero level on the hydrocarbon analyzer instrument 70 set in a manner appropriate to the instrument employed first valve means 30 is then switched to input 34 to accept samples being supplied thereto first valve means 30 is of course only a convenience item and sample air inputs could be applied directly into coupling 38 by removable lines to the various input sources with the sample being applied to system 37 second valve means 54 is then used to cause the sample gas to pass through reactor 46 or bypass it by switching to input 58 for gas through reactor 46 or input 52 to bypass when doing methane analysis the bleed line 60 is set at a rate which will allow the complete photo oxidation of all hydrocarbons except methane and a proportional photo oxidation of methane with second valve means 54 switched to the input 52 position a first reading is then made with instrument 70 which is the total hydrocarbon content of the sample second valve means 54 is then switched to the input 58 position and a reading taken on the sample now having passed through reactor 46 this reading is proportional to the methane content of the sample and must be rescaled to full value the exact rescaling factor will vary from reactor to reactor.