preferably has a 2 mm path length width with vessel 160b preferably having a 4 mm path length and vessel 160c preferably having a 10 mm path length each vessel includes windows not shown through which the light is directed the light is obtained from the light source 152 which preferably provides light in the near infrared nir spectrum the nir light from the light source 152 is carried via optical fibers 154b1 154c1 and 154d1 to the vessels 160a 160b and 160c respectively and light emerging from the vessels is carried by optical fibers 154b2 154c2 and 154d2 to a photo detector means 168 which is comprised of several arrays of photo detectors tuned to different frequencies of interest a microprocessor not shown coupled to the sample photo detector arrays is utilized to determine from which one or more of the arrays the frequency spectrum information is to be used light from the light source is also carried by fibers 154a to the photo detector means 168 and by fibers 154e1 to the known sample 172 and from the reference sample by fibers 154e2 to the photo detector means 168 if desired separate photo detectors means not shown can be provided for detecting light from fibers 154a and 154e2 in accord with yet another aspect of the invention analysis of the different hydrocarbon gas components of the gas stream is conducted by analysis of selected ch vibrational peaks in the near infrared range nir of.