{HEAD ~ The methane gas} within {TAIL ~ cell} weakly absorbs the infrared energy of laser beam , and thus the resulting output signal produced by a transducer serves as a reference signal for determining the methane concentration 
within
{ HEAD ~ The methane gas } within { TAIL ~ cell } weakly absorbs the infrared energy of laser beam , and thus the resulting output signal produced by a transducer serves as a reference signal for determining the methane concentration
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH