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