{HEAD ~ Beams} and produced by helium neon gas lasers and are directed at photoacoustic absorption cells and containing a flowing sample gas stream recovered from {TAIL ~ coal}, oil and natural gas well drilling 
{ HEAD ~ Beams } and produced by helium neon gas lasers and are directed at photoacoustic absorption cells and containing a flowing sample gas stream recovered from { TAIL ~ coal } , oil and natural gas well drilling
HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH