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