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