Once vaporized, {HEAD ~ the substance} is withdrawn from the chamber by {TAIL ~ the vacuum producing means} 
is withdrawn by
Once vaporized , { HEAD ~ the substance } is withdrawn from the chamber by { TAIL ~ the vacuum producing means }
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL