Once vaporized, {HEAD ~ the substance} is withdrawn from {TAIL ~ the chamber} by the vacuum producing means 
is withdrawn from
Once vaporized , { HEAD ~ the substance } is withdrawn from { TAIL ~ the chamber } by the vacuum producing means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