The method 1 including collecting {HEAD ~ gas molecules} containing the other of said isotopes traveling along said {TAIL ~ other path} 
The method 1 including collecting { HEAD ~ gas molecules } containing the other of said isotopes traveling along said { TAIL ~ other path }
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL