{HEAD ~ Ions} with a specific mass to charge ratio may selectively pass through {TAIL ~ the duct} and impinge on the substrate surface 
selectively pass through
{ HEAD ~ Ions } with a specific mass to charge ratio may selectively pass through { TAIL ~ the duct } and impinge on the substrate surface
HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH