{TAIL ~ An adsorption column} having completed the adsorption step and another adsorption column having completed {HEAD ~ the vacuum purge step} 
{ TAIL ~ An adsorption column } having completed the adsorption step and another adsorption column having completed { HEAD ~ the vacuum purge step }
TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD