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