▁exit ▁velocity , ▁that ▁is , ▁the ▁ratio ▁of ▁the ▁flow ▁rate ▁in ▁m l / second ▁to ▁the ▁cross - section al ▁area ▁of ▁the ▁‘ column - exit ▁flow ▁path .’ ▁For ▁a ▁pack ed ▁column , ▁the ▁cross - section al ▁area ▁of ▁the ▁column ▁exit ▁flow ▁path ▁is ▁usually ▁taken ▁as ▁ 0 . 6 ▁times ▁the ▁cross - section al ▁area ▁of ▁the ▁column . ▁ ▁Alternatively , ▁the ▁linear ▁velocity ▁can ▁be ▁taken ▁as ▁the ▁ratio ▁of ▁the ▁column ▁length ▁to ▁the ▁dead ▁time . ▁ ▁If ▁the ▁mobile ▁phase ▁is ▁a ▁gas , ▁then ▁the ▁pressure ▁correction ▁must ▁be ▁applied . ▁The ▁variance ▁per ▁unit ▁length ▁of ▁the ▁column ▁is ▁taken ▁as ▁the ▁ratio ▁of ▁the ▁column ▁length ▁to ▁the ▁column ▁efficiency ▁in ▁theoretical ▁pl ates . ▁ ▁The ▁van ▁De em ter ▁equation ▁is ▁a ▁hyper bol ic ▁function ▁that ▁predict s ▁that ▁there ▁is ▁an ▁optim um ▁velocity ▁at ▁which ▁there ▁will ▁be ▁the ▁minimum ▁variance ▁per ▁unit ▁column ▁length ▁and , ▁th ence , ▁a ▁maximum ▁efficiency . ▁The ▁van ▁De em ter ▁equation ▁was ▁the ▁result ▁of ▁the ▁first ▁application ▁of ▁rate ▁theory ▁to ▁the ▁chrom at ography ▁el ution ▁process . ▁ ▁Van ▁De em ter ▁equation ▁The ▁van ▁De em ter ▁equation ▁rel ates ▁ ▁height ▁equivalent ▁to ▁a ▁theoretical ▁plate ▁( H ET P ) ▁of ▁a ▁chrom at ographic ▁column ▁to ▁the ▁various ▁flow ▁and ▁kin etic ▁parameters ▁which ▁cause ▁peak ▁bro aden ing , ▁as ▁follows :