# How do you do HETP calculations?

Well basically that is another term for the height equivalent thoretical plates. The number of theoretical plates is a measure of the efficiency of a column. The formula used is HETP=L/N. Basically it is the length of the column divided by the number of theoretical plates. The formula for calculating N is N=5.55(RT/W1/2)^2 . Where RT is the retention time of the compound and W1/2 is the width of the peak at half height. Determine N the obtain the length of the column and you have HETP. Hope it helped.
HETP also known as simply plate height is defined as the variance (standard deviation squared) of a particular band divided by the distance the band has traveled. It relates the bandwidth to the distance traveled.

H = sigma^2/x where sigma is the standard deviation of the band from the chromatagraph obtained. To measure sigma, one typically takes the width of a particular peak at half of the height (w 1/2) and divides that number by 2.35. x as mentioned above is the distance traveled on the column.

A more efficient separation typically has a smaller plate height and a larger number of theoretical plates.

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