clarification needed in HETP calculation

[rrm1987]

in calculation theoretical plates the conversion faction factor(5.54-for half height method) and 16-for USP method). What does this conversion factor signifies?

[tom jupille]

The general formula is N = (tR/sigma)^2 where sigma is the square root of the peak variance (think of it as the standard deviation of the distribution). Assuming a normal (Gaussian) distribution, baseline width is 4*sigma, and width-at-half-height is 2.35*sigma (you can go to a standard statistical table and look up the sigma values at any fraction of the peak height).

[rrm1987]

thanks and can you explain how to calculate sigma value.

[tom jupille]

the formulas are here: http://en.wikipedia.org/wiki/Standard_deviation

From that you can see why we use width measurements!