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The Golay equation provides the following expresion for the theoretical plate height (H):
H = B/u + Csu + Cmu
where u is the flow rate (cm/s), B is a longitudinal diffusion term, Cs is a mass transfer term for the analyte in or on the stationary phase, and Cm is a mass transfer term for the analyte in the mobile phase. The longitudinal diffusion is the usual diffusion due to a concentration gradient, with analyte in a band moving through a column diffusing from the center of the band forward and backward. The mass transfer terms depend on the diffusion coefficients for the analyte in the stationary and mobile phases, and can be thought of as interactions that delay the analyte and lead to band broadening. The stationary phase mass transfer term, Cs, also depends on the thickness of the stationary phase, and the mobile phase mass transfer term, Cm, also depends on the particle size of the packing material due to its effect on eddy diffusion.
Plot of H (and components) vs. flow rate
Note that there is an optimum flow rate to obtain the minimum theoretical plate height.
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