elution time is likely to be impractical. Differentiating the van Deemter equation with respect to velocity, setting the resulting expression equal to zero, and solving for the optimum velocity yields the following: Plate count The plate height given as: with the column length and the number of theoretical plates can be estimated from a chromatogram by analysis of the retention time for each component and its standard deviation as a measure for peak width, provided that the elution curve represents a Gaussian curve. In this case the plate count is given by: By using the more practical peak width at half height the equation is: or with the width at the base of the peak: Expanded van Deemter The Van Deemter equation can be further expanded to: Where: H is plate height λ is particle shape (with regard to the packing) dp is particle diameter γ, ω, and R are constants Dm is the diffusion coefficient of the mobile phase dc is the capillary diameter df is the film thickness Ds is the diffusion coefficient of the stationary phase. u is the linear velocity Rodrigues equation The Rodrigues equation, named for Alírio Rodrigues, is an extension of the Van Deemter equation used to describe the efficiency of a bed of permeable (large-pore) particles. The equation is: where and is the intraparticular Péclet number. See also Resolution (chromatography) Jan van Deemter References Chromatography Equations