Dispersion in Chromatography Columns - The Van Deemter Equation > Page 56



Bearing in mind the values of (l) and (g) for a well packed column are 0.5 and 0.6 respectively,

                                     Hmin. = 1.48d

Thus, the approximate value of Hmin, for a well retained solute eluted from a well packed column and operated at the optimum linear mobile phase velocity, can be expected  to be about 1.48dp. Furthermore, to the first approximation, this value will be independent of the nature of the solute, mobile phase or stationary phase. For the accurate design of the optimum columns for a particular separation, however, this approximation should, in general, not be made, nevertheless, the value of 1.48 for Hmin is a useful guide for assessing the quality of a column. From d'Arcy's Law for fluid flow through a packed bed, at the optimum mobile phase velocity the length of the column is given by,


                      where  (P) is the inlet pressure to the column,

                                 (h) is the viscosity of the mobile phase,

                                 (j)  is the d'Arcy's Constant.