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the Van Deemter equation, the Giddings equation and the Knox equation fit experimental (H) versus (u) data accurately and only the Van Deemter equation and the Giddings equation correctly account for other physical properties of the chromatographic system. The Van Deemter equation appears to be a special case of the Giddings equation, which simplifies to the Van Deemter equation when the mobile phase velocity is close to, or around, the optimum mobile phase velocity. The form of the Van Deemter equation and, in particular, the individual functions contained in it, are well substantiated by experiment. The Knox equation is obtained from an empirical fit to experimental data and the individual functions of other pertinent variables contained in the equation are not all substantiated by experiment. The Golay equation accurately described dispersion in capillary or open tubular columns but in GC the compressibility of the mobile phase must also be taken into account (Golay in his original

The Golay Equation for Open Tubular Columns The corresponding equation describing dispersion in an open tubular column was developed by Golay (10) for GC columns but is equally applicable to LC columns and to dispersion in connecting tubes. The Golay equation differs from equation (10) in that, as there is no packing, there can be no multipath term. Consequently, the equation contains only three functions. One function describes dispersion from longitudinal diffusion and the other two describes dispersion from the resistance to mass transfer in the mobile and stationary phases, respectively. The Golay equation takes the following form:- (11) where (r) is the column radius, and other symbols have the meaning

nbsp;        (51) The equation of Horvath and Lin was very similar to that of Huber and Hulsman and, in fact, only differed in the magnitude of the power function of (u) in their (A) and (D) terms. These workers were also trying to address the problem of a zero (A) term at zero velocity and the fact that some form of "turbulence' between particles aided in the solute transfer across the voids between the particles. The Golay Equation The basicequation describing the dispersion that takes place in an open tubular column was developed by Golay (8) for GC but is equally, and directly, applicable to LC. The Golay equation differs in one important aspect from the equations for packed columns in that, as there is no packing, there can be no multipath term or coupling factor and thus, contains only two functions. One function describes the longitudinal diffusion effect and the other the combined resistance to mass transfer terms for the

Taking  a value of 2.5 x10-5 for Dm (the diffusivity of benzyl acetate in n-heptane) equation (52) can be employed to calculate the curve relating (H) and (u) for an uncoated  capillary tube. The results  are shown in figure 17. It is seen that the Golay equation produces a curve identical to the Van Deemter equation but with no contribution from a multipath term. It is also seen that the value of (H) is solely dependent on the diffusivity of the solute in the mobile phase and the linear mobile phase velocity. It is clear that the capillary column can, therefore, provide a simple means of determining the diffusivity of a solute in any given liquid. The Golay equation (equation (52)) can be put in a simplified form in a similar manner to the equations for packed columns:-     &

Equation (31), however, was developed for a GC column and in the case of an LC column, the resistance to mass transfer in the mobile phase should be included. Van Deemter et al. did not derive an expression for f1(k') for the mobile phase but Purnell (10) suggested that the function of (k'), employed by Golay (8) for the resistance to mass transfer in the mobile phase in his rate equation for capillary columns, would also be appropriate for a packed column. The form of f1(k') derived by Golay was as follows,                                                              Thus, Van Deemter's equation for LC becomes,            (32

the equation he derived took the following form,     (1)   Where (H) is the variance per unit length of the column , (k)' is the capacity factor of the solute, (DM) is the diffusivity of the solute in the mobile phase, (DS) is the diffusivity of the solute in the stationary phase, (r) is the radius of the column, and (u) is the average linear velocity of the mobile phase.   In the development of equation (1), Golay did not take into account that, due to the compressibility of the mobile phase (a gas), the linear velocity changed significantly along the length of the column. Due to the velocity change not being linear the average velocity can not be used in equation (1) to accurately described (H) (the variance of the dispersion that takes pace in the column