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, but the dependence of (Hmin) on diffusivity is extremely small for the solute benzyl acetate. The slight slope of the line for the solute hexamethylbenzene might well result from the fact that either the (A) term is not completely independent of the diffusivity (Dm) as shown by the results in figure  21, or the resistance to mass transfer in the stationary phase does make a small but significant contribution to the value of (H). The curves relating the optimum velocity with solute diffusivity are shown in figure 24 and it is seen that the straight lines predicted by the Van Deemter equation are realized for both solutes. It should be noted that similar treatment of the Knox equation does not predict that values of Hmin should be independent of the solute diffusivity neither does it predict that (uopt) should vary linearly with solute diffusivity. This is strong evidence, supporting the validity of the Van Deemter equation, as opposed to the Knox equation.   

mobile phase velocity and the solute diffusivity. The fit of the Van Deemter equation to the experimental data confirms the former condition and the plot of the (A) term against solute diffusivity (the data taken from tables 1 and 2 and shown in figure 20 confirms the latter. J.Chromatogr.,270(1983)62 Figure 20. Graph of A Term against Solute Diffusivity for Benzyl Acetate Figure 20 demonstrates that the magnitude of the (A) term appears, within experimental error, independent of the diffusivity of the solute in the mobile phase. Closer examination, however, indicates that there might be a slight residual dependence of (A) on diffusivity. This probably indicates that the velocity range over which the data was taken was not sufficiently high enough to ensure that the first term of the Giddings equation was reduced to a constant value as it is in the simple Van Deemter equation. This can be examined further by considering the detailed expression for the first term of 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:-                          

nbsp;         where (e) is a constant, probably close to unity. However that there are two ways in which the diffusivity of the solute in the mobile phase can be changed. It can be modified as a result of changing the solute which is being eluted, in which case the above assumptions are valid as (Ds) is likely to change linearly with (Dm). The diffusivity can also be modified by choosing an alternative mobile phase  in which case (Dm) will be changed but (Ds) will remain the same. Under these circumstances the above assumptions are not likely to be precisely correct. Nevertheless, if the

used. The permeability increases as the square of the particle diameter but the variance per unit length only increases linearly with the particle diameter. Thus, doubling the particle diameter will allow a column four times the length to be used but the number of plates per unit length will be halved. Consequently, the column efficiency will be increased by a factor of two. It is also seen that the higher efficiencies will be obtained with mobile phases of low viscosity and for solutes of low diffusivity. Solvent viscosity and solute diffusivity tend to be inversely proportional to each other and so the sensitivity of the maximum obtainable efficiency to either solvent viscosity or solute diffusivity will generally not be large. The approximate length of a column that will provide the maximum column efficiency when operated at optimum velocity is given by, l = nHmin

Effect of Mobile Phase Compressibility On the HETP Equation for a Packed GC Column  As the pressure falls along the column length, the velocity changes and, as the solute diffusivity depends on the pressure, the diffusivity of the solute will also change. The multi-path term, which contains no velocity or gas pressure dependent parameters, will be unaffected and the expression that describes it the same. The other terms in the  HETP equation, however, all contain parameters that are affected by gas pressure (solute diffusivity and mobile phase velocity) and, therefore, need to be modified to accommodate the compressibility of the mobile phase.   &