Читать книгу Chemical Analysis - Francis Rouessac - Страница 35

Term C, which is related to the resistance to mass transfer of the solute between the two phases, becomes dominant when the flow rate is too high for an equilibrium to be attained. Local turbulence within the mobile phase and concentration gradients slow the equilibrium process (C_S ⇔ C_M). The diffusion of solute between the two phases is not instantaneous, and thus the solute will be carried along out of equilibrium. No simple formula exists which takes into account the different factors involved in term C. The parameter C_G is dependent upon the diffusion coefficient of the solute in the gaseous mobile phase, while the term C_L depends upon the diffusion coefficient in the liquid stationary phase.

In practice, the values for the coefficients A, B, and C can be obtained by making several measurements of efficiency for the same compound undergoing chromatography at different flow rates, since flow rate and average linear speed are related. Next, the hyperbolic function that best satisfies the experimental values can be calculated, preferably by using the multiple linear regression method.