GC027 The Golay Equation: Capillary columns – Part 3

Mass transfer in the mobile phase can be related to the solute profile due to the nonturbulent flow through a tube. The molecules in the center of the tube move faster than those near the wall. Therefore, inadequate mixing (slow kinetics) result in band broadening. Columns with small diameter minimizes broadening because the mass transfer distances are reduced. The CM term for the Golay equation is:

CM = (1+6k+11k^2)*rc^2 / (24*(1+k)^2*DG)

Where rc is the radius of the column, k is the retention factor and DG is the diffusion coefficient.

The relative importance of the two terms CM or Cs is dependent on the column radius and film thickness. In columns of small inner diameter, the term CM is less dominant than Cs. On the other hand, for thin films (<0.2 µm) the mass transfer in the mobile phase (CM) controls the C term; in thick films (2-5.0 µm), the C term is controlled by the stationary phase mass transfer (Cs); for intermediate films (0.2-2.0 µm) both factors are important. For “wide-bore” columns, the mass transfer in the mobile phase is considerably greater.

The C terms are also multiplied by the linear velocity in the Golay equation. Therefore the terms are minimized at low velocities, which allow time for molecules to diffuse in and out of the liquid and also across the column in the mobile gas phase.

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