GC025 The Golay Equation: Capillary columns – Part 1

 Capillary columns are open tubes; hence their rate equation does not have the A term from Van Deemter equation. Golay introduced a new term to account for the diffusion process in the gas phase for the open tubular columns. His equation additionally has two C terms: one for mass transfer in stationary phase, Cs and another for mass transfer in the mobile phase, CM. The simple Golay equation is:

H = B/µ + (Cs + CM)µ

The term B accounts for molecular diffusion. The equation for molecular diffusion is:

B = 2DG

Where DG is the diffusion coefficient for the solute in the carrier gas. A small value for the diffusion coefficient is desired to decrease the value of B and for H. The usage of gases of high molecular masses like nitrogen or argon as carrier gases aids in this sense. Additionally, a high linear velocity also decreases the time a solute spends in the column and the time available for molecular diffusion drops. On the other hand, the C terms are related to mass transfer, either in the stationary phase or in the mobile phase. Fast solute sorption and desorption minimizes band broadening, because this keeps molecules together.

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