Mobile and Stationary Phase Contributions to Peak Dispersion in Gas Chromatography

Abstract

The independent contributions to the plate height from processes occurring in the mobile and stationary phases in GC have been found by determining the plate height as a function of velocity under different average column pressures, and with different carrier gases. It is confirmed that over a 100-fold range of reduced velocity the stationary phase contribution to the plate height obeys the simple equation H_s = C_sū. the values of C_s for solutes on 235 µm diameter Chromosorb G bearing 1% and 6% w/w of squalane lie between 2 and 5 × 10⁻³ s, but C_s does not decrease with retention as much as theory predicts for column capacity ratios, K, exceeding unity. The mobile phase contribution to the plate height shows a characteristic non-linear dependence upon fluid velocity confirming the importance of “coupling.” The dependence is best represented by an empirical equation H_m = B/u + (A + C/uⁿ)⁻¹ where n lies between 0.45 and 0.65 and increases with K. H_m at any velocity increases quite strongly with K to a maximum when K is large. The increase is about threefold at low reduced velocities, and about six-fold at high reduced velocities. The dependence cannot be accounted for entirely by shortrange interchannel velocity variations or by slow interchange of solute between streaming and stagnant carrier gas, and it seems most likely that non-uniform distribution of stationary phase across the column cross section is an important factor. An expression is derived for the plate height contribution from such non-uniformity.