GC029 Practical Implications

A comparison of the effect of the carrier gas on the rate equation for a capillary column (H) is made. The following analysis holds for isothermal operation. The process can be optimized for column efficiency (plate number) or the analysis time. For a given column, the solute diffusivity is minimized (term B) using a higher-molecular-weight gas and more plates are generated. Nitrogen shows the minimum H, at the expense of slower analysis. To optimize for speed, however, it is better to choose a lighter carrier gas, like hydrogen or helium. The minima in the figure of H versus average linear velocity for nitrogen, helium and hydrogen are around 12 cm/s, 20 cm/s and 40 cm/s, respectively. On the other hand, hydrogen has the smallest slope beyond the minimum. Thus, an increase in hydrogen flow rate produces only a small loss in efficiency, while considerably speeding up the analysis.

Regarding film thickness, high efficiencies are good to separate high-boiling compounds. However, the samples should be small because these columns cannot handle large sample sizes. Additionally, small radius columns are preferred, especially if they are coated with thin films.

Finally, even though hydrogen produces the faster analysis, special care must be taken for its safe use.