Effect of Various Binary Gas Mixtures as Mobile Phases on Theoretical Plate Number in Capillary Gas Chromatography

摘要

The effect of mixtures of hydrogen and helium and hydrogen and nitrogen on the theoretical efficiency of gas phase open-tubular column separations are reported. Two mixtures of N_2-H_2 and He_H_2, as well as the three pure gases, are used as the mobile phase. These 7 mobile phases provide a range of densities, diffusivities, and viscosities for studying the effects of these mass transfer properties on the height equivalent of a theoretical plate (HETP). A commercially available test mixture composed of various chemical classes is analyzed in triplicate with each carrier gas. The average HETP for each compound in the mixture is calculated and plotted against average linear velocity, mobile phase density, and inlet pressure. The He-H_2 mixes are very similar to each other in their effect on HETP at high gas velocities; their close agreement appears to be predicated on the narrow range of densities calculated for each mixture. Hydrogen, on the other hand, strongly affects the performance of nitrogen at all velocities. The wide spread of HETP among the N_2-H_2 mixes is also commensurate with their densities. The HETP of the 40% N_2-60% H_2 mobile phase at the lowest velocities is higher than the 60% N_2-40% H_2 mix. As the velocity increases to 40 cm/s, the 40% N_2 mix actually matched the performance of the He-H_2 mixes. Little plate height is sacrificed, even at 50 cm/s average linear velocity. This 40% N_2 mix is somewhat analogous to a multiviscosity motor oil; its N_2 component limits longitudinal diffusion at low velocities, and the H_2 component facilitates rapid mass transfer at higher velocities. The influence of the molecular volume of the probes on HETP is studied. The aromatic solutes produce lower HETP than the hydrocarbons at linear velocities greater than the optimum, but the aromatics produce higher HETP at suboptimum velocities. The responsible factor here is the binary diffusion coefficient, which is a function of the molar volume or size of the molecule.