DYNAMIC FILM FORMATION AND THE USE OF RETENTION GAPS WITH DIRECT INJECTION IN OPEN-TUBULAR SUPERCRITICAL-FLUID CHROMATOGRAPHY

Open-tubular supercritical fluid chromatography (SFC) is a useful technique for many analyses, but is somewhat limited, in current practice, with respect to trace analysis of liquid samples. Direct injection of the volumes required for trace analysis with ordinary detectors creates a meter or more length of flooded zone and may lead to unacceptable band broadening. We studied solvent mass transfer on short, uncoated columns for several solvents forming Type I binary mixtures with CO2, and found evidence in the solvent peak shapes of distinct liquid and vapor phases, with the liquid present as a dynamically formed film. This study suggests that selecting an injection temperature and pressure to reduce, but not eliminate, the surface tension of the liquid-vapor interface, and increasing the mobile phase velocity during injection increase the liquid film thickness and reduce the length of the resulting flooded zone. When a separate uncoated inlet tube (that is, a retention gap) is used to connect the injector with the column, decreasing the inlet tube radius improves the ratio of maximum effective injection volume to inlet tube volume. Just as in GC, dynamically formed films function as pseudo-stationary phases during injection. Without a liquid film, solutes can be transported by injection-solvent-modified mobile phase well beyond the length of a typical film, increasing the difficulty of refocusing the solutes before separation. Refocusing solutes from film-coated flooded zones is easily accomplished using either a solvent effect (specifically, solvent trapping) or phase-ratio focusing. We recommend the use of solvents with phase behavior like toluene and carbon tetrachloride, both of which form films at convenient temperatures and pressures. We discourage the use of solvents with phase behavior like n-pentane which tend to make single-phase, supercritical mixtures with CO2 under typical SFC injection conditions.