Anal. Chem. 1996, 68, 894-898
Effect of Helium in Helium Headspace Carbon Dioxide Cylinders on Packed-Column Supercritical Fluid Chromatography Eric Leichter,† J. Thompson B. Strode, III,† Larry T. Taylor,*,† and Frank K. Schweighardt‡
Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, and Air Products and Chemicals, Inc., Hamilton Boulevard, Allentown, Pennsylvania 19158
Supercritical fluid chromatography of PAHs was performed with pure carbon dioxide and helium headspace carbon dioxide at various cylinder fill levels. The retention times of the PAHs increased when helium headspace carbon dioxide was used as a carrier fluid relative to pure carbon dioxide. The increased retention times were affected by the level of the liquid phase present in the helium headspace carbon dioxide cylinder. As more liquid phase was removed from the cylinder, the effect of helium on the solvating power of CO2 was reduced because the relative amount of helium dissolved in the liquid phase decreased. Furthermore, the effect of helium headspace carbon dioxide was investigated with methanolmodified carbon dioxide mobile phases for the analysis of steroids. We observed that the relative solubility of helium in carbon dioxide resulted in longer retention times when compared to pure carbon dioxide as the liquid level of carbon dioxide decreased. Solvents used for supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC) are defined as supercritical fluids (SF). The most commonly used SF is carbon dioxide because of its low critical parameters (Tc ) 31.1 °C, Pc ) 7.37 MPa), low cost, and purity.1 Unfortunately, CO2 cannot be effectively pumped by standard high-pressure liquid chromatography pumps. Both syringe and piston pumps have difficulty filling because the CO2 enters the pump chamber as a compressible liquid.2 To decrease the compressibility of liquid CO2 entering the pumps, the temperature of the pump can be lowered by adding a device to cool the pump head (i.e., cooling jacket). This results in
