Use of various filters for generating modified supercritical fluid mobile phases

Adding various components to supercritical carbon dioxide in supercritical fluid chromatography can extend or significantly alter the solvating properties. Polar samples which are difficult to analyze with pure supercritical CO2 because of their high polarity can be separated by addidng polar modifiers. In this paper, a new mixing method using an HPLC filter for adding polar modifier to CO2 is described. Although several filters were tried, only one could keep the amount of modifier in the mobile phase constant for a long time. The amount of water or methanol dissolved in supercritical CO2 was measured by an amperometric microsensor made of a thin film of perfluorosulfonate ionomer (PFSI).