Determination of the column hold-up volume in supercritical fluid chromatography using nitrous-oxide

• Nitrous oxide is barely adsorbed by silica.
• Nitrous oxide absorbs UV around 190 nm.
• Nitrous oxide is a non-retained tracer useful in supercritical chromatography.

This study introduces a new tracer that is useful for the determination of the hold-up time or volume of packed columns, particularly of those used in supercritical fluid chromatography. The thermodynamic void volume of three columns packed with different adsorbents were determined using the weight difference method. These void volumes were used as the reference point in the further discussion. The hold-up volumes of these columns were determined under dynamic conditions, using nitrous oxide dissolved in methanol as the hold-up time marker. Changes in the hold-up volumes of these columns were monitored during changes of the volumetric flow rate of pure supercritical carbon dioxide and of dilute mixtures of organic modifier and supercritical carbon dioxide. The results suggest significant methanol enrichment on the adsorbent surface.