Plate height theory for compressible mobile phase fluids and its application to gas, liquid and supercritical fluid chromatography

Characterization of carbon dioxide mobile phase density profiles in packed capillary columns by Raman microscopy

Efforts to understand and optimize separations employing compressible mobile phases have been limited by a lack of understanding of the mobile phase density gradient. Mobile phase compressibility leads to gradients in linear velocity and solute retention and affects separation speed and efficiency, especially in packed columns. Gas chromatography (GC), supercritical fluid chromatography (SFC), and solvating gas chromatography (SGC) each rely on compressible mobile phases. This work describes the on-column density measurement of carbon dioxide, a common carrier fluid for SFC and SGC, in packed capillary columns using Raman microspectroscopy of the position of the Fermi doublet. Correlation of the spectrum with density was calibrated over a pressure range of 15 to 290 atm at 125 and 150 degrees C, which then allowed for determination of the density gradient of fluid flowing through a packed capillary column. The results of this work will be used to model the flow behavior of compressible fluids to understand the effects of mobile phase compressibility on separation speed and efficiency.

Fast supercritical fluid chromatography hydrocarbon group-type separations of diesel fuels using packed and monolithic columns

Two approaches for decreasing diesel hydrocarbon group-type separation times by normal phase supercritical fluid chromatography (SFC) are compared. Short (10-15 cm) columns with small 3 microm diameter packing are compared with monolithic Chromolith bare silica columns under high carbon dioxide flow rates approaching 5 ml min(-1). Elution times are reduced up to 13-fold on a 10 cm Chromolith column and 7-fold on the short packed columns compared with conventional length columns run at typical flow rates. Short packed columns, with their higher surface area and retention characteristics, offer higher resolutions compared with Chromolith columns. Diesel samples are separated into saturates, mono-, di-, tri-, and polyaromatics in as little as 2 min on a 10 cm packed silica column. Diesel group-type results on a 15 cm titania-silica coupled column compare favorably with results from longer columns.

Efficiency for unretained solutes in packed column supercritical fluid chromatography

A general theory for efficiency of nonuniform columns with compressible mobile phase fluids is applied to the elution of an unretained solute in packed-column supercritical fluid chromatography (pSFC). The theoretical apparent plate height under isothermal conditions is given by the Knox equation multiplied by a compressibility correction factor f1, which is equal to the ratio of the temporal-to-spatial average densities of the mobile phase. If isothermal conditions are maintained, large pressure drops in pSFC should not result in excessive efficiency losses for elution of unretained solutes.