Effect of pressure drop on solute retention and column efficiency in supercritical fluid chromatography Part 2: Modified carbon dioxide as mobile phase

Abstract

The effect of pressure drop on the performance of supercritical fluid chromatography systems using a modified mobile phase (carbon dioxide + ethanol) was studied. Experiments were performed on a Lichrospher-RP-18 column with phenanthrene as a solute. A wide range of back pressures (130 to 210 bar) and modifier concentrations (2 to 7% w/w) have been explored. Experiments yielding both small and large pressure drops were performed. From these experiments, parameters to describe pressure drop, retention, and column efficiency were extracted, and were used to simulate the dynamics of the chromatographic column. A good match between the experimentally measured and calculated values of pressure drop, retention times, and column efficiency was observed. At low back pressure and modifier composition, significant loss of column efficiency was observed.

Unexpected retention and efficiency behaviors in supercritical fluid chromatography: A thermodynamic interpretation

“…The relationship between the logarithm of the retention factor of the analytes and the acetonitrile content in CO 2 is usually strongly non-linear when increasing the volume fraction of the organic modifier from zero to a few pecents. For instance, under isopycnic conditions, the retention factor of phenanthrene was reported to decrease from 10 to 6 and from 6 to only 5.5 when increasing the volume fraction of ethanol from 0 to 2% and from 2 to 7%, respectively [13]. This justifies the selection of a non-linear solvation retention model regarding the impact of the content of acetonitrile on the retention factor of the alkylbenzenes (see Eq.…”

“…The adsorption isotherms of the seven n-alkylbenzenes is assumed to be linear (infinitesimally small amount injected) and competitive (with respect to the acetonitrile concentration C A ). The retention factor, k n , of the homologous compound C n (n is the number of carbon atoms in the alkyl chain), is best described by a curved non-linear solvation model [13,29]:…”

“…The efficiency of SFC columns may also be very sensitive to the surrounding thermal environment [9][10][11]. The reproducibility of SFC data depends strongly on the control of the inlet mass flow rate [12], the pressure set by the active back pressure regulator (BPR), the oven temperature, the nature (methanol, acetonitrile, ethanol,...) and concentration (0-40% in volume) of the organic modifier [13][14][15], the nature of the sample solvent [16,17], and the thermal environment in which the SFC column is placed [9][10][11]. SFC instruments, columns, and methods should then be well controlled for the sake of data robustness.…”

Supercritical Fluid Chromatography

“…This effect is dependent on the percentage of organic modifi er (Rajendran et al, 2005 ). For CO 2 -based binary fl uids, studies have shown that lower back -pressures produce lower plate counts and the effect is more pronounced with a lower percentage of modifi er (Rajendran et al, 2008 ).…”

Maximizing kinetic performance in supercritical fluid chromatography using state-of-the-art instruments

“…Because of these complex behaviours, the approaches used in HPLC to evaluate kinetic efficiency cannot be easily extrapolated to SFC conditions. Currently, three different methodologies are applied for evaluating the kinetic performance of SFC separations: (1) conventional method: changing the flow rate and column inlet pressure with fixed column length at constant outlet pressure, yielding changes in density [26][27][28], (2) isopycnic method: changing the flow rate at a given column length and keeping the average pressure (and thus average density) constant by adjusting the outlet pressure [29] and (3) variable column length method: changing the flow rate and column length by keeping the same inlet and outlet pressure (and thus the average density constant) [24].…”