Steven Hitchen

Selective extraction of organochlorine and organophosphorus pesticides using a combined solid phase extraction—supercritical fluid extraction approach

Supercritical fluid extraction (SFE) of pesticides has been investigated for the possibility of selective extraction between organochlorine (OCPs) and organophosphorus (OPPs) compounds. Three OCPs and OPPs were trapped onto a C18 EmporeTM extraction disk prior to SFE with CO2 only and methanol-modified CO2 at various pressures and constant temperature. The results indicate that OCPs can be quantitatively extracted using CO2 only whereas OPPs require a modifier for extraction.

Supercritical fluid extraction of organochlorine pesticides from an aqueous matrix

Abstract Supercritical fluid extraction (SFE) conditions were optimised for the removal of organochlorine pesticides (OCPs) from water. OCPs were collected and extracted from sofid-phase extraction disks (Empore) and also directly from a water sample using a modified extraction cell. High recoveries (>90%) were obtained for two of the three OCPs with Empore disks. Despite the good solubility of OCPs in pure CO2, the analyte recoveries decreased when they were extracted directly from water. Three different flow-rates were used in the direct SFE with no apparent change in recovery, indicating that extraction was diffusion-controlled. The effect of increasing the ionic strength of the aqueous sample on analyte recovery was investigated.

Experimental design approach for supercritical fluid extraction

Experimental design with multilinear regression has been used to examine the relative contribution of the main experimental variables during supercritical fluid extraction. Six steroidal compounds of various solubilities in supercritical CO2 were considered. The results indicate that the density of the supercritical fluid has the greatest effect on the solubilisation and transfer of steroid from extraction cell to collection device. The minimum number of cell volumes of supercritical CO2 required for effective extraction was experimentally determined.

Extraction of surfactants from aqueous media by supercritical fluid extraction

Two methods for the supercritical fluid extraction (SFE) of analytes from aqueous media were evaluated: direct extraction and solid-phase extraction (SPE) discs. SPE discs were used to isolate an alcohol phenol ethoxylate (APE) non-ionic surfactant from an aqueous matrix prior to extraction using supercritical CO2. This method was compared with the direct SFE of surfactant from the aqueous matrix. The second method allowed the continuous extraction of analyte from water using a modified extraction cell. The extraction cell was designed to maximize the exposure of the analyte to supercritical CO2 interactions and thereby allow the continuous extraction of the analyte from an aqueous sample. The results suggest that there is a difference in the effects of diffusion and equilibrium on the extraction process for the two methods of extraction studied.

Determination of octanol-water partition coefficients using gradient liquid chromatography

Abstract The use of liquid chromatography to estimate octanol-water partition coefficients, log Kow, has been demonstrated. The capacity factor with a mobile phase of 100% water, kw, has been found to be a useful Chromatographic parameter. Values of Kw were derived from a computer software package (Hipac-G), designed for optimization and simulation of gradient LC systems. The chromatographic parameter, Kw, correlates well with both literature and isocratic LC-generated log Kow values. The gradient LC method could be extended to a wider range of test compounds.

Properties of supercritical fluids

A supercritical fluid is a substance with both gas-and liquid-like properties. It is gas-like in that it is a compressible fluid that fills its container, and is liquid-like in that it has comparable densities (0.1–1 g ml-1) and solvating power. Supercritical behaviour only occurs when the substance is above its critical temperature and pressure. However, the converse is not always true—pressures and temperatures above the critical values do not always result in a supercritical fluid. At very high pressures (e.g. > 108 Pa) the freezing curve can rise into the supercritical fluid region and so both solid and supercritical phases can exist (Scholsky, 1989) (Figure 1.1). For analytical scale extraction the term supercritical generally refers to conditions above the critical temperature and close to the critical pressure.

Predicting solubility in supercritical fluid extraction using a neural network

A neural network has been constructed for prediction of the solubility of analytes in supercritical carbon dioxide. Preliminary studies for the input of molecular structure into the network indicates that connectivity indices are adequate to provide structural information in a condensed form. This allows neural networks, which would otherwise be very extensive, to have reduced training times; it also reduces the possibility of memorization of the training data and over-training of the network.

Interaction between carbon dioxide and naphthalene: a molecular modeling approach

The formation of a charge‐transfer complex between carbon dioxide and naphthalene was studied using a molecular modeling program, with the aim of studying the solubility of naphthalene in supercritical carbon dioxide. The orbitals involved in the formation of the complex were studied using MINDO/3 as the semi‐empirical method. A solvent cage was constructed, and the maximum number of carbon dioxide molecules to surround naphthalene was found to be 20. The heat of interaction of the complex was obtained using MINDO/3.