Liquid purification or separation – With means to add treating material – Chromatography
Patent
1993-03-10
1994-08-23
Therkorn, Ernest G.
Liquid purification or separation
With means to add treating material
Chromatography
210634, 210635, 210656, 210659, 96101, 96104, 422 70, 422 83, B01D 1508
Patent
active
053404750
DESCRIPTION:
BRIEF SUMMARY
The present invention relates generally to multidimensional chromatography, and in particular to an on-line multidimensional chromatographic system which incorporates supercritical fluid extraction to provide a method and system capable of rapid and efficient sample clean-up and analysis.
In order to analyze target compounds, such as trace pesticides residing on organic matter, substantial preparation is necessary to "clean-up" the sample. By clean-up, it is meant that the trace amount of pesticide or other target compound (e.g., chlorpyrifos) has to first be removed or extracted from the sample before the analysis can be performed. In this regard, a liquid is typically utilized as a solvent to extract the target compound from the sample matter (e.g., a blade of grass). In other words, the target compound is dissolved into the liquid solvent as the initial separation step. However, this procedure has several drawbacks, including the fact that it will be necessary to subsequently separate the extracted target compound from an excessive amount of the liquid solvent.
In contrast, supercritical fluid extraction offers several potential advantages over conventional liquid extraction methods as the initial sample preparation step. In this regard, a supercritical fluid may be defined using a phase diagram such as that shown in FIG. 1 for carbon dioxide. The regions corresponding to the solid, liquid, and gaseous states are well defined. However, at temperatures exceeding the critical temperature (T.sub.c), the densities of the liquid and vapor are identical and the fluid cannot be liquefied by increasing the pressure. The shaded area in the phase diagram illustrates the supercritical region. In this region, no phase change occurs, as the fluid is neither a liquid nor a gas. Rather, there is a transition from liquid to supercritical fluid as the temperature is increased at constant pressure, and there is also a transition from gas to supercritical fluid as the pressure is increased at constant temperature.
In general, extractions with supercritical fluids are faster and more efficient than conventional liquid or soxhelet extraction methods. Supercritical extraction is based upon the solubility of the target compound in the supercritical fluid, and this solubility property can be changed by varying the density of the particular supercritical fluid. In other words, a low density supercritical fluid approaching the qualities of a gas will typically not be as good an extraction fluid as one that approaches the densities of a liquid. Thus, the extraction strength of the supercritical fluid may be controlled by adjusting its density, which is in turn controlled by the temperature and pressure of the fluid. For example, because the compressibility of a supercritical fluid is large above the critical temperature, small changes in the pressure applied to the fluid will result in large changes in the density of the fluid.
Supercritical fluid densities can be two to three orders of magnitude larger than those of the gas. As a result of this larger density, molecular interactions in supercritical fluids increase due to shorter intermolecular distances. On the other hand, the viscosity and mass transport properties of supercritical fluids remain similar to those of a gas. The gas-like/liquid-like quality of supercritical fluids allow similar solvent strengths as liquids along with improved mass transport. Since supercritical fluids offer these two properties simultaneously, they provide the potential for rapid extraction rates and more efficient extractions. A further discussion of supercritical fluid extraction may be found in "Supercritical Fluid Extraction of Chlorpyrifos Methyl from Wheat at Part per Billion Levels", by Robert M. Campbell, David M. Meunier and Hernan J. Cortes, in the Journal of Microcolumn Separations, Volume I, No. 6, 1989, pages 302-308.
While supercritical fluid extraction ("SFE") offers several potential benefits as a tool to recover target compounds from complex sample matter, its utility would be su
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Campbell Robert M.
Cortes Hernan J.
The Dow Chemical Company
Therkorn Ernest G.
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