The extraction of analytes from aqueous solution using supercritical fluids
Hedrick, Joseph L.
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Supercritical fluid extraction (SFE) is becoming an attractive alternative to conventional solvent extraction for many reasons. These reasons include advantages of speed, the ability to be automated more easily than conventional solvent extractions and the lack of solvent disposal after the extraction has been performed. At this date SFE is performed in a multitude of ways with no one method out performing the others. The different ways in which SFE is performed is reviewed. Supercritical fluid extraction of analytes from aqueous solution has not received much attention. The design of a system which allows for the extraction of analytes from aqueous solution has been explored in this thesis. Several related areas (injection techniques for supercritical fluid chromatography and on-line SFE) were also developed. The injection port of a supercritical fluid chromatograph was modified to provide better ( more reproducible) sample introduction. For a 100 ppm 3,5-nitrobenzamide solution in methylene chloride the area reproducibility was increased from 3.2% RSD for the unmodified valve to 0.74% RSD for the modified valve. The method also resulted in a more narrow solvent front as well as an increase of 10% in the number of theoretical plates of the system. On-line SFE\SFC was explored as one possible configuration for the extraction of analytes from aqueous solution. Solvent elimination injection (SEI), was developed for SFC. The difference in vapor pressure between the analytes and solvent allowed for the solvent to evaporate and be transported form the system while the analytes were collected on various traps. After evaporation of the injection solvent the analytes were flushed onto a chromatographic column. SEI allowed for the reproducible injection of larger volumes of sample (solvent and analyte) into the system. SEI allowed for different hardware configurations to be tested without performing an actual supercritical fluid extraction. An off-line solid phase trapping system for SFE was developed. The system trapped the analytes from the SFE effluent onto a solid phase extraction cartridge. The cartridge could then be rinsed in a normal fashion to elute the analytes of interest. Trapping in this way was found to allow for faster extraction rates than liquid trapping. The efficiency of the trapping mechanism was found to be dependent upon the temperature of the trap, the chemical functionality of the phase bonded io the silica and the nature of the analytes. A system which allowed for the extraction of moderate volumes (3-5 mL) of aqueous solution was developed. A test solution of phenols was used to evaluate the system. The extractability of the phenols was found to be a function of pressure of the system and the chemical nature of the phenol. A decrease in extractability of the phenols was found to take place at pressures greater than 250 atm. The distribution coefficient of phenol was found to increase steadily through 400 atm. A decrease in surface area of the supercritica1 fluid passing through the aqueous solution was thought to be responsible for the apparent contradiction in behavior.
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