Short communicationEsterification of decanoic acid during supercritical fluid extraction employing either methanol-modified carbon dioxide or a methanol trap
Introduction
An advantage of supercritical fluid extraction (SFE) involves the ability to perform derivatization reactions during the extraction. Field [1] has published an excellent review of derivatization reactions coupled with SFE, which includes a discussion of the theories and approaches to derivatization with SFE, as well as many applications. In addition to organics, organometallics have also been derivatized [2], [3]. Hawthorne et al. [4] used trimethylphenylammonium hydroxide and boron trifluoride to enhance the extraction of microbial phospholipid fatty acids from whole cells and wastewater phenolics from water, both as their methyl esters.
Rochette et al. [5] investigated a variety of sample preparation methods to improve extraction recoveries of the pesticide (2,4-D) from soils. They used silylation, ion-pairing, methyl esterification and ionic displacement, finding the methyl esterification and ionic displacement to be the most promising for quantitative SFE work. Meyer and Kleibohmer [6] developed a rapid efficient method for the extraction of pentachlorophenol (PCP) from wood and leather products based on an in-situ derivatization method (acetylation with triethylamine and acetic anhydride). They found comparable results with conventional methods and SFE reduced analysis times from about two days to less than 3 h [7], [8]. Lee et al. [9] also developed an in-situ derivatization method for determining PCP in soils.
Hills et al. [10] used a silylation reagent, tri-sil concentrate, to derivatize samples of roasted coffee beans, roasted Japanese tea and marine sediment. They postulated that the derivatizing reagent not only made the compounds more soluble in the supercritical extraction fluid, but that it was involved in a competition for active matrix sites, resulting in displacement of the analyte from the matrix. King et al. [11] have reported the on-line derivatization of triglycerides under supercritical conditions. They used a solid catalyst, alumina pretreated with methanol, for the in-situ transesterification of the triglycerides. The methyl esters were then preferentailly eluted via supercritical fluid chromatography (SFC) from the alumina catalyst.
All of these papers have involved the derivatization reaction as a result of the addition of materials to the extraction chamber. Kawakura and Hiata [12] have recently reported the methylation of carboxylic acids in only methanol-modified supercritical fluid carbon dioxide using a flow through system. They investigated the methylation of substituted benzoic acids and the use of a cation exchanger as a catalyst for the reaction. They were able to achieve a maximum conversion of just under 60% for p-nitrobenzoic acid. They were able to apply this technique to phenoxy acids retained on a solid-phase extraction disk, although under the investigated conditions, recoveries were not quantitative (less than 50%). All of this work used methanol-modified carbon dioxide and a liquid trap of methylene chloride.
The objective of our work was to investigate the methylation (with no derivatization agent) of decanoic acid that occurs during the normal SFE process, and to determine the effects of both extraction and collection parameters on the reaction. Factors such as the chemical nature of the extraction fluid, collection fluid, collection temperature, and the presence of a catalyst in the collection vessel were all considered. Such reactivity to fatty acid methyl esters may lead to errors in the analytical SFE of free fatty acids.
Section snippets
Extraction
All extractions were performed using an Isco SFX 3560 (Lincoln, NE, USA) SFE system. Carbon dioxide with helium headspace (2000 p.s.i.; 1 p.s.i.=6894.76 Pa) from Air Products and Chemicals (Allentown, PA, USA) was used as the extraction fluid, and HPLC-grade methanol (Fisher Scientific, Fairlawn, NJ, USA) was used as the extraction fluid modifier.
Approximately 14 g of Ottawa Cement Testing Sand (Fisher Scientific, Houston, TX, USA) was placed in a 10-ml Isco special high crystalline polymer
Results and discussion
This work was divided into preliminary scouting experiments to determine whether the majority of the methylation reaction was occurring during the SFE, collection, or the chromatographic process, and specific experiments to determine the effects of various parameters on the methylation. During the scouting experiments no internal standard was added to the collection vials, as only qualitative data were desired.
Conclusions
The objective of this work was to investigate the methylation of decanoic acid occurring during the SFE process. The methylation was found to occur primarily during the collection process and was greatly enhanced (reaction rate increased almost 10-fold) by the presence of an acid catalyst (i.e., additional to any carbonic acid formed from the CO2 and residual water.) Increasing reaction time and collection temperature also increased the conversion rate to the methyl ester, but very little in
Acknowledgements
The authors would like to acknowledge the loan of the Isco SFX 3560 from the Isco/Suprex Corporation, and the donation of the SFE/SFC grade carbon dioxide from Air Products and Chemicals, Inc.
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