Sample preparation supercritical flow

This chapter reviews recent findings about the health benefits of phytochemicals present in fruits, vegetables, nuts, seeds, and herbs, including phenolics, carotenoids, sterols, and alkaloids. These phytochemicals are extracted using emerging technologies such as supercritical carbon dioxide (SC-CO2) extraction, PEF, MWE, HPP, UE, and OH. The impact of important parameters related to sample preparation (particle size and moisture content) and extraction process (temperature, pressure, solvent flow rate, extraction time, and the use of a cosolvent) on the efficiency of extraction and on the characteristics of the extracted products is evaluated based on an extensive review of recent literature. The future of extraction of phytochemicals is certainly bright with the... 

The robustness of a sample preparation technique is characterized by the reliability of the instrumentation used and the variability (precision) of the information obtained in the subsequent sample analysis. Thus, variations in controlled parameters and sequences are to be avoided. In sample preparation methods employing supercritical fluids as the extracting solvents, it has been our experience that minimal variations in efficient analyte recoveries are possible using a fully automated extraction system. The extraction solvent operating parameters under automated control are temperature, pressure (thus density), composition and flow rate through the sample. The precision of the technique will be discussed by presenting replicability, repeatability, and reproducibility data for the extraction of various analytes from such matrices as sands and soils, river sediment, and plant and animal tissue. Censored data will be presented as an indicator of instrumental reliability. 

Supercritical fluid extraction as an alternative sample preparation method applied to solid sample matrices of interest to TEQA became quite popular during the late 1980s and early 1990s. However, SEE requires that instrumentation be purchased. It has also been found that a significant matrix dependence exists and this matrix dependence contributes to differences in percent recoveries. The first generation of SEE instruments also suffered from problems with plugging of the flow restrictors that are located after the extraction vessels. SEE will be discussed in more detail later in the chapter. We next discuss the three variations of S-LSE introduced earlier (i.e., U-LSE, MAE, and ASE). 

The first step in preparing a solid sample for HPLC is usually homogenization in preparation for extraction [5]. Extraction of the relevant components of the solid can be accomplished by a variety of techniques including Soxhlet extraction, accelerated extraction methods, and supercritical flow [10]. Soxhlet extraction, which leaches a specific component from a sample by refluxing in an appropriate solvent, is the benchmark classical technique for extraction of small molecules from a matrix [5,10]. 

SEE is an instrumental approach not unlike PLE except that a supercritical fluid rather than a liquid is used as the extraction solvent. SFE and PLE employ the same procedures for preparing samples and loading extraction vessels, and the same concepts of static and dynamic extractions are also pertinent. SFE typically requires higher pressure than PLE to maintain supercritical conditions and, for this reason, SFE usually requires a restrictor to control better the flow and pressure of the extraction fluid. CO2 is by far the most common solvent used in SFE owing to its relatively low critical point (78 atm and 31 °C), extraction properties, availability, gaseous natural state, and safety. 

The basic idea of enantiomer resolution using supercritical fluid technology is that after the partial diastereo-mer formation of the racemate, only the free enantiomeric mixture is soluble in supercritical fluid,e.g., supercritical carbon dioxide (5CCO2). As illustrated in the flow sheet of the resolution process (Figure 1.43), it consists of three parts preparation of the sample, extraction, and processing the raffinate. 

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