Pressurised fluid extraction

PFE is based on the adjustment of known extraction conditions of traditional solvent extraction to higher temperatures and pressures. The main reasons for enhanced extraction performance at elevated temperature and pressure are (i) solubility and mass transfer effects and (ii) disruption of surface equilibria [487]. In PFE, a certain minimum pressure is required to maintain the extraction solvent in the liquid state at a temperature above the atmospheric boiling point. High pressure elevates the boiling point of the solvent and also enhances penetration of the solvent into the sample matrix. This accelerates the desorption of analytes from the sample surface and their dissolution into the solvent. The final result is improved extraction efficiency along with short extraction time and low solvent requirements. While pressures well above the values required to keep the extraction solvent from boiling should be used, no influence on the ASE extraction efficiency is noticeable by variations from 100 to 300 bar [122]. 

The efficiency of extraction is mainly dependent on temperature as it influences physical properties of the sample and its interaction with the liquid phase. The extraction is influenced by the surface tension of the solvent and its penetration into the sample (i.e. its viscosity) and by the diffusion rate and solubility of the analytes all parameters that are normally improved by a temperature increase. High temperature increases the rate of extraction. Lou et al. [122] studied the kinetics of mass transfer in PFE of polymeric samples considering that the extraction process in PFE consists of three steps  

Steps (i) and (ii) are controlled by molecular diffusion. Higher operating temperatures can improve the kinetics of mass transfer in all three steps. Vandenburg et al. [37] described the kinetics of PFE extraction using the hot-ball model [286] derived for SFE extractions. 

The static ASE mode may lead to incomplete extraction because of the limited volume of solvent used. Dynamic ASE yields faster extractions by continuously providing fresh extraction solvent to the sample, but obviously requires a larger solvent volume than static ASE and is therefore less suited for trace analysis. In practice, a combination of static and dynamic extraction is often considered to be the best choice. 

Selection of a suitable extraction solvent is probably the most difficult step in optimising PFE for polymers [122]. Solvent selection used in PFE of any particular analyte can be done in several ways  

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The influence of soil ageing on the recovery of POPs from spiked soil samples was also assessed. Spiked lettuce samples were subjected to in vitro gastrointestinal extraction to assess the bioavailability of Endosulfan compounds. All samples (soil and lettuce) were extracted using pressurised fluid extraction and analysed using gas chromatography with mass selective detection. 

Forced-flow leaching. 74 3.4.6 Pressurised fluid extraction. 117... 

More recently, some novel pressurised procedures for extraction of additives from polymers have been developed (Table 3.3). The principal objectives of all these techniques, such as SEE [89], MAE [90] and pressurised fluid extraction (PEE), is to replace the conventional extraction methods by shortening the extraction time, reduction in solvent use and automation. 

Figure 3.18 Schematic representation of a pressurised fluid extraction (PFE) system. After Ezzell [486], Reproduced by permission of Sheffield Academic Press/CRC Press...

Table 3.35 Main characteristics of pressurised fluid extraction Advantages...

Applications Pressurised fluid extraction (ASE , ESE ) is still in its early stage of development, both for polymeric and other samples. At present, most applications are found in the environmental, food, pharmaceutical and nutraceutical areas. Few reports describe the application of PEE to the extraction of monomers, oligomers and additives from polymers and most work is very recent. An application note [488] has provided some guiding principles for ASE applied to additives in polymers, namely as follows ... 

S, Soxhlet S , Soxtec R, reflux SF, shake-flask US, ultrasonics SFE, supercritical fluid extraction MAE, microwave-assisted extraction PFE, pressurised fluid extraction (ASE , ESE ) D/P, dissolution/precipitation. 

Boundaries in chromatography and extraction are blurring, as evident from the relation between GC, SFC and HPLC, the use of superheated/subcritical water for extraction and chromatography, and the role of enhanced fluidity solvents and pressurised fluid extractions [2]. Extraction is an extreme form of chromatography. Separation science recognises that there is unity in the... 

Although SFC fills a niche in what can be considered as a continuum of separation eluents from gases to liquids, it cannot claim a unique status subcritical water extraction (SWE, cf. Section 3.4.3) and pressurised fluid extraction (PFE, cf. Section 3.4.6) are other examples of eluents where altering the conditions cause a useful change in the solvation properties. 

Datta et al. [40] used pressurised fluid extraction to extract ground fish tissue, and the resulting extract was purified on aminopropyl silica (APS) extraction cartridges. With no further sample preparation, NP and its ethoxylates, up to nonylphenol pentaethoxylate, were quantified using normal phase (APS Hypersil) HPLC with fluorescence detection. 

Perraudin, E., H. Budzinski, and E. Villenave. 2005. Analysis of polycyclic aromatic hydrocarbons adsorbed on particles of atmospheric interest using pressurised fluid extraction. Anal. Bioanal. Chem. 383 122-131. 

EPA Method 3545, Pressurised Fluid Extraction, Test Methods for Evaluating Solid Waste EPA SW-846 US GPO, Washington, DC (1994). 

Alzaga R, Salgado-Petinal C, lover E et al (2005) Development of a procedure for the determination of perfluorocarboxylic acids in sediments by pressurised fluid extraction. 

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