Pressurized Liquid Extraction PLE

The commercial availability of ASE systems is limited nowadays. They are only available from the Dionex Corp., U.S.A. The ASE-200 model can reach temperatures up to 200°C and pressures up to 21 MPa in extraction cells of 1 ml, 5 ml, 11 ml, 22 ml or 33 ml. 

FIGURE 2.17 Automated focused microwave-assisted Soxhlet extractor. (Reprinted from Luque-Garcfa, J. L. and Luque de Castro, M. D., J. Chromatogr. A, 998, 21-29, 2003. With permission from Elsevier.) 

Employing a carousel, this extractor can operate with up to 24 extraction vessels. The speediness is a remarkable advantage over Soxhlet extraction. 

Raising the temperature increases diffusion rates, solubility of the analytes and mass transfer, and decreases the viscosity and surface tension of the solvents. These changes improve contact of the analytes with the solvent and enhance the extraction efficiency, which can be achieved more rapidly and with less solvent consumption compared with classical methods. For example, ASE reduces solvent consumption by up to 95% compared to Soxhlet extraction. The only hmitation is the thermal stability of the analyte of interest. 

ASE can be carried out in static or dynamic mode. Most of the applications found in the literature are in the static mode and, until now, analyte recoveries obtained under dynamic mode are not quantitative. 

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General trends are focused on reduced-solvent extractions or adsorption-based methods — enviromnentaUy friendly solvents for both solid and liquid samples. In recent decades, advanced techniques like supercritical fluid extraction (SFE), ° pressurized liquid extraction (PLE)," microwave-assisted extraction (MAE), ultrasound-assisted extraction, countercurrent continued extraction (www.niroinc.com), solid... 

The main drawback of this technique is solvent and time-consuming. In the last decade, there have been efforts to develop extraction techniques that allow efficient extraction and reduced solvent volumes in shorter times, incorporating high levels of automation, such as pressurized liquid extraction (PLE) [commonly known as accelerated solvent extraction (ASE)]. 

In mercury speciation studies, pressurized liquid extraction (PLE), microwave-assisted extraction (MAE), and supercritical fluid extraction (SEE) are employed [33]. In particular, methyl-mercury is extracted by the Westoo method [33,34], which consists in a leaching process with hydrochloric acid, the extraction of the metal chloride into benzene or toluene, the addition of ammonium hydroxide that converts the metal species to hydroxide and the saturation with sodium sulfate. Most of the HPLC methods reported in literature for the determination of organomercury compounds (mainly monomethyhnercury, monoethyhnercury, and monophenylmercury) are based on reversed... 

Some authors [467,468] use novel extraction techniques, such as pressurized liquid extraction (PLE) and SEE obtaining recovery higher than with saponification. 

Fig. 12 Chromatograms of a pressurized liquid extraction (PLE) wheat extract (5 g sample spiked with 100 ng g-1 of ZON and a-ZOL) (a) without and (b) with clean-up on the MIP. (c) Chromatograms resulting from clean-up on NIP [50]...

Comparison of simple methanol extraction, Soxhlet extraction, pressurized liquid extraction (PLE), and supercritical fluid extraction (SFE) shows (Clausen et al., 2003) that DEHP can be extracted relatively easily from dust and that the effectiveness does not differ significantly between the different extraction methods (see Figure 2.4). Selection of the optimal method depends on several circumstances, for example number of extraction cycles, instrument accessibility and the analysis method. However, PLE using cyclohexane/acetone was chosen as the preferred extraction method in the field study. 

Accelerated solvent extraction (ASE) is also known as pressurized fluid extraction (PFE) or pressurized liquid extraction (PLE). It uses conventional solvents at elevated temperatures (100 to 180°C) and pressures (1500 to 2000 psi) to enhance the extraction of organic analytes from solids. ASE was introduced by Dionex Corp. (Sunnyvale, CA) in 1995. It evolved as a consequence of many years of research on SFE [45], SFE is matrix dependent and often requires the addition of organic modifiers. ASE was developed to overcome these limitations. It was expected that conventional solvents would be less efficient than supercritical fluids, which have higher diffusion coefficients and lower viscosity. However, the results turned out to be quite the opposite. In many cases, extraction was faster and more complete with organic solvents at elevated temperature and pressure than with SFE. Extensive research has been done on the extraction of a variety of samples with ASE. ASE was approved by EPA as a standard method in 1996. 

Other extraction methods used in the lipid extraction include supercritical fluid extraction (SFE) and pressurized liquid extraction (PLE). With SEE, good extraction yields have been obtained for nonpolar lipids including ester-ified fatty acids, acylglycerols, and unsaponifiable matter. However, complex polar lipids are only sparingly soluble in supercritical carbon dioxide alone and polar modifiers, such as methanol, ethanol, or even water is required to improve the extraction of polar lipids (10). SFE has been used for the extraction of lipids especially from various food matrices, such as different nuts, edible oils, and seeds (11). The recoveries of lipids in SFE were on the same levels than with conventional solvent extraction methods (12,13), no significant differences between the fatty acids extracted were observed. PLE has also been used in lipid extraction, although only in very few applications (14). The elevated temperatures used in PLE can cause alteration of the lipid composition. 

As shown in Table 19.2, extraction with the mixture of methanol-water provides adequate extraction efficiency for most of the samples. However, this technique should be automated and the duration of sample preparation should be reduced. The usual methanolic extraction is carried out with the help of a shaking device and the total time of sample preparation including centrifugation and manual separation of the phases may be as long as 10-12 h. Recently, two new techniques have been introduced that can replace the traditional approach, that is, the accelerated solvent extraction (ASE), sometimes referred to as pressurized liquid extraction (PLE), and the MW assisted extraction (MAE). 

Table 2.1 Examples of extraction of antioxidant phytochemicals from various plant materials by pressurized liquid extraction (PLE). 

In the sample preparation of semi- and nonvolatile compounds, solvent extraction is typically used for extracting the analytes of interest from a sample matrix. For volatile analytes, head-space or thermal extraction are good alternatives to solvent-based techniques. Several novel extraction systems that utilize elevated temperatures of pressures in the extraction have been developed particularly for solvent-based extraction methods. These new methods typically are much faster and often more selective than older methods and consume smaller amounts of organic solvents and reagents. Commercially available systems with the ability to heat and pressurize liquids include pressurized liquid extraction (PLE), microwave-assisted extinction (MAE) and supercritical fluid extraction (SFE). Also sonication-assisted extraction (SAE) has given promising results. 

In pressurized liquid extraction (PLE), rapid extraction is performed with small volumes of conventional solvents by using high temperatures (up to 200°C) and high pressures (up to 20 000 kPa) to maintain the solvent in a liquid state. The extraction can also be done with a purely aqueous phase, a technique often referred to as pressurized hot water extraction (PHWE) or subcritical water extraction. Temperatures of up to 325 C have been used in PHWE 29-38). 

As a consequence of the toxicity related to the presence of aristolochic acid in plant preparations, several health institutions, such as the US Food and Drug Administration, Therapeutic Goods Administration have recently published safety information to prevent further cases of intoxication (information available at web address http //www.cfsan.fda. gov/ dms/ds-bot.html) [471], So detecting aristolochic acids in plant species that could be used in herbal remedies, and also in herbal preparations of uncertain composition, has attracted great priority in recent years to help prevent future adverse reactions. Aristolochic acids present in medicinal plants or herbs are analyzed by soxhlet extraction followed by TLC in the Chinese pharmacopoeia [412]. Another report used multiple ultrasonic extraction followed by HPLC analysis [472]. Ong s laboratory reported a method using a home made pressurized liquid extraction (PLE) system in dynamic mode to extract aristolochic acid in medicinal plants, followed by gradient elution HPLC [473]. Several scientific communities described various analytical methods for... 

Accelerated Solvent Extraction (ASE) or Pressurized Liquid Extraction (PLE)... 

An evaluation of the scientific literature reveals that over 500 papers on veterinary drug residue analysis were published in the 5-year period of 2005-2009. Liquid extraction (LE) and liquid-solid extraction (LSE) were found to be very popular sample treatment techniques that were used in 30% and 60% of the reported studies, respectively. Here, LE includes all liquid-based approaches such as liquid-liquid extraction (LLE), extrelut liquid-liquid extraction, liquid-liquid micro-extraction, and pressurized liquid extraction (PLE). LSE includes solid phase extraction (SPE) and all other sorbent-based extraction procedures, such as solid phase micro-extraction (SPME), stir bar sorptive extraction (SBSE), restricted-access materials (RAM), turbulent-flow chromatography (TEC), dispersive SPE (dSPE), and matrix solid phase dispersion... 

Currently, RDX is among the most widely used explosives. Many military installations have soils, sediments, surface water, and groundwater contaminated with RDX. To aid in the evaluation of the potential toxicity of A-nitroso derivatives of RDX, Pan el al. have described a pressurized liquid extraction (PLE) followed by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESTMS) method for determination of RDX and its A-nitroso derivatives hexahydro-l-nitroso-3,5-dinitro-l,3,5-triazine (MNX), hexahydro-l,3-dinitroso-5-nitro-l,3,5-triazine (DNX), and hexahydro-l,3,5-trinitroso-l,3,5-triazine (TNX) in soils (Figure 45) <2006JCH1107>. 

Pressurized liquid extraction (PLE) or accelerated solvent extraction (ASE) can be used e.g. to desorb PEGS bound to airborne particles collected on a quartz fibre filter (Sasaki et al. 2003). Nine pesticides were extracted from PM2.5 by a rapid PLE procedure using acetone (Coscolla et al. 2008). Regarding indoor dust, different solvents have been chosen for PLE extraction of several compounds such as parabens and triclosan (Canosa et al. 2007), PCBs (Harrad et al. 2009), PAHs (Naspinski et al. 2008), PBDEs (Toms et al. 2009 Sjodin et al. 2008 Stapleton et al. 2008), and hexabromocyclododecanes (HBCDs) (Harrad et al. 2009). 

This entry describes the classical and modem sample preparation methods that have been used prior to qualitative and quantitative analysis by thin-layer chromatography (TLC) and high-performance (HP) TLC. Extraction and cleanup methods that are covered include classical methods such as liquid-liquid extraction (LLE) and Soxhlet extraction, as well as modern methods such as solid-phase extraction (SPE), pressurized liquid extraction (PLE), and supercritical fluid extraction (SEE). Modern methods have not been as widely apphed in TLC as for other modes of chromatography, e.g., column high-performance liquid chromatography (HPLC). 

Most of the extraction techniques of phenolic compounds from vegetables are based on ultrasound-assisted extraction (UAE) [27,44,45], In addition, other techniques have been successfully applied to the pretreatment of phenolic compounds in fruits and vegetables, including pressurized liquid extraction (PLE) [46], solid-phase extraction (SPE) [47], supercritical fluid extraction (SFE) [48], microwave-assisted extraction (MAE) [49], rotary shaker-assisted extraction (RAE), [50] and QuEChERS (acronym of quick, easy, cheap, effective, rugged and safe) [51], as can be observed in Tables 16.3 and 16.4. In some cases, an acid treatment [52] was applied to hydrolyze the glycosides in order to determine the content of free and conjugated flavonoids as aglycons. 

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