Liquid extraction from solid matrices

Sorptive extraction techniques are based on the distribution equilibria between the sample matrix and a non-miscible liquid phase. Matrices are mostly aqueous and the non-miscible phase is often coated onto a solid support. Analytes are extracted from the matrix into the non-miscible extracting phase. Unlike adsorption techniques, where the analytes are bound to active sites on the surface, the total volume of extraction phase is important. Extraction of analytes depends on the partitioning coefficient of solutes between the phases (Ridgway et al., 2007). Two extraction techniques are commonly employed solid phase microextraction (SPME) and stir-bar sorptive extraction (SBSE). 

A relatively new automated extraction method is PLE, also called accelerated solvent extraction (ASE), which is based on an extraction under elevated temperature (50-200°C) and pressure (3-205 bar) during a short period of time (5-15 min). This technique has been used for the extraction of phenolic compounds from foods such as vegetables and fruits. In PLE, a solid sample is packed into the extraction cell and analytes are extracted from the matrix with conventional low-boiling solvents or solvent mixtures at elevated temperatures up to 200°C and pressure (30-200 bar) to maintain the solvent in the liquid state [61]. 

Proper sampling procedures have to be applied to obtain representative samples from analyzed biological material since mycotoxins are not homogenously distributed in contaminated grain stocks. Sample preparation usually involves a few cleanup steps to eliminate compounds present in the matrix that might coelute with the mycotoxins. The cleanup methods may involve either liquid-liquid extraction (LLE), solid-phase extraction (SPE) on columns that contain various types of solid phases, or immunoaffinity columns [39,42,43]. The recovery achieved with a cleanup method is critical for the final results. 

A challenging task faced by the analytical chemist when dealing with raw samples is extracting/isolating the analyte of interest from the sample matrix. It is fair to say that the majority of nonideal samples arrive in a format incompatible with most analytical instrumentation, and requires some degree of clean-up. Even well-defined samples (e.g. aqueous solutions) require basic filtration prior to analysis. Other desirable techniques may include liquid/liquid extraction and solid phase extraction. 

As suggested above, the presence of salt and fat on the solid matrixes may influence the extraction of PAHs from solid matrixes. We applied the SPME-DED method for analysing the effect of the presence of salt and fat in the solid matrix on the analysis of PAHs. It is known that the presence of lipids in the matrixes hinders the release of some compounds during the extraction by SPME (Keszler and Heberger, 1999) or by other traditional methods such as saponification or liquid-liquid partition (Phillips, 1999 Moret and Conte, 2000) since lipophilic compounds, as PAHs, interact with the non-polar groups of lipids. 

Matrix solid phase dispersion extraction with C18 (washed with hexane and ethyl acetate then LAS eluted with 1 1 ethyl acetate/MeOH) liquid-liquid extraction from alkaline tetrabutylammonium hydroxide/water into 3 1 CH2Cl2/MeOH Liquids SPE on Cg (MeOH elution) solids Soxhlet extraction with MeOH and SPE on anion exchange resin (methanolic HCl elution) and (MeOH elution) Extraction with MIBK back-extraction into water after dilution with hexane... 

Two approaches have been used to separate the analyte from its matrix in particulate gravimetry. The most common approach is filtration, in which solid particulates are separated from their gas, liquid, or solid matrix. A second approach uses a liquid-phase or solid-phase extraction. 

Volatile analytes can be separated from a nonvolatile matrix using any of the extraction techniques described in Ghapter 7. Fiquid-liquid extractions, in which analytes are extracted from an aqueous matrix into methylene chloride or other organic solvent, are commonly used. Solid-phase extractions also are used to remove unwanted matrix constituents. 

To date most of the work which has been done with supercritical fluid extraction has concentrated on the extraction of analytes from solid matrices or liquids supported on an inert solid carrier matrix. The extraction of aqueous matrices presents particular problems [276-278]. The co-extraction of water causes problems with restrictor plugging, column deterioration, and phase separation if a nonpolar solvent is used for sample collection. Also, carbon dioxide isay have limited extraction efficiency for many water soluble compounds. 

An analyte may be present in one material phase (either a solid or liquid sample) and, as part of the sample preparation scheme, be required to be separated from the sample matrix and placed in another phase (a liquid). Such a separation is known as an extraction—the analyte is extracted from the initial phase by the liquid and is deposited (dissolved) in the liquid, while other sample components are insoluble and remain in the initial phase. If the sample is a solid, the extraction is referred to as a solid-liquid extraction. In other words, a solid sample is placed in the same container as the liquid and the analyte is separated from the solid because it dissolves in the liquid while other sample components do not. 

Extraction efficiency. Recovery of the analyte from biological matrix after sample pretreatment (i.e., liquid-liquid extraction, solid-phase extraction, protein precipitation, etc.) to remove endogenous substances. 

If online extraction techniques are used, calculation of the different types of recovery could be particularly challenging. Different approaches has been used in literature to overcome this problem, such as using an external injection valve for adding the neat solution to the extracted blank from the online extraction column just on the head of the analytical column [9,10]. Although liquid matrix can be extracted directly, a solid matrix such as tissue needs to be disrupted in... 

SFE manifests its best advantages when extracting analytes from solid and semisolid rather than liquid samples. A primary limitation in extracting analytes from liquid sample matrices is the mechanical difficulty of retaining the liquid matrix in the extraction vessel. To extract a liquid sample by SFE successfully, analysts must first mix it with a solid material, such as diatomaceous earth or alumina, so that the sample is no longer free-flowing. Control of sample matrix effects is critical in SFE to limit coextractives, moderate the influence of moisture, and improve the efficiency of the extraction. Recent studies have shown that the addition of both inert and active sorbents to the sample matrix can improve the efficiency of SFE (153). 

Diethylstilbestrol is particularly difficult to quantitate below 1.0 ppb in bovine tissues, especially in liver, which is among the last tissues to contain diethystilbestrol after cattle are withdrawn from receiving tire drug (101, 102). Interferences from tissue matrix constitute a major problem that might be due to nonspecific interference of lipids and fatty compounds (103, 104). In addition, problems with false-positive results often appear in urine analysis unless a chromatographic step such as a solid-phase extraction cleanup (105, 106) is introduced. Simple sample preparation procedures such as those based on solvent extraction and liquid-liquid partitioning do not usually give satisfactory results (107, 108). 

For liquid matrices such as milk, a pretreatment step for fat removal that is accomplished by centrifugation (1 -3) or hexane extraction (4) may be required. Solid samples such as muscle, kidney, and liver necessitate usually more intensive sample pretreatment through use of a mincing and/or a homogenizing apparatus. In some cases, as in the analysis of apramycin in swine kidney tissue, protein digestion with concentrated ammonium hydroxide may be needed to achieve better recovery of the analyte from the matrix (5). 

The feasibility of extracting substituted phenols from an aqueous solution with supercritical CO2 is reported A special extraction vessel was used in order to overcome the mechanical difficulty in retaining the liquid matrix in the extraction vessel. Solid phase trapping was utilized with a diol silica bonded phase. Methanol was used to rinse the trap. Below 300 atm extraction recovery paralleled CO2 pressure at fixed temperature. Phenol was least extractable while, 2,4-dichlorophenol yielded the greatest percent recovery. Above 300 atm extraction yield declined with pressure. It is theorized that at high CO2 density there is less mixing with the aqueous phase because of increased fluid-fluid interaction. 

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