Solid-supported LLE

Several manufacturers introduced products amenable for this solid-supported LLE and for supported liquid extraction (SLE). The most common support material is high-purity diatomaceous earth. Table 1.8 lists some commercial products and their suppliers. The most widely investigated membrane-based format is the supported liquid membrane (SLM) on a polymeric (usually polypropylene) porous hollow fiber. The tubular polypropylene fiber (short length, 5 to 10 cm) is dipped into an organic solvent such as nitrophenyl octylether or 1-octanol so that the liquid diffuses into the pores on the fiber wall. This liquid serves as the extraction solvent when the coated fiber is dipped... 

Figure 1.25 illustrates the principle underlying LLE in the solid-supported LLE format. In order to facilitate elution with a water-immiscible organic solvent, it is imperative that analytes are in their neutral form during sample load. Thus, for basic analytes, loading should be done in a high pH (9 to 10) buffer and for acidic analytes, a low pH (2 to 3) buffer. 

Is analyte recovery using a solid-supported liquid phase classified as LLE or LSE In Section 2.2.4, a process described as solid-supported LLE [49,50] was discussed in which the liquid sorbent phase was distributed on the surfaces of individual particles (Figure 2.18). The solid-supported phases in the LSE section have been arbitrarily distinguished as liquids mechanically supported on solid devices, such as the liquid-coated fused silica fibers used for SPME or the liquid-coated glass sheath of a stirring bar in used SBSE, rather than liquids supported on finely divided solid particles. 

In solid-supported LLE (SS-LLE) or liquid-liquid cartridge extraction, the aqueous sample is applied on to a dry bed of inert diatomaceous earth particles in a flow-through tube or in 96-well plate format. After a short equihbration time (3-5 nun), organic solvent is added. The organic eluate is collected, evaporated to dryness, and reconstituted in mobile phase. Compared to conventional LLE procedures, SS-LLE avoids the need for vortex-mixing, phase separation by centrifugation, and phase transfer by aqueous layer freezing. 

Solid-supported LLE (SS-LLE, Ch. 1.5.2) in 96-well plate format was applied in the extraction of simvastatin from human plasma [47] and in several other studies. 

Method development for the LC-MS analysis of HAA in meat was performed by the gronp of Galceran [83-90]. Initially, complex mnlti-step extraction and sample pretreatment methods were applied, e.g, hqnid extraction from meat, clean-up by solid-supported LLE, and ion-exchange SPE. Later on, different SPE procednres for sample clean-np were evalnated [85]. A combination of solid-supported-LLE and SPE on C18 material was most efficient in selective elution of the polar and less polar HAA. LC was first performed on a lOOxl-mm-lD Cig column (5 pm) with 50% acetonitrile in 5 nunol/1 aqueous ammonium acetate (pH 6.7) at a flow-rate of 50 pEmin. ESl-MS was applied in SIM mode, enabling detection of HAA at 1-6 pg/kg in beef extracts [83]. 

The many disjointed mixing and centrifugation steps of traditional LLE can be eliminated by performing LLE in a flowthrough column filled with inert diatomaceous earth particles. This technique is referred to as solid-supported LLE (SS-LLE). The high surface area of the diatomaceous earth particle facilitates efficient, emulsion-free interactions between the aqueous sample and the organic solvent. Essentially, the diatomaceous earth with its treated aqueous phase behaves as the aqueous phase of a traditional LLE, yet it has the characteristics of a solid support. 

Table 3.46 compares SPME and SPE. Although SPME has in common with SPE that the analytes are concentrated by adsorption into a solid phase, SPE involves absorbing the analyte from the sample onto a modified solid support. In practice, the two techniques are quite different. SPME differs from conventional SPE in that SPE isolates the majority of the analyte from a sample (>90%) but injects only about 1 to 2% of the sample onto the GC. SPME isolates a much smaller quantity of analyte (2-20%), but that entire sample is injected into the GC. SPME is easy-to-perform and often significantly more rapid and simpler than SPE, but its quantitative aspect is exacting. Both conventional SPE and SPME minimise the use of solvents for sample preparation and free analysts from tedious sample clean-up. Where SPE can replace LLE... 

The simplest technique is the use of the 96-well collection plate format (analogous to the format used in SPE) in conjunction with a liquid handling robotic system it follows the same principle as bulk scale LLE. However, immobilization of the aqueous plasma sample on an inert solid support medium packed in a cartridge or in the individual wells of a 96-well plate and percolating a water-immiscible organic solvent to extract the analyte from this medium evoked significant enthusiasm from the pharmaceutical industry. 

One advance in the area of LLE is the use of solid supports that facilitate the partitioning of the analyte(s) of interest. LLE extraction methods involving nonpolar matrices often suffer from the formation of emulsions, and using the solid support is a possible solution. In one study, polychlorinated biphenyls, dioxins, and furans were extracted from the lipid fraction of human blood plasma [32], using diatomaceous earth as the solid support. Long glass columns (30 cm) were packed with several layers of Chem-Elut (a Varian product) and sodium chloride. The plasma samples were diluted with water and ethanol and passed over the columns. A mixture of isopropanol and hexane (2 3) was passed over the column and the LLE was performed. It can be concluded that the LLE with the solid support is easier to perform and can be applied to other lipid heavy matrices such as milk [32]. 

However, conventional LLE requires precise removal of the aqueous layer, which is not amenable to large number of samples. To solve this problem, solid supported liquid-liquid extraction (SLE) was developed. Instead of using separation funnels, the reaction mixture is loaded on a cartridge packed with diatomaceous earth, which is pretreated with an aqueous buffer and contains an aqueous layer. A water-immiscible solvent, usually methylene chloride or ethyl acetate, is then applied to elute the products off the cartridge, leaving more water-soluble impurities on the column. 

The best results obtained through each extraction method are described below. In LLE, 60 ml of dichlo-methane was used to remove analytes from the aqueous matrix. In SPE, 1.0 g of Cig (octadecyl silane) sorbent was used as solid support, and ethyl acetate was used for both phase condition and compound elution. In SFE, the Cig phase was used as support for the sample and CO2 in a supercritical state modified with acetone in the compound extraction at 60°C, while a pressure of 300 atm was used as the mobile phase. 

Figure B3.5.12 Effect of mutations detected by CD. The far-UV CD spectra (A) show that the secondary structure of p-lactamase PC1 (solid line) from Staphylococcus aureus is essentially unaffected by point mutations P2 (Thr 140—>lle dashed line) and P54 (Asp 146->Asn dotted line). The crystallographic structure of P54 (Herzberget al., 1991) confirms that, apart from a loop region, the main body of the molecule that contains the thirteen tyrosine residues is very closely similar to that in the wild-type enzyme. The intensity of the tyrosine ellipticity (B) is, however, markedly decreased in each of the mutants, the lower thermodynamic stabilities of which support the interpretation of increased dynamics (Craig et al., 1985).

The dual-phase (DP) membrane used in analytical separation usually consists of a polymer, or in some cases a ceramic solid-phase support impregnated with a fluid (i.e., gaseous or liquid phase). If the fluid is air the DP membrane is known as a gas-diffusion membrane. DP membranes incorporating a liquid phase can be considered in a broader sense as liquid membranes. The liquid phase in a liquid DP membrane can be identical to the feed and/or receiver solution (e.g., dialysis membranes, membrane-assisted LLE (MALLE)) or it can form a third immiscible liquid phase in the membrane separation system (e.g., supported and polymer liquid membranes). Membranes incorporating a liquid phase immiscible with the feed and receiver solutions are usually referred to as liquid membranes. This narrower definition of liquid membranes, currently accepted in the literature, will be used in subsequent discussions. The... 

Accelerated solvent extraction (ASE), focused microwave soxhiet extraction (FMSE), immuno affinity cleanup (im-Cu), liquid-liquid extraction (LLE), low-temperature lipid precipitation (LTLP), matrix solid-phase dispersion (MSPD), microwave-assisted extraction (MAE), nanofiltration (NF), pressurized fluid extraction (PEE), single drop microextraction (SOME), solid-phase extraction (SPE), solid-phase microextraction (SPME), steam distillation (SD), stir bar sorptive extraction (SBSE), surpercritical fluid extraction (SFE), subcritical fluid extraction (ScFE), supported liquid membrane extraction (SLME), ultra-sonication (US), size exclusion chromatography (SEC), liquid chromatography-fraction collection (LC)... 

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