Solid-phase extraction hydrophobic sorbents

Prepai ative isolation of nonvolatile and semivolatile organic compounds fractions (hydrophobic weak acids, hydrophobic weak bases, hydrophobic neutrals, humic and fulvic acids) from natural and drinking waters in optimal conditions was systematically investigated by solid-phase extraction method with porous polymer sorbents followed by isolation from general concentrate of antropogenic and/or toxic semivolatile compounds produced in chlorination and ozonation processes. 

Agonists are particularly suited to reversed-phase solid-phase extraction due, in part, to their relatively nonpolar aliphatic moiety, which can interact with the hydrophobic octadecyl- and octyl-based sorbents of the cartridge (472, 473, 475, 480,486, 487). By adjusting the pH of the sample extracts at values greater than 10, optimum retention of the analytes can be achieved. Adsorption solid-phase extraction using a neutral alumina sorbent has also been described for improved cleanup of liver homogenates (482). 

Reversed-phase solid-phase extraction (SPE) involves the partitioning of organic solutes from a polar mobile phase, such as water, into a nonpolar solid phase, such as the C-18 sorbent (Fig. 4.1). Partitioning involves the interaction of the solute within the chains of the stationary phase, which may be a C-18 hydrocarbon, C-8 hydrocarbon, or the polymeric sorbents (such as styrene-divinylbenzene). The word hydrophobic mechanism is commonly... 

The first example of the utilization of MOFs for solid-phase extraction (SPE) was reported by Yan s group in 2006. Copper(II) isonicotinate [Cu(4-C5Fl4N- 00)2(1320)4] MOF was dry-packed in a precolumn for flow injection SPE on-line coupled with high-performance liquid chromatography (FIPLC) for the determination of trace polycyclic aromatic hydrocarbons (PAFIs) in coal fly ash and local water samples. Enhancement factors from 200 to 2337 for the studied PAFIs ranged were achieved, depending on the shape, size, and hydrophobic property of the PAHs. Recently, Zn(BTA)2 (BTA = benzotriazole) and MOF-5 were also explored as novel sorbents for the SPE of trace benzo[a]pyrene in edible oils and PAFIs in environmental matrices, respectively. 

In matrix solid-phase dispersion (MSPD) the sample is mixed with a suitable powdered solid-phase until a homogeneous dry, free flowing powder is obtained with the sample dispersed over the entire material. A wide variety of solid-phase materials can be used, but for the non-ionic surfactants usually a reversed-phase C18 type of sorbent is applied. The mixture is subsequently (usually dry) packed into a glass column. Next, the analytes of interest are eluted with a suitable solvent or solvent mixture. The competition between reversed-phase hydrophobic chains in the dispersed solid-phase and the solvents results in separation of lipids from analytes. Separation of analytes and interfering substances can also be achieved if polarity differences are present. The MSPD technique has been proven to be successful for a variety of matrices and a wide range of compounds [43], thanks to its sequential extraction matrices analysed include fish tissues [44,45] as well as other diverse materials [46,47]. 

Procedures. Sorbent Conditioning. To facilitate partitioning of organics in aqueous solution onto hydrophobic nonpolar sorbents, the sorbents must first be conditioned with methanol to increase their wettability. This solvation of the solid phase is necessary to provide efficient and reproducible extractions. The conditioning process was carried out according to the instructions accompanying tbe extraction columns. 

Thus, the mechanisms of interaction include hydrogen bonding and dipole-dipole forces (polar interactions), van der Waal s forces (nonpolar or hydrophobic interactions), size exclusion, and cation and anion exchange. Some sorbents combine several interactions for greater selectivity. The extensive line of sorbent chemical structures facilitates one of the most powerful aspects of SPE, that is, selectivity. Selectivity is the degree to which an extraction technique can separate the analyte from interferences in the original sample. The number of possible interactions between the analyte and the solid phase facilitates this selectivity. 

These compounds are another example of nonionic, nonpolar compounds. They are found in trace levels in water and result from combustion processes and hydrocarbon spills. They are trace enriched from water by sorption onto CN, C-8, or C-18 and elution with ethyl acetate/toluene. Toluene is added to the ethyl acetate eluent to increase solubility of the polyaromatic hydrocarbons (PAHs) and to enhance elution from the solid phase. The more hydrophobic PAHs, such as pyrene (Fig. 7.11), will recover more efficiently from a more polar reversed phase, such as CN or C-8 due to less strong van der Waals interactions between the analyte and the sorbent. The PAHs may be analyzed by either GC/MS or by HPLC. Soil samples may be processed as in Section 7.10.2 with 90% methanol extraction, followed by dilution with... 

Some advances have been accomplished exploiting SIA and an additional peristaltic pump, in order to mix by confluence the sample and the disperser/extractant mixture. However, additional steps are required such as the need of a solid support for the retention/collection of the dispersed organic micro-droplets, and the re-elution of the retained microdroplets from the polytetrafluoroethylene (PTFE) support prior to detection. Other SIA systems have been proposed by using a conical tube and adding an auxiliary solvent to adjust the density of the extraction solvent when this is lighter than water density [17—19]. Other authors use micro columns packed with hydrophobic sorbent materials after extraction as phase separators requiring a second extraction [20—22]. 

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