Solid-phase extraction adsorbents for

Selected Adsorbents for Solid-Phase Extraction of Liquid Samples... 

This chapter has four sections that focus on laboratory-scale capture steps solids removal, solvent extraction, solid phase adsorption, and expanded bed adsorption. Although these techniques are widely applicable, most of this chapter is aimed at extraction of microbial fermentation broth. Techniques specific to initial extraction of plants and marine organisms can be found in Chapters 12 and 13, respectively. The first section describes laboratory-scale procedures for batch filtration and centrifugation. The second section describes solvent-extraction procedures with either water-miscible or -immiscible solvents. The third section describes using adsorbents for solid-phase extraction. The fourth section describes a technique known as expanded bed adsorption, which is unique in that it enables resin-column recovery of product directly from unclarified fermentation broth. 

The arylurea hetbicides wete also used for MIPs synthesis. Two arylureas, fenuron and isoproturon, wete polymerized svith MAA as functional monomet and used as adsorbents for solid phase extraction of pollutants. The isoprotuton-imprinted polymer did not demonstrate the selectivity towards the template but had a high affinity towards all other arylurea herbicides. The corresponding blanks (nonimprinted polymers) did not retain any of these compounds, which suggest that the imptinting was achieved. With fenuron, very good specificity for this herbicide was observed. This result can be explained by the small size of the fenuron molecule. Indeed the other arylureas were not able to enter in the small cavities formed around fenuron due to steric interactions of the meta- or para-substituents in the aromatic ring. ... 

Other simulation experiments performed in our laboratory include design of MIP adsorbents for solid-phase extraction of atrazine [77], DDT, lindane, aflatoxin Bl, ochratoxin A, and tylosin (unpublished data) and the development of assay/ sensor recognition elements for biotin (unpublished data) and creatinine [78]. In all these cases, molecular modeling proved to be a useful tool for MIP design. It would... 

Many different adsorbents are applicable in this context (see Table 8). The most common adsorbents for solid-phase extraction are based on silica gel, the surface of which has been modified in some way. 

Solvent extraction in combination with TD-GC-FID gives the opportunity to concentrate the extract directly on the adsorbent tube (solid phase extraction) by injection of the sample extract and purge off the solvent with for example, helium. Methanol extracts of house dust can be concentrated on adsorbent tubes by injecting up to 50 jllI and analyze the tubes by TD-GC (Kofoed-Sorensen and Clausen, 2004). 

The selection of adsorbent packing material is based on the polarity of pollutants to be analyzed. The nonpolar hydrophobic adsorbents retain the nonpolar analytes and allow the polar substances to pass through the column. The hydrophilic adsorbents adsorb the polar components, allowing the nonpolar materials to pass through. Various stationary phases for solid phase extraction are listed below in Table 1.5.1. 

Methods for the extraction of pesticides from water exploit the partitioning of analytes between the aqueous phase and a water-immiscible solvent (Liquid-Liquid Extraction [LLE]) or an adsorbent material (Solid-Phase Extraction [SPE]). The SPE technique has been shown to offer several advantages over LLE and it is included in most of recent analytical protocols devoted to analyzing contaminants in water samples. Nevertheless, the LLE technique is still used in many environmental laboratories. 

Neutral nonfimctionalized macroporous adsorbents were suggested for the removal of organic pollutants from all types of waste and drinking waters and industrial gas emissions. On an analytical scale macroporous polystyrene adsorbents, both functionafized and nonfunctionafized, gained wide acceptance as stationary phases in gas and liquid chromatography, as well as sorbents for solid-phase extraction of organic components from... 

For solid phase extraction, an ideal MIP adsorbent should have the following characteristics High binding affinity, specificity and capacity fast association and disassociation kinetics broad solvent compatibility and long-term stability. 

Molecular imprinting for solid phase extraction has attracted increasing interest in the past few years. MIP-based SPE is now considered to be the first commercialization effort to market the imprinting technology. As in other related applications, success in this aspect largely depends on the performance of MIP adsorbents under real conditions. In addition to strive for very high binding affinity and specificity, perhaps more important issues, but not so much foreseen by many of the research... 

Admittedly, todays MIP often do not perform yet at par with established materials like, for example, P-cyclodextrins for enantioselective separations. However, they did improve considerably over the last 5-10 years and already constitute useful alternatives in some areas, for example as adsorbents in solid phase extraction. As such they can be applied for sample preparation or preconcentration in case of complex matrices or a high number of other contaminating substances. 

Solid-Phase Extractions In a solid-phase extraction the sample is passed through a cartridge containing solid particulates that serve as the adsorbent material. For liquid samples the solid adsorbent is isolated in either a disk cartridge or a column (Figure 7.17). The choice of adsorbent is determined by the properties of the species being retained and the matrix in which it is found. Representative solid adsorbents... 

Fig. 18.1 Systems used to absorb aroma compounds from samples for analytical purposes, a Traps loaded with various adsorbents [4]. b Solid-phase extraction (disk in a holder assembly) [5]. c Solid-phase microextraction (coated needle inserted in sample) [5]. d Twister (1 -cm length) [4]. (Courtesy of GERSTEL GmbH and Co. KG)...

Besides solid-phase extraction, column chromatography is also often used for cleanup and purification of polyphenolics from plant material. Ionic adsorbants (polyvinylpyrrolidone or PVP, polyamides, and Sephadex LH-20) and Amberlite XAD-2 resin have been used to isolate and purify polyphenolics from crude extracts. For the separation of polyphenolics from plant material, column chromatography using Sephadex LH-20, a gel-filtration matrix, is often used with various eluting solvents (Park and Lee, 1996). The most widely used solvents for column chromatography are aqueous methanol and aqueous ethanol. 

After several decades of research, fundamental aspects of the chemical composition and structure of marine organic matter remain elusive. Advances in the chemical characterization of marine organic matter are, in large part, dependent on the development of quantitative methods for its concentration and isolation from seawater. Each of the major methods currently used for the isolation of marine DOM recovers around one-third of the DOM in seawater (solid-phase extractions, using XAD resins or C18 adsorbents, and ultrafiltration). A coupled reverse osmosis-electrodi-alysis method has recently been used to recover an average of 75% 12% of marine DOM from 16 seawater samples however, the method has emerged too recently to have been well tested at this time. 

Analytes may accumulate in the sorption phase either by adsorption onto the surface of solid sorbent materials or by absorption in absorbent liquids or polymers that behave like subcooled liquids.The advantage of solid adsorbents is the potential to select materials with a high affinity and selectivity for target analytes. However, the sorption capacity of adsorbents is usually limited, and the description of adsorption/desorption kinetics of analytes to adsorbents is complex. Typically, the adsorbent materials used in passive samplers are similar to those used in solid-phase extraction techniques. 

A promising solid-phase extraction adsorbent for metals and more particularly As is nanometer Ti02 material and immobilized nanometer Ti02.70,71 Both As(III) and As(V), or As(III) alone, were quantitatively absorbed on immobilized nanometer TiOz depending on the pH (for LODs, see Table 7.2). 

Another major drawback of classical extractions is that additional clean-up procedures are frequently required before chromatographic analyses. Solid phase extraction (SPE) avoids the emulsion problems often encountered in liquid-liquid extraction. A wide range of adsorbents are commercially available and may be divided into three classes polar, ion-exchange, and nonpolar adsorbents. Solid-supported liquid-liquid extraction on Extrelut columns is frequently reported for efficient cleanup of crude tropane alkaloid mixtures. Basifled aqueous solutions of alkaloids maybe transferred to Extrelut columns and the bases recovered in dichloromethane-isopropanol mixture [13]. 

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