Silica gels adsorbant, solid phase extraction

A cleanup procedure is usually carried out to remove co-extracted matrix components that may interfere in the chromatographic analysis or be detrimental to the analytical instrument. The cleanup procedure is dependent on the nature of the analyte, the type of sample to be analyzed, and the selectivity and sensitivity of the analytical instrument used in the analysis. Preliminary purification of the sample extracts prior to chromatographic separation involves liquid-liquid partitioning and/or solid-phase extraction (SPE) using charcoal/Celite, Elorisil, carbon black, silica, or aminopropyl-silica based adsorbents or gel permeation chromatography (GPC). 

In principle, there are no major differences between solid-phase extraction and liquid-liquid extraction, but SPE can avoid or reduce some of the disadvantages of liquid-liquid extraction. Thus, SPE can handle small samples and very dilute solutions, it overcomes the formation of emulsions, and can be easily automated. Furthermore, the sorbents that are commonly used are commercially available as cartridges. These sorbents are alumina, silica gel, reversed-phase silica gel, and various ion-exchange resins. It is also possible to pack different adsorbents in layers inside the same cartridge to give a sandwich-type extraction column. 

In fluorous solid phase extraction (FSPE Figure 3.4), a reaction mixture containing organic and fluorous components is placed on a fluorous silica gel column in a fluorophobic solvent, for example acetonitrile, methanol or a methanol-water mixture. Then, with a fluorophobic mobile phase, first the organic compounds are eluted while the fluorous compounds remain adsorbed by the solid phase. In the second step a fluorophilic solvent such as THF, diethyl ether, or benzotrifluoride (BTF) is used to elute the fluorous compounds. 

In addition to electrophoresis-based collection of DNA in microchannels (see Section 7.7.3), adsorbents have been used to collect and purify DNA. In these cases, microchannels are generally used because of the small quantity of material requiring collection. Using solid phase extraction, sorbent particles of nano- and micro-silica and micro-sized octadecylsilica were immobilized using sol-gel chemistry to hU the microchannels of the microfluidic device [194]. DNA as well as several organic compoimds were evaluated for adsorption and desorption. They showed excellent adsorption, but poor recovery because they were difficult to extract. 

The method of preparation of the crude extract pnor to loading on the solid phase extraction column will depend on the compatibility of the column and the solvent. For example, if an aqueous methanol extract is loaded onto a normal phase column (e.g., silica gel, diol, or alumina), the sample must first be evaporated to dryness to remove water, which is incompatible with normal phase adsorbents, and then the residue must be dissolved in a nonpolar solvent such as ethyl acetate, toluene, or hexane that has limited solubility for the product. Conversely, if a toluene extract is loaded onto a reverse phase column (e.g., C8, Cl8), the sample should be evaporated to dryness and the residue dissolved in a minimal volume of solvent compatible with the mobile phase. 

Liquid-solid extraction procedures are accompanied by a serious risk of contamination. For example, blank tests with commercial extraction columns have revealed traces of plasticizers and antioxidants as well as various alkanes and alkenes [154]. For this reason certain manufacturers now offer solid-phase extraction columns made from glass. The extent of the po-tential contamination depends on the manufac-turer, the adsorbent load, conditioning steps, and the solvent used for the extraction [154], [155], If no satisfactory commercial column is available, one can easily be prepared in the laboratory from a suitable adsorbent and an appropriate glass tube. For example, XAD can be purified to an extent sufficient for this purpose by Soxhiet extraction [ 156], [ 157]. which reduces the level of impurities by a factor of 30. Silica gel and aluminum oxide can be purified... 

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. 

The first step is to preconcentrate and separate the material of interest from the matrix by solid-phase extraction. The solid-phase material used is hydro-phobic silica gel (octadecyltrichlorosilane derivative). After preconditioning with methanol, 0.1-1 L of sample solution is passed through the cartridge or empore disk. After rinsing with water, the adsorbed surfactant is eluted with methanol. Recovery is in the 95-100% range. In the extract, alkyl polyglycosides are determined by HPLC or GC methods identical to or comparable with those already described. 

Calculation procedures for adsorption and ion exchange differ only in detail from liquid-liquid extraction since an ion-exchange resin or adsorbent is analogous to the solvent in extraction. All coordinate systems used to represent solvent-solute or liquid-vapor equilibria may be used to display three-component solid-liquid, or solid-gas phase equilibria states. For the case of gas adsorption, equilibria are usually a function of pressure and temperature, and so isobaric and isothermal displays such as Fig. 3.21, which represents the propane-propylene-silica gel system, are convenient. 

Apart from modified silica gel, the most frequently used solid-phase adsorbents are activated charcoal, macroreticular resins (XAD), ordinary silica gel, aluminum oxide, and Florisil. Activated charcoal is a universal adsorbent for concentrating trace organic materials in aqueous solutions and air, XAD resins are also commonly employed for extracting organic trace constituents, such as alkoxyacetic acids [1.57], organochlorine pesticides [200], [218], carbamates [198], 202j, and triaz.ines [218] from aqueous matrices. 

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