Silica-based sorbents

The sorbents that are most frequently used in environmental analysis are Cig-silica based sorbents, polymeric sorbents (usually styrenedivinilbenzene) and graphitized carbon. In order to increase the selectivity of these sorbents, immunosorbents (35, 36) have been developed and used with good results, while recently, molecularly imprinted polymers have started be to used (35, 36). 

Studies on solid supports have shown that tlie pore size of the silica-based sorbents is of little importance in MSPD, but it should be considered as could vary with the sample. Particle size is of greater importance since particles as small as 3-20 m can lead to extended solvent elution times and plugged MSPD columns. However, 40 m particles with 60 Angstrom pores have been used extensively and successfully. Sorbents that have a blended range of particle size such as 40-100 m work equally well and can be used in most applications (101, 103). These materials also tend to be less expensive. Depending on the application, analysts can also use non-end-capped materials and materials with a range of carbon loading. Different applications may benefit, suffer, or be unaffected by these parameters, but workers should consider them to obtain the best extraction efficiency and the cleanest sample. 

Thin-tjayer Chromatography. Chiral stationary phases in tic have been primarily limited In phases based on normal nr inicrocrystalline cellulose, triaeelyleellulo.se sorbents or silica-based sorbents that have been chemically modified or physically coated to incorporate chiral selectors such as amino acids or macrocyclic antibiotics into the stationary phase. The cost of many chiral selectors, as well as the accessibility and success of chiral additives, may have inhibited widespread commercialization. 

A vast range of materials have been bonded to silica such as phenyl, cyano, nitro, amino, and diol functionalities. The variety of silica-based sorbents are listed in the United States Pharmacopeia (USP) [23] where it is noted that of the 33 column packings, 19 are silica based. The synthesis and characteristics of over 80 nonconventional bonded silicas have been reviewed [24]. In spite of the number of chromatographic packings, 50% of the chromatographers develop methods using a C18 sorbent [1]. Sorbents... 

TABLE 9 Comparison of Some of the Different Properties between Soft Gels and Silica-Based Sorbents Used in the Purification of Polypeptides and Proteins... 

The main downside of silica-based sorbents is pH compatibility they are stable only at pH values from 2 to 8 at lower pH, the H3O+ ions catalyze the hydrolysis of the alkyl ligand at higher pH, the nucleophilic attack of Si-O bonds by OH results in the erosion of the silica surface. The formation of Si(OH)4 is signaled by a back pressure increase. Recently the chemical resistance of the silica particles has been improved by hybrid particle technology. Organic moieties such as ethyl groups were inserted close to the particle surfaces and columns stable between pH 1 and 12 under ultra-high pressure (1000 bar) were produced. 

Make pH adjustments to the eluting solvent to change the hydrophobicity or ionic state of the compound. Hydrogen-bonding interactions may also be occurring between the compound and active sites on the silica-based sorbent, requiring a basic solvent for elution. 

Sorption Ions or their complexes may be collected in microcolumns on activated carbon, hydrous iron(III) oxide or reversed phased silica based sorbents (Das et al., 1983 Mizuike, 1983 Fang, 1993). The techniques are easily automated. Either elution or direct analysis may be performed. However the problem with blank values (by direct analysis of the sorbent) deserves serious attention. 

Laskorin, B.N. Strelko, V.V. Strazhesko, D.N. Denisov, V.I. Silica Based Sorbents in Radiochemistry Atomizdat Moscow, 1977. (In Russian). 

Although C-18 silica having high carbon content provides the stronger retention for hydrophobic analytes, it also traps more interference. Other silica-based sorbents, like C-2, can extract the highly hydrophobic analytes more specifically. 

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