Sorbents graphitized carbon

Supelco Bonded sorbents, graphitized-carbon sorbents... 

The most widely used carbon-based SPE sorbent graphitized carbon black (GCB) with specific surface areas up to 210m /g. Applications of the GCB sorbent in SPE have been extensively studied for polar pesticides in water. Table 10.4 shows the results of recovery values for extraction of 2L of water samples using Ig of GCB comparing with recoveries using C-18 sorbent and liquid-liquid extraction (LLE) with methylene chloride. The recoveries for GCB reach 90% to 100% for most of the compounds. 

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). 

Various forms of carbon are used to sample those analytes whose breakthrough volume is too low on Tenax for sufficient preconcentration [8,395-399]. Charcoal, graphitized carbon blacks, and ceurbosieves with wface areas from 5 to 900 w /g are commercially availablJ Bhe high surface area sorbents are used... 

QhEChERS 3 g homogenized sample mixed with water, addition of ACN, NaCl, an. MgS04, centrifugation Clean-up Mix with PSA sorbent, an. MgS04, Cl8, graphitized carbon black, 0.45 pm PTFE filtering... 

In some instances, combinations of Cig and silica columns are also used for better purification of the crude extracts (431, 445). A combination of Cg, silica, and amino solid-phase extraction columns has been successfully employed to fractionate anabolic and catabolic steroid hormone residues from meat in polar and nonpolar neutral and phenolic compounds, and to purify further each fraction effectively (452). Another combination of two solid-phase extraction columns, one using a graphitized carbon black sorbent and the other Amberlite resin in the hydroxyl form, allowed neutral anabolics to be isolated and separated from acidic anabolics and their metabolites (453). A combination of basic alumina column placed in tandem with an ion-exchange column has also been applied for the purification of the crude extracts in the determination of diethylstilbestrol and zeranol (427), and estradiol and zeranol in tissues (450). 

Nonporous mono- and polycrystalline sorbents, (e.g., graphitized carbon black, NaCl). 

One extraction phase developed for triazines is graphitized carbon black, which has been shown to be versatile for a wide range of pesticides (Di Corcia et al, 1987 Di Corcia and Marchetti, 1992 Di Corcia et al., 1993 Bucheli et al, 1997 Crescenzi et al, 1997). It has also been shown to be very effective for retaining the more polar metabolites (e.g., deisopropylatrazine (DIA), deethylatrazine (DEA), and hydroxyatrazine (ATOH) see Table 20.1, also Appendix, Table A3) (Berg et al, 1995 Di Corcia et al, 1997 Pichon et al, 1995 Schulein et al., 1995). The methods published require considerably more column preparation and elution steps than traditional Cl8 methods. Pichon et al. (1995) also included the polar triazine metabolites ammeline, ammelide, and cyanuric acid, with good recoveries using a graphitic carbon sorbent. 

Graphitized carbon black (GCB) materials are frequently used for off-line SPE as well. GCB behaves as a nonspecific sorbent, but because positively-charged adsorption sites are present, it also acts as an anion exchanger, which is useful for the enrichment of acidic compounds [75]. Examples involve the enrichment of chlorinated phenoxy acids [42], aryloxyphenoxypropionic herbicides [45], sulfonylurea herbicides [50], and imidazolinone herbicides [65]. 

Graphitized carbon polymeric sorbents (styrene-divinylbenzene)... 

Graphitized carbon and reversed-phase polymeric sorbents also are frequently used in environmental applications, such as trace enrichment, for... 

Examples of the use of these sorbents and their applications will be given in Chapter 7. Finally, a good review of examples of multiresidue extractions of compounds of environmental interest using polymeric and graphitized-carbon sorbents is given by Font and co-workers (1993). 

Chapter 1). The widespread use of the C-18 for reversed-phase sorption is a result of the popular use of C-18 columns in high-pressure liquid chromatography (HPLC). For this reason, it gained immediate popularity for SPE applications. Other reversed phases include, C-8, C-4 (wide pore), C-2, C-1, cyclohexyl, and phenyl groups. The most commonly used reversed-phase polymeric sorbent in SPE is the styrene-divinylbenzene, or SDB. Graphitized carbon is also used. 

Hennion and Pichon (1994) recommend C-18 for compounds in the range of log Poet from 1 to 3. For compounds with log Poet greater than 3, a sorbent with a lower carbon coating could be used (i.e., C-2, C-4, or C-8). Another choice is to add methanol to the mobile phase to decrease the k of the nonpolar solute for the C-18 sorbent. Finally, for the most polar compounds, with a log Poet of less than 1, the C-18 may not have adequate capacity for trace enrichment. In this case, the polymeric sorbents and the graphitized carbon are useful (Hennion and Pichon, 1994 also see discussion in Chapter 2 on polymeric sorbents). 

For moderately polar compounds in aqueous solutions, the use of the C-18 sorbent is the most efficient. The sorbent has good capacity for these compounds and is easy to elute with a solvent such as ethyl acetate. For most polar compounds (such as phenol in Table 7.1), the C-18 sorbent is typically not hydrophobic enough to sorb the compounds from a water sample. Therefore, a nonpolar sorbent with greater surface area is required. Typically there are two choices, a styrene-divinylbenzene (SDB) sorbent and graphitized carbon. The SDB sorbents are easier to use, cleaner, and easier to elute. The graphitized-carbon sorbents contain active sites of anionic exchange, which give more... 

The generic approach to SPE of aqueous samples follows the following method (Fig. 7.1/ First look at the polarity of the compound or its solubility and whether it can form an ion. Thus, is it ionic or not If nonionic, then what is its polarity Is the molecule nonpolar, moderately polar, or polar If the molecule is nonpolar or moderately polar, then the generic C-18/C-8 method described below will work. Alternatively, either a less polar sorbent may be used (i.e. either a C-8 or even a CN sorbent) to isolate the most nonpolar analytes. If the compound is polar (i.e., aqueous solubility greater than 1000 mg/L), then either the SDB or graphitized carbon will work best as a sorbent. 

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