Sorbents in SPE

Appropriate SPE sorbent selection is critical to obtaining efficient SPE recovery of semivolatile organics from liquids. Henry [58] notes that an SPE sorbent must be able to sorb rapidly and reproducibly, defined quantities of sample components of interest. Fritz [73] states that successful SPE has two major requirements (1) a high, reproducible percentage of the analytical solutes must be taken up by the solid extractant and (2) the solutes must then be easily and completely eluted from the solid particles. The sorption process must be reversible. In addition to reversible sorption, SPE sorbents should be porous with large surface areas, be free of leachable impurities, exhibit stability toward the sample matrix and the elution solvents, and have good surface contact with the sample solution [68,73], 

The earliest applications of chromatography, a term coined by Tswett in 1906, used polar sorbents to separate analytes dissolved in nonpolar solvents. Using light petroleum as the nonpolar mobile phase, Tswett separated 

Silica particles used for SPE sorbents are typically irregularly shaped, 40 to 60 pm in diameter. Silica particles used for sorbents in high-performance liquid chromatographic (HPLC) columns are generally spherical and 3 to 5 pm in diameter. Due to the differences in size and shape, SPE sorbents are less expensive than HPLC sorbents. Much greater pressures are required to pump solvents through the smaller particle sizes used in HPLC. 

Synthetic styrene divinylbenzene and other polymers, particularly the trade-marked XAD resins developed by Rohm Haas, were used for SPE in the late 1960s and early 1970s. However, the particle size of the XAD resins is too large for efficient SPE applications, and therefore the resins require additional grinding and sizing. Also, intensive purification procedures are needed for XAD resins [73,75], 

The enhanced performance of PS-DYB resins is due to their highly hydrophobic character and greater surface area as compared to the bonded silica sorbents, which are discussed in the following section. The strong sorption properties of PS-DYB resins may arise from the aromatic, poly- 

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Bielicka-Daszkiewicz, K., A. Voelkel, M. Szejner, and J. Osypisk. 2006. Extraction properties of new polymeric sorbents in SPE/GC analysis of phenol and hydroquinone from water samples. Chemosphere 62 890-898. 

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. 

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. 

Caro et al. [155] s)mthesized three polymers using 4-chlorophenol (4-CP) as the template, following different protocols (noncovalent and semicovalent), and used different functional comonomers, 4-vinylpyridine (4-VP) and methacrylic acid (MAA). They have evaluated the selectivity of the polymers as MIPs sorbent in SPE coupled online to LC. They found out that the 4-VP noncovalent polymer was the only polymer that showed a clear imprint effect. This MIP also showed cross-reactivity for the 4-chloro-substituted phenols and for 4-NP from a mixture containing the 11 priority EPA phenolic compounds and 4-CP. The MIP was applied to selectively extract the 4-chloro-substituted compounds and 4-NP from river water samples. Figure 16.4 shows the chromatograms obtained by online MISPE with the 4-VP noncovalent 4-CP imprinted polymer of 10 mL standard solution (pH 2.5) spiked at 10 mg/L with each phenolic compoimd. 

Eor the selective pre-concentration of deactivated phenols a new silica-based material with the grafted 2,3,5-triphenyltetrazole was proposed. This method is based on the formation of molecular chai ge-transfer comlexes of 2,3,5-triphenyltetrazole (7t-acceptor) with picric acid (7t-donor) in the phase of the sorbent. Proposed SPE is suitable for HPEC analysis of nitrophenols after their desorption by acetonitrile. Test-system for visual monitoring of polynitrophenols under their maximum concentration limits was developed using the proposed adsorbent. 

LC-MS with on-line SPE using a RAM pre-column with an internal ODS phase was described by van der Hoeven et al. (95) for the analysis of cortisol and prednisolone in plasma, and arachidonic acid in urine. The samples were injected directly and the only off-line pretreatment required was centrifugation. By using the on-line SPE-LC-MS system, cortisol and related compounds could be totally recovered and quantified in 100 p.1 plasma within 5 min with a typical detection of 2 ng/ml (Figure 11.6(b)). The RAM-type of sorbents, in which the outer surface of the particles is covered with aj-acid glycoprotein, also appear to be useful for direct SPE of... 

METAL-LOADED SORBENTS AND MOLECULARLY IMPRINTED POLYMERS IN SPE-LC... 

The low selectivity of the SPE columns currently in use can be increased with more selective sorbents such as the immunosorbents, which have been quite extensively used in SPE-LC (72). Immunoaffinity-based solid-phase extraction (lASPE) sorbents have also been used in coupled gas chromatography for determining... 

The most common and diverse approach to cleanup (and extraction of water samples) in pesticide residue analysis is SPE. Over the last 20 years, improvements and diversifications in SPE formats, sorbent types, and apparatus have made SPE a widely used approach for a variety of applications, including the analysis of pesticide residues. SPE cartridges or disks can be likened to low-resolution HPLC columns in that similar stationary and mobile phases are used. A typical particle size in SPE is 40 pm, and the plastic cartridges are generally packed with 0.1-1 g of sorbent in plastic tubes. The choice of reversed-phase, normal-phase, and ion-exchange media in SPE is very diverse, and Table 2 lists some of the more popular SPE applications for the cleanup of pesticides. 

This technique is based on the same separation mechanisms as found in liquid chromatography (LC). In LC, the solubility and the functional group interaction of sample, sorbent, and solvent are optimized to effect separation. In SPE, these interactions are optimized to effect retention or elution. Polar stationary phases, such as silica gel, Florisil and alumina, retain compounds with polar functional group (e.g., phenols, humic acids, and amines). A nonpolar organic solvent (e.g. hexane, dichloromethane) is used to remove nonpolar inferences where the target analyte is a polar compound. Conversely, the same nonpolar solvent may be used to elute a nonpolar analyte, leaving polar inferences adsorbed on the column. 

For each sample, condition Cig SPE (6-cc/l-g) cartridges with 6mL of methanol and 3 x 6mL of 0.005% aqueous H3PO4 in 10% MeOH-HaO. Stop Flow as the solution reaches the top of the sorbent (the SPE sorbent must not be allowed to go... 

Although SPE can be done in a batch equilibration similar to that used in LLE, it is much more common to use a small tube (minicolumn) or cartridge packed with the solid particles. SPE is often referred to as LSE, bonded phase or sorbent extraction SPE is a refinement of open-column chromatography. The mechanisms of retention include reversed phase, normal phase, and ion exchange. 

The sample volume initially introduced onto the sorbent, the choice of sorbent and solvent system and careful control of the amount of solvent used are of paramount importance for effective pre-concentration and/or clean-up of the analyte in the sample. The number of theoretical plates in an SPE column is low (/V = 10-25). SPE is a multistage separation method and as such requires only a reasonable difference in extractability to separate two solutes. In SPE concentration factors of 1000 or more are possible, as compared to up to 100 for LLE with vortex mixing. 

The sorbents used for SPE are available in three basic formats, namely disks, prepacked cartridges, and syringe barrels, each with certain advantages. Solid media employed in SPE may be classified as nonpolar, polar and ion-exchange phases with mixed retention mechanisms (Table 3.41). The large sorbent selection ranges also... 

Interactions of Sorbent and Analyte in SPE and Selective Extractions Based on Sorbent Chemistry... 

Analysis was performed on an ES-Ovomucoid column for stereoselectivity assessment, and for MS/MS, an X-Terra MS C18 column (2.1 x 100mm, 5 fan) was used. Figure 1.17 shows the wash and elution fractions from the SPE in a 384-well plate. The SPE conditions evaluated are listed in the table below the figure. The binding of the drug to the affinity sorbent in a 96-well plate was less efficient than the 384-well plate because the sorbent formed a disk on the former and a column on the latter. The efficiency is reflected in the >95% recoveries achieved with the 384-well format. 

The complexity of the method in terms of number of steps and solvents needed depends on the sorbent chemistry. The development in a simplified scenario involves running an analyte in several concentrations in multiple replicates and assaying for recovery and performance. This procedure is described in detail for several silica and polymeric sorbents by Wells.42 However, if a number of sorbents are to be evaluated, the process becomes time-consuming if multiple 96-well plates (each with one sorbent packed in all the wells) must be screened separately. This process may take a week or more and consume an analyst s precious time as well. The most plausible solution is to pack different sorbents in the same well plate and use a universal procedure that applies to all of them. An example of such a multisorbent method development plate is the four-sorbent plate recently introduced by Phenomenex demonstrated124 to require only 1 to 2 hr to determine optimal sorbent and SPE conditions. 

Method development in SPE begins with an analysis of the sample matrix as well as the sample analytes. One of the most efficient mechanisms of SPE is to use mixed-mode ion exchange sorbents. If the analytes are bases, cation exchange is the most selective SPE mode. If the analytes are acids, anion exhange is the most efficient. However, it is sometimes easier to capture the matrix components and allow the analytes to pass through the sorbent unretained. 

In SPE, the extraction is carried out using a small column (syringe-type or cartridge) containing 0.1 to 1 g of sorbent. The sorbent is typically a modified silica gel or one of many copolymers. This chemical filter can only be used once. The low-cost SPE process, which can be readily automated, is generally more useful for hydrophobic or apolar compounds than it is for ionic substances. 

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