Solid Phase Extraction

C. F. Poole, Wayne State University, Detroit, MI,

Copyright Ā© 2000 Academic Press

Solid-phase extraction is a method used to isolate analytes from a gas, fluid or liquid by their transfer to and retention on a solid-phase sorbent. After separation of the sorbent from the sample the analytes are recovered by elution using a liquid or fluid, or by thermal desorption into the gas phase. If the analytes are recovered from the sorbent in a final volume that is only a fraction of the sample volume, then concentration as well as isolation is achieved. In addition, if the sorption step, any subsequent rinse steps, and the elution conditions are selective for retention and recovery of the analyte, then matrix simplification is achieved. Isolation, concentration and matrix simplification are the primary goals of solid-phase extraction.

Probably the earliest application of solid-phase extraction was the use of charcoal-filled columns in the 1950s to isolate organic contaminants from surface waters for toxicity evaluation. The large volume of water generally sampled (more than 1000 L over several days) precluded the use of liquid-liquid extraction techniques. The subsequent evolution of solidphase extraction techniques is summarized in Figure 1.

The introduction of macroreticular porous polymers in the early 1970s was responsible for rekindling interest in solid-phase extraction and extending its scope to air sampling and the isolation of drugs from biological fluids. These sorbents had reasonable mechanical strength compared with gels, a large surface area and sample capacity, low water retention, and gave high sample recoveries by solvent desorption. Compared with carbon the overall analyte recovery was generally better and irreversible adsorption and catalytic activity greatly diminished. These

Figure 1 Time line showing the general evolution of solidphase extraction techniques.
Figure 2 Schematic diagram showing the typical construction of a solid-phase extraction cartridge and a vacuum manifold for parallel sample processing.

properties, together with a reduction in the amount of material needed for identification due to improved instrumentation, resulted in the general use of small columns, similar in size to those in use today. Porous polymers with high thermal stability and low water retention revolutionized the room temperature sorbent extraction of volatile organic compounds from air or purge gas from water samples. Trapped compounds were thermally desorbed directly into a gas chromatograph for analysis. Automated systems based on the above process are used for routine analysis today.

Solid-phase extraction for liquid samples became a widely used laboratory technique with the introduction of disposable sorbent cartridges containing porous, siloxane-bonded silica particles, sized to allow sample processing by gentle suction (Figure 2). A typical solid-phase extraction cartridge consists of a short column (generally an open syringe barrel) containing a sorbent with a nominal particle size of 50-60 |im, packed between porous metal or plastic frits. A large number of sorbents are in use today corresponding to the desire for general purpose, class-specific and even compound-specific extractions.

Slow sample processing rates for large sample volumes, low tolerance to blockage by particles and sorbed matrix components, and problems arising from the low and variable packing density of cartridge devices spawned the development of alternative sampling formats based on disc technology. At least three different designs for solid-phase extraction discs are offered commercially today. The particle-loaded membranes consist of a web of polytetraf-luoroethylene (PTFE) microfibrils, suspended in which are sorbent particles of about 8-10 |im diameter. The membranes are flexible with a homogene ous structure containing 80% (w/w) or more of sorbent particles formed into circular discs 0.5 mm thick with diameters from 4 to 96 mm. For general use they are supported on a sintered glass disc (or other support) in a standard filtration apparatus using suction to generate the desired flow through the membrane (Figure 3). Particle-embedded glass fibre discs contain 10-30-|im sorbent particles woven into a glass fibre matrix. The small diameter discs are rigid and self-supporting, while the larger diameter discs require a supporting structure. SpeediscsĀ® (Figure 4) consist of a sandwich of 10-|im sorbent particles held between two glass-fibre filters, with a screen to hold the filters in place. Disc technology has contributed directly to the automation of solid-phase extraction through the development of the multiwell extraction plate (Figure 5), which is used for the clean-up of samples in high-throughput screening techniques for drug development. Direct coupling of solid-phase extraction and high pressure liquid chromatography for on-line sample processing and analysis is now routine and the direct coupling of solid-phase extraction and gas chromatography for the analysis of liquid samples has moved beyond the research phase. Several research groups have demonstrated the direct coupling of solid-phase extraction and electrophor-etic and thin-layer chromatographic separation techniques.

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