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Normal-phase sorbents have polar functional groups, e.g. cyano, amino and diol (also included in this category is unmodified silica). The polar nature of these sorbents means that it is more likely that polar compounds will be retained.
Suggest some typical functional groups that would be characteristic of polar compounds.
Typical functional groups which are capable of polar interactions include hydroxyls, amines, carbonyls and groups containing heteroatoms such as
oxygen, chlorine, nitrogen, sulfur and phosphorus. Common polar environmental compounds include organochlorine pesticides, organophos-phorus pesticides and polychlorinated biphenyls.
In contrast, reversed-phase sorbents have non-polar functional groups, e.g. octadecyl, octyl and methyl, and conversely are more likely to retain non-polar compounds, e.g. polycyclic aromatic hydrocarbons. Ion-exchange sorbents have either cationic or anionic functional groups and when in the ionized form attract compounds of the opposite charge. A cation-exchange phase, such as benzene-sulfonic acid, will extract analytes with positive charges (e.g. phenoxyacid herbicides) and vice versa. A summary of the commercially available silica-bonded sorbents is given in Table 8.1.
8.3.2 Cartridge or Disc Format
The design of the SPE device can vary, with each design having its own advantages related to the number of samples to be processed and the nature of the sample and its volume. The most common arrangement is the syringe barrel or cartridge. The cartridge itself is usually made of polypropylene (although glass and polytetrafluorethylene (PTFE) are also available) with a wide entrance, through which the sample is introduced, and a narrow exit (male luer tip). The appropriate sorbent material, ranging in mass from 50 mg to 10 g, is positioned between two frits, at the base (exit) of the cartridge, which act to both retain the sorbent material and to filter out particulate matter. Typically, the frit is made from polyethylene with a 20 ^m pore size.
Solvent flow through a single cartridge is typically carried out by using a side-arm flask apparatus (Figure 8.5), whereas multiple cartridges can be simultaneously processed (from 8 to 30 cartridges) by using a commercially available vacuum manifold (Figure 8.6). A variation on this type of cartridge system or syringe filter is when a plunger is inserted into the cartridge barrel. In this situation, the solvent is added to the syringe barrel and forced through the SPE unit by using the plunger. This system is effective if only a few samples are to be processed, for early method development, the SPE method needs to be simple, or if no vacuum system is available.
The most distinctly different approach to SPE is the use of a disc, not unlike a common filter paper. This SPE disc format is referred to by its trade name of Empore™ (discs). The 5-10 ^m sorbent particles are intertwined with fine threads of PTFE which results in a disc approximately 0.5 mm thick and a diameter in the range 47 to 70 mm. Empore™ discs are placed in a typical solvent filtration system and a vacuum applied to force the solvent-containing sample through (Figure 8.7). To minimize the dilution effects that can occur, it is necessary to introduce a test-tube into the filter flask to collect the final extract. Manifolds are commercially available for multiple sample extraction using such Empore discs.
Table 8.1 Some commercially available silica-bonded sorbents used in solid-phase extraction
Silica-based - anion Silica-based - anion Silica-based - cation Silica-based - cation
Silica-based - cation Silica-based - cation
Cl8, octadecyl C8, octyl C6, hexyl C4, butyl C2, ethyl CH, cyclohexyl
CN, cyanopropyl ENV+
CN, cyanopropyl Si, silica
NH2, aminopropyl FL, florisil Al, alumina
NH2, aminopropyl SAX, quaternary amine CBA, propylcarboxyhc acid SCX, benzenesulfonic acid
SCX-2 (PRS), propylsulfonic acid SCX-3, ethylbenzenesulfonic acid
-Si-(CH2)3CN Hydroxylated polystyrene divinylbenzene
-Si-OH -Si-(CH2)3 -OCH2CHOHCH2OH -Si-(CH2)3NH2 Mg036(Si02)o.iOH
Methods for Environmental Trace Analysis
Figure 8.5 A typical system used for solid-phase extraction, employing a cartridge and a side-arm flask apparatus. From Dean, J. R., Extraction Methods for Environmental Analysis, Copyright 1998. © John Wiley & Sons Limited. Reproduced with permission.
Figure 8.6 Schematic of a commercially available vacuum manifold system used in solid-phase extraction.