Low Specificity Sorbents and Their Applications

Low specificity sorbents include silica-based, chemically bonded sorbents, macroreticular porous polymers and various forms of carbon. Silica-based, chemically bonded sorbents are derived from materials developed for high pressure liquid chromatogra-phy. They are generally prepared by reaction of monofunctional or trifunctional silanes with silica gel followed by end-capping in some cases. Trifunctional reagents result in sorbents with a polymeric-bonded layer of higher carbon loading and greater acid stability and are the more common type of sorbent in general use. Chemically bonded sorbents can be prepared with a wide range of bonding densities, pore sizes and functional group types. Some common examples are given in Table 3. Chemically bonded sorbents with large surface areas, long alkyl chains and high phase loading maximize retention of small analytes from aqueous solution, while wide-pore materials with low phase loading and short alkyl chains are generally used to isolate macromolecules. Chemically bonded sorbents with immobilized polar functional groups are used to isolate analytes from organic solutions, based on their selective interactions with analyte polar functional groups (see Table 1).

The macroreticular porous polymers are co-polymers of styrene-divinylbenzene or acrylic esters, prepared by suspension polymerization to yield particles consisting of agglomerates of randomly packed microspheres permeated by a network of holes and channels (Table 4). They are used exclusively for extraction from aqueous solution and are more retentive than most chemically bonded phases. They possess a high sample capacity and are frequently used in large-scale isolation studies and for the purification of industrial products. Tenax®, a polymer based on 2,6-diphenyl-4-phenylene oxide, revolutionized the sorbent trapping of volatile organic compounds from air and the purge-and-trap analysis of volatile organic compounds in water. It exhibits strong retention of semivolatile organic compounds (> C7) at room temperature with little adsorption of water vapour and can be rapidly heated to high temperatures, without thermal breakdown, for the recovery of analytes by thermal desorption. Since no single adsorbent is ideal for trapping all analytes it is common practice to use cartridges packed with several adsorbent beds in series, so that a broad range of compounds with different molecular weight and polarity can be trapped on a single cartridge. Besides Tenax, different forms of carbon, silica gel and liquid-coated sorbents are used. In a multiple bed cartridge, each bed protects the next, increasingly active bed, by preventing compounds from being held so strongly that they cannot be desorbed quickly without decomposition. During thermal desorption the carrier gas passes through the trap in the reverse direction to the sample flow and the desorbed compounds are swept onto the separation column in a gas chromatograph. A cryogenic interface may be used to refocus the desorbed sample to improve the chromatographic separation. The complete processes of desorption and separation can be automated for sample cartridges stored in an autosampler.

The common forms of carbon used in solid-phase extraction are granular activated carbon, graphitized carbon blacks and carbon molecular sieves. Granular activated carbons are prepared by the low temperature oxidation of vegetable charcoals. They have large surface areas (300-2000 m2 g"1), a wide pore size distribution, and a heterogeneous surface containing active functional groups. Their use in solidphase extraction is largely confined to the isolation of dissolved organic compounds in surface waters, and as the sorbent material in personal monitors for sampling workplace atmospheres. The most common form of personal monitor makes use of a sorbent cartridge filled with activated charcoal in conjunction with a small pump to maintain a fixed flow of air through the cartridge. Trapped volatile compounds

Table 3 Structures of silica-based chemically bonded sorbents

Type

Functional group

Structure

DIOL

Octadecyl

Octyl

Ethyl

Cyclohexyl

Phenyl

Cyanopropyl

Aminopropyl

2,3-Dihydroxypropoxypropyl

Trimethylaminopropyl (quaternary amine)

Carboxypropyl

Benzenesulfonic acid

Propylsulfonic acid

-Si—CH2CH2CH2OCH2CH—CH2 OH OH
—Si

are then eluted with carbon disulfide or another solvent, or can be thermally desorbed by microwave heating, for separation by gas chromatography. Poor reproducibility of activated carbons and their variable chemical and catalytic activity result in limited laboratory use. Graphitized carbon blacks are more refined and generally nonporous, with surface areas between about 5-100 m2g_1. They are used primarily for the room temperature trapping of volatile organic compounds (>C4), either separately or in combination with Tenax®. Carbon molecular sieves have small pores and large surface areas (> 500 m2 g~1

with some > 1200 m2 g"1). They are used primarily for the room temperature trapping of volatile organic compounds (C1 and C2), usually as a component of a multiple-bed sorbent trap for air sampling and purge-and-trap analysis.

Foamed polyurethanes, composed of agglomerated spherical micrometer-sized particles bonded to one another in a rigid and highly permeable structure, are suitable for sampling semivolatile organic compounds (e.g. airborne pesticides and polychlorinated biphenyls) at high flow rates. They are frequently used in conjunction with high-volume air samplers on

Table 4 Characteristic properties of some macroreticular porous polymer sorbents

Amberlite sorbents Mean pore diameter Specific surface area Pore volume Sample molecular

XAD-2010 (STY-DVB) 28 660 1.80

STY-DVB, styrene-divinylbenzene; MMA, methylmethacrylate.

account of their low pressure drop compared with standard sorbent cartridges. They are used less frequently for water analysis where macroreticular porous polymers are considered a better choice.

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