should be bulky organic hydrophobic groups to provide high solubility in
nonpolar solvents. The metal ion can be a cationic or an anionic complex, and
can be an inorganic species such as FeCl 4 - , MnO 4 - or a chelated organic complex
such as Fe(1,10-phenanthroline) 32 + or UO 2 (oxine) 3 -. Suitable counter-ions of
opposite charge to the metal-complex ions include (C 4 H 9 ) 4 N+, ClO 4 -
(C 6 H 5 CH 2 ) 3 NH+, [(C 4 H 9 O) 3 P=O]H+and an oxonium ion such as [(C 2 H 5 ) 2 O] 3 H+.
A list of metal chelate complexing agents and ion-association complexes is
given in Table 1. Most complexing agents react with a large number of metals
(up to 50 or more), but pH control, the use of masking agents and a variety of
ion-association systems can enable the selective extraction and separation of just
one or two metals to be accomplished.These are generally either silica or chemically-modified silica similar to the
bonded phases used in high-performance liquid chromatography (Topics D6
and D7) but of larger particle size, typically 40– 60 mm diameter. Solutes interact
with the surface of the sorbent through van der Waals forces, dipolar interac-
tions, H-bonding, ion-exchange and exclusion. The four chromatographic sorp-
tion mechanisms described in Topic D2 can be exploited depending on the
sorbent selected and the nature of the sample. Sorbents can be classified
according to the polarity of the surface. Hydrocarbon-modified silicas are
nonpolar, and therefore hydrophobic, but are capable of extracting a very wide
range of organic compounds from aqueous solutions. However, they do not
extract very polar compounds well, if at all, and these are best extracted by
unmodified silica, alumina or Florisil, all of which have a polar surface. Ionic
and ionizable solutes are readily retained by an ion-exchange mechanism using
cationic or anionic sorbents. Weak acids can be extracted from aqueous solu-
tions of high pH when they are ionized, and weak bases from aqueous solutions
of low pH when they are protonated. It should be noted that this is the opposite
way around compared to solvent extraction into non-polar solvents. However,
by suppressing ionization through pH control, extraction by hydrocarbon-
modified silica sorbents is possible. Sorbents of intermediate polarity, such as
cyanopropyl and aminopropyl modified silicas may have different selectivities
to nonpolar and polar sorbents. Some SPE sorbents are listed in Table 2along
with the predominant interaction mechanism for each one.Compared to solvent extraction, solid-phase extraction(SPE),is a relatively
new technique, but it has rapidly become established as the prime means of
sample pre-treatment or the clean-up of dirtysamples, i.e. those containing high
levels of matrix components such as salts, proteins, polymers, resins, tars etc. In
addition to being potential sources of interference with the detection and quan-
titation of analytes, their presence can be detrimental to the stability and perfor-
mance of columns and detectors when a chromatographic analysis is required.
The removal of interfering matrix components in general and the pre-concentra-
tion of trace and ultra-trace level analytes are other important uses of SPE which
is versatile, rapid and, unlike solvent extraction, requires only small volumes of
solvents, or none at all in the case of solid phase microextraction(vide infra).
Furthermore, SPE sorbents are cheap enough to be discarded after use thus
obviating the need for regeneration. The analysis of environmental, clinical,
biological and pharmaceutical samples have all benefited from the rapid growth
in the use of SPE where it has largely replaced solvent extraction. Specific exam-
ples include the determination of pesticides and herbicides in polluted surfaceSolid-phase
extraction
Solid-phase
sorbents
D1 – Solvent and solid-phase extraction 115