ReviewSurfactant cloud point extraction and preconcentration of organic compounds prior to chromatography and capillary electrophoresis
Introduction
In recent decades the development of preconcentration steps to be implemented prior to analytical determinations of trace level compounds has been explored in considerable depth. With a view to eliminating or at least minimising the use of organic solvents used in conventional liquid–liquid extraction, other methodologies have been developed, such as membrane extraction [1], [2], [3], solid-phase extraction (SPE) [4], [5], [6], [7], solid-phase microextraction (SPME) [8], [9], [10], etc.
Alternative extraction approaches that make use of liquid phases have also been proposed; among them are those that employ polymers and those that use surfactants. Extractions with polymers may be of two types: the aqueous biphasic system (ABS) [11] and extractions using thermoseparating polymer systems (TPS) [12], [13]. In the case of ABS, two or more water-soluble polymers are added in the presence of a given salt concentration. When the concentrations of the polymers added are above their critical concentration, one or two phases are formed when working at room temperature. In TPS it is necessary to increase the temperature of the solution containing the polymer up to a suitable level in order to obtain two phases; this is what is known as the cloud point temperature. Both techniques have mainly been used in the separation of biomolecules [14], [15], [16], [17].
Aqueous solutions of some surfactants are used in micellar extraction (ME) and cloud point extraction (CPE). In ME, the selective separations can be achieved owing to the fact that the micellar aggregates have a size that prevents them from crossing certain ultrafiltration membranes. This, together with the capacity of micelles to solubilise different compounds, has been used for the separation of nitrates from underground water using cellulose membranes and the surfactant cetyltrimethylammonium bromide (CTAB) [18].
The aqueous micellar solutions of some surfactants exhibit the cloud point, or turbidity, phenomenon when the solution is heated or cooled above or below a certain temperature. The temperature at which this phenomenon occurs is known as the cloud point temperature. This methodology is known as CPE or micelle-mediated extraction.
The aim of the present work is to explore the different analytical possibilities of this modality (CPE) in the extraction and preconcentration of organic compounds other than biomolecules (polycyclic aromatic hydrocarbons, polychlorinated compounds, pesticides, phenolic derivatives, aromatic amines, vitamins, etc.) that, owing to their high analytical interest, continue to be the objective of many investigations.
Different authors have offered reviews [19], [20] concerning the fundamental characteristics of the CPE technique such as the basic features, experimental procedures and general applications. Here we report the most recent analytical applications of CPE when used as an isolation and trace enrichment step prior to the analysis of organic compounds by capillary electrophoresis (CE), or liquid and gas chromatography (HPLC and GC). In the final part of the work, we briefly discuss the future trends of the technique.
Section snippets
Micellar systems
Surfactants are amphiphilic molecules, one of whose parts (the head) is polar or hydrophilic in nature and the other (the tail) hydrophobic. This latter part is generally a hydrocarbon chain with different numbers of carbon atoms and may be linear or branched. It may also contain aromatic rings.
In aqueous solution, and at low concentrations, surfactant molecules are found in monomer form, although dimers and trimers have also been detected. When the surfactant concentration is increased above a
High-performance liquid chromatography
Most analytical applications of CPE methodology for the extraction of organic compounds (Table 4) make use of reversed-phase high-performance liquid chromatography (HPLC); the surfactant-rich phase obtained in the extraction process is compatible with the hydro–organic phases usually employed in this chromatographic mode.
Conclusions and future trends
The use of micellar systems for the separation and preconcentration of organic compounds is a useful alternative with the following characteristics: (a) a high capacity to preconcentrate analytes of different polarities; (b) the preconcentration factor can be optimised by modifying the type and concentration of surfactant as well as the experimental conditions under which extraction and phase separation are carried out; (c) surfactants are less toxic and cheaper than the extractants used in
Acknowledgements
This work was supported by the DGICYT (Projects PB97-1322 and PB98-0278) and the Consejerı́a de Cultura y Turismo of the Junta de Castilla y León y la Unión Europea (Fondo Social Europeo, Projects SA19/99 and SA63/99).
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