Solid-phase microextraction
Introduction
There have been major advances in the area of organic micro-component analysis in the last few decades, mainly owing to developments in instrumental analysis. Unfortunately, sample-preparation methods have often lagged behind, which is partly the responsibility of the regulatory bodies' slowness in changing the established analytical procedures [1].
The extraction of organic compounds is still usually carried out with organic solvents, both for liquid (liquid–liquid extraction, LLE) and for solid samples (Soxhlet extraction, ultrasonic extraction). Usually, micro-components are extracted, so the solvents must be of high purity, with a concomitant high price. Also, many are toxic, even carcinogenic, and their disposal is problematic. Classical extraction procedures usually consist of many steps, including purification of the extract, with consequent greater analyte losses and longer sample preparation times.
The disadvantages of conventional extraction techniques, and especially their use of large quantities of organic solvents, have led to the development of new methods, whose main advantages are their speed and the small volume of solvent used.
Extraction methods using little or no organic solvent can be divided into three groups according to the extraction medium: gas-phase extractions, membrane extractions, and sorbent extractions [1]. Gas-phase extractions are static headspace techniques, using the equilibrium gas phase above the sample, and purge-and-trap or dynamic headspace methods, where a gas is bubbled through the sample, to purge the volatile components, which are subsequently trapped. Also, among gas-phase extractions is supercritical fluid extraction (SFE). Even the less volatile compounds can be separated from solid samples using this method. The disadvantage of the last two methods is the complicated and expensive equipment necessary, while there is essentially no pre-concentration of the analytes in the static headspace method. Membrane extraction techniques use a polymer membrane to extract analytes from the sample. The method is effective for volatile analytes, which are desorbed from the membrane by the gas, but it is not suitable for more polar analytes because of the lack of specific membranes [1].
Sorbent extractions are solid-phase extraction (SPE) and solid-phase microextraction (SPME). SPE is an established method, in which a liquid sample is passed through a specific sorbent, similar to the stationary phases used in HPLC, where the analytes are retained. SPE can be applied to extract hydrophobic, but also more hydrophilic compounds, which is an advantage over LLE. The sorbents have high capacities and are available in the form of cartridges or extraction discs. The more usual approach is to use SPE in an off-line procedure: analytes are desorbed from the sorbent with a small volume of organic solvent and an aliquot of the final extract is subsequently analysed, which is still a drawback, and has a persistent risk of contamination [2]. Both major disadvantages are eliminated by the use of an on-line procedure coupled to high performance liquid or gas chromatography [3]. For subsequent LC analysis a very mobile phase is usually used to extract analytes from the sorbent. Another advantage of the on-line approach is the possibility of complete automation of the process. However, some difficulties encountered when using SPE can be the low breakthrough volumes for more hydrophilic analytes, the need to pre-filter the real-life samples to avoid clogging and subsequent analyte loss, and the possibility of interferences such as plasticisers present in the sorbent material [4].
SPME is a novel approach in sorbent extractions. It completely eliminates the use of organic solvents and has the advantage of simplicity. It has, in general, higher detection limits than SPE, because the extraction is not exhaustive, and a narrower application range because of difficulties encountered when analysing hydrophilic and non-volatile compounds. The many aspects of SPME are the subjects of this review.
Section snippets
General considerations on SPME
In SPME, a fused silica fibre is coated with a stationary phase. The fibre is exposed to an aqueous or gaseous sample until equilibrium is established between the analyte in the sample and on the fibre. The analyte is then desorbed from the fibre at a high temperature in a gas chromatograph injector, and subsequently analysed by GC.
The method was first reported by Arthur and Pawliszyn in 1990 [5]and is now widely accepted, with a constantly rising number of new papers. The first experiments
Theory of SPME
The theory of SPME has been amply presented by Pawliszyn and his co-workers 6, 7, 8and the following discussion is mainly based on these papers. SPME can be used for aqueous or gaseous samples. In both cases, there is a partition of analytes between the sample matrix and the stationary phase. The partition coefficient is expressed as the concentration of the analyte in the stationary phase versus the concentration of analyte in the sample:cf is the analyte concentration in the
Choice of stationary phase
Several types of stationary phase, of different thicknesses and polarities, are available commercially (Supelco, Gland, Switzerland), which show great selectivity for different analytes (Fig. 1). Generally speaking, for non-polar analytes a polydimethylsiloxane phase is preferable, and for more polar compounds a polyacrylate phase. As an example, for a group of triazine pesticides the extraction efficiency can be as much as 10–20 times higher with a polyacrylate rather than with a
SPME applications
SPME is very effective for screening purposes because of its speed and ease of use. Quantitative analysis is feasible when using internal or external calibration. It should be borne in mind that this is an equilibrium method, and that extraction is rarely quantitative, so the extraction conditions for both the standard solution and the sample should be kept under strict control. For a more complex matrix, the standard-addition method is recommended. When extracting from the gaseous phase,
Conclusions
Solid-phase microextraction is a new sample-preparation technique, which is very suitable for the analysis of volatile and semi-volatile compounds in water and gaseous matrices. It offers complete elimination of organic solvents from the extraction process and thus greatly simplifies it; it is also rather inexpensive and rapid. Because of these features, it is very suitable as a fast screening technique, especially in the field. As extraction and concentration are combined, the whole of the
Acknowledgements
Many thanks are offered to Prof. Udo A.Th. Brinkman of the Vrije Universiteit, Amsterdam, for his support and editorial comments.
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