Solid phase microextraction (SPME) was first introduced by Arthur and Pawliszyn in the early 1990s [19]. It uses a fused silica fiber coated along a length of ca. 1 cm with an appropriate stationary phase to extract target analytes from aqueous samples. Since it became commercially available in 1993 [20,21], SPME has been widely applied to a large variety of compounds, especially volatile and semi-volatile organic compounds.
SPME procedure is based on the partition of analytes between the sample and the coated fiber. During extraction, SPME can be performed by direct immersion (DI) mode, in which the fiber is directly immersed into the sample solution, or headspace (HS) mode, in which the fiber is exposed to the headspace of a sample placed in a closed vessel. Extraction by DI-SPME is relatively fast since the analytes move from
the sample solution onto the fiber directly. However, the fiber usually suffers from the effects of salts and pH of the sample solution, and also interferences in complex sample matrices, which decrease the lifetime of the fiber. This problem can be avoided in HS-SPME. In HS-SPME, the fiber is protected from the interferences which are non-volatile or of high molecular masses. It is also noted that the analytes extracted by HS-SPME should be volatile or semi-volatile in order for them to partition to the headspace [22].
In SPME, the selection of fiber coating is essential to the extraction; it should be based on the principle of “like dissolves like” and the properties of the analytes. There is no universal coating that can extract all kinds of analytes. Different types of coatings, including a solid porous sorbent or a high molecular weight polymeric liquid, or both, have been developed for SPME. The commonly used commercially available sorbents (from nonpolar to highly polar) are: polydimethylsiloxane (PDMS), carboxen (CAR)-PDMS, divinylbenzene (DVB)-CAR-PDMS, polyacrylate (PA), PDMS-DVB, carbowax (CW)-DVB, and CW-templated resin (TPR). The thickness of the fiber coating, usually 7-150 àm [23], determines the surface and volume of the extraction phase, thus, the amount of analytes adsorbed.
After extraction, the analytes are desorbed from the fiber into a suitable chromatographic system for analysis. Most conveniently, the fiber is directly inserted into the injection port of a GC for thermal desorption. In order to analyze thermally
labile or weakly volatile analytes which are not amenable to GC, solvent desorption has also developed for SPME to couple to an HPLC system [24,25].
Generally, SPME is a simple, sensitive, and solvent free (for coupling to GC) sample preparation technique. It combines sampling, extraction and preconcentration in one step. Since its introduction, the application of SPME has covered a variety of fields.
However, there are also some limitations. The carryover effect is the main problem in SPME, which is very hard to be eliminated [26]. In addition, the limited commercially available fiber coatings, the limited extraction capacity, the fragility and limited lifetime of fibers, and the relatively high cost of fibers are considered, in some cases, as drawbacks in SPME.
In-tube SPME is another configuration of SPME, initially reported by Eisert and Pawliszyn in 1997 [27], in which the stationary phase is immobilized on the interior wall of a tube or a capillary, or is packed inside a tube or a capillary, instead of the surface of a fiber. In-tube SPME is based on the distribution of analytes between the sample solution and the stationary phase. After extraction, the analytes can be desorbed by a flow of an approximate mobile phase. In-tube SPME is fast and inexpensive, and it can overcome the drawbacks of fibers used in SPME, such as fragility and low extraction capacity. In addition, in-tube SPME is suitable for convenient automation which provides fast analysis and better precision and accuracy compared to manually operated techniques [20]. Moreover, a short length of a
capillary GC column coated with a common stationary phase can be used for the technique.
1.2.2 Stir bar sorptive extraction
Stir bar sorptive extraction (SBSE) was introduced by Baltussen et al. in 1999 [28]. A 1-4 cm magnetic stir bar is coated with a layer of stationary phase, and then is placed into an aqueous sample to extract analytes. After extraction, the analytes absorbed on SBSE can be desorbed thermally or by solvent [29].
As its name indicates, SBSE is based on sorptive extraction [30]. In a typical SPME PDMS fiber (100 àm thickness coating), the volume of stationary phase is about 0.5 àL. In SBSE, the thickness of stationary is typically 0.5 to 1 mm, and the volume of stationary is 50 to 250 times larger than that of SPME, therefore resulting in higher sample capacity, higher extraction efficiency, and better detection sensitivity [31-33].
Like SPME, SBSE can be performed by direct immersion in which the stir bar is directly added into an aqueous sample solution, or in headspace mode in which the stir bar is supported by a special device and placed in the headspace of a solid or aqueous sample.
The stir bar can be reused for 20-50 consecutive extractions, depending on the matrix [30]. The technique has been applied to environmental, food, and biological samples.
However, the limited range of stationary phases is the main drawback of SBSE. Up to now, the commercially available coating for stir bars include PDMS, ethylene glycol -silicone, and polyacrylate, still a limited range [29]. Also, because of the higher sample capacity, solvent desorption in SBSE usually requires more solvent and over a longer period of time.
1.2.3 Micro solid-phase extraction
Basheer et al. [34] reported the first application of micro-solid-phase extraction (à-SPE) in 2006, in which multi-walled carbon nanotubes (MWCNTs) as sorbent held in a porous polypropylene membrane envelope (2 cm × 1.5 cm) was used to extract organophosporous pesticides from a sewage sludge sample. After extraction, analytes were desorbed in organic solvent and analyzed by GC–MS. Good linearity and limits of detection were obtained. They [34] reported that no analyte carryover was observed, and the à-SPE device could be used for up to 30 extractions. In subsequent studies, the same authors also developed C18 sorbent to extract acidic drugs from wastewater [35]. Since then there have additional independent studies of à-SPE (see below).
In à-SPE, device tumbles freely in the sample solution, facilitating extraction. The porous membrane acts as a filter to prevent the extraction of interferences and afford protection of the sorbent. Thus, no further cleanup of the extract is necessary. In comparison with conventional SPE, à-SPE consumes much less organic solvent.
à-SPE has also been demonstrated to address some drawbacks associated with SPME,
such as fiber fragility, analyte carryover, and relatively high cost.
Since the à-SPE device consists of the sorbent enclosed in a porous polypropylene membrane envelop, its main advantage is that a wider range of different sorbent materials can be tailored to the extraction of different analytes. The selection of a suitable sorbent is essential to determine the selectivity of the extraction.
Different materials have been employed as sorbent for the à-SPE of a variety of compounds in different samples, such as C18 to extract carbamate pesticides in soil samples [36], HayeSep A/C18 sorbent to extract persistent organic pollutants in tissue samples [37], ethylsilane modified silica to extract estrogens in ovarian cyst fluid samples [38], C2 to extract aldehydes in rainwater [39], hydrazone-based ligands to extract biogenic amines in orange juice [40], multiplewalled carbon nanotubes [41] to extract PAHs in environmental water samples, hybrid organic-inorganic silica monolith to extract sulfonamide residues from milk [42], functionalized fiberglass with apolar chains to extract illicit drugs in oral fluids [43], molecularly imprinted polymer to extract phenolic compounds in environmental water [44], and graphite fiber to extract PAHs from soil sample [45].