SPME Fiber Coating

Solid phase micro extraction (SPME) is a widely used extraction technique that was developed by Pawlistyn and co-workers in 1990 (74). SPME uses a fused silica fiber that is coated on the outside with an appropriate stationary phase (a S to 100 pm thick coating of different polymers, e.g. polydimethylsiloxane, PDMS). The small size of the SPME flber and its cylindrical shape enables it to fit inside the needle of a syringe-like device. Target molecules from a gaseous or a liquid sample are extracted and concentrated to the polymeric fiber coating. SPME has been used coupled to GC and GC-MS (75), as well as to HPLC and LC-MS 7ll>). Figure 7 shows a conunercially available SPME device. 

Solid phase micro extraction (SPME) is a techniques in which a silica fiber coated with a thin film of polymer is brought into contact with an aqueous matrix where the organics in solution partition onto the fiber. The fiber is subsequently placed into the injector of a GC where the heat causes the release of analyte onto the column. This has been applied to endosulfan (a- and (3-) and endosulfan sulfate in water with limits of detection of less than 0.3 pg/L reported (Magdic and Pawliszyn 1996). 

The development of new fiber coatings in the near future should further improve the specificity of SPME and overcome some of the observed matrix effects. Quantification by stable isotope dilution gas chromatography/mass spectrometry (GC/MS) may assist in improving analytical performance. Along with the possible application of micro LC and capillary LC columns to in-tube SPME, the development of novel derivatization methods and the potential for the analysis of fumigant pesticides, SPME appears to be a technique with a future in the analysis of pesticide residues in food. 

A newer addition is in-tube SPME that makes use of an open capillary device and can be coupled online with GC, HPLC, or LC/MS. All these techniques and their utilization in pharmaceutical and biomedical analysis were recently reviewed by Kataoka.45 Available liquid stationary fiber coatings for SPME include polydimethylsiloxane (PDMS) and polyacrylate (PA) for extracting nonpolar and polar compounds, respectively. Also in use for semipolar compounds are the co-polymeric PDMS-DVB, Carboxen (CB)-PDMS, Carbowax (CW)-DVB, and Carbowax-templated resin (CW-TPR). A few examples of in-tube SPME extractions from biological matrices are shown in Table 1.19 and drawn from Li and coworkers.166... 

In the 1990s, Pawliszyn [3] developed a rapid, simple, and solvent-free extraction technique termed solid-phase microextraction. In this technique, a fused-silica fiber is coated with a polymer that allows for fast mass transfer—both in the adsorption and desorption of analytes. SPME coupled with GC/MS has been used to detect explosive residues in seawater and sediments from Hawaii [33]. Various fibers coated with carbowax/divinylbenzene, polydimethylsiloxane/divinylbenzene, and polyacrylate are used. The SPME devices are simply immersed into the water samples. The sediment samples are first sonicated with acetonitrile, evaporated, and reconstituted in water, and then sampled by SPME. The device is then inserted into the injection port of the GC/MS system and the analytes thermally desorbed from the fiber. Various... 

SPME Fiber. A 65 pm poly(dimethylsiloxane)/divinyl benzene (PDMS/DVB) fiber coating (Supelco, Bellefonte, PA) was used in this method. This fiber coating was selected for its ability to retain the derivatizing agent and for its affinity for the PFBOA-aldehyde oxime (7). 

Taking into account these results, our objective was to develop a SPME procedure that improved the release of PCBs from the sample to the fiber coating irrespective of the fat content of the samples. Saponification of fats to their corresponding glycerols and carboxylates facilitates the release of PCBs from fatty matrixes and also can... 

Arthur and Pawliszyn introduced solid-phase microextraction (SPME) in 1990 as a solvent-free sampling technique that reduces the steps of extraction, cleanup, and concentration to a unique step. SPME utilizes a small segment of fused-silica fiber coated with a polymeric phase to extract the analytes from the sample and to introduce them into a chromatographic system. Initially, SPME was used to analyze pollutants in water - via direct extraction. Subsequently, SPME was applied to more complex matrixes, such as solid samples or biological fluids. With these types of samples, direct SPME is not recommended nevertheless, the headspace mode (HSSPME) is an effective alternative to extracting volatile and semivolatile compounds from complex matrixes. (Adapted from Llompart et ah, 2001)... 

SPME makes use of a small segment of fused-slllca fiber coated with a polymeric phase to take out the analytes from the sample and to Introduce them Into a chromatographic system. Initially, SPME was used to look at pollutants In water via direct extraction. (Adapted from Elompart et ah, 2001)... 

Solid-phase microextraction (SPME). used as a sample introduction technique for high speed gc, utilizes small-diameter fused-silica fibers coated with polymeric stationary phase for sample extraction and concentration. SPME lias been utilized for determination of pollutants in aqueous solution by the adsorption of analyte onto stationary-phase coated fuscd-silica fibers, followed by thermal desorption in the injection system of a capillary gas chromatograph. Full automation can be achieved using an autosampler. 

Headspace SPME is a solventless extraction method where a silica fiber coated with adsorbant or absorbant polymer material is exposed to a gas phase to extract analytes. The food of interest is placed in a closed or open container (such as a mouth simulator). After extraction, the fiber is desorbed in a GC injection port for separation and detection of the extracted analytes. 

Expose coated SPME fiber to headspace for 5 min, monitoring precisely with a timer. 

Expose coated SPME fiber in the injection port. Leave for 2 min. 

SPME is a sample-preparation technique based on absorption that is useful for extraction and concentration of analytes either by submersion in a liquid phase or exposure to a gaseous phase (Belardi and Pawliszyn, 1989 Arthur et al., 1992). Following exposure of the fiber to the sample, absorbed analytes can be thermally desorbed in a conventional GC injection port. The fiber behaves as a liquid solvent that selectively extracts analytes, with more polar fibers having a greater affinity for polar analytes. Headspace extraction from equilibrium is based on partition coefficients of individual compounds between the food and headspace and between the headspace and the fiber coat-... 

GC combined with mass spectroscopic (MS) detection provides very accurate identification and quantification of FFAs. Pinho et al. (2003) monitored changes in the FFA content during the ripening of ewe cheese. Sampling was done by headspace solid-phase microextraction (SPME). An excellent correlation was observed between the initial concentration of the sample and the amount absorbed on the SPME fiber. SPME sampling was done at 65 °C with a fiber coated with 85-p.m polyacrylate film. After equilibration at 65 °C for 40 min, the fiber was exposed to the sample headspace for 20 min and inserted into the GC port. Despite its accuracy, the GC-MS method is not widely used, presumably because of its cost and complexity. 

TWA-SPME Glass fibers coated with a thin layer of various sorbent materials Water boundary layer BTEX, PAHs, organometallic compounds Integrative Up to several days Thermal desorption in GC inlet 22,38,77... 

One of these methods is called kinetic calibration, in which analyte absorption from the sample to the liquid coating (PDMS) on the fiber is related to analyte desorption from the coating to the sample. The isotropy of absorption and desorption in the kinetic calibration has been described by Chen et al.31 In kinetic calibration, also called in-fiber standardization, desorption of a radio-labeled standard (preloaded on the fiber coating) into the sample is used to calibrate the extraction (absorption/adsorption in the case of a liquid/solid coating) of analyte from the sample into the fiber. This calibration approach considerably facilitates the use of SPME for the on-site field sampling of water, where the control of flow velocity or addition of a standard to the matrix is very difficult. 

The conventional polymeric coating, PDMS, employed in SPME has a film thickness of 100 pm, which corresponds to a volume of about 0.5 pL for the whole fiber. In SPME, the thin PDMS film provides the highest enrichment when equilibrium between the film and sample is realized, the attainment of which depends largely on analyte hydrophobicity and distribution to the coating. With thin PDMS films, SPME does not generally afford quantitative exhaustive extraction, which renders SPME a less sensitive technique even for nonpolar compounds and an unsatisfactory sampling device that fails to extract polar analytes. Implementing SPME for total exhaustive extraction is conceivably plausible if the PDMS film thickness is increased dramatically. 

Solid-phase microextraction capillary gas chromatography (SPME-GC) is also an interesting preconcentration method. After derivatization with tetraethylborate, tetrapropylborate, or tetraphenylborate, the ethylated compounds are extracted by SPME on a silica fiber coated with polydimeth-ylsiloxane (PDMS). SPME can be performed either in the aqueous phase or in the headspace. After SPME extraction, species are thermally desorbed, separated by GC, and analyzed.106... 

SPME (Figure 2.48) can be conducted as a direct extraction in which the coated fiber is immersed in the aqueous sample in a headspace configuration for sampling air or the volatiles from the headspace above an aqueous sample in a vial (headspace SPME analyses are discussed elsewhere) or by a membrane protection approach, which protects the fiber coating, for analyses of analytes in very polluted samples [136]. The SPME process consists of two steps (Figure 2.49) (a) the sorbent, either an externally coated fiber or an internally coated tube, is exposed to the sample for a specified period of time (b) the sorbent is transferred to a device that interfaces with an ana-... 

Fiber coating thickness is a second consideration in selecting a fiber for both direct immersion and headspace SPME. The PDMS coating is avail-... 

Figure 4.11. Graphical scheme for choosing a SPME fiber coating. [Reprinted with permission from Ref. 36 (Fig. 4.3, p. 99). Copyright John Wiley Sons.]...

The sample volume also has an effect on both the rate and recovery in SPME extractions, as determined by extraction kinetics and by analyte partition coefficients. The sensitivity of a SPME method is proportional to n, the number of moles of analyte recovered from the sample. As the sample volume (Vs) increases, analyte recovery increases until Vs becomes much larger than the product of K, the distribution constant of the analyte, and Vf, the volume of the fiber coating (i.e., analyte recovery stops increasing when KfeVf Vs) [41]. For this reason, in very dilute samples, larger sample volume results in slower kinetics and higher analyte recovery. 

The sample matrix may also be modified to enhance extraction recovery. This is typically done by either dissolving a solid sample in a suitable solvent, usually water or a strongly aqueous mixture, or by modifying the pH or salt content of a solution. Modifying the pH to change the extraction behavior works the same way in SPME as it does for classical liquid-liquid extraction. At low pH, acidic compounds will be in the neutral form and will be extracted preferentially into the fiber coating at high pH, basic compounds are extracted favorably. Neutral compounds are not affected appreciably by solution pH. 

---