Fibers, PDMS fused-silica fiber

In the early 90s, a new technique called solid-phase-micro extraction (SPME), was developed (Arthur and Pawliszyn, 1990). The key-part component of the SPME device is a fused silica fiber coated with an adsorbent material such as polydimethylsiloxane (PDMS), polyacrylate (PA) and carbowax (CW), or mixed phases such as polydimethylsiloxane-divinylbenzene (PDMS-DVB), carboxen-polydimethylsiloxane (CAR-PDMS) and carboxen-polydimethyl-siloxane-divinylbenzene (CAR-PDMS-DVB). The sampling can be made either in the headspace (Vas et al., 1998) or in the liquid phase (De la Calle et al., 1996) of the samples. The headspace sampling in wine analyses is mainly useful for quantifying trace compounds with a particular affinity to the fiber phase, not easily measurable with other techniques. Exhaustive overviews on materials used for the extraction-concentration of aroma compounds were published by Ferreira et al. (1996), Eberler (2001), Cabredo-Pinillos et al. (2004) and Nongonierma et al. (2006). Analysis of the volatile compounds is usually performed by gas chromatography (GC) coupled with either a flame ionization (FID) or mass spectrometry (MS) detector. 

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) first became available to analytical researchers in 1989. The technique consists of two steps first, a fused-silica fiber coated with a polymeric stationary phase is exposed to the sample matrix where the analyte partitions between the matrix, and the polymeric phase. In the second step, there is thermal desorption of analytes from the fiber into the carrier gas stream of a heated GC injector, then separation and detection. Headspace (HS) and direct insertion (DI) SPME are the two fiber extraction modes, whereas the GC capillary column mode is referred to as in-tube SPME. The thermal desorption in the GC injector facilitates the use of the SPME technology for thermally stable compounds. Otherwise, the thermally labile analytes can be determined by SPME/LC or SPME/GC (e.g., if an in situ derivatization step in the aqueous medium is performed prior to extraction). Different types of commercially-avarlable fibers are now being used for the more selective determination of different classes of compounds 100 /rm polydimethylsiloxane (PDMS), 30 /rm PDMS, 7 /rm PDMS, 65 /rm carbowax-divinylbenzene (CW-DVB), 85 /rm polyacylate (PA), 65 /rm PDMS-DVB, and 75 /rm carboxen-polydimethyl-siloxane (CX-PDMS). PDMS, which is relatively nonpolar, is used most frequently. Since SPME is an equilibrium extraction rather than an exhaustive extraction technique, it is not possible to obtain 100% recoveries of analytes in samples, nor can it be assessed against total extraction. Method validation may thus include a comparison of the results with those obtained using a reference extraction technique on the same analytes in a similar matrix. 

Unlike the other techniques, SPME has a considerable concentration capacity and is very simple because it does not require especial equipment. The principle involves exposing a silica fibre covered with a thin layer of adsorbent in the HS of the sample in order to trap the volatile components onto the fibre. The adsorbed compounds are desorbed by heating and introduced into the detection system A SPME sampler consists of a fused silica fiber that is coated by a suitable polymer (e.g. PDMS, PDMS/divinylbenzene, carboxen/PDMS) and housed inside a needle. The fiber is exposed to headspace volatile and after sampling is complete, it is retracted into the needle. Apart from the nature of the adsorbent deposited on the fiber, the main parameters to optimize are the equilibration time, the sample temperature and the duration of extraction. Compared with other sampling methods, SPME is simple to use and reasonably sensitive, so it is a user-friendly pre-concentration method. 

SPE dominates in extraction of pesticides from water samples and it represents presumably the most flexible technique for clean-up of food extracts. SPME and SBSE use a fused-silica fiber or a stir bar, respectively, coated by a sorptive material (usually polydimethylsiloxane, PDMS, or its modifications) for partition (i.e., not complete isolation) of analytes. These techniques are convenient and fast however, their application in quantitative MMRMs for analysis of pesticides in complex food samples is rather limited. The main disadvantages relate to strong matrix effects (matrix-dependent partition of analytes and deterioration of the coating by irreversible adsorbed matrix components), insufficiently wide polarity range to extract diverse pesticide residues, and variability of method sensitivity for different analytes depending on their partition coefficients. 

Two SPME devices were used one contained a 1-cm fused silica fiber coated with a 75-pm layer of Carboxen/PDMS the second contained a 1-cm Stable-flex fiber coated with 50/30-pm DVB/Carboxen on PDMS. Both... 

This method utihzes a fused silica rod coated with a thin layer of stationary phase mounted in a holder. During extraction, the fiber is exposed to the sample, and analytes are adsorbed onto the stationary phase and concentrated. After a defined extraction time, the fiber is withdrawn in the holder and then analytes are thermally desorbed in the GC injector. Several types of coatings are commercially available, such as PDMS, polyamide, Carbowax-DVB, Carboxen-PDMS, and PDMS-DVB. Coatings prepared with three kinds of materials are also available (e.g., DVB-Carboxen-PDMS) [71]. Selection of the fiber is mainly based on the principle like dissolves like. For example, PDMS sorbent is suitable for the extraction of hydrocarbons, and the sorbent should be polar for the extraction of alcohols or ketones. The thickness of the coating film determines the sorption capacity of the fiber. Changing the temperature, pH, or ionic strength of the hquid... 

SPME was coupled with cyclodextrin-modified CE in the development of a method for the EPAjg. A PDMS-coated SPME fiber was contacted with a low ppb level aqueous solution of PAHs and then placed directly in the inlet of the separation capillary. The mnning buffer at pH 9 contained 35 mM SB(3CD, 10 mM M(3CD, and 4 mM MaCD. At 30 kV and a 60 cm X 50 /rm i.d., 350 /rm o.d. fused-silica capillary, ACE, NAP, and FLU coeluted. With UV detection, sensitivity was only slightly less than with LIF detection. 

The SPME apparatus looks like a modified syringe (see Fig. 5) consisting of a fiber holder needle and a fiber assembly, the latter equipped with a 1-2 cm long retractable SPME fiber. The fiber itself is a thin fused-silica optical fiber, coated with a thin polymer film (such as polydimethylsiloxane, PDMS), as shown... 

A Varian Saturn 3 GC/MS was used. The GC was equipped with a split/splitless model 1078 injector. The injector was operated in the split mode (6 1 split ratio) at a temperature of 275°C. The SPME fiber used was 75-pm Carboxen/PDMS. For thermal desorption, the SPME fiber remained in the injector for 3 minutes. Helium was used as the carrier gas. A 30 m X 0.25 mm I.D. DB-5 fused-silica capillary column with a film thickness of 1 pm was used, and the flow rate of the helium carrier gas was 1.0 niL/minute. The following column temperature programming sequence was used An initial temperature of 150 C was maintained for 4 minutes, increased to 180 C at a rate of 15 C/minute, and held at 180°C for an additional 2 minutes. All milk volatile peaks eluted within 7 minutes, with many components coeluting. 

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