Polydimethylsiloxane fiber

Amphetamine Urine, hair Polydimethylsiloxane fiber HPLC... 

Lidocaine Blood serum, urine Polydimethylsiloxane fiber GC-FID, HPLC-UV... 

Before applying solid-phase microextraction to the water extracts obtained from hot water extraction of rosemary, optimization of experiments were conducted to determine the optimum working conditions for SPME, including water volume, SPME time, and carry over effect. Polydimethylsiloxane fiber (PDMS, 100 tun, Supelco, Bellefonte, PA) was used for performing solid-phase microextraction. 

Figure 10.14 shows a typical SPME application. Twenty common pesticides and an internal standard at the 200 ppt level in drinking water are extracted by a 100 /i.m polydimethylsiloxane fiber immersed for 15 min. in 4 mL of drinking water. A recent review [19] and a new book [20] can be consulted for other applications. 

Since unmetabolized cocaine in urine can be an indicator of recent cocaine use, an improved method for the analysis of cocaine to the exclusion of any metabolites was reported [25]. A single-step LLE with petroleum ether of buf fered urine removed cocaine while leaving benzoylecgonine unextracted. The underivatized extract was analyzed by El with SIM. In conjunction with deuter ated cocaine as an internal standard, a LOD of about 5 ng/ml was determined, even in the presence of a large excess of benzoylecgonine [25]. Unmetabolized cocaine was also analyzed preferentially from urine using SPME because ben zoylecgonine was not effectively extracted with the polydimethylsiloxane fiber used [26]. This method employed El and SIM, and resulted in a LOD of 50 ng/ ml [26]. 

Low-molecular-weight carboxylic acids are difficult to extract from aqueous solutions using SPME techniques. Formic through butyric acids are miscible in water, and even caprylic acid (Cs) is soluble to the extent of 68 mg/100 g (7). The low capacity factors of carboxylic acids to nonpolar phases used in capillary gc columns lead to severe fronting of acid peaks, which often can be used to identify their presence in mixtures with other flavor compounds. The same phenomenon also has an effect on the absorption of acids by SPME phases. It is possible to enhance their extraction by SPME fibers, however. Figure 3 shows the relative extraction efficiencies for several carboxylic acids, each at a concentration of 10 ppm in water. The results show the effectiveness of headspace extractions using both the 85- a,m polyacrylate and lOO-pm polydimethylsiloxane fibers alone, and after the addition of 25% NaCl to the solutions. With the excep-... 

Figure 5 Stacked chromatograms showing the GC analysis of a series of n-hydrocarbons isolated by headspace SPME at various temperatures. The sample was prepared by transferring l-pl of individual hydrocarbon standards into a 4-mL vial and sealing with a Teflon-coated septum. After 10 minutes equilibration at the indicated temperature, the headspace vapor was extracted using a 100-lim polydimethylsiloxane fiber and was analyzed by GC with flame ionization detection. The injector temperature for this sample was set at 300°C, and the fiber was desorbed for 1 minute.

Figure 7 A calibration curve for ethanol prepared using n-propanol as an internal standard at 1.0% and GC analysis following headspace SPME extraction. The upper curve (solid circles) was obtained with an 85- xm poly aery late fiber and the lower curve (open squares) was obtained using a 100- xm polydimethylsiloxane fiber. The linear correlation coefficients for ethanol concentrations from 0.1% to 20.0% using both fibers is in excess of 0.999.

Silk (qv) suture is made from the threads spun by the silkworm Bombjx mori. The fiber is composed principally of the protein fibroin and has a natural coating composed of sericin gum. The gum is usually removed before braiding the silk yams to make sutures in a range of sizes. Fine silk sutures may be made by simply twisting the gum-coated silk yams to produce the desired diameter. White silk is undyed. Silk is either dyed black with logwood extract or blue with D C Blue No. 9. The suture may be uncoated or coated either with high molecular weight polydimethylsiloxane or with wax. 

The most widely used sUicones are polymers of methyl(hydrogen)sUoxane and of dimethylsiloxane. Polydimethylsiloxane is the basic polymer used in sUicone repeUents. If the polymer is terminated with methyl groups it is inert however, if it is terminated with hydroxyl groups, it can be cross-linked. Continuous, durable coatings result from the use of curable blends of polydimethyl siloxane and polymethyl(hydrogen)sUoxane. The sUicone finish encapsulates individual fibers. 

Takahasi, T., Murayama, T., Higuchi, A., Awano, H. and Yonetake, K. (2006) Aligning vapor-grown carbon fibers in polydimethylsiloxane using dc electric or magnetic field. Carbon, 44, 1180-1188. 

Solid-phase microextraction (SPME) consists of dipping a fiber into an aqueous sample to adsorb the analytes followed by thermal desorption into the carrier stream for GC, or, if the analytes are thermally labile, they can be desorbed into the mobile phase for LC. Examples of commercially available fibers include 100-qm PDMS, 65-qm Carbowax-divinylbenzene (CW-DVB), 75-qm Carboxen-polydimethylsiloxane (CX-PDMS), and 85-qm polyacrylate, the last being more suitable for the determination of triazines. The LCDs can be as low as 0.1 qgL Since the quantity of analyte adsorbed on the fiber is based on equilibrium rather than extraction, procedural recovery cannot be assessed on the basis of percentage extraction. The robustness and sensitivity of the technique were demonstrated in an inter-laboratory validation study for several parent triazines and DEA and DIA. A 65-qm CW-DVB fiber was employed for analyte adsorption followed by desorption into the injection port (split/splitless) of a gas chromatograph. The sample was adjusted to neutral pH, and sodium chloride was added to obtain a concentration of 0.3 g During continuous... 

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

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... 

Nitrobenzene, 2,4-dinitrotoluene and 2,6-dinitrotoluene were determined in water by GC-EC or GC-CLD thermal energy analyzer (TEA) and by EI-MS, CI-MS and NICI-MS455, after solid-phase microextraction (SPME) with polydimethylsiloxane coated fiber. SPME is a technique to concentrate organic compounds dissolved in an aqueous matrix by adsorption on a solid stationary phase immobilized on a fused silica fiber. The analytes were thermally desorbed directly into the GC injector LOD was 9 pg/L for nitrobenzene and 15 pg/L for the dinitrotoluenes456. 

In regard to the potential effects of silicone membrane or film thickness (e.g., polydimethylsiloxane [PDMS]) on partition coefficients, Paschke and Popp (2003) have shown that that at equilibrium an SPME fiber with a 7 xm thick film of PDMS had about a 6-fold higher ATpw than a similar fiber with a 100 xm thick film. However, this could be the result of interactions with the silica core. Recent research (Smedes, 2004) has shown that silicone sheeting with PRCs can be employed for water sampling with good results. 

Triacetone triperoxide (TATP) is a powerful explosive manufactured in clandestine laboratories and used by terrorists. As TATP subHmes easily, analysis was performed by SPME trapping of its vapor, using polydimethylsiloxane/divinyl benzene (PDMS/DVB) fiber, followed by desorption into a GC/MS injector [10]. Figure 6 shows the TIC, mass chromatogram and the El mass spectmm of headspace from a debris sample containing TATP [11]. The El mass spectmm contains a molecular ion at m/z 222 and several fragment ions. In the chemical ionization mass spectmm of TATP [12], the m or ions were at m/z 223 (100%), 222 (20%), 133 (20%), 117 (40%), 115 (20%), 103 (75%) and 100 (50%). 

Concentrator materials of choice are often polymers. Polydimethylsiloxane (PDMS), or a polydimethylsiloxane/divinylbenzene copolymer (PDMS/DVB) are favored choices for explosive molecules. PDMS are often used in the form of solid-phase microextraction (SPME) fibers. PDMS/DVB is often used in the form of microspheres with diameters in the 50- to 75-pm range. Detailed considerations for use of SPME fibers is given on a website maintained by the University of Western England [19], It references a more complete treatise [20], Other geometries, such as stacked spheres, have also been used successfully [21],... 

Commercial fibers must be conditioned prior to initial use. Each coating requires different conditioning parameters, and the manufacturer s recommendations should always be followed. As an example, conditioning of Su-pelco s 100-pm polydimethylsiloxane (PDMS) fiber is performed by heating the fiber to 250°C for 1 hour. Conditioning can be carried out in the injection port of a GC or in a special conditioning apparatus. 

For structural integrity, SPME sorbents are most commonly immobilized by coating onto the outside of fused silica fibers or on the internal surface of a capillary tube. The phases are not bonded to the silica fiber core except when the polydimethylsiloxane coating is 7 pm thick. Other coatings are cross-linked to improve stability in organic solvents [135], De Fatima Alpendurada [136] has reviewed SPME sorbents. 

Koster et al. [140] conducted on-fiber derivatization for SPME to increase the detectability and extractability of drugs in biological samples. Amphetamine was used as a model compound. The extraction was performed by direct immersion of a 100-pm polydimethylsiloxane-coated fiber into buffered human urine. On-fiber derivatization was performed with pentafluorobenzoyl chloride either after or simultaneously with extraction. 

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