In-tube SPME

Lord and Pawliszyn" developed a related technique called in-tube SPME in which analytes partition into a polymer coated on the inside of a fused-silica capillary. In automated SPME/HPLC the sample is injected directly into the SPME tube and the analyte is selectively eluted with either the mobile phase or a desorption solution of choice. A mixture of six phenylurea pesticides and eight carbamate pesticides was analyzed using this technique. Lee etal. utilized a novel technique of diazomethane gas-phase methylation post-SPE for the determination of acidic herbicides in water, and Nilsson et al. used SPME post-derivatization to extract benzyl ester herbicides. The successful analysis of volatile analytes indicates a potential for the analysis of fumigant pesticides such as formaldehyde, methyl bromide and phosphine. 

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... 

SPME uses a polymer-coated fused-silica fiber, typically 1 cm 100 m, that is fastened into the end of a fine stainless steel tube contained in a syringelike device and protected by an outer stainless steel needle. In use, the plunger of the device is depressed to expose the fiber to the sample matrix so that the organic compounds to be sorbed onto the fiber. The plunger is retracted at the end of the sampling time, and then it is depressed again to expose the fiber to a desorption interface for analysis typically by GC or LC. In a recent variation of this technique, the so-called in-tube SPME, the polymer is not coated on a fiber but on the inside of a fused-silica capillary before analysis by LC. 

Sorbent/ solid phase Solid phase extraction (SPE) Solid phase microextraction (SPME) Stir bar sorptive extraction (SBSE) INCAT/OTT/in-tube-SPME SPDE Headspace-solid phase microextraction (HS-SPME) Headspace stir-bar sorptive extraction (HS-SBSE) Purge-and-sorbent trapping Spray-and-sorbent trapping... 

In-tube SPME In-tube solid-phase Internal polymer-coated fused silica (GC) Relatively small sample... 

Another important mode of operation in SPME is in-tube SPME.65 In this system, usually coupled on-line to HPLC, a finite portion of sample is drawn through an internally coated capillary tube and then ejected into the sample vial. This technique requires more complex instrumentation than that used for standard SPME, but a greater sensitivity is obtainable with a longer tube (and consequently more sorbent). Two solvent desorption modes—are usually applied for introducing species into HPLC off-line desorption and on-line desorption. In the latter, the HPLC mobile phase is used for desorbing the analytes. 

SPME is a powerful tool in GC-MS. Adaptation to LC-MS involves two operational modes that are in use, i.e., fiber SPME and (automated) in-tube SPME [110]. 

A method for analysis of polar pesticides in wine by the use of automated in-tube SPME coupled with LC/ESI-MS was proposed (Wu et al., 2002). In-tube SPME is a microextraction and preconcentration technique that can be coupled on-line with high-performance liquid chromatography (HPLC), suitable for the analysis of less volatile and/ or thermally labile compounds. This technique uses a coated open tubular capillary as an SPME device and automated extraction. Using a polypyrrole coating, six phenylurea pesticides (diuron, fluometuron, linuron, monuron, neburon, siduron) and six carbamates (barban, car-baryl, chlorpropham, methiocarb, promecarb, propham) were analyzed in wine. Structures of compounds are reported in Fig. 9.4. Due to the high extraction efficiency of the fiber toward polar compounds, benzene compounds, and anionic species, LODs ranging between 0.01 and 1.2pg/L were achieved, even if the sample ethanol content affects the recoveries of analytes. 

An automated in-tube solid-phase microextraction HPLC method for NNK and several metabolites have been developed by Mullett et al. ° In-tube SPME is an on-line extraction technique where analytes are extracted and concentrated from the sample directly into a coated capillary by repeated draw-eject steps. A tailor-made polypyrrole-coated capillary was used to evaluate their extraction efficiencies for NNK and several metabolites in cell cultures. This automated extraction and analysis method simplified the determination of the tobacco-specific A-nitrosamines, requiring a total sample analysis time of only 30 min. 

Most SPME applications involve GC. Following extraction, the analytes are thermally desorbed in the chromatograph injector. More recently, the scope of apphcation has been extended to nonvolatile and thermally unstable compounds by coupling SPME to LC. Desorption is performed at an appropriate interface consisting of a standard six-way HPLC injector with a special fiber-desorption chamber used instead of the sample loop. A different approach to SPME-LC called in-tube SPME has also been developed, it uses an open tubular fused-silica capillary column instead of the typical SPME fiber. This latter method has been used for the determination of carbamate pesticides in water with relative standard deviations of... 

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. 

Advantages of SPME to traditional extraction methods should promote advances in the field of herbicide chemistry. However, SPME has some limitations, such as analyte carryover, fiber damage at extreme pH, salt-related problems, and low sensitivity in some complex soil samples. Advancements are being made in the refinement of the SPME technique. The HPLC/SPME interface has then been improved, and new mixed phases based on solid/liquid sorption (e.g., CW-DVB and PDMS-DVB) have been developed in recent years for the analysis of compounds by HPLC. A modified accessory to the HPLC system, called in-tube SPME, was developed. This device aspirates and dispenses samples from vials with the syringe in the inject position and then desorbs with aspirated solvent in the load position. Returning the valve to the inject mode will transfer analytes to the analytical column. ... 

Automated in-tube solid-phase microextraction (SPME) has recently been coupled with liquid chromatography/electrospray ionisation mass spectrometry (LC/ESI-MS), e. g. for the determination of drugs in urine [60, 62]. In-tube SPME is an extraction technique in which analytes are extracted from the sample directly into an open tubular capillary by repeated draw/eject cycles of sample solution. The analyte is then desorbed with methanol and transferred to an analytical HPLC-column. 

Figure 5 illustrates the two fundamental approaches to in-tube SPME (1) active or dynamic, when the analytes are passed through the tube and (2) passive or static, when the analytes are transferred into the sorbent using diffusion. In both of these approaches, the coating may be supported on a fused silica rod, or coated on the inside of a tube or capillary. The theoretical aspects of the extraction processes that use these geometric arrangements will be discussed below. 

Dynamic in-tube SPME In this system, we assume the use of a piece of fused silica capillary, internally coated with a thin film of extracting phase (a piece of open tubular capillary gas chromatography (GC) column), or that the capillary is packed with extracting phase dispersed on an inert supporting material (a piece of micro-LC capillary column). 

It should be emphasized that the above discussion is valid only for direct extraction when the sample matrix passes through the capillary. This approach is limited to particulate-free gas and clean water samples. The headspace SPME approach can broaden the application of in-tube SPME. In that case, careful consideration of the mass transfer between sample and headspace should be given in order to describe the process properly. Also, if the flow is very rapid... 

Static in-tube SPME time-weighted average sampling An integrated sampling is possible with a simple SPME system. This is particularly important in field measurements when analyte concentrations change with time and location. 

Further recent developments in fiber SPME have extended applications to compounds with low volatilities and/or low thermal stability. Thus, fiber-(or tube-) based SPME-LC has a considerable future potential, particularly as it has been recognized that GC capillary columns (available with a very wide range of internal coatings) can be used for this purpose. Samples are pumped through the tube using a micropump and then eluted onto the EC column using appropriate solvents. Applications of in-tube SPME (in combination with LC) include phthalates, chlorinate phenoxy acid herbicides, tributyl tin compounds, polyaromatic hydrocarbons (PAEIs), and polar aromatic compounds in water. 

The need for maximum sample throughput and minimal human interaction within analytical procedures has provided considerable impetus to the development of integrated systems. SPE-LC in-tube SPME followed by ultraviolet (UV) or MS detection and membrane introduction mass spectrometry (MIMS) have both been used to this end. Submersible MIMS systems capable of extended underwater deployment down to 200 m and with a mass range of up to 200 amu have recently come onto the market. Elow injection coupled with MIMS allows fast, near-real-time determination of, for example, phenols in water. Derivatization of the phenols with acetic anhydride can be used to enhance both the selectivity and sensitivity of this method. Other online derivatization procedures are under development with a view to increasing the scope for rapid determination of highly polar compounds that have previously proved difficult to analyze. Large volume injection techniques and developments in enzyme-linked immunosorbent assay (ELISA) technologies... 

The in-tube SPME method is suitable for the extraction of less volatile or thermally labile HCAs compounds [89]. The food sample is treated with HCl followed by centrifugation, the sample supernatant is neutralized with NaOH, and the HCAs are extracted by the Blue Rayon adsorption method. This method can selectively adsorb compounds having polycyclic planar molecular structures, such as HCAs, in order to concentrate them from the aqueous solution. The extract is passed through a syringe micro filter, and a capillary colunm is used as a SPME device. This column is placed between the injection loop and the injection needle of the auto sampler. The method is simple, rapid, automatic, and gives 3-20 times higher sensitivity in comparison with the direct liquid injection method [89]. 

There are also two different modes for the desorption of retained pesticides on the fiber and their determination by LC [91], namely (i) conventional fiber coupling, where the interface consists of a special desorption chamber and switching valve [106-109] and (ii) in-tube SPME, where the combination of in-tube SPME and LC can be done by fixing the capillary column as the SPME device between the injection loop and injection needle of the LC autosampler [110,111]. 

Campins-Falco et al. (2008) Comunidad Valenciana Mussels, tellins Miniaturized MSPD-samples C18 phase-Florisil, in tube SPME LOD 0.05-0.6 ng/g dw Optimization of method HPLC-FLD... 

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