Preconcentration microextraction

In recent decades the development of preconcentration steps to be implemented prior to analytical determinations of trace level compounds has been explored in considerable depth. With a view to eliminating or at least minimising the use of organic solvents used in conventional liquid-liquid extraction, other methodologies have been developed, such as membrane extraction, solid-phase extraction, solid-phase microextraction, etc. 

Both HPLC and GC-MS were employed for the separation, identification and quantitation of the decomposition products of indigo and indigo carmine. The chemical structures of the dyes are shown in Fig. 3.73. Carboxylic acids were preconcentrated before HPLC analysis either by ion-exchange SPE or by solid-phase microextraction. HPLC measurements were performed in a Sarasep column (300 X 7.8 mm i.d.) using 5 mM H2S04 at a flow rate of 0.7 ml/min. Analytes were detected at 215 nm. The main intermediates formed during the photocatalytic decomposition are compiled in Table 3.26. The results demonstrated that... 

Solid-phase microextraction (SPME) for preconcentration, followed by GC/ Ion Trap MS, was used for trace analysis of explosives and their metabolites in seawater [9]. NICI was used with methane as reagent gas. Compounds of interest included RDX, TNT and two of its metabofrtes 2-amino-4,6-dinitrotoluene (2ADNT) and 4-amino-2,6-dinitrotoluene (4ADNT). Although the instrument sensitivity was in low-ppb range, the detection limits for SPME with GC/ITMS... 

Ion mobility spectrometry (IMS [43]). Solid phase microextraction (SPME) using a 100 pm polydimethylsiloxane (PDMS) SPME fibre was used for head-space sampling and preconcentration of volatile markers of cocaine, MDMA and marijuana (methyl benzoate, piperonal and terpenes, respectively) in cargo containers. Analysis was then performed by IMS after thermal desorption of the drug markers from the fibre into the IMS analyser. 

HSSPIVIE Headspace solid-phase microextraction a preconcentration technique that concentrates volatile analytes on a fiber than can be inserted directly into a GC... 

The fabrication of imprinted monolithic solid-phase microextraction fibres has been developed for the selective extraction and preconcentration of diacetylmorphine and its structural analogues, triazines, bisphenol A, anaesthetics, and antibiotics followed by GC or HPLC analysis [156,163,179,196,197]. In addition, the on-line coupling of the imprinted monolith as a preconcentration column with a conventional analytical column has been proposed for the enrichment and cleanup of environmental and food samples [163]. However, at present, the capacity of the imprinted fibres and thus the degree of recovery of analytes are very variable and obviously need some improvement. For example, the recoveries of triazines after SPME with an imprinted monolith prepared by in situ polymerisation of MAA as... 

Supercritical fluid extraction [153,154], accelerated solvent extraction [68] and subcritical fluid extraction [107,155] have been studied. To reduce the equipment cost and the analysis time in the extraction process and sample preconcentration, a solid-phase microextraction method was proposed by Pawliszyn and coworkers [156-158]. 

Yang et al. [47,48,53,54] developed a HWG sensing system for liquid and soil analyses based on an extractive polymer membrane coated onto the inside of the HWG. The polymer coating performs a solid-phase microextraction of the analyte from the headspace of the sample and preconcentrates the analyte prior to IR analysis. 

Fattahi, N., S. Samadi, Y. Assadi, and M. R. M. Hosseini. 2007. Solid-phase extraction combined with dispersive liquid-liquid microextraction-ultra preconcentration of chlorophenols in aqueous samples. J. Chromatogr. A 1169 63-69. 

Several techniques have been used to overcome the problem of low column loadings on capillary columns. Capillary columns have also been used after preconcentration of alkyl derivatives on a wide-bore fused silica column103 or by solid-phase microextraction (SPME).106... 

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... 

Perez-Sirvent, C. and M.-J. Martlnez-Sanchez. 2007. Comparison of two derivatizing agents for the simultaneous determination of selenite and organoselenium species by gas chromatography and atomic emission detection after preconcentration using solid-phase microextraction. J. Chromatogr. A 1165 191-199. 

Ma, M. and F.L. Cantwell. 1999. Solvent microextraction with simultaneous back-extraction for sample cleanup and preconcentration Preconcentration into a single microdrop. Anal. Chem. 71 388-393. 

Solid-Phase Microextraction Solid-phase microextraction (SPME) is a preconcentration technique based on the sorption of analytes present in a liquid phase or, more often, in a headspace gaseous phase, on a microbber coated with a chromatographic sorbent and incorporated in a microsyringe [15]. The analytes sorbed in the coating are transferred to a GC injector for thermal desorption. 

Solid-phase microextraction (SPME) — is a procedure originally developed for sample preconcentration in gas chromatography (GC). In this procedure a small-diameter fused silica optical fiber, coated with a liquid polymer phase such as poly(dimethylsiloxane), is immersed in an aqueous sample solution. The -> analytes partition into the polymer phase and are then thermally desorbed in the GC injector on the column. The same polymer coating is used as a stationary phase of capillary GC columns. The extraction is a non-exhaustive liquid-liquid extraction with the convenience that the organic phase is attached to the fiber. This fiber is contained in a syringe, which protects it and simplifies introduction of the fiber into a GC injector. Both uncoated and coated fibers with films of different GC stationary phases can be used. SPME can be successfully applied to the analysis of volatile chlorinated organic compounds, such as chlorinated organic solvents and substituted benzenes as well as nonvolatile chlorinated biphenyls. 

Bosch Ojeda, C., Rojas, F.S. Separation and preconcentration by dispersive liquid-liquid microextraction procedure. Chromatographia 69, 1149-1159 (2009)... 

A related technique, called solid-phase microextraction, uses a fused silica fiber coated with a nonvolatile polymer to extract organic analytes directly from aqueous samples or from the headspace above the samples. The analyte partitions between the fiber and the liquid phase. The analytes are then desorbed thermally in the heated injector of a gas chromatograph (see Chapter 31). The extracting fiber is mounted in a holder that is much like an ordinary syringe. This technique combines sampling and sample preconcentration in a single step. 

Preconcentration of analytes in aqueous solution may be performed by a miscible organic phase followed by salting out. Thus, microextraction of anionic solntes snch as phenol, cresols and xylenols in industrial effluents can be carried ont with a small amonnt of isopropyl alcohol, followed by demixing of the phases with ammoninm snlfate. End analysis of the extract by GC-MS in the selected ion monitoring (SIM) mode allowed a LOD of 1 ppb for 50 mL samples . The best conditions for eliminating petrolenm prodncts from the concentrate were found for the GC determination of volatile phenols in natnral waters. Losses of volatile phenols due to preconcentration were insignificant and cansed no increase in the relative error of determination by the internal-standard method. The concentration of phenol in the atmosphere can be determined by sorption on Chromosorb 102, desorption with benzene and 0.1 M NaOH and GC nsing a capillary colnmn. LOD was abont 1 p,gm, with accuracy within 15% . 

Generally, preconcentration of pollutants from water samples and sample preparation steps are accomplished by extraction techniques based on enrichment of liquid phase (liquid/liquid extraction) or solid phase (solid/liquid extraction) ". Historically, liq-uid/liquid extraction (LEE) was used exclusively to enrich phenols from water samples. LEE is still used as a preconcentration step . However, there is an increasing tendency to replace LEE by solid phase extraction (SPE) and solid phase microextraction (SPME). Among the reasons for replacing LEE are foam formation, the large volume of organic solvents needed, the length of the analysis time and difficulties in the automation of LEE procedures. On the other hand, SPE requires incomparable smaller amounts of solvents (SPME requires no solvent at all) and can be easily automated . Finally, SPE and SPME are cheaper in comparison with LEE. 

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. 

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