Sample preconcentration extraction

The procedure described earlier for sample preconcentration can be easily extended for the online extraction of solid samples, e.g., powdered plant materials. Horizontal conbguration of the chromatographic plate in the chamber facihtates this procedure, because the sample to be extracted is then placed on a carrier plate at the begiiming part of the adsorbent layer (or in the scrapped channel of the adsorbent layer), which should be directed upward [15,26]. The chamber is covered with a narrow plate, and the development is started with a snitable extracting solvent. In some cases, it is advantageous to put the narrow plate directly on the adsorbent layer to press the sample to be extracted. Extracted components are preconcentrated on the adsorbent layer at the end of the narrow plate, as shown in Fignre 6.26 [15]. 

Speciation involves a number of discrete analytical steps comprising the extraction (isolation) of the analytes from a solid sample, preconcentration (to gain sensitivity), and eventually derivatisation (e.g. for ionic compounds), separation and detection. Various problems can occur in any of these steps. The entire analytical procedure should be carefully controlled in such a way that decay of unstable species does not occur. For speciation analysis, there is the risk that the chemical species can convert so that a false distribution is determined. In general, the accuracy of the determinations and the trace-ability of the overall analytical process are insufficiently ensured [539]. 

Sample preconcentration was performed by means of an automated on-line SPE sample processor Prospekt-2 (Spark Holland, Emmen, The Netherlands). Oasis HLB cartridges (Waters, Barcelona, Spain) were used to preconcentrate cannabi-noids present in the water samples whereas isolation of the rest of the compounds was done in PLRPs cartridges (Spark Holland). Before extraction, influent samples were diluted with HPLC water (1 9, v/v) to reduce matrix interferences and to fit some analyte concentrations, e.g., cocaine (CO) and benzoylecgonine (BE), within the linear calibration range. A sample volume of 5 mL was spiked with the internal standard mixture (at 20 ng/L) in order to correct for potential losses during the analytical procedure, as well as for matrix effects. Elution of the analytes to the LC system was done with the chromatographic mobile phase. 

Analytical methods for detecting phenol in environmental samples are summarized in Table 6-2. The accuracy and sensitivity of phenol determination in environmental samples depends on sample preconcentration and pretreatment and the analytical method employed. The recovery of phenol from air and water by the various preconcentration methods is usually low for samples containing low levels of phenol. The two preconcentration methods commonly used for phenols in water are adsorption on XAD resin and adsorption on carbon. Both can give low recoveries, as shown by Van Rossum and Webb (1978). Solvent extraction at acidic pH with subsequent solvent concentration also gives unsatisfactory recovery for phenol. Even during carefully controlled conditions, phenol losses of up to 60% may occur during solvent evaporation (Handson and Hanrahan 1983). The in situ acetylation with subsequent solvent extraction as developed by Sithole et al. (1986) is probably one of the most promising methods. 

Solvent extraction is a very widely used and simple preconcentration technique. After the sample is extracted with a suitable solvent (such as methylene chloride), the extract is concentrated by evaporation and subjected to analysis. One important requirement is extremely clean solvents fortunately these are now commercially available. Because of the evaporation step, solvent extraction cannot be used for the analysis of very volatile compounds. Depending on sample size, sensitivities of 0.1 ppb can easily be achieved. 

Pushing detection limits of nitroaromatic explosives into the parts per trillion (ppt) level requires sample preconcentration. Collins and coworkers used solid-phase extraction (SPE) of explosives from sea water which was followed by rapid on-chip separation and detection [18]. Explosives were eluted from SPE column by acetonitrile and were injected in the microchip separation channel. Lab-on-a-chip analysis was carried out in nonaqueous medium. The mixed acetonitrile/methanol separation buffer was used to produce the ionized red-colored products of TNT, TNB and tetryl [27,28]. The chemical reaction of the bases (hydroxide and methoxide anions) with trinitroaromatic explosives resulted in negatively charged products, which were readily separated by microchip... 

Horwitz, E. P, Dietz, M. L., and Fisher, D. E., Separation and preconcentration of strontium from biological, environmental, and nuclear waste samples by extraction chromatography using a crown ether, Anal. Chem., 63, 522-525, 1991. 

As techniques for chemical analysis are used in continually smaller domains, experimental challenges for inherently insensitive methods such as NMR spectroscopy become increasingly severe. Among the various schemes to boost the intrinsic sensitivity of an NMR experiment, the development of small-volume RF probes has experienced a renaissance during the past decade. Commercial NMR probes now allow analyses of nanomole quantities in microliter volumes from natural product extracts and combinatorial chemical syntheses. Figure 7.3.1.9 illustrates the range of volumes that can be examined by NMR probes and accessories such as microsample tubes and inserts. With recently reported advances in sample preconcentration for microcoil NMR analysis [51], dilute microliter-volume samples can now be concentrated into nanoliter-volume... 

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]. 

Saito, Y., M. Imaizumi, T. Takeichi, and K. Jinno. 2002. Miniaturized fiber-in-tube solid-phase extraction as the sample preconcentration method for microcolumn liquid-phase separations. Anal. Bioanal. Chem. 372 164-168. 

The effectiveness of analyte preconcentration using SPME depends on a number of parameters, for example, the type of liber, sample stirring, extraction time, and ionic strength. 

The techniques of sample preparation, extraction (isolation), and/or preconcentration of analytes are usually applied in the analysis of trace components of gaseous, liquid, and solid samples. During this operation, transport of analytes from primary matrices (donors) to the secondary matrix (the acceptor) takes place. It should be remembered, however, that the extraction and preconcentration steps could be a source of environmental pollution. The techniques of sample preparation introduced in this chapter have the following advantages253 ... 

Both liquid-phase and solid-phase extractions have been used for sample preconcentration or cleanup. For instance, an OTS-coated glass channel was employed for liquid-phase extraction, leading to an enrichment of samples (the... 

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. 

Cooper et al. [86] analyzed environmental matrices derived from soil, plant, and animal extracts to study an insecticide, a fungicide, and their metabolites. The clean-up steps consisted simply of removal of the solid debris by centrifugation and filtration. The authors recognized that while retention time reproducibility was satisfactory for a study of pesticides metabolism, in order to achieve the sensitivity required for this type of analysis a sample preconcentration step was required before the CEC separation. 

An interlaboratory comparison of the performance of thermospray and PBI LC-MS interfaces for the analysis of chlorinated phenoxyacid herbicides was reported by Jones et al. [94]. Except for Silvex, statistically significant differences were observed in the results from the two interfaces. PBI LC-MS exhibited a high positive bias, but a better %RSD at the highest concentration (500 pg/ml). A comparison of the official US-EPA method 515.1 for CPA analysis with on-line solid-phase extraction (SPE) in combination with GC with electron-capture detection (GC-ECD), LC-UV, and PBI LC-MS was reported by Bruner et al. [95]. In this method, liquid-liquid extraction (LLE), as prescribed in the US-EPA method, was replaced by SPE for sample preconcentration. In the LC methods, no derivatization was necessary. Detection limits were in the range of 0.07-0.8 ng/1 for GC-ECD, 0.7-7 ng/1 for PBI-LC-MS, and 6-80 ng/1 for LC-UV. The most accurate methods were LC-UV and GC-ECD, although PBI LC-MS is still more accurate than the US-EPA 515.1 method. 

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. 

Many investigations have been carried ont of procednres for improving the analytical qnality of GC methods by changing the matrix, increasing the concentration of the pertinent analytes and redncing the interference of other componnds present in the sample. Preconcentration by LLE, before or after derivatization, is most freqnently apphed in GC trace analysis however, other techniqnes, snch as SPE, sample stacking (see Section V.A.l) and some of their modifications, snch as simnltaneons distillation and extraction (SDE) and SPME, are also mentioned. Application of microwave-assisted processes (MAP) dnring sample preparation seems to improve recoveries. 

S. Motomizu, T. Sakai, On-line sample pretreatment extraction and preconcentration, in S.D. Kolev, I.D. McKelvie (Eds.), Advances in Flow Injection Analysis and Related Techniques, Wilson and Wilson s Comprehensive Analytical Chemistry, vol. 54, Elsevier, Amsterdam, 2008, pp. 159—201. Chapter 7. 

L.N. Moskvin, J. Simon, P. Loffler, N.V. Michailova, D.N. Nicolaevna, Photometric determination of anionic surfactants with a flow-injection analyzer that includes a chromatomembrane cell for sample preconcentration by liquid-liquid solvent extraction, Talanta 43 (1996) 819. 

C. Lu, G.-Q. Song, J.-M. Lin, C.-W. Huie, Enhancement in sample preconcentration by the on-line incorporation of cloud point extraction to flow injection analysis inside the chemiluminescence cell and the determination of total serum bilirubin, Anal. Chim. Acta 590 (2007) 159. 

When large volumes of water are sampled, the extraction of analytes is directly performed in the field water is passed through a cartridge containing a suitable stationary phase, for instance XAD-2 resin. Generally, before the preconcentration cartridge, a filtering system, with a pore size lower than 1 /rm, is positioned, and the particulate matter recovered is stored and analyzed separately. 

As a matter of fact, the use of LLE as an extraction technique shows several drawbacks, i.e., the long time necessary to perform the extraction, the use of relatively large volumes of expensive and potentially toxic solvents, formation of emulsions resulting in analyte losses, extensive use of glassware which can contaminate the sample, magnification of solvent impurities, need of sample preconcentration prior to analysis, evaporative losses of analytes, not always satisfactory repeatability, and loss of sensitivity as a consequence of the injection of only fractions of the extracted compounds. 

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