Cleanup and preconcentration

Solvent extraction has also played a major role in sample pre- or posttreatment to improve selectivity and sensitivity. Initially simple schemes of liquid-liquid extraction were used as separation methods for the cleanup and preconcentration of samples, mainly because of its simplicity. 

Liquid-liquid extraction is by far the most popular separation method for the cleanup and preconcentration of samples because it is simple, reproducible, and versatile. There are several ways to achieve these objectives, from the original discontinuous ( batch ) and nonautomatic techniques to continuous separation techniques incorporated with automated methods of analysis. The methodologies can be classified into two general types ... 

SPEs offer distinctive advantages over conventional liquid-liquid extractions. They are relatively fast, require small sample size, eliminate emulsification problems, provide the possibility of performing both cleanup and preconcentration of the extract in one analytical step, and offer high precision. Another great advantage of SPEs over liquid-liquid extractions is solvent savings. Unlike liquid-liquid extractions that often require hundreds of milliliters for single or multiple extractions, SPEs require only a few milliliters of solvents for analyte extraction and cleanup. 

Due to the high sensitivity of the UV-detection and less problematic UV-absorption maximum values (around 365 nm), simpler sample cleanup and preconcentration techniques were developed, as compared to the other antibiotic groups. 

Several countries have fixed the maximum allowed concentration for some of the BAs in wine e.g., Switzerland recommends 4 mg/L for His, Netherlands 5 mg/L, Germany 2 mg/L, and France 8 mg/L. High-performance LC techniques are largely used to determine BAs content in wines, with ion exchange (96,107) or RP columns withpre- (100,108-110) or post- (10) column derivative formation or without derivatization (98,4,111,112) with different detection means, mainly UV (50,110,113,114) or fluorescence (97,100,50,108,109). Nevertheless, if BAs are to be determined at low levels with no interference from other compounds, e.g., AA, previous cleanup and preconcentration steps are required. 

Audunsson, G. 1986. Aqueous/aqueous extraction by means of a liquid membrane for sample cleanup and preconcentration of amines in a flow system. Anal. Chem. 58 2714—2723. 

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. 

CZE-UVD, measuring at 210 nm, was applied for separation of a mixture of HA A of the types found in Table 2.B-C, extracted from cooked protein food. After cleanup and preconcentration by the technique described in Section III.A.5, samples were run in solution at pH 2.20 for optimal resolution (better than 1.7). The order of emergence of five such analytes was MelQ (25b), Trp-P-2 (28a), Glu-P-1 (29b), MelQx (26a) and PhIP (23a). Best overall results were obtained for hydrodynamic injection, with LOD from 35 to 50 ppb, linearity from 0.06 to 10 ppm and good reproducibility214. 

G. Audunsson, Aqueous/Aqueous Extraction by Means of a Liquid Membrane for Sample Cleanup and Preconcentration of Amines in a Flow System. Anal. Chem., 58 (1986) 2714. 

Solid phase extraction procedures are now commonplace as a means of sample cleanup and preconcentration in chromatography. Espinosa et al. (1999) used strong anion exchange (SAX) solid phase cartridges to concentrate and isolate phosphorus-... 

Figure 2 Instrument arrangement for online solid-phase extraction with column switching. Pa and Pb, pumps Ci, extraction column for the online SPE C2, analytical column Vi and V2, injection and switching valves position 1, switching valve is positioned for online SPE cleanup and preconcentration position 2, switching valve is positioned to transport sample to the analytical column.

Liquid chromatography in combination with CIS or CS columns avoid the need to derivatize phenols and enables the online coupling of cleanup and preconcentration systems. The UV—visible, or diode array detection, is the most frequently used in this context. 

Due to the complexity of environmental real samples and the low relative abundance of pollutants, preparative cleanup and preconcentration steps are usually required. Although sample preparation techniques are widely developed in conventional formats and off-line, it is important to point out that from the different steps related to the analytical process, sample preparation is notably the least developed in miniaturized devices. However, very exciting unique opportunities could be expected because of... 

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