Sample preconcentration techniques approach

CE presents potential advantages in forensic science to carry out the analysis of opium alkaloids, as it can be in the different applications being published. Nevertheless, the lack of the sensitivity required for this type of analysis is always a great problem. One of the most basic approach for sensitivity enhancement is based on increasing analyte mass loading via online sample preconcentration techniques. The most widely employed in the analysis of major alkaloids is FASI that basically consists in a mismatch between the electric conductivity of the sample and that of the miming buffer. It is achieved by injecting the sample diluted in a solvent of lower conductivity than that of the carrier electrolyte. Upon the application of the... 

Determination of inorganic anions by capillary electrophoresis is critically compared with ion chromatographic determinations on the basis of recent literature in the field. After a very brief summary of the theoretical background, the selection and optimization of the running electrolyte system are discussed, especially in connection with modification of the electroosmotic flow. Preconcentration techniques are surveyed, as are the approaches to the sample introduction and analyte detection. The principal analytical parameters of the determinations are evaluated and illustrated on selected applications described in the literature. 1997 Elsevier Science B.V. 

The use of pattern recognition techniques in conjunction with sensor arrays constitutes a promising approach for multicomponent analysis and for improved selectivity [26]. Similarly, a smart sensor system that employs a temperature-controlled array of SAW sensors, automated sample preconcentration, and pattern recognition has been described [27], The proposed technology seems to offer a satisfactory solution for the problems associated with non-selective adsorption by coating materials. 

Because the luminol detection system is also sensitive to NO2, chemical amplification methods have been attempted to further decrease detection limits for PANs below the pptv range for trace-level measurements. With this approach, the PANs are thermally decomposed to NO2 in the presence of large amounts of NO (6 ppm) and CO (8%). Thermal decomposition of the PANs yields peroxy radicals which initiate a free-radical chain oxidation of NO to NO2, producing several NO2 molecules (approximately 180 (20) for each PAN decomposed. This technique has been used as a gas chromatography detector to achieve ultratrace detection limits without sample preconcentration. The detector exhibits a slightly nonlinear response relative to conventional BCD, attributed to the nonlinear response of the luminol reaction in the presence of NO at 6 ppm. 

Both techniques, SIA and LOV, in a sequential injection approach, offer the means of performing automated handling of sample pretreatment, e.g. sample preconcentration and matrix removal, exploiting SPE achieving high enrichment factors (between 20 and 125). 

Several research groups have reported ways to preconcentrate samples in Lab-on-a-Chip devices. While most on-chip preconcentration approaches evolve from conventional capillary electrophoresis and chromatographic column techniques, these preconcentration techniques play an increasingly important role in chip-based system. The basic preconcentration strategies applied on microfluidic devices can be classified into three large categories electrokinetic preconcentration, chromatographic preconcentration and membrane preconcentration. 

As pointed out above, IC is a well-established method for the analysis of inorganic anions and has become the method of choice in many application areas. Many techniques are available using singlecolumn [46] or dual-column systems with various detection modes. IC can be used both for analytical and preparative purposes. Large sample volumes, up to 1300 pul, can be injected to determine trace anions and cations and to attain detection limits of 10-400 ng/1. For determinations at a pig/1 to mg/1 level, a sample size of 10-50 xl is sufficient. Preconcentration is necessary for lower concentrations (an additional column, a sample pump, an extra valve and an extra time are the disadvantages of this approach [47]). With an IEC column and isocratic... 

It is clear that the use of mPC-CE and lA-CE afford a powerful approach for preconcentration and on-line sample eleanup of antilyte mixtures prior to separation by CE. While these techniques continue to be refined, they overcome the current limitations of poor CLOD in conventional CE. Finally, the use of these devices as microreactors affords enhanced chemical derivatization or enzymatic reactions a lower analyte concentrations than is currently possible by conventional solution chemistries. 

A number of approaches have been devised to improve the limit of detection of online CE detectors. These include increasing the length of the OLP, using more sensitive optical techniques, and preconcentrating the sample. 

The presence of trace organics in natural and waste waters can often cause a problem in terms of the selected analytical technique. In order to be able to quantify the concentrations of trace organics in aqueous samples, appropriate methods of pre-concentration therefore need to be selected. This present chapter has summarized the main methods available for such preconcentration procedures. The traditional approach has utilized liquid-liquid... 

Perhaps one of the most promising approaches to enhancing the performance of an electronic nose is based in the choice of sampling system. The concentration of an odour in a dynamic headspace will always be much lower than in the static headspace. Consequently, the use of a headspace preconcentrator when trying to detect very low concentrations of a particular odour is often a logical decision. There are a variety of methods that can be used to enrich the headspace and some of these are now commonly used. The four main techniques are ... 

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