Pre-concentration of analytes

Pre-concentration methods using online trace enrichment by applying chromatographic principles are also reported [66,69]. As described by Guzman and Meyers [71,72], this can be achieved by incorporating e.g. a solid-phase CE-concentrator tip at the inlet of the capillary. Undesired sample components can be flushed out prior to the CE separation run, providing faster and more specific analyses. Especially in the field of bioanalysis, where sample clean up and pre-concentration of analytes is usually critical, this approach may be preferred. 

Methods involve extractions of analytes into organic solvents, as well as treatments with acidic or basic reagents. Solid-phase extraction can be used for removal and pre-concentrations of analytes in aqueous solutions. Applications of low-power focused microwave technology have been investigated as a means of dissolution, and good results have been reported for extractions of organometal-lic compounds of tin and mercury (Schmitt et al., 1996 Szpunar et al., 1996). Analyses of CRMs were used for verification. The time necessary for quantitative isolations of the analytes was greatly reduced, e.g. 24 h to 5 min. In addition, there were reductions in solvent volumes, and improvement in analyte recoveries. Some of the analytical procedures for speciation of particular elements such as mercury, described later in this chapter, include microwave-assisted sample preparation. 

Part 111 describes in detail the use of hypercrosshnked sorbents, both industrial resins and laboratory-made polymers, for the sorption of organic compounds from air, water, and biological liquids and for the pre-concentration of analytes in SPE, as well as the use of hypercrosshnked resins as matrices for ion-exchange resins and as separating media in gas and hquid chromatography. 

While ion exchange membranes have been studied for many decades and have been applied to many areas, they are only beginning to be applied to microfluidic applications. The primary area that ion exchange membranes have been applied to in microfluidics has been for the pre-concentration of analytes in microfluidic systems. Pre-concentration is of critical importance in microfluidic systems because the analyte concentrations are typically too low for the detection methods used. This requires that the analytes be concentrated prior to separation and/or detection in order to enhance the detection limits of the systems. 

Thin films (mono- and multistacked layers) as well as membranes may serve for various purposes [108] protective layers with size exclusion, charge barrier or diffusion-inhibiting effects immobilization and anchoring structures for modifiers, catalysts, and mediators pre concentrators of analytes modifiers of surface characteristics wetting aids and adhesion or insulation layers. 

Electroanalysis and Electrocatalysis Polymer-film electrodes can revolutionize electroanalytical techniques because of their ability to provide a matrix for selective pre-concentration of analytes, and to immobilize high concentrations of electrocatalysts. Many reports have appeared to demonstrate such effects. For example, Guadalupe and Abruna reported preparation of copolymers of vinylferrocene with vinylpyridine or vinylbipyridine (48). 

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

Adsorptive stripping analysis involves pre-concentration of the analyte, or a derivative of it, by adsorption onto the working electrode, followed by voltanmietric iiieasurement of the surface species. Many species with surface-active properties are measurable at Hg electrodes down to nanoniolar levels and below, with detection limits comparable to those for trace metal detemiination with ASV. 

Application of rotating coiled columns has become attractive for preparative-scale separations of various substances from different samples (natural products, food and environmental samples) due to advantages over traditional liquid-liquid extraction methods and other chromatographic techniques. The studies mainly made during the last fifteen years have shown that using rotating coiled columns is also promising for analytical chemistry, particularly for the extraction, separation and pre-concentration of substances to be determined (analytes) before their on-line or off-line analysis by different determination techniques. 

There ai e noted the most convenient, simple and chip methods, which ensure the high quality of specimens and can be easily combined with different techniques for analytical pre-concentration of impurities. In particulaidy, it is proposed to make specimens in the form of gel, film or glass in the case of XRF analysis of concentrates obtained by low-temperature crystallization of aqueous solutions. One can prepai e film or organogel specimens from organic concentrates obtained by means of extraction of impurities by organic solvent. Techniques for XRF analysis of drinking, natural and wastewater using considered specimens ai e adduced. 

The great attraction of SV lies in the effect of pre-concentration of the analyte at the electrode with, as a consequence for the stripping current, a very high ratio of faraday current to charging and impurity currents it is this high ratio which has made SV the most sensitive voltammetric analysis method to date. 

Increasingly used for sample clean-up prior to chromatographic analysis and pre-concentration of trace and ultra-trace levels of analytes. Largely replacing solvent extraction. 

To leam how the analytical sensitivity can be further enhanced to about 10 " mol dm by pre-concentration of the analyte in the technique of stripping voltammetry. 

Where none of the above techniques is capable of providing a sufficiently low limit of detection (LOD), it may be necessary to pre-concentrate the analyte relative to the drug substance as part of the method. Alternatively, a cheaper, less powerful detector may be suitable for the analysis given this enrichment. This section describes various approaches. 

Some compounds are fluorescent, in that they have the ability to re-emit part of the light absorbed from the excitation source (Chapter 12). In practice, fluorescence is measured perpendicular (at 90°) to the exciting radiation source (Fig. 3.16). The intensity of fluorescence is proportional to the concentration of analyte, as long as this concentration is kept low. The application of this very sensitive and selective detector (Fig. 3.17) can be extended by using derivatisation, either pre- or postcolumn. 

Both Cr111 and Cr concentrations in natural water samples were measured by flame AAS after pre-concentrations of the chromium species on microcolumns packed with activated alumina (acidic form) (Sperling et al., 1992). An FI manifold was used in this work to obtain conditions for species-selective sorption and subsequent elution of the chromium species directly to the nebuliser of the spectrometer. In this procedure, water samples were maintained at a safe pH of 4 prior to analysis. Analytical conditions of pH 2 and 7 were attained by adding buffers on-line only fractions of a second before the corresponding chromium species was sorbed into the column. In this manner, any risk of losses of analytes and/or shifts in equilibria between the species at pH 2 and 7 were minimised. The detection limits were 1.0 and O.Smgdm 3 for Cr111 and Cr, respectively. 

Another common sample pre-concentration method is dialysis which serves to remove small molecules. For instance, affinity dialysis and pre-concentration of aflatoxins were achieved in a copolyester chip (see Figure 5.10). After affinity binding to the aflatoxin Bi antibody, various aflatoxins (Bj, B2, Gi> G2, G2J in a sample were retained, while the other small molecules passed through a PVDF dialysis membrane. Thereafter, the sample solution was exposed to a countercurrent flow of dry air, leading to water evaporation and analyte concentration. The concentrated and desalted sample was used in subsequent MS analysis [821], More details for MS analysis are described in Chapter 7, section 7.3. 

In specific cases, electrode bulk as opposed to surface modification can be employed, as with carbon paste electrodes23. The modifier, a substance that reacts preferentially in some way with a species to be determined, is mixed directly with the carbon paste. The mode of action is either by catalyzing the analyte reaction or pre-concentrating the analyte on the surface before determination. 

Fig. 13 shows an example of how important it is to avoid contamination, and its influence on the final result of the analysis. The analytical procedure was a pre-concentration of aluminum by adsorption of its complex with an organic reagent (chrome azurol S) onto a polymeric material (polyethylene powder) packed into a column. Standard solutions were prepared and the procedure carried out after adopting stepwise precautions to avoid contamination. It can be seen that, only after adopting all steps, was the contamination controlled. The graph also shows that the lower the aluminum content in the sample the higher is the contribution of the contamination sources. 

Samples are pre-concentrated in order to allow detection to be achieved. This is required when the concentration of analyte in the sample is too low or when as a result of pre-treatment the concentration in the resulting solution is too low for detection. Care is needed in any pre-concentration procedure to ensure that it does not lead to contamination from, for example, the equipment or chemicals used. 

The authors recommend using 5-10 relatively low concentrations of analyte spotted on a TLC plate. DL can be increased by using pre- or postchromatographic derivatization. ... 

The main reasons are (a) pre-concentration of the analyte of interest from a relatively large volume of sample to a small extract volume, and (b) clean-up of the sample matrix to produce a particle-free and chro-matographically clean extract. 

Blow-down Removal of liquids and/or solids from a vessel by the use of pressure often used to remove solvents to pre-concentrate the analyte. 

Competition assays can also be performed by immobilisation of the analyte to the crystal surface. A fixed concentration of the specific antibody is pre-incubated with the free analyte in a test solution, and this solution added to the crystal. Any free antibody will specifically bind to the immobilised analyte on the crystal surface. The lower the concentration of analyte in the test solution the more antibody will bind to the surface and thus a larger frequency change will be observed. This format eliminates the need for a label (but the immobihsed analyte is often a conjugate) but involves an extra incubation step. A diagrammatic representation of a capture is seen in Fig. 1. 

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