Sample preconcentration techniques

Winge et al. [730] have investigated the determination of twenty or more trace elements in saline waters by the inductively coupled plasma technique. They give details of experimental procedures, detection limits, and precision and accuracy data. The technique when applied directly to the sample is not sufficiently sensitive for the determination of many of the elements at the low concentrations at which they occur in seawater, and for these samples preconcentration techniques are required. However, it has the advantages of being amenable to automation and capable of analyzing several elements simultaneously. 

Sample preconcentration techniques are used with two purposes (1) to increase concentration in order to achieve detection and (2) to eliminate disturbances of the electrophoretic system during hydraulic or electrokinetic sample introduction when the conductivity of the sample is significantly different from that of the analysis buffer. It is important to keep sample manipulations and modifications to a minimum, and a rule of thumb is to prepare the sample so that its composition is at the same pH as the analysis buffer. It is also advantageous... 

There are many books, book chapters, or review articles on fundamental MEKC [4-11], In particular, online sample preconcentration techniques are becoming popular in CZE to improve concentration detection sensitivity and so are in MEKC, which will be introduced rather in detail. Although it is generally stated that CE requires minimal amounts of sample (certainly true), it should be mentioned that the volume of sample solution required is much greater than that actually introduced into the capillary because the injection end of the capillary must be dipped into the sample solution— this is typically at least a few microliters. 

As mentioned before, when pressure-based sample injection is employed, the maximum injection volume must be less than the effective length of the capillary. At least 10% of the effective capillary length must remain available for separation. To inject a larger sample volume, electrokinetic injection must be employed. In most online sample preconcentration techniques, the maximum amount that can be injected without loss of separation efficiency is certainly less than the capillary volume. However, with a large volume sample injection under cathodic EOF conditions in SDS MEKC, a sample devoid of the micelle can be continuously electrokinetically injected for a volume equivalent to seven times the capillary volume without significant loss of separation efficiency under favorable... 

Kim, J.B., and Terahe, S. On-line sample preconcentration techniques in micellar electrokinetic chromatography. J. Pharm. Biomed. Anal., 30, 1625, 2003. 

In this chapter, we will identify the role and the scaling of dispersive effects in determining the ultimate limits of sample preconcentration techniques. Where possible, we will offer simple quantitative theory for estimating the effects of dispersion, and in all cases we will provide scaling arguments that can help guide empirical optimization of preconcentration techniques. First, we will... 

UV-Vis absorption Aii anaiytes 10- 3-io-i6 Universai easily interfaced and spectral information using DAD Low sensitivity and substances UV-transparent Work at wavelengths less than 200 nm wide-bore capillaries bubble or z-shaped cells sample preconcentration techniques and indirect by BGE or complexation... 

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... 

Sueyoshi K, Kitagawa F, Otsuka K (2008) Recent progress of online sample preconcentration techniques in microchip electrophoresis. J Sep Sci 31 2650-2666... 

Theoretical and applied aspects of microwave heating, as well as the advantages of its application are discussed for the individual analytical processes and also for the sample preparation procedures. Special attention is paid to the various preconcentration techniques, in part, sorption and extraction. Improvement of microwave-assisted solution preconcentration is shown on the example of separation of noble metals from matrix components by complexing sorbents. Advantages of microwave-assisted extraction and principles of choice of appropriate solvent are considered for the extraction of organic contaminants from solutions and solid samples by alcohols and room-temperature ionic liquids (RTILs). 

Many process mixtures, notably fermentations, require sample preconcentration, microdialysis, microfiltration, or ultrafiltration prior to analysis. A capillary mixer has been used as a sample preparation and enrichment technique in microchromatography of polycyclic aromatic hydrocarbons in water.8 Microdialysis to remove protein has been coupled to reversed phase chromatography to follow the pharmacokinetics of the metabolism of acetaminophen into acetaminophen-4-O-sulfate and acetaminophen-4-O-glucu-ronide.9 On-line ultrafiltration was used in a process monitor for Aspergillus niger fermentation.10... 

At present, inductively coupled plasma mass spectrometry provides a unique, powerful alternative for the determination of rare earths in natural samples [638,639]. Nevertheless, its application to the determination of rare earths at ultratrace concentration level in seawater is limited, because highly saline samples can cause both spectral interferences and matrix effects [640]. Therefore, a separation of the matrix components and preconcentration of the analytes are prerequisites. To achieve this goal, many preconcentration techniques have been used, including coprecipitation with... 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

Flow injection analysis is based on the injection of a liquid sample into a continuously flowing liquid carrier stream, where it is usually made to react to give reaction products that may be detected. FIA offers the possibility in an on-line manifold of sample handling including separation, preconcentration, masking and color reaction, and even microwave dissolution, all of which can be readily automated. The most common advantages of FIA include reduced manpower cost of laboratory operations, increased sample throughput, improved precision of results, reduced sample volumes, and the elimination of many interferences. Fully automated flow injection analysers are based on spectrophotometric detection but are readily adapted as sample preparation units for atomic spectrometric techniques. Flow injection as a sample introduction technique has been discussed previously, whereas here its full potential is briefly surveyed. In addition to a few books on FIA [168,169], several critical reviews of FIA methods for FAAS, GF AAS, and ICP-AES methods have been published [170,171]. 

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. 

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