Design techniques preconcentration methods

The attractive features of splitless injection techniques are that they allow the analysis of dilute samples without preconcentration (trace analysis) and the analysis of dirty samples, since the injector is easily dismantled for cleaning. Success with individual samples, however, depends on the selection of experimental variables of which the most important sample size, sample solvent, syringe position, sampling time, initial column temperature, injection temperature and carrier gas flow rate, often must be optimized by trial and error. These conditions, once established, are not necessarily transferable to another splitless injector of a different design. Also, the absolute accuracy of retention times in splitless injection is generally less than that found for split injection. For splitless injection the reproducibility of retention times depends not only on chromatographic interactions but also on the reproducibility of the sampling period and the evaporation time of the solvent in the column inlet, if solvent effects (section 3.5.6.2) are employed. The choice of solvent, volume injected and the constancy of thermal zones will all influence retention time precision beyond those for split injection. For quantitative analysis the precision of repeated sample injections is normally acceptable but the method is subject to numerous systematic errors that may... 

Precipitation is one of the oldest separation techniques used in classical chemical analysis. However, its importance in modem analytical chemistry has declined due to the development of more versatile and efficient separation techniques such as solvent extraction and ion-exchange which are also more easily automated. Conventional operations for precipitation in the batch mode are both labour and time consuming, and require considerable operator skill. When coprecipitation methods are used to separate or preconcentrate trace constituents, the long manual procedures are particularly undesirable, as they may introduce contamination risks which are difficult to overcome. >espite the obvious drawbacks of the precipitation-dissolution manual batch procedure, little has been attempted for its automation, presumably owing to difficulties in designing efficient automated procedures for aging, quantitative transfer of precipitates on to a filter, and its subsequent dissolution or weighing [1]. 

In traditional electrophoresis, electrically charged analytes move in a conductive liquid medium under the influence of an electric field. Introduced in the 1960s, the technique of CE was designed to separate species based on their size to charge ratio in the interior of a small capillary filled with an electrolyte. The technique uses fused silica capillaries under an electrical field, and separation occurs either solely on the basis of molecular weight and charge, electrophoretic mobility, or associated with differential solubilisation in surfactant micelles. Associated with preconcentration, pre-derivatisation or incapillary derivatisation techniques in order to overcome the problem of inherent low sensitivity, this method has shown potential for excellent separation efficiency (Asthana et al. 2000 Cavallaro et al. 1995 Sun et al. 2009). 

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