Reaction methods of trace analysis

In spite of the successful proliferation of non-reaction methods of trace analysis, the application of chemical techniques in the GC determination of trace substances permits a much better accomplishment of such general problems as the determination of trace amounts of components in the zone of the main component, selective concentration of trace substances, improved separation of the latter and the main substance, decreased detection limits. 

The importance of chemical methods in GC trace analysis has long been recognized. The earliest analytical work based on chemical conversions in the chromatographic system was carried out as far back as 1955 by Ray [6], Green [7] and Martin and Smart 

The chief analytical purpose of resorting to chemical reactions is to simplify the solution of specific analytical problems and to extend the area of application of gas chromatography. In analytical reaction GC the column efficiency and the characteristics of the detector used generally remain invariable. However, as a result of chemical conversions of the sample components, derivatives are formed with different separation characteristics and detection limits, which generally leads to changes in the basic chromatographic parameters with respect to the initial compounds. 

The application of chemical methods offers the following potential advantages  

This chapter deals only with those methods of analytical reaction GC which are apphcable to solving specific problems in trace analysis. The apphcation of chemical chromatographic methods for the identification of the components of mixtures and the solution of other general chromatographic problems is considered in other chapters. 

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Sometimes the analyte is in such low concentration that it is impossible to isolate. It can be noted from equations (17.1) and (17.3) that it is not necessary to know Ax and As individually if their ratio can be determined. To achieve this, a reproducible reaction can be conducted on the labelled standard (analytical blank) and, in an identical fashion, on the sample in order to obtain the same quantity of derivatised compound. Thus the sub-stoichiometric method is similar to the immunochemical method for trace analysis. 

As any method of anion analysis may be applied if isolation techniques such as evaporation, precipitation, ion exchange, or solvent extraction are employed, we shall limit the discussion to direct methods and admit isolation techniques only if they are simple and rapid. The methods apparently best suited to the direct analysis of trace amounts of anions therefore are limited to selective membrane potentiometric, atomic absorption, fluorescence, and spectrophotometric methods following oxidation-reduction or complexometric reactions, or solvent extraction. Most of the traditional analytical methods—gravimetric, titrimetric, emission spectrometric, and electrical methods involving oxidation and reduction are less suitable, as are most radioactive procedures including neutron activation analysis, except in special cases. 

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

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