Subtraction method reagents

In addition techniques, the test substance concentration is determined from the difference in the ISE potentials obtained before and after a change in the sample solution concentration. The main advantage Ues in the fact that the whole measurement is carried out in the presence of the sample matrix, so that results with satisfactory accuracy and precision can be obtained even if a substantial portion of the test substance is complexed. Several addition techniques can be used, namely, single, double or multiple known addition methods, in which the sample concentration is increased by additions of a test substance standard solution single, double or multiple known subtraction methods, in which the sample concentration is decreased by additions of a standard solution of a substance that reacts stoichiometrically with the determinand and analyte addition and subtraction methods, in which the sample is added to a test substance solution or to a reagent solution. 

The application of chemical methods makes it possible in some instances to eliminate one of the typical difficulties encountered in trace analysis, namely masking of the peaks of trace components by the broad zone of the main component. To separate the zone of trace components from that of the main components, which may contain various types of compounds, the subtraction method is often recommended. This consists in incorporation into the chromatographic system of a reactor containing a reagent that forms nonvolatile compounds with the main component the zones of trace components pass... 

A special form of the subtraction method is obviously one in which one or several components of the test mixture form with the reagents not involatile but, on the contrary, super-volatile products whose retention times are substantially shorter than that of the Ught components of the test mixture ... 

Molecular sieves, as physical reagents, are also used in the subtraction method for analysing relatively high-boihng reagents. Thus, rapid methods with the use of molecular sieves 5A [40—47] have been developed for the GC determination of n-alkanes in... 

An advantage of the subtraction method, which, however, can be used in only a limited number of instances, is the possibility of selective determination of the adsorbed compounds after their desorption. The adsorbed compounds are regenerated either by increasing the temperature or by the action of a new reagent, which destroys the compounds formed or displaces the components absorbed. In this instance the selectivity of determination increases considerably, and so do the reliability and sensitivity. An example of the utilization of this method is the determination of n-alkanes in a mixture with other hydrocarbons n-alkanes absorbed at 350 C can be eluted at 550°C, whereupon their individual composition can be determined with temperature programming of the chromatographic column, which is located after the subtraction reactor [48]. 

The possibility of identifying cis- and trans-isomers of 1,3-dienes, based on the fact that the trans-isomer reacts faster than the c/s-isomer with the dienophilic chloromaleic anhydride, was demonstrated by Gil-Av and Herzberg-Minzly [93]. They proposed the use of comparatively slow chemical reactions for identifying the peaks of unknown compounds. As a result of these reactions the zones of the reacting compounds are subtracted only partially ( partial subtraction method ). The reagent used, as a Uquid stationary phase, was chloromaleic anhydride, which reacts selectively with dienes. Later this method was also used successfully by Alekseeva and Solomatina [94]. 

In conclusion, we shall mention some examples of the utilization of selective reagents in the subtraction method (see Table 5.7). Note that in applying the indicated methods to new types of samples one must make a preliminary evaluation and consider possible side-reactions, irreversible absorption, etc. 

EXAMPLES OF THE UTILIZATION OF SOME REAGENTS IN THE SUBTRACTION METHOD... 

Simplicity and flexibility are the attractive features of the subtraction method. Despite the utilization of this method in practice it has not been developed thoroughly enough, even for identification purposes. Numerous publications on this method mainly contain, with rare exceptions, a description of its application to several classes of organic compounds, although its practical value essentially depends on how comprehensively the reagent used has been studied (selectivity, interaction with many classes of organic compounds, volatility, maximum temperature of utilization, reaction rate, completeness of reaction under different conditions, side-reactions, etc.). 

Naturally, in place of a comparison solution with constant measured ion activity, a reagent solution can be added which lowers the measured ion activity by a specific amount through precipitation or complexation of the indicated ion (subtraction method). 

In all three methods a blank is obtained. To ascertain the blank, excess sodium thiosulfate is added to the potassium iodide reagent at a concentration of 4.0 x 10 14 mol/1. Samples were re-analysed and the appropriate blank subtracted from the sample signal. 

A method blank is a sample containing all components except analyte, and it is taken through all steps of the analytical procedure. We subtract the response of the method blank from the response of a real sample prior to calculating the quantity of analyte in the sample. A reagent blank is similar to a method blank, but it has not been subjected to all sample preparation procedures. The method blank is a more complete estimate of the blank contribution to the analytical response. 

SUBTRACTIVE PRECOLUMNS. For many applications the mixture to be analyzed is so complex that the only reasonable method of analysis requires the removal of certain classes of compounds. This process can be easily implemented by the use of a reactive precolumn. For example, a precolumn of potassium hydroxide can be used to remove acid vapors. The mixture could then be chromatographed with and without the precolumn to identify which peaks had acid character. A discussion of precolumn reagents is given by Littlewood (7). Potential packing materials for precolumns may also be found in the trace analysis literature, (see Chapter... 

Volumetric methods are largely employed for the rapid estimation of iron in solution. As a rule, the iron must be present in the ferrous condition, and any ferric iron must first be reduced, the reagents employed varying somewhat with the method of titration. If both the ferrous and ferric contents of a solution are required, the ferrous iron is first determined by titration, then the whole is reduced, and the total iron determined. Subtraction gives the amount of ferric iron originally present. 

The content of the components in the total unresolved peak, one of which is subtracted with the use of the reagent, can be calculated by the following equation when applying the internal-standard method ... 

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