Standard addition method, application chemical analysis

There is one more, very important and relatively simple method to use when an interference effect is difficult to explore but its occurrence is probable and poses a threat to the reliability of analytical results. This refers to the case when an analyzed series of samples have similar chemical composition (at least in terms of the composition of interferents) and the determined component is present in all samples in a similar quantity. In this situation, the standard addition method can be used for analysis of one selected sample and the constructed calibration graph employed for interpolative determination of analyte in the remaining samples. This combined procedure is depicted in Fig. 3.16. Thus obtained results are, as a mle, more accurate than those obtained after application of the set of standards method to all the samples. In addition, the analyses are conducted faster than when all the samples are analyzed using the standard addition method. 

Perhaps the simplest of all applications of radioisotopes in chemical analysis is the use of a yield monitor . The element to be measured is labelled by the addition of a known amount of carrier-free radioisotope, in the same chemical form, to the sample. After chemical separation of the element, a comparison of the activity of the recovered material with the original activity gives the chemical yield. The method is used frequently to correct for the chemical losses of carrier separations in activation analysis and is, of course, a standard procedure for checking gravimetric analytical methods. Considerable amounts of time can be saved in certain gravimetric analyses if less than quantitative recovery is accepted and radiometric yield correction is used. 

Compositional analysis is concerned with determining structural relationships in the molecules present in a sample. Inhared spectroscopy is the most commonly used tool for qualitative chemical analysis of viscous oils. Descriptions and tables of characteristic absorbance for a variety of organic functional groups are readily available in many textbooks. Techniques for quantitative anal3rsis for many additives and some hydrocarbon types are available, although few have been issued as ASTM standards. Reports on new methods are commonly reported in the chemistry literature. To locate information on new analytical methods, a most useful reference is the bi-aimual Application Review published by the American Chemical Society. These have appeared recently in the June 15 issue of Analytical Chemistry in odd-numbered years. Recent reviews cover coal, crude oil, shale oil, heavy oils (natural and refined), lubricants, natural gas, and refined products and source rocks. Extensive references to original research papers are provided. A complimentary Fundamental Review covering the basic analytical techniques is published in even-numbered years. 

Research into new analytical techniques for foodstuffs continues, striving for greater accuracy, sensitivity or simplicity, for more rapid methods, for simultaneous multielement analysis, etc. Chromatographic techniques, e.g., LC, GLC, GC-MS, have led to great improvements in the levels of accuracy, sensitivity, and detection that can be achieved for many analytes including carbohydrates, certain vitamins, chemical residues, and additives. Work is still required, for instance, in the area of vitamin analysis in order to provide standard techniques that are applicable to all food types and that would enable concurrent multi-vitamin analysis to take place. Many of the microbiological assays currently used for vitamin determination involve long incubation times and more rapid techniques are needed. 

Research the type of additives and normal concentrations. Learn about their properties. Do they have a unique element or chemical tag that can be used for quantitative measurements Is an analysis method available from the supplier or in the literature Many sources are available scientific journals, trade journals, technical application sheets from instrument manufacturers and standards compendia. Any method selected must be validated in your own laboratory. Learn as much as possible about the polymer matrix. Thermoplastics can be hot pressed into thin films. Many polymers are solvent resistant, so extraction techniques can be used to isolate the additives. 

Perhaps the most common application of VS in the determination of chemical makeup in polymeric systems is the identification of components in complex polymer mixtures. Polymeric products are rarely composed of a single component. There are always additives present that aid in processing, appearance, adhesion, chemical stability or other properties important to the function of the final product. In an industrial setting, it is important to be able to determine both the identity and quantity of polymers and additives in a specific formulation for quality control purposes. This can be a fairly routine operation if tools such as spectral libraries are utilized. In this method, a computer search algorithm compares a spectrum with a catalogue of standard spectra to determine the identity of the compovmd or compounds present. Advanced statistical techniques, such as partial least squares (PLS) and principal-component analysis (PCA), are also often used to identify known and unknown components in polymeric systems. The details of these methods are described elsewhere in the Encyclopedia. 

The unavailability of suitable standards can severely restrict the quantitative applications of this method. For some sample forms standards can be prepared from the sample by the method of additions. Preconcentration methods that utilize chemical separation are also useful for spectrographic analysis these methods facilitate the preparation of standards because the matrix has been simplified. On the other hand, such preconcentration methods or the preparation of standards by the method of additions increase the likelihood of contamination of the sample. Common matrix methods, involving dilution of the samples with a material such as germanium or gallium oxide, are not applicable to the analysis of the purest materials, since the dilution process raises the limits of detection. 

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