Every organic compound contains the element carbon together with one or more of the other elements in the periodic table, in definite proportions. Elemental analysis serves to measure the proportion of each element present in the compound. Thus, when a new compound is synthesized in the laboratory or when an unknown compound is obtained from nature, the product is isolated and carefully purified. Milligram amounts of the pure sample are subjected to elemental analysis. The percentage of every element is separately determined if necessary, and the sum of all elements should total 100%. In this way the composition of the synthetic compound is confirmed, while an unknown compound is characterized by the quantitative relationship of its constituents and its empirical formula can be deduced.

Besides being used to determine the elements in pure organic compounds, elemental analysis is performed on mixtures of organic matter to determine the content of certain elements therein. For instance, foodstuffs are analyzed for nitrogen, which gives an indication of the nutritional value of the material. Shale is analyzed for carbon and hydrogen to estimate its fossil oil content. Coal is analyzed for sulfur and nitrogen because these two elements are responsible for environmental pollution due to the burning of coal.

The general principle of elemental analysis of organic compounds involves the total breakdown (decomposition) of the organic molecules so that the individual elements are converted into their respective elemental forms or simple inorganic compounds. So that the objective of quantitative analysis can be accomplished, two conditions must be met: (1) the decomposition of the sample should be complete and (2) the element to be determined should be transformed into one specific product that can be measured accurately.

At present, it is possible to determine any element that may be present in organic materials. There are a great variety of techniques for decomposing the organic sample. After decomposition, there may be more than one method of measuring the product. The latter process is known as the mode of finish. In the following sections, the elements most commonly determined in organic compounds are discussed, and selected methods for decomposition and finishing are described.

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