Elemental analysis

Elemental analysis is significant in establishing the identity and purity of a known compound or the empirical (stoichiometric) formulae of a new one. The Elemental composition is generally quoted as percent by mass, by which the stoichiometry can be determined from atomic mass (RAM) values. Refer to a compound (X) with the following composition by mass:

Dividing each mass percent by the subsequent RAM (Appendix 1) gives the following relative molar quantities

These are almost in the proportions 1:9:6:3, suggesting the stoichiometric formula CrC₉H₆O₃. Traditional techniques of elemental analysis depend on particular chemical reactions for given elements, either in solution using titrations (termed as volumetric analysis) or precipitation of solids that can be weighed (gravimetric analysis). Even though such techniques are still used for particular and very accurate purposes, they have been changed in routine work by automated instrumental methods. Combustion analysis is employed to determine C, H, N, and sometimes S through complete oxidation of the compound forming CO₂, H₂O, N₂ and SO₂. The gases are determined and separated automatically by gas chromatography. The method is most valuable for organic compounds, but is also employed for the organometallic compounds of inorganic elements.

Methods for determining the majority of elements depend on measuring the line spectra of atoms, from an example that has been heated sufficiently to give complete atomization. Every element gives a characteristic series of lines, the intensities of that can be calibrated against samples of known composition to determine the amounts exist in the unknown sample. A long-established method is atomic absorption spectroscopy using samples sprayed into a hot flame. A more recent development which gives greater reliability and sensitivity is inductively coupled plasma atomic emission spectroscopy (ICPAES). The sample is injected into plasma (hot ionized gas) at a temperature around 10000°C that ensures more complete atomization than in the flame method.

In the method of X-ray fluorescence (XRF) characteristic X-ray wavelengths are produced from a solid sample and may be employed to identify elements available. The technique is less correct than those based on the atomic spectra of gases, but is helpful for solid samples, particularly minerals that may consist of several elements. X-rays may be excited by the electron beam in an electron microscope and the resultant energy dispersive X-ray analysis

(EDAX) can be used to provide estimated atomic analyses of individual grains of a powdered solid and to estimate the chemical homogeneity of a sample.