Analytical chemistry is aimed at determining the composition of a sample. It means the identity, molecular structure, quantity, and concentration of in principle all, but in practice some, components of the sample. In chemical and biochemical analysis, first a given compound needs to be identified and its structure determined. Structural studies are most often performed by spectroscopy (mostly nuclear magnetic resonance (NMR) but also IR or UV), X-ray diffraction, or mass spectrometry, although a large number of other techniques are used as well. There are techniques (notably NMR and X-ray diffraction) capable of determining the structure of molecules with no or minimal prior information (up to approximately 1000 Da molecular mass), but these typically require a relatively large amount of pure compound. Following identification of a given compound, its amount (or concentration) needs to be determined as well (quantitation). As a given sample may contain thousands of different compounds in widely differing amounts, this is not a trivial task. Instead of structure identification and quantitation, often the biological effect (such as enzyme activity) is measured in the biomedical field.
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