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HPLC, which is short for High Performance Liquid Chromatography, is primarily suited for separation of substances that are not volatile, that are thermally unstable, or possess some kind of reactive or electrically charged functional groups. The sample is injected via a loop to the mobile phase (a liquid), and the analytes are separated from each other as the mobile phase is transported through a column. By varying the material in the column (i e the stationary phase) and/or the mobile phase, different separation systems can be designed. Unlike in GC, the mobile phase plays a major role in the separation in HPLC. Methanol, water, and acetonitrile are examples of common mobile phases.
Most detectors in liquid chromatography are based on some optical principle, responding to the variation in intensity of a beam of light which passes through a flow-through cell.
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The UV detector, where UV stands for ultraviolet radiation, is the detector which is most generally useful. It is used to detect compounds with strong UV absorption, e g aromatic compounds. There are different types of UV detectors, e g diode array detectors and scanning UV detectors, the latter recording an entire spectrum.
A fluorescence detector is used for fluorescent compounds, or compounds made fluorescent by derivatization. It can be up to 1000 times more sensitive than the UV detector. Its high specificity also results in less interferences from the background (i e from the solvent or from impurities). A common application within environmental chemistry is the detection of plane polycyclic aromatics (PAH).
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