Photo-ionisation detector PID

The PID is well suited and quite widely used in environmental analysis, particularly where the main interest is in aromatic and other hydrocarbons. HNU Systems, Inc. developed the first PID in 1976 (Driscoll Spaziani 1976). Molecules are ionised by absorbing UV photons of sufficient energy. The positively ionised molecules are accelerated away from a positively charged electrode towards a collector electrode at which the current is measured. 

The PID is a non-destructive concentration sensitive detector that is 10-100 times more sensitive than the FID. The sensitivity increases with carbon number and aromatics olefins aliphatics. Certain solvents including water, methanol and pentane in particular, have a low sensitivity in a PID and therefore can be used as extractants. The dynamic range and good sensitivity towards certain compounds, reasonable selectivity, ease of use and robustness, have therefore made the PID a successful detector in some environmental situations, particularly those related to the petroleum industry. 

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As GC techniques became more refined and were generally accepted, due to the superior separation and resolution potential of modern capillary columns, they took preference, especially in the analysis of PAH in car exhaust emissions and air particulates. In combination with an FID-detector, which, in contrast to the UV-detectors used frequently in HPLC analysis, has a nearly uniform response factor for hydrocarbons, or coupled to mass spectrometry, this technique must now be considered the method of first choice for a reliable and reproducible determination of PAH traces in a wide range of matrices. The introduction of more sensitive (photo-ionisation detector - PID) and specific detectors, such as the nitrogen-phosphorus (NPD)... 

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