Chromatographic fingerprinting techniques

Liang, Y. Xie, P. Chau, F. (2010). Chromatographic fingerprinting and related chemometric techniques for quality control of traditional Chinese medicines. Journal of Separation Science, Vol.33, No.3 (February 2010), pp. 410-421, ISSN 1615-... 

Tistaert C, Dejaegher R, Vander Heyden Y. Chromatographic separation techniques and data handling methods for herbal fingerprints a review. Anal Chim Acta... 

Chromatographic techniques are often used to separate free radicals formed in complex gas reactions, and ESR is used to identify them. The extreme complexity of the spectra results in a unique fingerprint for the substance being analysed. With ESR it is possible to detect radicals and other absorbing species in very, very low amounts such as 10 11 to 10 12 mol, making it an ideal tool for detecting radical and triplet intermediates present in low concentrations in chemical reactions. 

The OPCW requires two independent techniques for identification. In the case of GC/MS, El, and Cl spectra, acquired from separate chromatographic runs, are regarded as independent techniques. A combination of the two is accepted as unequivocal identification - one as a fingerprint, the other to confirm molecular mass. LC/MS provides an alternative to GC/CI/MS for confirmation of the molecular mass and LC/MS/MS provides a partial fingerprint. However, LC/MS/MS spectra, which are normally acquired under identical chromatographic conditions as LC/MS spectra, are not currently considered as a second independent identification. Further development of these criteria may be required with instrumental development. LC retention time is accepted as a second technique if the retention time falls within a window of 0.2 min of the retention time for an authentic chemical, with a signal-to-noise ratio of at least 5 1. However, great care has to be exercised if identification is based solely on LC/MS. 

Analytical pyrolysis can be coupled with different analytical techniques for providing information on polymers. Among analytical pyrolysis techniques, Py-GC and Py-GC/MS are probably the most common. The pyrolysis process typically generates a very complex mixture of molecules. For this reason, a chromatographic technique is very important for the separation of pyrolysate components. The fingerprint generated by Py-GC can be used for polymer identification. However, the detection associated with compound identification provided by GC/MS is invaluable in many applications. The exceptional sensitivity and identification capability of mass spectrometric analysis make Py-GC/MS technique the most important analytical pyrolysis technique. 

An important aspect of the use of analytical pyrolysis is its capability to provide complementary information to other analytical techniques used for polymer characterization. One such technique is IR analysis of polymers. Although IR spectra can be used as fingerprints for polymer identification, the success of this technique can be questionable when the polymer is not pure or is in a mixture with other compounds. The IR spectra are particularly difficult to use when a polymer is present only at a low level in a particular material and cannot be easily separated. The use of Py-GC/MS allows identification of polymers even in low concentration in specific mixtures because it couples pyrolysis with a chromatographic technique. On the other hand, some polymers generate by pyrolysis a low proportion of easily identifiable molecules, producing mainly char and small uncharacteristic molecules such as HF, H2O, CO2, etc. In these cases, IR is the technique of choice. Since for an unknown sample each technique can be misleading, the use of both types of information is always beneficial. 

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