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Identifying the separated substances

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  306

2026-11-03

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Identifying the separated substances

The R_F value is affected too adversely by measurement difficulties and by variations in experimental and environmental conditions to be a useful identification parameter on its own. When standard substances are available, it is common practice to run standards and samples in the same system for improved confidence in identification based on RF values. If scanning densitometry is used, an acceptable agreement in RF values is generally supported by the auto mated matching of specific absorbance ratios or full spectra for the samples and standards. In drug-screening programmes, in which simultaneous separation of standards and samples is impractical, the certainty of drug identification is improved by simultaneous separation of a series of related standard substances that allow the experimental RF values to be corrected to standardised RF values from auto mated library searches:

and hRF (X) is the RF value for substance X, hRF (A) and hRF (B) are the RF values for the standard substances that bracket hRF (X), and ^c indicates the corrected value for X and the accepted values for A and B.

Alternatively, a calibration curve of experi mental RF values against the accepted RF values for the standards can be prepared and used to transpose experimental RF values to corrected RF values. Typically, four evenly spaced standard substances with the sample origin (hr_f=0) and solvent front (hr_f =100) are included as additional reference points.