Chemometric approach to selectivity in TLC with densitometric detection.

A recent study showed that spectral correlation cannot be considered as a reliable method for spot purity of thin-layer chromatography (TLC). The current paper discusses limitations of possible chemometric techniques performed on diode-array detection or multiwavelength densitograms of spots. The primary difference between high-performance liquid chromatography and TLC is the nonlinearity of the detector response. The discussion is based on two examples: a spot of decomposed aspirin and sixteen spots with different concentrations and contaminations of ciprofibrate and clofibric acid. It is concluded that only visual inspection of principal component analysis (PCA) scores can detect inhomogeneity in the cases of nonlinearity, spectral similarity and high overlap. The objective values and other methods can be affected by these phenomena and detect nonlinear artifacts as other compounds, or can be insensitive to a second compound with similar spectra. Therefore, PCA analysis of the spot is the recommended procedure for spot purity checking at the stage of method development.