Differences among Epilobium and Hypericum species revealed by four IR spectroscopy modes: transmission, KBr tablet, diffuse reflectance and ATR.

INTRODUCTION: Quality control of dried plant material in assessments of suitability of herbal medicinal products is of extreme importance. Commonly used procedures for identification of species are time consuming and expensive. The development of multivariate statistical methods has enabled application of vibrational spectroscopy for establishing plant species membership. OBJECTIVE: To determine which infrared spectroscopy mode gives most informative spectra for plant species identification. METHODOLOGY: Different modes of infrared spectroscopy were applied for investigation of differences among Epilobium and Hypericum species: diffuse reflectance (DR), attenuated total reflectance (ATR), direct transmission of whole leaves and KBr tablet transmission with comminuted leaves. The same chemometrical methods were applied to all spectra. The informative wave numbers were chosen by one-way analysis of variance. Afterwards the colinearity was reduced with principal component analysis. In the final step the species identification was determined with discriminant analysis. RESULTS: Transmission and diffuse reflectance mode did not give satisfactory results. Best results for discrimination among Epilobium species were obtained with ATR mode (98%), while best results for Hypericum species were obtained with KBr transmission mode (97%). This might be explained by the morphological properties of the two genera. Epilobium species differ in distribution and morphology of trichomes on the surface of the leaves. Hypericum species differ in structures and secondary metabolites in the interior of the leaves. CONCLUSION: For best results in using infrared spectroscopy for plant species identification in quality control, the morphological properties of plant material should be taken into consideration.

Identification of herbarium whole-leaf samples of Epilobium species by ATR-IR spectroscopy.

A simple, high-accuracy FT-IR method based on attenuated total reflection (ATR) was developed for the rapid determination of leaf samples of Epilobium species. The method is superior to other analytical techniques, since there is no need of laborious sample preparation such as grinding or extraction and solvent removal. A total of 70 herbarium specimens, belonging to all 13 Epilobium and to 2 Chamerion species growing in Slovenia, were analyzed. With the 100 most-informative wavenumbers in the range 700-1800 cm(-1), we obtained over 90% accuracy of species identification, with discriminant multivariate statistical analysis on the measurements made on whole dried leaves.