Raman and surface enhanced Raman spectroscopic investigation on Lamiaceae plants P. Ro ¨sch, J. Popp, W. Kiefer * Institut fu ¨r Physikalische Chemie, Universita ¨t Wu ¨rzburg, Am Hubland, D-97074 Wu ¨rzburg, Germany Received 25 August 1998; accepted 30 September 1998 Abstract The essential oils of Thymus vulgaris and Origanum vulgaris are studied by means of micro-Raman spectroscopy. The containing monoterpenes can be identified by their Raman spectra. Further the essential oils are investigated in their natural environment, the so-called oil cells of these Lamiaceae plants, with surface enhanced Raman spectroscopy (SERS). This method has the advantage to enhance Raman signals and furthermore the SERS effect leads to fluorescence quenching.  1999 Elsevier Science B.V. All rights reserved. Keywords: Micro-Raman spectroscopy; Surface enhanced Raman spectroscopy; Essential oils 1. Introduction The Lamiaceae plants and their essential oils are well known for their therapeutical effects and medical applications [1]. Most of the common spice plants like thyme and origanum belong to this family. In order to investigate the chemical composition of these spice plants in vivo, several biological and spec- troscopic obstacles have to be ruled out. First of all biological raw materials show qualitative and quanti- tative differences in their chemical composition on account of annual variations, different soils, solar radiation, rain and fertilizing. Furthermore, each species consists of various subspecies or hybrids with a specific chemical composition. Raman spectroscopy yields detailed information about biological molecules, especially about their structure and conformation. Therefore, it is possible to characterize and identify these substances in their natural environment. Especially the micro-Raman spectroscopy, as a non-invasive technique, is a valu- able tool for the in vivo investigation of biological samples, such as single cells and tissues [2,3]. A micro-Raman setup – a combination of a common Raman set-up with a conventional microscope – allows the investigation of even smallest samples, which makes it possible to identify for instance essen- tial oils in the cross section of a plant. In in vivo Raman spectroscopic investigations there arise two main problems. First, the low scattering cross-section of biological samples, which results in low signal levels, and second, the excitation of fluorescence can often be observed, which makes it difficult or even impossible to obtain Raman spectra at all. Surface enhanced Raman spectroscopy (SERS) is known for its sensitivity towards a wide range of Journal of Molecular Structure 480–481 (1999) 121–124 0022-2860/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0022-2860(98)00624-3 * Corresponding author. Tel.: + 49-931-888-6330; fax: + 49- 931-888-6332. E-mail address: wolfgang.kiefer@mail.uni-wuerzburg.de (W. Kiefer)