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discussed mass spectra originate from fragmentation of these heavier fractions. Further, one can speculate that the signal at m/z = 316 (present in Figs 2(b), 3(d) and 4(c) can be attributed to the fragmentation of galdosol (see the [344 - CO]" or [344 + He - 2CH3]⁺ ions), and similarly, the signal at m/z = 350 (present in Figs 2(c) and 4(c)) can also be attributed to galdosol ([344 + H + He + H2O - CH3]⁺). In most spectra, the signal at m/z = 305 is repeatedly observed (see Figs 2(a), (c), (b), Figs 3(a), (d) and Figs 4(a), (c)) and it can probably be attributed to the [350 - 3 CH₃]" or [350 - 2-propyl - He]⁺ ions. It is also possible that the signals at m/z = 302 and 305 (present in the majority of the mass spectra) originate from certain abietane type compounds that are commonly met in Sa/via.

Many terpenes can furnish very similar mass spectra as a result of structural similarity thereof and hence, of similar fragmentation pattems and similar rearrangements also, as a result of ionization. Consequently, identification of terpenes based on the mass spectra alone cannot be very conclusive, and especially not in the case of complex mixtures. As direct recognition of individual parent ions is hardly possible with a complex mixture of the essential oil components, one can only attempt to identify certain molecular fragments by very simple mass subtractions, although even with this rather cautious approach, a wide variety of numerical combination exists and remains unsolved.

CONCLUSIONS

Low-temperature thin-layer chromatography (LT TLC) enables an efFicient essential oils fractionation, as it was demonstrated upon an example of the essential oil derived from the selected sagę species (i.e., Salvia lewandulifolia). Separated fractions can be fmgerprinted with use of the TLC-MS interface to give collective mass spectra of the obtained fractions. However, identification of individual compounds upon these fmgerprints which are valid for the still relatively complex mixtures of structurally related compounds (mainly terpenes and terpenoids, and many of them appearing in several isomeric forms) is very difficult. To partially alleviate an identification problem with complex mixtures of natural origin, a novel two-dimensional chromatographic system was proposed, composed of LT TLC coupled off-line with high-performance liquid chromatography with mass spectrometric detection (2D LT TLC-LC-MS). In that way, a two-step fractionation of a complex mixture is possible and mass spectra recorded for the bands separated with use of the liquid chromatographic column are morę informative than those originating from the ID LT TLC-MS system. Upon the example of Salvia lavanduli/olia presented in this study, a rough assessment of its essential