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Low-Temperature TLC—MS of Essential Oiis

from the piąte before elution of the chromatographic bands and recording of the MS fingerprints. Finally, it must be added that although the predominant signals in the mass spectra shown in Figures 1-5 are labeled with the respective m/z values, this has been done for generał orientation of the reader rather than for identiflcation purposes (which was not the objective of this study).

In contrast with the separation performaitce of GC, TLC in most cases cannot achieve resolution of individual spccies from com-plcx mixtures and, hence, attempts to identify such specics by usc of external standards seem rather futile. This evident bot-tleneck of TLC is madę even morę acute by the isomerism of many terpenes (which to a large extent constitute essentiaJ oils). Isomeric terpenes (for examp!e the pairs a and (3-phellandrene, caryophyllenc/isocaryophyllene, and a and (3-chamigrene) have identical molecular weights and similar Chemical structures. In these circumstances, the diversity of mass spectra obtaincd from each separatcd fraction dcrivcd from a givcn esscntial oil and, simultaneously, the repcatability thereof is a rcal bonus. In that way the uniqueness of each fraction is strongly emphasized.

However, visual inspection alone can be regarded as a prelimi-nary step only toward differcntiation of the plant species on the basis of thcir fingerprints. From multiple fingerprints (i.e. those comprising densitograms and mass spectra), it is evident that the wealth of relevant Information encodcd in each individual fin-gerprint laigcly escapes visual inspection and, hence, is lost for analytical purposes. EPfective extraction of this Information can be achieved by use of chcmometric approaches, as we demon-strated elsewiicre for the different sagę species [12J. In that paper, exhaustive information was provided about the strategies used for preprocessing instrumental signals (i.e., chromatographic fingerprints), followed by exploration of the differences among them (e.g., by means of principal-components analysis, PCA). Such chemometrically based analysis of differences among the plant specics can be regarded as an advanced approach to chemotaxonomic research. In futurę studies, the refom, we will make use of multiple fingerprints (comprising densitograms and mass spectra) of the essential oils of the different sagę spccies to perform chemometric differentiation among them.

4 Conclusion

Basic conclusions from the results presented in this study can be

formulated as follows:

(a)    Lx)w-temperature thin-layer chromatography of volatile essential oils provides reproducible planar separations that can further be analyzed by mass spectrometry.

(b)    Hvcn in an absence of GC-MS system (which is preferably dedicated to the analysis of the volatile compounds), mass spectrometry can be used for fingerprinting (and, in favor-able circumstances, identiflcation also) of essential oil com-ponents by use of TLC and the MS interfacc.

(c)    Thin-layer chromatographic fractionation of esscntial oils followed by mass spectrometry of the separated fractions provides multiple fingerprints that can better characterize an analyzed sample than the single (e.g., densitometric) fingerprints.

(d)    Fingerprinting of the essential oil fractions is particularly helpful because of the presence of many isomeric terpenes (e.g. the pairs a and p-phellandrene, caryophyllene/iso-caryophyllene, and a and p-chamigrene) with identical molecular weights and very similar structures which can neither be effectively separated by TLC nor identified by usc of extemal standards.

(e)    Multiple fingerprints of plant materiał provide multidimen-sional quantitative characteristics that can further prove use-ful in chemotaxonomic research supported by chcmometric approaches.

Acfcnowtodgments

The work of one author (Ł.W.) was partiaJly supported by a PhD scholarship granted to him in 2009 within the framework of the ^łUnivcrsity as a Partner of the Economy Based on Science’ (UPGOW) projcct, subsidized by the European Social Fund (EFS) of the European Union. 1 hc authors wish to express thcir gratitude to Mrs Małgorzata Zych from Donau Lab Sp. Z o.o. (Polish repre9entative of the firm CAMAG) for kindly lending them the CAMAG TLC-MS interface and in that way cnabling this research.

References

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Ms received: Decembcr 2, 2009 Accepted: March 16,2010

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