Two-dimensional thin-layer chromato,mphy is useful for microchemical studies on mixtures difficuit to resolve by the st2ndardized one-dimensional thin-layer chro-matogr2phic method now commonly used for lichen products. A modified two-dimensional technique uses the large body of standardized RF data already accumulated for these compounds. In addition, correlations of RF values with chemical structures permit tentative identifications of many trace constituents, including new natura1 products, resolved from microextracts by the two-dimensional method. The standardized two-dimensional procedure also allows more reliable comparisons of chromatograms and the determination of RF classes of components of complex mixtures. The method is illustrated for the orcinol-type depsides of two closely related specks, Parmelia loxodes and P. verrucul$era.
LNTRODUCTLON
The relative ease with which most characteristic lichen products can be identified by 2 commonly used standardized thin-layer chromatographic (TLC) methodla has allowed ffie identific&ion of major constituents of many species. En addition, TLC can resolve the previously unstudiable minor constituents, and these lesser products may enable the prediction of the total chemical variation within species group9. Consequently, we need a simple TLC method for identifying minor constituents something of the ease and reliability with which we now survey major products.
-Recently Maas~~~ successfully used two-dimensionaI TLC to study several species of Stictaceae. It would appear that labile lichen products did not decompose by this method 2s might hzve been suspected. Indeed, his resufts are so impressive that the two-dimensional me'thod should receive wide application in studying chemical variation in fichens.
The principal adv2ntage of two-dimensional chromatography is its ability to resolve compounds poorly separated by one-dimensional methods. The principal disadvantages are (1) &at samples cannot be compared on the same [email protected] except by co-&romatogr2phy,
(2) that artifacts can be introduced if labile compounds decompose before chromatography in the second dimension, and (3) that e2ch anal-