Below detection, limit structural identification

Three samples (4, 26, and 27) demonstrated the complementary rather than parallel functions of IR and TLC analyses in organic archaeometry. Organic materials were not revealed by IR in sample 4, but both abietic and dehydroabietic acids were detected by TLC in the same sample. Clearly, the resin acid concentrations were below the limit of detection of whole-resin IR spectroscopy but were sufficient for detection by TLC, Conversely, samples 26 and 27 showed neither acid by TLC but gave a clear indication of the isopropyl structure in the IR spectra. In this case, the IR technique was superior although the resin acids have been lost evidently (most likely by decarboxylation), the remaining neutral compounds retained the isopropyl group, a fact that allowed the identification of the samples as pine pitches. 

The amino acid sequences of the subunits of bovine heart enzyme have been determined primarily by Buse and his colleagues (Buse et al., 1986). Many of the sequences have been determined not only by DNA sequencing but also by peptide analysis. Peptide analysis is an indispensable method for the detection of post translational modifications, and an example of such a modification has been recently shown in this enzyme, and will be described below. Hydropathy plots for these amino acid sequences have provided an astonishingly successful predictions of which regions contain a-helices. This is quite remarkable considering the limited accuracy of this method for structural identification (Hosier et al., 1993 Tsukihara et al., 1996). 

In practice, identification ( yes ) means establishment of chemical structure, determination of detection and quantification limit. If the impurity is unusually toxic then it has to be qualified in all above cases, meaning the demonstration of biological safety. An impurity may stay below the threshold, making any action superfluous. 

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