Application of histograms in evaluation of large collections of gas chromatographic retention indices.

The effective use of gas chromatographic retention data presented in the form of retention indices (RI) requires the development of a comprehensive structure-based digital archive of retention parameters. Development of such an archive includes the collection of all available RI values for a variety of compounds including replicates measured under slightly different conditions. Review of retention data often shows a relatively wide range of RI values for certain well studied compounds that is larger than expected on the basis of the simple reproducibility of experimental measurements. The finding of unusual RI data distributions and their examination presents a possible way to detect and correct errors during the development of comprehensive RI libraries. Our approach involves the construction of histograms representing the distribution of data-points in various RI intervals. The observed shape of the distribution is compared to the expected and observed shapes for well-identified compounds. Significant systematic deviations represent anomalies in the sets of RI data. The occurrence of more than a single maximum on a histogram generally indicates the presence of erroneous data. For some compounds such multimode RI distributions may be caused by differences in experimental conditions of the RI determination. The construction and interpretation of histograms for compounds with multiple RI measurements is illustrated by several examples. Thus, the RI sub-set for the diterpene alcohol, isophytol, was separated from the RI data set for phytol and four additional sub-groups of published RI data for one of the sesquiterpenes, gamma-elemene, were re-identified as alpha-, beta-, delta-elemenes and germacrene B.