Prediction of thermodynamic parameters in gas chromatography from molecular structure: hydrocarbons

Theoretical prediction of gas-chromatographic retention times could be used as an additional method for a more accurate identification of organic compounds during GC/MS analysis. Two separate quantitative structure-property relationship models were introduced for the calculation of thermodynamic values (DeltaH degrees, DeltaS degrees) for aliphatic and aromatic hydrocarbons. These values are required for the calculation of retention times in temperature programmed gas chromatography. Seven-descriptor and five-descriptor MLR models were selected for the calculation of DeltaH degrees and DeltaS degrees values, respectively, based on the best cross-validation abilities. The final prediction capabilities of the models were evaluated by a test set procedure. RMS errors calculated from the test set were 207 cal mol(-1) and 0.58 cal mol(-1) K(-1) for DeltaH degrees and DeltaS degrees prediction models, respectively. To evaluate the error of the models represented in the time scale, several chromatograms were simulated using experimental Pro ezGC and theoretically calculated thermodynamic data. Afterward a standard deviation of retention time residuals was calculated. It was found out that, although the standard deviation varies from one chromatographic condition to another, the ratio between the standard deviation and the maximum available separation space for the particular set of organic compounds remains constant and was around 5% of the maximum separation space available at selected chromatographic conditions. Our prediction model was able to accurately differentiate between the retention times of the consecutive compounds in the n-alkanes, 1-alkenes, and 2-alkenes homological series.