Theoretical analysis of the retention behavior of alcohols in gas chromatography

Theoretical prediction of the Kovat's retention index for oxygen-containing organic compounds using novel topological indices

For the retention index of polar compounds, polar groups in molecules would participate in polar interactions between eluents and stationary phases and thus would be expected to make large and separate contributions to the total retention index (RI). The characterization of the structural feature will help to elucidate the quantitative structure-retention relationship (QSRR). In this paper, on the basis of the PEI index previously developed by Cao, two novel molecular polarizability effect index, modified molecular polarizability index (MPEI(m)) and modified inner molecular polarizability index (IMPEI(m)) were proposed to predict the GC retention of a variety of oxygen-containing organic compounds with diverse chemical structures on OV-1 and SE-54 stationary phases. The sets of molecular descriptors were derived directly from the structure of the compounds based on graph theory. Simple linear regression equations between the RI and the topological indices were established for each stationary phase separately (R>0.99). Statistical analysis showed that the QSRR models have high internal stability and good predictive ability for external groups. The molecular properties known to be relevant for GC retention data, such as molecular size, branching and polar functional groups were well covered by the generated descriptors. The models with topological indices were compared with those based on quantum-chemical descriptors. It is observed that topological indices produce better correlations with Kovat's retention index. The results indicate the efficiency of presented indices in the structure-retention index correlations of complex compounds with polar multi-functional groups.

Quantitative structure-(chromatographic) retention relationships

Since the pioneering works of Kaliszan (R. Kaliszan, Quantitative Structure-Chromatographic Retention Relationships, Wiley, New York, 1987; and R. Kaliszan, Structure and Retention in Chromatography. A Chemometric Approach, Harwood Academic, Amsterdam, 1997) no comprehensive summary is available in the field. Present review covers the period of 1996-August 2006. The sources are grouped according to the special properties of kinds of chromatography: Quantitative structure-retention relationship in gas chromatography, in planar chromatography, in column liquid chromatography, in micellar liquid chromatography, affinity chromatography and quantitative structure enantioselective retention relationships. General tendencies, misleading practice and conclusions, validation of the models, suggestions for future works are summarized for each sub-field. Some straightforward applications are emphasized but standard ones. The sources and the model compounds, descriptors, predicted retention data, modeling methods and indicators of their performance, validation of models, and stationary phases are collected in the tables. Some important conclusions are: Not all physicochemical descriptors correlate with the retention data strongly; the heat of formation is not related to the chromatographic retention. It is not appropriate to give the errors of Kovats indices in percentages. The apparently low values (1-3%) can disorient the reviewers and readers. Contemporary mean interlaboratory reproducibility of Kovats indices are about 5-10 i.u. for standard non polar phases and 10-25 i.u. for standard polar phases. The predictive performance of QSRR models deteriorates as the polarity of GC stationary phase increases. The correlation coefficient alone is not a particularly good indicator for the model performance. Residuals are more useful than plots of measured and calculated values. There is no need to give the retention data in a form of an equation if the numbers of compounds are small. The domain of model applicability of models should be given in all cases.

Theoretical analysis on retention behavior of pigments in reversed-phase high-performance liquid chromatographic (HPLC)

Quantitative structure-retention relationship (QSRR) models have been used successfully to predict and explain retention behavior of pigments in reversed-phase high-performance liquid chromatography (HPLC). The semi-empirical quantum chemical method (PM3) in Gaussian98 was employed to calculate a set of molecular descriptors of pigments. Using multiple linear regression (MLR), we obtained empirical functions with high correlation coefficient between retention times and quantum-chemical descriptors. This analysis indicated that the proposed QSRR models were satisfactory.

Experimental and theoretical study of the electrochemical behavior of N-protonated dopamine at Nafion multiwalled carbon nanotubes (MWNTs) modified glassy carbon (GCE) electrode

The electrochemistry of N-protonated dopamine (DAH+) was studied by cyclic voltammetry (CV) at a glassy carbon electrode modified by Nafion multiwalled carbon nanotubes (MWNTs) in phosphate buffers (pH 5.50) and the results show that the standard electrode potential of half reaction for DAH+(O),H+/ DAH+(R) is 0.74 V. Calculations are performed using DFT B3LY/6-31G(d,p) for nine conformers of N-protonated dopamine (DAH+(R)) and three conformers of oxidized N-protonated dopamine (DAH+(O)). The energies of solvation and sum of electronic and thermal free energies of DAH+(R), DAH+(O), p-benzoquinone (p-BQ), and p-hydroquinone (p-HQ) are calculated at same level. The theoretical weighted average of the standard electrode potential (0.732 V) for DAH+(O),H+/ DAH+(R), using the transformed Gibbs free energies of the stable DAH+(R) and DAH+(O) with a p-BQ, H+/p-HQ reference electrode, is consistent with the experimental one of 0.74 V.Key words: dopamine, DFT, cyclic voltammetry, standard electrode potential.