Statistical Analysis of Temperature Dependence of Retention indices. A new polarity scale?

Károly Héberger¹, Miklós Görgényi²

¹Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences H-1525 Budapest, P. O. Box 17, Hungary, E-mail: heberger@chemres.hu;

Fax No: +36 1 325 75 54; Phone: +36 1 438 04 90

²Institute of Physical Chemistry, University of Szeged,

H-6701 Szeged, P. O. Box 105, Hungary, E-mail: gorgenyi@chem.u-szeged.hu;

Fax No: +36 62 544652; Phone: 36 62 544628

Our earlier study revealed that molar heat capacity difference could also be determined from retention data [1]. Temperature dependence of Kováts retention indices has also been measured for carbonyl compounds [2,3]. Present study deals with temperature dependence of Kováts indices for McReynolds test compounds on four stationary phases of different polarity.

Linear, non-linear and multilinear (cf. Eq. 1) models were compared to describe the temperature dependence of Kováts indices. A detailed statistical analysis showed that the novel empirical relationship:

I = A + B/T + C*lnT (1)

is much better than the earlier suggested linear and Antoine type relationships [4].

The temperature for minimum (T_min = B/C) was correlated with various quantum chemical parameters (HOMO, LUMO, dipolmoment, SASA, polarizability, Vm, etc.). However, from among the quantum chemical parameters only polarizability can be accepted showing a definite trend.

McReynolds probes show interesting pattern in their temperature dependence. n-Alkanes were used for comparison. It can be established that the enthalpy of solution for n-alkanes decreases with increasing polarity of the stationary phase while this tendency cannot be observed for McReynolds probes owing to the various types of intermolecular interactions between the various functional groups.

The new enthalpy term defined in ref. [4] and calculated from the temperature dependence of retention indices for nitropropane holds all the necessary information on polarity as defined by McReynolds.

References

[1] K. Héberger and M. Görgényi: Estimation of Molar Heat Capacities in Solution from Gas Chromatographic Data J. Chromatogr. Sci. 38, (2001) 113-120.

[2] K. Héberger, M. Görgényi: Principal Component Analysis of Kováts Indices for Carbonyl Compounds in Capillary Gas Chromatography J. Chromatogr. A, 845, (1999) 21-31.

[3] K. Héberger, T. Kowalska and M. Görgényi: Determination of Gibbs Free Energy of One Methylene Unit From Kováts Retention Index - Boiling Point Correlations on DB-210 Stationary Phase Acta Chromatographica, No. 9, (1999) 25-37.

[4] K. Héberger, M. Görgényi and T. Kowalska, Temperature Dependence of Kováts Indices in Gas Chromatography Revisited. J. Chromatogr. A, 973, (2002) 135-142.