An Accurate Method for the Estimation of Equivalent Temperature in Programmed Temperature Gas Chromatography

Abstract

Equivalent temperature (T_eq) is the temperature at which the isothermal retention time (t_R(iso)) equalto the temperature-programmed retention time (t_R(TPGC)). The correct Teq is useful for the identification ofcompounds analyzed with temperature-programmed gas chromatography (TPGC) by using isothermal database(e.g. Kováts retention index (I) or equivalent chain length (ECL)).

The equation (1) proposed by Krisnangkura et al. can be used to calculate T_eq of fatty acid methyl esters(FAMEs) from Chinese mustard seed oil (Brassica juncea) analyzed on BPX-70 in one-, two-, three- and four stepsTPGC correctly.

In(t_R-t_M/t_M) = a + bz +c/T +dz/T        (1)

where a, b, c and d are thermodynamically related to column constants; T (K) is the T_eq; z is the carbon numberor equivalent chain length (ECL); t_R and t_M are retention time of solute and gas hold-up time, respectively.

From these experimental results, all the t_R values obtained at the T_eq (t_R(Teq)) are very close to thetR(TPGC) values. The highest difference for each chromatogram is less than 0.5 %. The ECL values calculated fromdifferent methods; including ECL_(TPGC), ECL_(Teq) and the ECL from the graphical method at the T_eq (ECL_(G)), arealso in good agreement. Therefore, it can be proposed that the presented method is suitable for and can facilitatein the gas chromatographic identification of FAMEs as well as other general organic compounds in the TPGCfrom the database of ECL or Kováts retention index.

Keywords : Equivalent Chain Length / Equivalent Temperature / Fatty Acid Methyl Ester /Retention Index / Temperature-programmed Gas Chromatography