Logical criteria for estimating gas chromatographic retention indices of organic compounds using their...

There is no unified equation that would correlate gas chromatographic retention indices of any organic compounds with their normal boiling points (at atmospheric pressure). Nevertheless, RI estimation with acceptable accuracy is possible using principal physicochemical properties, namely, boiling points, relative densities, and indices of refraction. The main feature of the algorithms proposed is the application of a logical criteria for comparing so-called indices of boiling points I(T_b), molecular weights I(M), and molar refractions I(MR_D). If the comparison of these indices confirms the approximate equality of any two or all three of them (that is typical for nonpolar organic compounds), then as the best estimate of GC retention indices (RI_calc) we can accept I(T_b) values without any additional calculations. Such comparison is formally equivalent to the application of logical criteria for I(T_b), I(M), and I(MR_D) values. For polar analytes, the relation I(T_b) > I(MR_D) > I(M) is valid, and in these cases the inequality I(T_b) > RI_calc > I(M) appeared to be correct. Instead of large intervals between I(T_b) and I(M) for estimating RI_calc, the use of the relation RI_calc ≈ (1 - a)I(T_b) + aI(MR_D) + b is proposed. The indices of molecular weights are not used for the calculations directly, but they are needed just for application of the logical criteria. The average accuracy of RI estimation by the method proposed is 9-16 index units for nonpolar compounds of any chemical nature and up to 27 for polar analytes. This high accuracy is comparable to that of the more sophisticated modern algorithms. The restriction of this approach is the impossibility of applying it to poly- and perfluoroorganic compounds.