Estimation of retention indices of higher dimethyl paraffins by a graphical method

A graphical method for estimating retention indices in gas chromatography of higher dimethyl paraffins than C10 was investigated. In this method, the retention indices of these compounds are indirectly obtained in the form of "retention index component of methyl radical", (ΔIMeR), of methyl paraffins, which are calculated by subtracting linear carbon number, (x), multiplied by a factor of 100 from retention indices of mono- or dimethyl paraffins. Three rules are derived from the relationship between ΔIMeR and x, those are as follows; (1) ΔIMeR values of monomethyl paraffin homologues in regard to substituted position of a methyl group tend to decrease exponentially with increase of the x, and to approach to respective converged values. These values of 2-, 3-, 4-, 5-, and 6-methyl paraffin homologues, for example, are 63.5, 68.8, 57.9, 50.9, and 46.4, respectively, for a squalane column at 80°C. The same tendency was observed in the case of dimethyl paraffins. (2) when a dimethyl paraffin has one or more methylene groups between the two methyl groups, its ΔIMeR value is approximately equal to a sum of the converged ΔIMeR of the two corresponding monomethyl paraffins, and the difference between them approaches to zero with increase of x. (3) the ΔIMeR value of k, k-dimethyl paraffin (k≥3) is approximately equal to the double of ΔIMeR of k-monomethyl paraffin having the same carbon number, and the difference between them approaches to zero with increase of x. For this work, the retention indices of lower monoor di-methyl paraffins than C10 were cited from literatures, and calculated their ΔIMeR for a squalane column at 80°C. These values are plotted against x, and extrapolated on the basis of the rules mentioned above. The ΔIMeR-x curves can be drawn accurately by drawing the straight lines which denote the rate of variation of ΔIMeR-x curves. In the case of monomethyl paraffin homologues such as 6- or 7-methyl, of which retention data is not at hand, their ΔIMeR-x curves can be estimated by using ΔIMeR values of another lower monomethyl paraffin homologues such as 5- or 4-methyl. The similar estimation technique can be applied to dimethyl paraffins. By comparing the retention indices of C13 dimethyl isomers calculated by means of this method with experimental values in a literature, the standard deviation of the differences between them is within 2 i. u.