The linearity of the log-plot of adjusted retention times versus carbon number

Abstract

The linearity of the log (adjusted retention time of normal alkanes) versus carbon number is of interest because of the possibility to calculate retention indices by logarithmic interpolation over a wider range of carbon atoms than as defined by Kovats who considered the adjusted retention times of two neighbouring (even) normal alkanes. Although by some authors this logplot is considered to have a certain curvature, at least in a part of the plot it appears from our data that no curvature can be observed at all. One of the reasons that curvature has been found is due to the general assumption in Gas Chromatography that the adjusted retention time of methane equals zero. This induces an error which is responsible for the observed curvature in the initial region of the logplot. There is a way to get around this error viz by calculating the column dead volume point from three “standard” normal alkane retention times. On this base retention indices of other normal alkanes were calculated and appeared to be multiples of 100 within a small non systematic experimental error. An Algol 60 programme as used for the calculation of the retention indices is added. The use of a standard programme is almost imperative in this sort of calculations and will add to a more accurate standardisation of retention index determinations as carried out in different laboratories.