Important features of retention indices determination in reversed-phase high performance liquid chromatography

The features of the retention indices determination in reversed phase high perfor­mance liquid chromatography (RP HPLC) are discussed, namely the different possibilities of the application of the recurrent relations. At first, not the experimental, but the precalculated values are recommended as the retention parameters of non-sorbable compound (dead time, t0) in RP HPLC. It is because just the precalculated values provide the linearity of rela­tionships lg (tR – t0) = anC + b, where nC is the number of carbon atoms in molecules of ho­mologs selected as reference compounds (n-alkyl phenyl ketones). Besides the known Peterson and Hirsch equation for the theoretical evaluation of t0 using the retention times of three consecutive homologs, the following recurrent relations are also recommended: tR(nС – 1) = atR(nC) + b, from where it follows that t0 = lim (tR)½(nC ® 0) = b / (1 – a) under the condition that 0 < a < 1. Another area of the effective application of the recurrent relations is caused by the following fact: the ranges of the concentrations of the eluent’s organic constituents during the separation of hydrophilic analytes and hydrophobic reference n-alkyl phenyl ketones may not overlap, or over­lap only partially. In such cases, expanding the data arrays seems to be reasonable using the extrapolated values both for the target analytes, and (preferably) for the reference compo­unds. For these purposes, another kind of recurrent relations can be recommended, name­ly tR(C ± DC) = atR(C) + b, DC = const. All recommendations on the application of recurrent relations in RP HPLC are illustrated with examples.