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Fig. 1. Plot of logarithm-adjusted retention timeversusKovats retention index: isoamyl
acetate at 120
◦
C (Grob&Barry, 2004).
Selectivityof stationaryphases canbedeterminedby comparing thevalues I of a soluteon a
non-polar phase, such as squalane OV 101 (I = 872) with corresponding I values of 1128
associatedwithmorepolar columns containingCarbowax 20M, for example. This difference
of 256units shows the superior retentionof theCarbowax 20M column. Specifically, isoamyl
acetate elutes between n-C11 and n-C12 on a Carbowax 20M columns, but much faster on
OV 101, where it elutes after n-octane. Retention indices in GC normalize instrumental
variables, allowing retention data obtained by various systems to be compared. For
example, isoamyl acetatewith a retention index of 1128will elute between C11 andC23 in
the same chromatographic conditions. Retention indices are also very useful for comparing
the relative elution order of the series of analytes on a specific column to a given
temperature, and comparing the selectivebehaviour of twoormore columns.
5.McReynoldsclassificationof stationaryphases
As shown, the most important criteria upon which stationary phases choice is made for a
certain analysis is the polarity of the phase and of analyzed compounds. Numerical
expression of this polarity, whichwould facilitate the choice, is not possible because there is
no physical size that can be associated directlywith the polarity (to some extent the dipole
moment couldbe this size). For this reason, to express the stationaryphase polarity they are
using some reference compounds, which are arbitrarily chosen. Such a system was
developed by Rohrschneider and later developed by McReynolds, based on the Kovats
retention indices andusing squalane as reference stationaryphase, assigning it null polarity.
A total of five reference compounds were selected: benzene, 1-butanol, 2-pentanona,
