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To evaluate the behaviour of a stationary phase, a comparison of the Kovats indexes for five reference compounds belonging to different structural classes is made on the studied phase as well as on squalane, chosen as the reference phase for this calculation. The five indexes on a column using squalane, the only reproducible nonpolar phase since it is formed from a pure material, have been established once and for all (Table 2.2).

The five McReynolds constants for a given stationary phase are obtained (Eq. (2.7)) by calculating the differences observed for each of the substances tested between their Kovats indexes on squalane (I_Squalane) and on the stationary phase being studied (I_Phase):

(2.7)

Table 2.2 McReynolds constants (∆I) for several stationary phases normalized to squalane.

Stationary phase	Benzene X′	1‐butanol Y′	2‐pentanone Z′	Nitropropane U′	Pyridine S′
Squalane	0	0	0	0	0
SPB‐Octyl	3	14	11	12	11
SE‐30 (OV‐1)	16	55	44	65	42
Carbowax 20M	322	536	368	572	510
OV‐210	146	238	358	468	310
Kovats index for the five reference compounds above (X′, Y′, Z′, U′, S′) on squalane
Isqualane	653	590	627	652	699

The sum of the five calculated values, using Eq. (2.7), has been used to define the overall polarity of the phase under study.

Each of the test compounds yields specific information regarding the stationary phase: benzene for the inductive effect, pyridine for H⁺ proton accepting, butanol for hydrogen bonding, nitropropane for dipolar interactions, etc.

These constants, which are related to molecular structures, allow an appreciation of the interactive forces between stationary phase and solute as a function of compound class. A high index suggests that the stationary phase strongly retains the compounds that contain the corresponding organic functions. This generally leads to improved selectivity for this type of compound. Thus, to separate an aromatic hydrocarbon from a mixture of ketones, we would select a stationary phase whose McReynolds constant for benzene is sufficiently different from that for butanone. These differences in retention indexes are provided by manufacturers for use by chromatographers (Table 2.2). McReynolds constants have replaced Rohrschneider constants, which were based upon the same principle but used certain different reference compounds.