Various manufacturers and suppliers of GC columns provide lists of stationary phase properties and characterization parameters. One reference we provide here is to a document from Supelco that contains McReynolds constants for many stationary phases, as well as information on how such values are obtained and can be used.
As explained in Module 2.8 it is easy to characterize a stationary phase in terms of five McReynolds’ constants. All it requires is to measure isothermal retention indices (at 120oC) of five probes (i.e. benzene, 1-butanol, 2-pentanone, 1-nitropropane, and pyridine).
Apolar columns show low McReynolds’ constants, whereas polar columns exhibit high values. The average of the five McReynolds’ constants is a common indication for column polarity. Columns with similar McReynolds’ constants are expected to exhibit similar retention and selectivity.
In contrast it is difficult to characterize analytes Establishing their McReynolds’ constants would require measuring retention indices on six different columns, including one with squalane as stationary phase – a rare type of column nowadays.
In (mathematical) detail #
The basic equation is a linear-free-energy model of the form
\Delta I_{i,sp}= a_i x_{sp} +b_i y_{sp} +c_i z_{sp} +d_i u_{sp} +e_i s_{sp}where the parameters a_i through e_i are characteristics of the analyte and x_{sp} through s_{sp} characterize the stationary phase. \Delta I_{i,sp} is the (isothermal) Kováts retention index of analyte \i on the stationary phase in question at 120oC, minus its retention index on squalane.
Rohscneider, and later McReynolds, chose a set of reference analytes. The McReynolds set is more practical and consists of benzene (retention index on squalane 653), 1-butanol (590), 2-pentanone (627), nitropropane (652) and pyridine (699). The smart choice instigated by Rohrschneider to assign one term specifically to one of the reference analytes makes the characterization of a new stationary phase extremely easy, as is obvious from the following equation.
The final result does not require any matrix notation:
Each of the stationary phase parameters follows directly from the retention-index increment of one of the references. We only need to inject these five analytes and a mixture of n-alkanes to determine the retention indices and from these the parameters x_{sp} through s_{sp} .
The characterization of additional analytes is more complicated. We then have to deal with a full set of five parameters and five unknowns as follows.
where SP denotes the matrix of coefficients of five previously characterized stationary phases.
The solution of this equation requires inversion of the SP matrix:
The difficulty here is not so much the inversion and manipulation of the matrix, as well as collection the data on six different columns, i.e. the five columns that make up the SP matrix, plus a squalane column. The latter has become largely obsolete, as it shows limited stability due to bleeding and oxidation.
For more on GC stationary phases see Section 2.1.4
For more on column characterization see Module 2.8
