Rtx015-16-628

13 / 629

11
GC COLUMNS
Selecting a GC Column
Separation Factor (α)
Choosing the right stationary phase is the first step toward optimizing your GC separation. It is the most
important decision you will make because separation factor (α) has the greatest impact on resolution, and it
is strongly affected by stationary phase polarity and selectivity.
Stationary phase polarity is determined by the type and amount of functional groups in the stationary phase.
Structures for Restek stationary phases are presented in order of polarity on When choosing a col-
umn, consider the polarity of both the stationary phase and your target analytes. If the stationary phase and
analyte polarities are similar, then the attractive forces are strong and more retention will result. Greater re-
tention often results in increased resolution. Stationary phase polarity strongly influences column selectivity
and separation factor, making it a useful consideration when selecting a column.
Stationary phase selectivity is defined by IUPAC as the extent to which other substances interfere with the
determination of a given substance. Selectivity is directly related to stationary phase composition and how it
interacts with target compounds through intermolecular forces (e.g., hydrogen bonding, dispersion, dipole-
dipole interactions, and shape selectivity). As methyl groups in the stationary phase are replaced by different
functionalities, such as phenyl or cyanopropyl pendant groups, compounds that are more soluble with those
functional groups (e.g., aromatics or polar compounds, respectively) will interact more and be retained lon-
ger, often leading to better resolution and increased selectivity. In another example of the effect of stationary
phase-analyte interactions, an Rtx®-200 stationary phase is highly selective for analytes containing lone pair
electrons, such as halogen, nitrogen, or carbonyl groups, due to interactions with the fluorine pendant group
in this phase. Selectivity can be approximated using existing applications or retention indices (Table I), mak-
ing these useful tools for comparing phases and deciding which is most appropriate for a specific analysis.
Stationary phase polarity and selectivity also affect how much sample loading capacity the column will have
for a particular analyte; the more soluble an analyte is in the stationary phase, the greater the sample loading
capacity will be for that analyte. For example, a nonpolar stationary phase will have higher sample loading
capacity for a nonpolar compound (e.g., pentane) than for a polar compound (e.g., ethanol).
Phase Benzene Butanol Pentanone Nitropropane
Rtx-1 651 651 667 705
Rtx-5/Rtx-5MS 667 667 689 743
Rtx-20 711 704 740 820
Rtx-1301/Rtx-624 689 729 739 816
Rtx-35 746 733 773 867
Rtx-200 738 758 884 980
Rtx-50 778 769 813 921
Rtx-1701 721 778 784 881
Rtx-65TG 794 779 825 938
Rtx-225 847 937 958 958
Stabilwax 963 1,158 998 1,230
Table I: Retention Indices for Restek Phases
Restek’s Learning Network
Learn more at
Solutions to fit every level of expertise, budget,
schedule, and analytical topic.
eminars