Restek GC Column Selection Guide

Section II: Stationary Phase Selectivity

When purchasing a new column, the most important consideration is the stationary phase. There are many different interactions that occur between the analytes and the functional groups of the stationary phase. These interactions contribute more to the overall results of the analysis than any other factor in the column. That is why it is important to understand as much about your column and sample as possible.

Table I shows the chemical structure of most common stationary phases. Changes in selectivity can be observed by using a column with different functional groups as well as increasing the percentage of substitution of those functional groups. The non-polar Rtx®-1 phase will preferentially retain non-polar compounds compared to polar compounds such as alcohols. As non-polar methyl units are substituted with polar functionalities such as phenyl and cyanopropyl units, the selectivity of the column shifts towards more polar compounds. In turn, non-polar compounds are retained less as there are less overall methyl units for the non-polar compounds to interact with. The Rtx®-200 stationary phase contains triflouropropyl units which provide high selectivity for analytes containing lone pair electrons, such as nitro and carbonyl groups. Polyethylene glycol columns, such as Stabilwax® and Rtx®/MXT®-WAX, are polar and are highly selective towards polar compounds such as alcohols.

Table I: Stationary Phase Structures and Properties

Rtx®/MXT®-1
100% dimethyl polysiloxane
Stable to 360°C
Polarity: non-polar
Uses: solvents, petroleum products, pharmaceutical samples, waxes

Rtx®/MXT®-1301, Rtx®/MXT®-624
6% cyanopropylphenyl - 94% dimethyl polysiloxane
Stable to 280°C
Polarity: slightly polar
Uses: volatile compounds, insecticides, residue solvents in pharmaceutical products

Rtx®/MXT®/XTI®-5
5% diphenyl - 95% dimethyl polysiloxane
Stable to 360°C
Polarity: non-polar
Uses: flavors, environmental samples, aromatic hydrocarbons

Rtx®/MXT®-35
35% diphenyl - 65% dimethyl polysiloxane
Stable to 300°C
Polarity: intermediately polar
Uses: pesticides, Aroclors, amines, nitrogen containing herbicides

Rtx®/MXT®-20
20% diphenyl - 80% dimethyl polysiloxane
Stable to 310°C
Polarity: slightly polar
Uses: volatile compounds, alcohols

Rtx®/MXT®-50
50% phenyl - 50% methyl polysiloxane
Stable to 340°C
Polarity: intermediately polar
Uses: triglycerides, phthalate esters, steroids, phenols

Rtx®/MXT®-1701
14% cyanopropylphenyl - 86% dimethyl polysiloxane
Stable to 280°C
Polarity: intermediately polar
Uses: pesticides, Aroclors, alcohols, oxygenates

Rtx®/MXT®-200
trifluoropropylmethyl polysiloxane
Stable to 360°C
Polarity: selective for lone pair electrons
Uses: environmental samples, solvents, Freons ,drugs, ketones, alcohols

Rtx®/MXT®-65TG
65% diphenyl - 35% dimethyl polysiloxane
Stable to 370°C
Polarity: intermediately polar
Uses: triglycerides, rosin acids, free fatty acids

Rtx®-2330
90% biscyanopropyl - 10% cyanopropylphenyl polysiloxane
Stable to 275°C
Polarity: very polar
Uses: FAMEs, cis/trans and dioxin isomers, rosin acids

Stabilwax®/MXT®-WAX
Carbowax® PEG
Stable to 250°C
Polarity: polar
Uses: FAMEs, flavors, acids, amines, solvents, xylene isomers

Rtx®-225
50% cyanopropylphenyl - 50% phenylmethyl polysiloxane
Stable to 260°C
Polarity: polar
Uses: FAMEs, carbohydrates

Table II (below) shows retention indices for the stationary phases shown in Table I. Retention indices are mathematical derivations indicating the elution point of a probe with respect to two hydrocarbons. For example, if the retention index for benzene was 650, then it would elute halfway between C6 (RI=600) and C7 (RI=700).

Table II: The retention indices for each phase illustrate the differences in selectivity for a variety of compounds.

Phase	Benzene	Butanol	Pentanone	Nitropropane
Rtx/MXT-1	651	651	667	705
Rtx/MXT-5/XTI-5/Rtx-5MS	667	667	689	743
Rtx/MXT-1301/624	689	729	739	816
Rtx/MXT-20	711	704	740	820
Rtx/MXT-35	746	733	773	867
Rtx/MXT-1701	721	778	784	881
Rtx/MXT-50	778	769	813	921
Rtx/MXT-65TG	794	779	825	938
Rtx/MXT-200	738	758	884	980
Rtx-225	847	937	958
Stabilwax/MXT-WAX	963	1158	998	1230