fastfacts07 - page 3

When choosing a stationary phase for capillaryGC separations, remember the saying
“like dissolves like.” The stationary phase is a nonvolatile liquid coated on the inside
of the column and acting as a solvent for the sample. Themore soluble the solute (your
analyte) is in the stationary phase, themore it is retained in the column.
Separations inGC are the result of the relative solubility and selective interactions of
the sample solute and column stationary phase. Table I shows the four main forces
responsible for solute-stationary phase interactions. The sum of all four serves as a
measure of the polarity of the stationary phase. Selectivity is the ability of a phase to
preferentially retain one compound over another based on specific solute-stationary
phase interactions and is determined by the type and amount of substituted functional
groups in the stationary phase.
Choosing the Best Phase for Your Sample
Retention indices (RI) are used tomeasure the overall stationary phase polarity. Reten-
tion indices onRtx®/MXT®-1701 columns are listed inTable II. They aremathematical
calculations used to indicate the elution point of a probe with respect to two straight-
chainhydrocarbons. Theprobes used tomeasureRI areof different functionalities, each
one designated tomeasure a specific solute-stationary phase interaction.As theRI for a
probe on a given phase increases, the degree of specific interaction increases, relative to
hydrocarbons.
Rtx®/MXT®-1701 columns are intermediate in polarity and are coated with a 14%
cyanopropylphenyl/86% dimethyl polysiloxane stationary phase. The 14% cyano
substitution imparts a dipole moment to this stationary phase. Solute-stationary phase
interactions occurring in the Rtx®/MXT®-1701 column include a strong dispersion
interaction, plus a selectivity toward aromatic compounds, proton donors or acceptors,
and compounds possessing a dipole moment. Rtx®/MXT®-1701 columns can provide
separationsof alcohols, nitriles, andhalogenatedmaterials that anonpolar phasecannot.
In summary, when selecting a stationary phase, choose a phase with functional
groups similar to those present in your analyte. For a versatile, intermediate-polarity
stationary phase, select an Rtx®/MXT®-1701 column. These columns are the best
choice for pesticides, solvents or drug compounds.
Table I: Selective Solute-Stationary Phase Interactions
Dispersion forces arise from electric intermolecular fields, which result in the
induction of in-phase dipoles. They are present in all stationary phases.
Orientation interactions occur between a stationary phase and a compound,
both of which possess a permanent dipole.
Induction interactions occur between a stationary phasewith a permanent
dipole and a compound that forms a dipole as a result of the interaction
with the stationary phase.
Hydrogen bonding occurs between a strong polar group (OH, NH) and a
compoundwith strong electronegativity (F, O, N atoms). Hydrogen bonding is the
strongest interaction force.
At-a-Glance
Product
Information
fromRestek
Rtx® -1701/MXT®-1701 Columns
Rtx®-1701/MXT ®-1701
14% cyanopropylphenyl/
86% dimethyl polysiloxane
14% 86%
C = N
(CH2) 3
– Si – O
CH3
– Si – O
CH3
Similar Phases
J&W:
DB®-1701
Supelco:
SPBTM-1701
Hewlett-Packard:
HP-1701, HP-624
Alltech:
AT®-1701
SGE:
BP-10
Chrompack:
CP-Sil® 19CB
Quadrex:
007-1701
Ohio Valley:
OV®-1701
PerkinElmer:
Elite® -1701
USPNomenclature:
G46
Table II: 14%Cyanopropylphenyl/86%Dimethyl Polysiloxane
Stationary Phase Retention Indices
RI probe RI Measured interaction
benzene 721 Electron density for aromatic& olefinic
hydrocarbons
n-butanol 778 Proton donor & acceptor capabilities
(alcohols and nitriles)
2-pentanone 784 Proton acceptor interaction (ketones,
ethers, esters, aldehydes)
nitropropane 881 Dipole interactions