Strategic column choices can improve lab productivity by assuring that speed and
resolution are optimized. While the number of choices available can be daunting,
consideration of the resolution equation variables—selectivity, retention (capacity),
and efficiency—simplifies the decision. Selectivity determines which stationary phase is
most appropriate, and it can be approximated using retention indices or existing appli-
cations. Once the phase has been chosen, physical dimensions (internal diameter, film
thickness, length) can be selected based on retention and efficiency. Understanding how
selectivity, retention, and efficiency influence separations allows analysts to make
effective, informed choices and quickly select the best column for specific separations.
www.restek.com 23
Retention
Selectivity
R = resolution
L = column length
H = HETP
k = capacity factor
α
= selectivity
R =
X X
1 L k
α
-1
4 H k+1
α
Efficiency
Chroma
BLOG
raphy
Topical and timely insights from top
chromatographers.
Visit us at
blog.restek.com
Selectivity,
α
The selectivity of the capillary column is directly related to how the analyte molecule
interacts with the stationary phase being considered. If the analyte strongly interacts
with the stationary phase, it can be said that strong intermolecular forces exist. These
intermolecular forces of attraction between the analyte and the stationary phase are a
function of the structure of both the analyte molecule and the stationary phase. If these
two structures are similar, then the attractive forces are strong. If they are dissimilar,
then analyte to stationary phase attraction is weak, and less retention is observed.
Therefore, when selecting a stationary phase, knowledge of the structure of the analytes
of interest and the stationary phase is crucial. The reference table on page 27 provides
the chemical structure of Restek’s most common stationary phases.
An example of selectivity can be shown using benzene and butanol (both have nearly
the same boiling point) eluting through the 20% diphenyl/80% dimethyl polysiloxane
stationary phase (Rtx®-20). The benzene molecule will dissolve into the stationary
phase more readily than the butanol based on the concept that “likes dissolve likes”.
Since benzene solvates more readily with the stationary phase, it has more interactions
with the stationary phase as it elutes through the column. Therefore, the elution order
of these two compounds on the Rtx®-20 stationary phase will be butanol first and
benzene second.
As methyl groups are replaced by different functionalities such as phenyl or cyanopropyl
pendant groups, the selectivity of the column shifts towards compounds that will have
a better solubility in the stationary phase. For example the Rtx®-200 stationary phase
provides high selectivity for analytes containing lone pair electrons, such as halogens,
nitrogen, or carbonyl groups. Polyethylene glycol columns, such as the Stabilwax® and
Rtx®-Wax columns are highly selective towards polar compounds such as alcohols.
Again using the example above, the butanol more readily solvates into the polyethylene
glycol stationary phase; therefore, the butanol will have more interaction with the phase
and elute after benzene.
Website : www.chromtech.net.au E-mail : info@chromtech.net.au TelNo : 03 9762 2034 . . . in AUSTRALIA Mar