Over the past several years, HPLC columnmanufac-
turers have been creating new stationary phases in
attempts to address some of the separation prob-
lems encountered by analytical chemists.
Stationary phases in traditional reversed phase
columns are strictly non-polar alkyls, like C18. In
contrast, many newer specialty reversed phase
columns have stationary phases that are primarily
alkyls, but with some secondary polar functionality.
The polar functionality offers several advantages,
including: unique selectivity, enhanced retention of
polar compounds, and compatibility with complete-
ly aqueousmobile phases.
These specialty reversed phase columns can differ
either in the type of polar group they incorporate
or in how the polar group is incorporated into the
stationary phase. Restek offers two specialty
reversed phase columns that represent two different
approaches to introducing secondary polar groups
into a straight chain alkyl ligand (Figure 1). The
stationary phase in the Ultra Aqueous C18 column
has small polar groups attached to the silica sur-
face, between the C18 chains. In contrast, the Ultra
IBD stationary phase is a “polar embedded” type
• 4 •
800-356-1688
by Terrence S. Reid, HPLC Innovations Chemist
SpecialtyReversedPhaseHPLC
Columns for Polar Analytes
Ultra Aqueous C18 andUltra IBDColumns
Solve RetentionDilemmas
✔
Both columns provide sharp peaks for basic analytes.
✔
Both columns compatiblewith 100% aqueousmobile phases.
✔
Complementary selectivity for acidic and basic analytes.
stationary phase, because its polar groups are
embeddedwithin a straight alkyl chain.
One drawback to these specialty columns is that
their potential formixedmode interactionsmakes
it more difficult to predict which columnwill per-
form best for a particular application. With this in
mind, we used a series of simple tests to directly
compare the performance characteristics of Ultra
Aqueous C18 and Ultra IBD columns. From the
results of these tests, we can offer some useful
guidelines for selecting a specialty reversed phase
column.
The first test measured the hydrophobic retention
of each column, using a samplemixture of com-
pletely nonpolar analytes and amobile phase con-
taining a high proportion of organic solvent. For
pure alkyl stationary phases, hydrophobic retention
usually is directly proportional to the percent car-
bon (%C) in the bonded phase silica, if the phases
are bonded on silica particles of comparable sur-
face area. Figure 2 shows that the hydrophobic
retention of Ultra Aqueous C18 columns is approxi-
mately twice that of Ultra IBD columns, based on
capacity factors for pyridine, despite the two bond-
ed phases' similar surface area and%C (Ultra
Aqueous C18: 100Å pores, 14%C; Ultra IBD: 100Å
pores, 12%C). The hydrophobic retention of Ultra
Aqueous C18 columns is equivalent to that of con-
ventional C18 columns with the same surface area
and%C. The considerably reduced hydrophobic
retention of Ultra IBD columns can be attributed to
the embedded polar group in the stationary phase
shielding the lower portion of the alkyl chain from
the nonpolar analytes.
We compared the columns’ base deactivation by
measuring the peak shape for a basic analyte, pyri-
dine (Figure 3). Both Ultra Aqueous C18 and Ultra
IBD columns show excellent base deactivation, with
pyridine peak symmetry values better than those for
highly base-deactivated C18 phasesmade through
conventional chemistry. Although they are similarly
base-deactivated, Ultra Aqueous C18 columns
exhibit much greater retention of pyridine than do
Ultra IBD columns.
Next, we compared Ultra Aqueous C18 and Ultra
IBD columns’ ability to separate small carboxylic
acids. It is difficult for conventional reversed phase
columns to retain thesemolecules. A very weak,
highly aqueousmobile phase is required. Many C18
phases are not compatible with highly aqueous
mobile phases, and show a gradual or sudden loss
of retention that is attributed to “chain folding” or
“phase collapse.” Both Ultra Aqueous C18 and Ultra
IBD columns are completely compatible with 100%
aqueousmobile phases, as shown in Figure 3A.
Neither column showed any loss of retention, even
aftermobile phase flowwas temporarily stopped.
(Absence of pressuremaximizes the potential for
phase collapse, thus exposure to 100% aqueous
mobile phase under no flow is themost extreme
test of phase integrity.) This comparison did reveal
Conventional Alkyl
Ultra Aqueous C18
Ultra IBD
Figure 1
The stationary phase in anUltra Aqueous
C18 column has small polar groups attached
to the silica surface; in the Ultra IBD station-
ary phase polar groups are embedded in the
alkyl chain.
Peak List:
Conc. (mg/mL)
1. uracil
0.02
2 benzene
3.00
3. naphthalene
0.50
4. biphenyl
0.06
LC_0233
1
2
3
4
Sample:
Inj.:
1.3µL
Solvent: methanol:water
(75:25, v/v)
Mobile Phase:
A: water
B: methanol
Isocratic: 80%B
Flow:
1.0mL/min
Temp.:
ambient
Det.:
UV@ 254nm
LC_0234
Column:
Ultra Aqueous C18
Ultra IBD
Catalog #:
9178565
9175565
Dimensions:
150 x 4.6mm
150 x 4.6mm
Particle Size:
5µm
5µm
Pore Size:
100Å
100Å
Figure 2
Hydrophobic retention for Ultra Aqueous C18 columns is approximately twice that for Ultra IBD
columns, despite similar surface areas and% carbon.
polar group
polar group
