32
LCGC LC COLUMN TECHNOLOGY SUPPLEMENT APRIL 2006
www.chromatographyonline.com
Recent Developments in Ion-Exchange
Columns for Inorganic Ions and Low
Molecular Weight Ionizable Molecules
Ion exchange is one of the older of the chromatographic techniques yet
each year new products continue to hit the market. In this paper, Chris
Pohl of Dionex will summarize some of the stationary phases that have
been developed for modern ion-exchange and ion chromatography. He
will focus on phase design and then turn his attention to new anion
and cation columns introduced in the last couple of years.
I
on-exchange chromatography is the
chromatographic technique most
widely used for the separation of
ionic and ionizable compounds. Classical
ion-exchange techniques have been used
for many years for the separation of inorganic cations and anions, amino acids,
organic acids, amines, and proteins. This
review will discuss general stationary phase
architecture and covers new columns introduced in the last couple of years intended
for the analysis of inorganic ions and low
molecular weight ionizable molecules.
Stationary Phase Architecture
Stationary phase construction for columns
in this category consists of eight basic
architectures: silane-based modification of
porous silica substrates; electrostaticagglomerated films on nonporous substrates; electrostatic-agglomerated films on
ultrawidepore substrates; polymer-grafted
films on porous substrates; chemically
derivatized polymeric substrates; polymerencapsulated substrates; ionic molecules
adsorbed onto chromatographic substrates; and step-growth polymers on polymeric substrates. I will now delve into the
differences of the eight approaches.
Silane-Based modification of porous silica substrates: Of the eight approaches,
Chris Pohl
Dionex Corp., Sunnyvale, California
Please direct correspondence to Chris
Pohl at [email protected].
silane-based modification of porous silica
substrates, although one of the first
approaches to be employed is now rarely
employed in the separation of small ions
when used in conjunction with conductivity
detection. However, this type of stationary
phase has seen a resurgence in the last few
years in the form of mixed-mode stationary
phases. Early versions of this approach simply mixed particles of silica separately
bonded with reversed phase ligands or ionexchanger ligands. Later versions used
simultaneous bonding of ligand mixtures.
The newer generation of mixed mode
phases makes use of a hydrophobic ligand
also containing an ionizable site (see the following for more information about ligand
structure used in mixed-mode columns).
Electrostatic-agglomerated films on
nonporous substrates: These were among
the first types of materials to be employed
in separation of small ions. Developed in
the early 1970s and first described by
Hamish Small of Dow Chemical (Midland,
Michigan), these materials have been the
mainstay of suppressor-based ion chromatography for many years. Originally
developed as a convenient means of producing low-capacity hydrolytically stable
materials when first generation suppressors
had limited capacity, this type of stationary
phase is now only used in guard columns
and concentrator columns where sample
capacity is not a major factor in column
design. Materials of this sort are constructed
with a nonporous polymeric substrate.
Early materials used a low cross-linked substrate but modern materials of this type
make use of high cross-linking to render the
stationary phase compatible with all common high performance liquid chromatography (HPLC) solvents. In principle, the
substrate could be composed of inorganic
materials such as silica, alumina, zirconia or
titania but, to date, no commercial exam-
APRIL 2006 LCGC LC COLUMN TECHNOLOGY SUPPLEMENT
www.chromatographyonline.com
Figure 1: Separation of ions using an anion-exchange phase designed for use with carbon
ples of such materials are known. The substrate is then derivatized to introduce
charged groups onto the surface of the substrate. Following derivatization, the substrate is brought into contact with a suspension of oppositely charged colloidal
particles to produce the final product.
Although this construction might sound
like something that would be inherently
unstable, in fact, such materials are nearly
indestructible when constructed using a
styrenic substrate with a colloidal film of
vinyl–aromatic ion-exchange material.
For the most part, materials of the second type of architecture have been
replaced by a higher capacity version of
the material: electrostatic agglomerated
films on ultrawide pore substrates.
Electrostatic-agglomerated films on
ultrawide pore substrates: Using architec-
ture similar to that described previously, but
making use of substrates with pore sizes in
the 1000–3000 Å range, it is possible to
construct substantially higher capacity materials. For example in a typical application
with a ultrawide pore substrate with pore
sizes large enough to accommodate a coating of ion-exchange colloid on the interior
and exterior surfaces, the resulting material
will exhibit six to eight times the capacity
achievable on an identical particle size nonporous analog (that is, 30–150 mequiv/mL
for the ultrawidepore format vs. 5–30
mequiv/mL for the nonporous format).
Given the increasing importance of high
capacity chromatographic materials and the
increasing use of high-capacity suppressor
devices for ion chromatography, this stationary phase architecture has seen wide use.
Polymer-grafted films on porous substrates: This type of material is widely used
to prepare high capacity packings where
cross-linking is not required for selectivity
control. Packing materials of this sort are
prepared through attachment of polymer
strands to the surface of a substrate. To prepare such materials, the substrate is either
prepared with polymerizable groups on the
surface or the surface is modified to introduce polymerizable groups. Resin,
monomer(s), and initiator are then allowed
to react to produce the grafted composite.
Incorporation of a cross-linking monomer
in the reaction mixture will produce a gel
with substrate particles suspended in the
gel much like fruit cocktail suspended in a
gelatin fruit salad. Because such a gel could
not be used as column packing material,
this type of stationary phase synthesis precludes the use of cross-linking agents for
selectivity control (unless cross-linker is
added after the graft step). In theory, such
materials could be prepared from either
polymer or silica substrates but in practice
only polymeric substrates are in commercial use. The IonPac CS18 column
(Dionex) described in the following is a stationary phase using this architecture.
Chemically derivatized polymeric substrates: This type of material tends to
involve proprietary chemistry, so the actual
chemistry used for the derivatization reaction is generally unknown. In general, packing materials of this sort are of rather substantial capacity, so they have come into
vogue in recent years with the general shift
toward materials of increasing capacity. The
Metrosep A Supp 8 column and the Metrosep A Supp 10 column (Metrohm,
Herisau, Switzerland) (see the following)
are both believed to be examples of materials of this type, although, no specific information on the preparation chemistry is
available from the manufacturer. The critical difficulty with such materials is the
requirement that the derivatization be con-
33
Figure 2: Separation of ions using an anionexchange phase designed for use with carbonate eluent systems. Column: IonPac AG22, AS22,
4 mm; eluent: 4.5 mM sodium carbonate–1.4
mM sodium bicarbonate; flow rate: 1.2 mL/min;
injection volume: 10 mL; detection: suppressed
conductivity, ASRS ULTRA II 4 mm, AutoSupression recycle mode; temperature: 30 °C. Peaks: 1
5 fluoride (5 ppm), 2 5 acetate (20 ppm), 3 5
chloride (10 ppm), 4 5 nitrite (15 ppm), 5 5
bromide (25 ppm), 6 5 nitrate (25 ppm), 7 5
phosphate (40 ppm), 8 5 sulfate (30 ppm).
strained to the surface. Reactions that take
place beneath the surface in the dense polymer matrix of the substrate will exhibit
sluggish mass transport and relatively poor
chromatographic performance. Early examples of this stationary phase architecture
exhibited relatively poor performance but
newer materials such as the IC SI-52 4E
column (Showa Denko, Kawasaki, Japan)
illustrate that high performance materials
can indeed be constructed in this manner.
Polymer-encapsulated substrates: Professor Gerard Schomberg of the Max
Planck Institute (Mulheim-Ruhr, Germany) pioneered this type of material as a
means of preparing reversed-phase materials
using alumina as the base material. Synthesis of polymer-encapsulated materials is
accomplished by combining the substrate, a
preformed polymer with residual double
bonds and a suitable free radical initiator
dissolved in solvent, stripping off the solvent to leave a polymer film on the substrate and then curing the film at elevated
temperature to yield a cross-linked film permanently encapsulating the substrate. The
technique was later adapted by Schomberg’s
group as a means of preparing a weak cation
exchange phase using a preformed butadiene–maleic acid copolymer as the encapsulating polymer. Introduction of this material fundamentally changed the focus of
stationary phase design for inorganic
cations, shifting the emphasis from strong
cation-exchange materials to weak cationexchange materials for most applications.
34
LCGC LC COLUMN TECHNOLOGY SUPPLEMENT APRIL 2006
www.chromatographyonline.com
6
5
2
3
7
4
8
1
9
Figure 3: Separation of common anions
along with disinfectant by-product anions.
Columns: IonPac AG23, AS23, 4 mm; eluent:
4.5 mM sodium carbonate–0.8 mM sodium
bicarbonate; flow rate: 1.0 mL/min; injection
volume: 25 mL; detection: suppressed conductivity, ASRS ULTRA II 4 mm, AutoSupression
recycle mode; temperature: 30 °C. Peaks: 1 5
fluoride (3 ppm), 2 5 chlorite (10 ppm), 3 5
bromate (20 ppm), 4 5 chloride (6 ppm), 5 5
nitrite (10 ppm), 6 5 chlorate (25 ppm), 7 5
bromide (25 ppm), 8 5 nitrate (25 ppm), 9 5
phosphate (40 ppm), 10 5 sulfate (30 ppm).
An example of this architecture is the Universal Cation column (Grace Alltech, Deerfield, Illinois) described in the following.
Ionic molecules adsorbed onto chromatographic substrates: A number of
such columns were developed in the early
1990s for anion-exchange separations. Such
columns have the advantage of providing
exceptional resolution of highly hydrated
anions such as fluoride from the column
void volume. However, because the stationary phase is an adsorbed coating, it slowly
leaches from the substrate through continued use and is rapidly removed when even
low percentages of solvents are incorporated
in the eluent. The latter disadvantages significantly have limited the popularity of
such phases. The Metrosep A Dual 4 column (Metrohm) described in the following
is the most recent commercial product to
utilize this stationary phase architecture.
Step-growth polymers on polymeric
substrates: This simple yet versatile synthe-
sis method has seen wide use in recent years.
Over the last few years, six anion-exchange
columns (see below) have been introduced
using this stationary phase architecture. It is
essentially a hybrid of the third and fourth
architectures described previously. Stationary phase preparation begins with sulfonation of a wide-pore substrate to introduce
anionic surface charges. Then, an epoxyamine copolymer is formed in the presence
of this material, producing an amine rich
“basement” polymer electrostatically bound
Figure 4: Separation of petrochemical industry analytes. Column: IonPac CS18, 2 mm; eluent: 0.5 mM MSA, gradient to 1 mM at 20 min,
gradient to 4 mM at 28 min, gradent to 11 mM
at 34 min, isocratic to 40 min, back to 0.5 mM
at 40.1 min; eluent source EGC II MSA; flow
rate: 0.3 mL/min; temperature: 50 °C; injection
volume: 5 mL; detection: suppressed conductivity, CSRS ULTRA II 2 mm, AutoSuppression recycle mode. Peaks: 1 5 lithium (0.05 ppm), 2 5
sodium (0.2 ppm), 3 5 ammonium (0.25 ppm),
4 5 ethanolamine (3.0 ppm), 5 5 methylamine (3.6 ppm), 6 5 diethanolamine (3.6
ppm), 7 5 potassium (0.5 ppm), 8 5 ethylamine (3.0 ppm), 9 5 dimethylamine (1.4
ppm), 10 5 N-methyldiethanolamine (3.0
ppm), 11 5 mopholine (3.2 ppm), 12 5 1dimethylamino-2-propanol (3.7 ppm), 13 5 Nmethylmorpholine (7.5 ppm), 14 5 butylamine
(1.5 ppm), 15 5 magnesium (0.25 ppm), 16 5
calcium (0.5 ppm), 17 5 strontium (0.5 ppm),
18 5 barium (0.5 ppm).
to the resin surface. Finally, in a repetitive
series of reactions, this polymer-coated substrate is allowed to react with first an epoxy
monomer and then an amine monomer. By
using a primary amine for the amine
monomer, it is possible to introduce branch
sites through subsequent reaction with
additional epoxy monomer. The resulting
surface composite can be exceptionally
hydrophilic, containing only aliphatic substituents and yet it is completely compatible
with high-pH mobile phases which tend to
damage most hydrophilic stationary phases.
New Anion-Exchange
Chromatography Columns
The analysis of trace anions in drinking
water is an important area in environmental
analysis and is the focus of the United States
Environmental Protection Agency’s Method
300.1 Part A. In the area of anion-exchange
column development, a new anionexchange phase for analysis of drinking
water, the IonPac AS22 column (Dionex),
was introduced this year at the Pittsburgh
Conference. This is the first column based
upon Type 8 stationary phase architecture
(see Figure 2) designed specifically for use
with carbonate eluent systems. An advan-
0
5
10
15
20
25
Time (min)
Figure 5: Separation of transition metals and
nonmetal cations. Column: 100 mm 3 4.6 mm
Universal Cation; mobile phase: 2 mM tartaric
acid, 1 mM oxalic acid; flow rate: 1 mL/min;
detection: conductivity. Peaks: 1 5 lithium (0.5
ppm), 2 5 sodium (0.5 ppm), 3 5 ammonium
(0.5 ppm), 4 5 potassium (0.8 ppm), 5 5
nickel (5 ppm), 6 5 zinc (5 ppm), 7 5 cobalt (5
ppm), 8 5 magnesium (0.7 ppm), 9 5 calcium
(0.7 ppm). (Courtesy of Grace Alltech.)
tage of the column selectivity is that the
position of carbonate relative to other common anions can be readily adjusted by altering the ratio of carbonate and bicarbonate in
the mobile phase. While carbonate is typically not an analyte of interest in drinking
water analysis, its proximity to other anions
of interest can often compromise analytical
performance. With the IonPac AS22, carbonate can be moved from an early portion
of the chromatogram to just after nitrate,
improving the analytical performance of the
column when utilized for determination of
trace levels of nitrite, chlorate, and bromide.
The high-capacity column based upon
ultrawide pore substrate (210 mequiv/column in the case of the 250 mm 3 4 mm
column) allows most drinking water samples to be directly injected without overloading the column. In addition, the column
exhibits an unusually wide application range
with good chromatographic performance
for analytes ranging from fluoride to perchlorate, which generally does not exhibit
good chromatographic performance with
columns designed for analysis of the common anions.
In the area of disinfectant byproduct
analysis, two new columns, the IonPac
AS23 (Dionex) and the Metrosep A Supp
7 column (Metrohm) have been introduced to add to the field of commercially
available anion exchange columns designed
for this application available from Dionex,
APRIL 2006 LCGC LC COLUMN TECHNOLOGY SUPPLEMENT
www.chromatographyonline.com
Figure 6: Separation of ionic analytes using
an LC–MS-compatible eluent system. Column:
150 mm 3 4.6 mm Primesep 200; mobile
phase: 20:80:0.1 methanol–water–formic
acid; flow rate: 1.0 mL/min; detection: UV
absorbance at 250 nm. Peaks: 1 5 DOPA, 2 5
tyrosine, 3 5 phenylalanine. Courtesy of SIELC
Technologies, Prospect Heights, Illinois.
Showa Denko, and Grace/Alltech. The
IonPac AS23 column, constructed in a
manner analogous to the IonPac AS22
described previously, can accomplish the
separation of the common anions along
with the disinfectant byproduct anions
mandated in EPA Method 300.1 Part B in
less than 23 min (see Figure 3). The IonPac
AS23 column also shares with the IonPac
AS22 the unique feature of being able to
adjust the selectivity of the column for carbonate over a wide range. The Metrosep A
Supp 7 column, another column designed
specifically for this application is based
upon chemical modification of a polyvinyl
alcohol substrate (Type 5 architecture).
The Metrosep A Supp 7 column offers an
exceptionally high-efficiency separation of
the standard anions along with the oxyhalides. The two columns each have somewhat different selectivity with regard to the
oxyhalides and carbonate. Another column
recently added to this category is the IC SI91 4C column (Showa Denko, Kawasaki,
Japan) specifically designed for analysis of
bromate when used in conjunction with
EPA Method 317, which utilizes a postcolumn reaction and UV detection for a
determination of bromate. Because of the
specificity of the method, it is not necessary
to use a high-resolution column for this
application. Thus, the IC SI-91 4C column is only 100 mm in length.
Another relatively new area of environmental interest is the analysis of perchlorate.
Perchlorate was generally thought to be rare
in the environment but the relatively recent
discovery of perchlorate in food crops (1),
dairy products (2) and human milk samples
(3) has elevated greatly the interest in this
drinking water contaminant, especially in
United States. While perchlorate is a highly
retained anion and typically well resolved
35
Figure 7: Adjustment of solvent content using
a mixed-mode column to affect elution order of
analytes. Column: 150 mm 3 3.2 mm Primesep
B2; flow rate: 0.5 mL/min; detection: UV
absorbance at 210 nm; eluent (left): 20:80:0.2%
acetonitrile–water–trifluoroacetic acid; eluent
(right): 20:80:0.1% acetonitrile–water–trifluoroacetic acid. Courtesy of SIELC Technologies
from anions commonly found in drinking
water, the analysis of perchlorate at low
part-per-billion levels in a high total dissolved solids groundwater matrix tends to
be a demanding application. Analysis of
perchlorate at such levels in the presence of
more than 3000 ppm total dissolved solids
which can be found in groundwater samples necessitated the development of customized anion-exchange materials exhibiting good chromatographic performance for
perchlorate while still providing high loading capacity. In recent years, several new
products have been introduced to address
this application area. The IonPac AS20
(Dionex) is constructed using Type 8 architecture. The high capacity of the stationary
phase (310 mequiv/column in the case of
the 250 mm 3 4 mm column) combined
with the highly hydrophilic polymer backbone result in an ideal material for the
analysis of perchlorate in environmental
samples. The column is specified as the confirmation column in EPA Method 314.1
for the determination of perchlorate in
drinking water samples because its selectivity is significantly different from the IonPac
AS16 (Dionex), specified as the primary
column in EPA Method 314.1. In addition,
a lower capacity version of this the AS20
column, the IonPac AS21 (Dionex) has
been introduced for the analysis of perchlorate in EPA Method 331.0 via electrospray
LC-MS. The lower capacity of the column
renders it compatible with a volatile eluent
system based upon methylamine suitable
for use in electrospray applications while
the high sensitivity of the detection technique allows achievement of the necessary
detection limits with much smaller injection volumes, thus, avoiding column overload for this relatively low capacity column.
Two new anion-exchange columns have
been introduced recently for the analysis of
Figure 8: Separation of ions obtained using a
monolithic column at two flow rates. Column:
100 mm 3 4.6 mm Metrosep Dual 4; eluent: 12
mM p-cyanophenol, 5 mM potassium hydroxide; temperature: 30 °C; injection volume: 5 mL;
detection: suppressed conductivity, ASRS ULTRA
II 4 mm, AutoSupression recycle mode. Peaks: 1
5 fluoride (2 ppm), 2 5 chloride (3 ppm), 3 5
sulfate (5 ppm), 4 5 iodide (20 ppm), 5 5 thiosulfate (10 ppm), 6 5 thiocyanate (20 ppm).
phosphoric acid in soft drinks: Metrosep A
Supp 1 HS (Metrohm) is a special column
for the rapid determination of phosphoric
acid using a carbonate-based eluent system.
It is prepared from a styrene divinylbenzene copolymer which has been derivatized
chemically (Type 5 architecture). This column enables the analysis of phosphate in
cola drinks in the presence of chloride and
sulfate in less than 3 min. The other recent
entry to this product category is the IonPac
Fast Anion IIIA column (Dionex) (4). This
column, based upon Type 8 architecture, is
designed for use with hydroxide eluents. It
allows rapid elution of both phosphate and
citrate with total analysis time being less
than 4 min for samples of minimal complexity. Citrate is commonly found in soft
drinks and represents an analytical challenge when using conventional carbonate
eluent systems due to its high selectivity
relative to phosphate. This column chemistry is compatible with carbonate eluent
systems as well, permitting the rapid analysis of both phosphate and citrate with
either eluent system.
In addition, Metrohm has introduced
two new high capacity anion columns for
use in high ionic strength matrices. The
Metrosep A Supp 8 (Metrohm) allows
determination of nitrite, bromide, and
nitrate in concentrated salt solutions. UV
detection at 215 nm allows the determination of the above analytes in the single digit
36
LCGC LC COLUMN TECHNOLOGY SUPPLEMENT APRIL 2006
www.chromatographyonline.com
Table I:
Column name
Primesep A
Primesep 100
Primesep P*
Primesep 200
Primesep C
Primesep 500
Primesep D
Primesep B2
Primesep AB
Acclaim Surfactant
Company
Ionic retention mode
pKa of ionic site
SIELC Technologies
SIELC Technologies
SIELC Technologies
SIELC Technologies
SIELC Technologies
SIELC Technologies
SIELC Technologies
SIELC Technologies
SIELC Technologies
Dionex
Cation exchange
Cation exchange
Cation exchange
Cation exchange
Cation exchange
Cation exchange
Cation exchange
Cation exchange
Anion and cation exchange
Anion exchange
0
1
1
2
3.5
5
10
5
not stated
10
*Utilizes a phenyl group in the hydrophobic portion of the ligand
parts-per-billion range. A sodium chloride
eluent is used for these applications. The
column which appears to be an example of
Type 5 architecture with a styrene–divinylbenzene backbone, has an extremely high
capacity (700 mmol/column in the case of
the 150 mm 3 4 mm column). The other
new column in this category is the Metrosep A Supp 10 column, apparently, also
an example of Type 5 architecture with a
styrene–divinylbenzene backbone (210
mmol/column in the case of the 250 mm 3
4 mm column) is suitable for high-ionicstrength samples. The column can be operated at elevated temperature using simple
carbonate eluent systems or at ambient
temperature with a carbonate–bicarbonate–perchlorate eluent system. The addition
of perchlorate to the eluent system helps
mask high-energy sites often found in
styrenic stationary phases.
New Cation-Exchange Chromatography Columns
In the area of cation-exchange columns,
Dionex recently introduced a new cationexchange column (the IonPac CS18) optimized for the separation of common inorganic cations along with polar amines (5).
This column utilizes hydrogen abstraction
grafting technology (Type 4 architecture)
with a two layer graft coating on a highly
cross-linked high surface area substrate
based upon styrenic monomers. The resin is
first grafted with a hydrophilic monomer
system to block hydrophobic interactions
with the underlying substrate and then subsequently grafted with a carboxylic acid
monomer system to incorporate cation
exchange sites. This new column is useful in
the analysis of a wide variety of polar amines
commonly used in various industrial applications. Especially notable is the ability to
simultaneously resolve mono-, di-, and triethanolamine all along with the common
inorganic cations single analysis. The column can be used in isocratic mode with
either suppressed or nonsuppressed conductivity detection and can readily separate a
wide variety of amines without resorting to
solvent containing eluents (see Figure 4). In
addition, the high hydronium selectivity of
the column makes it a good choice for
analysis of multiply charged species. Separations of diamines as well as biogenic amines
were both demonstrated using the new column, neither separation requiring any
organic solvent in the mobile phase.
A significant fraction of cation-exchange
columns in use today for the analysis of
inorganic cations utilize the stationary
phase architecture developed by Professor
Schomberg (Type 6 architecture). While
the most common application for such
columns is determination of alkali metal
and alkaline earth metal cations, this stationary phase architecture also finds frequent use for the determination of transition metal cations. Although spectroscopy
remains the preferred methodology for the
analysis of transition metal cations in environmental samples, chromatographic techniques still play an important role when
simultaneous determination of transition
metals along with non-metal cations such
as ammonia or small aliphatic amines is
required. A representative separation utilizing the Universal Cation column from
Grace Alltech is shown in Figure 5. Under
the conditions shown, lithium, sodium,
potassium, ammonium ion, nickel, copper,
zinc, magnesium, and calcium are separated using a complexing eluent system
and detected via conductivity detection.
Generally, however, detection of transition
metal cations at levels likely to be found in
real environmental samples requires the use
of a colorimetric metal complexing postcolumn reagent such as pyridylazoresorcinol to boost sensitivity.
New Mixed-Mode Columns
One product category which has seen a
fairly large number of new products in
recent years is silica-based mixed-mode stationary phases. While commercial products with mixed-mode stationary phases
have been around since the late 1980s,
there has been considerable new-product
activity in this area based upon new synthetic approaches. The company leading
this new wave of activity is SIELC Technologies. Table I lists a number of columns
from SIELC Technologies as well as a new
entry into this product category from
Dionex. SIELC Technologies utilizes an
embedded ionizable group in their ligand
design. Their approach borrows from the
architecture of polar embedded reversed
phase ligands. Polar embedded phases are
widely utilized to overcome the problem of
stationary phase dewetting in reverse phase
HPLC. For such columns, a polar group is
incorporated into the ligand situated near
the silica surface to prevent stationary
phase dewetting in highly aqueous eluent
systems. The proprietary ligands utilized by
SIELC Technologies make use of a similar
architecture but substitute an ionizable
embedded group. This combination overcomes the reproducibility issues associated
with mixed ligand architectures employed
in first-generation mixed-mode stationary
phases by assuring a homogeneous distribution of the two retention sites utilized in
mixed-mode stationary phases. It also
allows for better control of stationary phase
selectivity through direct control of the ligand architecture.
APRIL 2006 LCGC LC COLUMN TECHNOLOGY SUPPLEMENT
www.chromatographyonline.com
Mixed-mode ion-exchange phases offer a
number of advantages over conventional
ion-exchange media or conventional
reversed-phase media. For samples containing mixtures of analytes, some of which are
ionic and some of which are neutral, it is
often difficult to arrive at chromatographic
conditions suitable for retention of all analytes. While the addition of an ion-pair
reagent is commonly used to solve this sort
of problem, ion-pair reagents cause a number of significant problems including: long
equilibration times, reagent purity problems
that frequently result in chromatographic
artifacts, low loading capacity, abnormal
chromatographic peak shapes under overload conditions, incompatibility with
LC–MS and limited ability to tailor the
selectivity of the system. Mixed-mode
columns allow retention of both neutral and
ionic analytes under conditions that overcome most of the problems mentioned previously with ion-pair reagents. For example,
Figure 6 shows an example separation on a
Primesep 200 column illustrating the ability
of a mixed-mode column to retain ionic
analytes with good retention and selectivity,
utilizing a LC–MS-compatible eluent system. Figure 7 illustrates the ability to tailor
the selectivity of a mixed-mode separation
through independent control of the two
retention modes. By adjusting the amount
of trifluoroacetic acid in the mobile phase, it
is possible to adjust the retention time of an
anionic component (in this case
bromide),which is retained via an anionexchange retention mechanism. At the same
time, adjusting the amount of trifluoroacetic
acid in the mobile phase has the opposite
effect on the retention of a cationic solute.
The combined effects of these two retention
modes allow control of the elution order. In
the case of systems containing mixtures of
ionic and neutral components, it is possible
to adjust the retention of the neutral components through adjustment of the solvent
content without affecting the retention of
ionic components. Likewise, changing the
ionic strength allows control of the retention
of ionic components without affecting the
retention of neutral components. As a result,
a wide variety of different elution orders can
be achieved with a mixed-mode system.
This sort of selectivity control is difficult to
achieve using only reversed-phase or only
ion-exchange retention mechanisms.
37
New Monolithic Columns
Conclusions
An emerging area in ion exchange stationary phases is the use of monolithic structures. Three academic groups are actively
pursuing research in this area: Dr. Charles
Lucy’s group (6) at University of Alberta,
Dr. Paul Haddad’s group at the University
of Tasmania (7,8) and Dr. Brett Paull’s
group at Dublin City University (9–13).
While the three groups have investigated a
wide variety of different monolithic materials for use in ion chromatography, until
recently, there have not been any commercial products available for use in ion chromatography based upon their research.
However, one such material recently has
been introduced: the Metrosep Dual 4 column (Metrohm). Although the supplier
does not reveal the nature of the stationary
phase, the monolithic structure (based
upon silica with 2-mm macropores and 13
nm mesopores), the eluent system (based
upon cyanophenol) and the maximum solvent constraint (.5% acetonitrile) all
match an experimental material first
described by Dr. Lucy (14). The column
exhibits excellent efficiency (8000 plates for
nitrate with a 100 mm 3 4.6 mm column
operated at 2 mL/min) along with excellent
permeability (5.4 MPa at 2 mL/min). Figure 8 illustrates the minimal performance
penalty associated with operation of monolithic materials at elevated flow rates. Only
very minor changes in resolution can be
seen with a twofold change in flow rate.
Figure 8 also illustrates two other interesting properties of this architecture. First, the
selectivity of this column is unusual in
terms of the relatively modest separation
factor for fluoride and chloride. This selectivity is distinctly different from most aion
exchange materials, which exhibit a much
larger separation factor for these two ions.
Second, the fluoride peak shows greater
asymmetry than other analytes in Figure 8,
even when compared with an analyte with
similar retention time (for example, chloride). This is no doubt a consequence of
chemical interaction between fluoride
anion and the underlying silica substrate
because fluoride is prone to reacting with
silicate to form fluorosilicate. Nonetheless,
this first commercial product illustrates the
potential of this technology. No doubt this
column represents just the first of a whole
new class of ion chromatography materials
which can be expected in the coming years.
New ion-exchange columns for small ionizable molecules and inorganic ions continue
to be introduced each year, as improvements
in column selectivity continue at a steady
pace. The growth in ion exchange is spurred
by environmental regulations and food
safety analysis. Due to their ruggedness,
most new columns have been packed with
polymeric-based materials and this trend
will undoubtedly continue. Ion exchangers
based upon polymeric monoliths have made
their appearance, mainly for biomolecule
separations, and with the appearance of the
first such column based upon a silica monolith, the promise of monolithic media in ion
chromatography has been demonstrated. A
significant fraction of future ion chromatography stationary phases will likely include
polymer-based monolithic media.
References
(1) C. Sanchez, K. Crump, R. Kreiger, N. Khandaker, and Gibbs, J. Environ. Sci. Technol. 39,
24, 9391–9397 (2005).
(2) A.B. Kirk, E.E. Smith, K. Tian, T.A. Anderson, and P.K. Dasgupta, Environ. Sci. Technol.
37, 4979–4981 (2003).
(3) A. Kirk, P. Martinelango, K. Tian, A. Dutta,
and P. Dasgupta, Environ. Sci. Technol. 39, 7,
2011–2017 (2005).
(4) B. De Borba, C. Pohl, J. Rohrer, C. Saini, and
B. Thompson, paper 130-3, 2005 Pittsburgh
Conference.
(5) M. Rey, A. Bordunov, and C. Pohl, poster
750-9P, 2006 Pittsburgh Conference.
(6) P. Hatsis and C.A. Lucy, Analyst 127, (4)
451–454 (2002).
(7) P. Zakaria, J.P. Hutchinson, N. Avdalovic, Y.
Liu, and P.R. Haddad, Anal. Chem. 77(2),
417–423 (2005).
(8) J.P. Hutchinson, P. Zakaria, A.R. Bowie, M.
Macka, N. Avdalovic, and P.R. Haddad,
Anal. Chem. 77(2), 407–416 (2005).
(9) E. Sugrue, P.N. Nesterenko, and B. Paull,
Anal. Chim. Acta 553(1–2), 27–35 (2005).
(10) E. Sugrue, P.N. Nesterenko, and B. Paull, J.
Chromatogr. A 1075(1–2) 167–175 (2005).
(11) B. Paull and P.N. Nesterenko, Trends in Anal.
Chem. 24(4), 295–303 (2005).
(12) E. Sugrue, P. Nesterenko, and B. Paull, J. Sep.
Sci. 27(10–11) 921–930 (2004).
(13) E. Sugrue, P. Nesterenko, and B. Paull, Analyst 128(5) 417–420 (2003).
(14) P. Hatsis and C.A. Lucy, Anal. Chem. 75(4)
995–1001 (2003). n