ION CHROMATOGRAPHY PART 1

“Chromatography” is the general term for a variety of physico-chemical separation
techniques, all of which have in common the distribution of a component
between a mobile phase and a stationary phase. The various chromatographic
techniques are subdivided according to the physical state of these two phases.
The discovery of chromatography is attributed to Tswett, who in 1903
was the first to separate leaf pigments on a polar solid phase and to interpret
this process. In the following years, chromatographic applications were limited
to the distribution between a solid stationary and a liquid mobile phase (Liquid
Solid Chromatography, LSC). In 1938, Izmailov and Schraiber laid the foundation
for Thin Layer Chromatography (TLC). Stahl refined this method
in 1958 and developed it into the technique known today. In their noteworthy
paper of 1941, Martin and Synge proposed the concept of theoretical plates,
which was adapted from the theory of distillation processes, as a formal measurement
of the efficiency of the chromatographic process. This approach not
only revolutionized the understanding of liquid chromatography, but also set the
stage for the development of both gas chromatography (GC) and paper chromatography.
In 1952, Martin and James published their first paper on gas chromatography,
initiating the rapid development of this analytical technique.
High Performance Liquid Chromatography (HPLC) was derived from the
classical column chromatography and, besides gas chromatography, is one of
the most important tools of analytical chemistry today. The technique of HPLC
flourished after it became possible to produce columns with packing materials
made of very small beads (10 µm) and to operate them under high pressure.

Ion Chromatography (IC) was introduced in 1975 by Small, Stevens, and Bauman
as a new analytical method. Within a short period of time, ion chromatography
evolved from a new detection scheme for a few selected inorganic
anions and cations to a versatile analytical technique for ionic species in general.
For a sensitive detection of ions via their electrical conductance, the separator
column effluent was passed through a “suppressor” column. This suppressor
column chemically reduces the eluant background conductance, while at the
same time increasing the electrical conductance of the analyte ions.
In 1979, Fritz et al. described an alternative separation and detection
scheme for inorganic anions, in which the separator column is directly coupled
to the conductivity cell. As a prerequisite for this chromatographic setup, low
capacity ion-exchange resins must be employed, so that low ionic strength eluants
can be used. In addition, the eluant ions should exhibit low equivalent
conductances, thus enabling sensitive detection of the sample components.
At the end of the 1970s, ion chromatographic techniques were used to analyze
organic ions for the first time. The requirement for a quantitative analysis of
organic acids brought about an ion chromatographic method based on the ionexclusion
process that was first described by Wheaton and Bauman in 1953.
The 1980s witnessed the development of high efficiency separator columns
with particle diameters between 5 µm and 8 µm, which resulted in a significant
reduction of analysis time. In addition, separation methods based on the ionpair
process were introduced as an alternative to ion-exchange chromatography,
because they allow the separation and determination of both anions and cations.
Since the beginning of the 1990s column development has aimed to provide
stationary phases with special selectivities. In inorganic anion analysis, stationary
phases were developed that allow the separation of fluoride from the system
void and the analysis of the most important mineral acids as well as oxyhalides
such as chlorite, chlorate, and bromate in the same run. Moreover, highcapacity
anion exchangers are under development that will enable analysis of,
for example, trace anionic impurities in concentrated acids and salinary samples.
Problem solutions of this kind are especially important for the semiconductor
industry, sea water analysis, and clinical chemistry. In inorganic cation analysis,
simultaneous analysis of alkali- and alkaline-earth metals is of vital importance,
and can only be realized within an acceptable time frame of 15 minutes by using
weak acid cation exchangers . Of increasing importance is the analysis of
aliphatic amines, which can be carried out on similar stationary phases by adding
organic solvents to the acid eluant.
The scope of ion chromatography was considerably enlarged by newly designed
electrochemical and spectrophotometric detectors. A milestone of this
development was the introduction of a pulsed amperometric detector in 1983,
allowing a very sensitive detection of carbohydrates, amino acids, and divalent
sulfur compounds.
A growing number of applications utilizing post-column derivatization in
combination with photometric detection opened the field of polyphosphate, polyphosphonate,
and transition metal analysis for ion chromatography, thus providing
a powerful extension to conventional titrimetric and atomic spectrometry
methods.
Types of Ion Chromatography
These developments made ion chromatography an integral part of both modern
inorganic and organic analysis.
Even though ion chromatography is still the preferred analytical method for
inorganic and organic ions, meanwhile, ion analyses are also carried out with
capillary electrophoresis (CE) , which offers certain advantages when analyzing
samples with extremely complex matrices. In terms of detection, only spectrometric
methods such as UV/Vis and fluorescence detection are commercially
available. Because inorganic anions and cations as well as aliphatic carboxylic
acids cannot be detected very sensitively or cannot be detected at all, applications
of CE are rather limited as compared to IC, with the universal conductivity detection
being employed in most cases.
Dasgupta et al. as well as Avdalovic et al. independently succeeded to
miniaturize a conductivity cell and a suppressor device down to the scale required
for CE. Since the sensitivity of conductivity detection does not suffer from
miniaturization, detection limits achieved for totally dissociated anions and low
molecular weight organics compete well with those of ion chromatography techniques.
Thus, capillary electrophoresis with suppressed conductivity detection
can be regarded as a complementary technique for analyzing small ions in simple
and complex matrices.

Types of Ion Chromatography
Modern ion chromatography as an element
of liquid chromatography is based on three different separation mechanisms,
which also provide the basis for the nomenclature in use.
Ion-Exchange Chromatography (HPIC)
(High Performance Ion Chromatography)
This separation method is based on ion-exchange processes occurring between
the mobile phase and ion-exchange groups bonded to the support material. In
highly polarizable ions, additional non-ionic adsorption processes contribute to
the separation mechanism. The stationary phase consists of polystyrene, ethylvinylbenzene,
or methacrylate resins co-polymerized with divinylbenzene and
modified with ion-exchange groups. Ion-exchange chromatography is used for
the separation of both inorganic and organic anions and cations. Separation of
anions is accomplished with quaternary ammonium groups attached to the polymer,
whereas sulfonate-, carboxyl-, or phosphonate groups are used as ionexchange
sites for the separation of cations.
Ion-Exclusion Chromatography (HPICE)
(High Performance Ion Chromatography Exclusion)
The separation mechanism in ion-exclusion chromatography is governed by
Donnan exclusion, steric exclusion, sorption processes and, depending on the
type of separator column, by hydrogen bonding. A high-capacity, totally sulfonated
cation exchange material based on polystyrene/divinylbenzene is employed
as the stationary phase. In case hydrogen bonding should determine selectivity,
significant amounts of methacrylate are added to the styrene polymer. Ion-exclusion
chromatography is particularly useful for the separation of weak inorganic
and organic acids from completely dissociated acids which elute as one
peak within the void volume of the column. In combination with suitable detection
systems, this separation method is also useful for determining amino acids,
aldehydes, and alcohols.
Ion-Pair Chromatography (MPIC)
(Mobile Phase Ion Chromatography)
The dominating separation mechanism in ion-pair chromatography is adsorption.
The stationary phase consists of a neutral porous divinylbenzene resin of
low polarity and high specific surface area. Alternatively, chemically bonded octadecyl
silica phases with even lower polarity can be used. The selectivity of the
separator column is determined by the mobile phase. Besides an organic modifier,
an ion-pair reagent is added to the eluant (water, aqueous buffer solution,
etc.) depending on the chemical nature of the analytes. Ion-pair chromatography
is particularly suited for the separation of surface-active anions and cations, sulfur
compounds, amines, and transition metal complexes.
Alternative Methods
In addition to the three classical separation methods mentioned above, reversedphase
liquid chromatography (RPLC) can also be used for the separation of
highly polar and ionic species. Long-chain fatty acids, for example, are separated
on a chemically bonded octadecyl phase after protonation in the mobile phase
with a suitable aqueous buffer solution. This separation mode is known as ion
suppression.
Chemically bonded aminopropyl phases have also been successfully employed
for the separation of inorganic ions. Leuenberger et al. described the separation
of nitrate and bromide in foods on such a phase using a phosphate buffer
solution as the eluant. Separations of this kind are limited in terms of their
applicability, because they can only be applied to UV-absorbing species.

Moreover, applications of multidimensional ion chromatography utilizing
multimode phases are very interesting, too. In those separations, ion-exchange
and reversed-phase interactions equally contribute to the retention mechanism
of ionic and polar species .
TO BE CONTINUED
ADAPTED FROM "Handbook of Ion Chromatography", Joachim Weiss