NEUTRON ACTIVATION ANALYSIS

Neutron Activation Analysis (NAA) is a highly sensitive method for the accurate determination of elemental concentrations in material. Sensitivities are sufficient to measure certain elements at the nanogram level and below.

The NAA method is based on the detection and measurement of characteristic gamma rays emitted from radioactive isotopes produced in the sample upon irradiation with neutrons. Samples with unknown elemental concentrations are irradiated with thermal neutrons in a nuclear reactor together with standard materials of known elemental concentrations. Neutrons are absorbed in the nuclei of constituent atoms, and later these nuclei emit radiation with energy and quantity characteristic of the particular element. This emitted radiation is a 'fingerprint' of the element, and the amount of radiation given off at a certain energy is indicative of the amount of the element present in the sample. A comparison between specific activities induced in the standards and unknowns provides the basis for computation of elemental abundances. From this analysis, a report is issued giving elemental concentrations in the unknown sample.

In the NAA process, a nucleus absorbs a neutron. The nucleus becomes excited, and immediately releases a gamma ray and decays to a lower energy level, although it still is in an excited state. Then after a period of time (dependent on the nucleus) the excited nucleus emits a beta particle and a gamma ray, at which time the gamma ray is detected by a detector (not shown). Analysis of the spectrum of gamma rays emitted allows determination of the elemental composition of the sample.

Basic steps of NAA
The sample is first carefully weighed into a plastic or quartz container.
The sample is then sealed with a high-speed friction sealer.

The sample, along with appropriate standards and blanks, is placed near the core of the reactor and irradiated for a predetermined length of time. While it is in the reactor, it is exposed to a high intensity neutron field. If a neutron approaches the nucleus of an atom, it may be absorbed. When this happens, the element will become a different isotope of the same element. This "new" isotope is almost always unstable (radioactive) and usually decays by emitting a gamma ray(s).

The sample is then pulled out of the reactor and then it is allowed to decay for a predetermined length of time.

The sample may be counted immediately, for elements with short half-lives, or counted after a delay for samples with longer half-lives. The reason for the delay for isotopes with longer half-lives is to allow the isotopes with a short half-life to decay away, thereby preventing interference and allowing the isotopes with longer half lives to be more easily measured.

After the sample decays, it is "counted" using gamma ray detectors.

Gamma rays are very penetrating, so the gamma rays emitted from the center of the sample easily reach the detector. The resulting gamma-ray spectra looks something like a gas chromatograph spectra with "peaks" at different "retention times".

The position of each peak determines the energy of the gamma ray (identifying the responsible element), and the area under the peak is proportional to its concentration. Final results are obtained after correcting for detector efficiency, decay time, size of the sample, and counting and irradiation times. The comparator standard approach is normally employed with this method. A "standard" is irradiated and counted along with the sample(s). This standard contains a known amount of the element(s) to be determined. Although matrix problems are not usual, standards are usually selected to be similar to the sample(s).

Applications of NAA

The technique of Neutron Activation Analysis (NAA) can be applied to virtually all sample types without any pretreatment of the sample. This includes:

solids such as:

coal, metals, sediments, ores, tissues, bone, synthetic fibers, alloys, bullets, crystals, gems, glass, hair, gunshot residues, moon samples, wheat spores, wood, tree leaves, tree needles, plastic films, rocks, salts, shells, skin, soil, sugar, teeth, minerals, metals, meteorites, ocean sediments oxides, and fingernails,

liquids such as:
blood, gasoline, manufacturing wastes, oil, urine, and water,
gases such as:
argon, chlorine, and fluorine,
suspensions and slurries such as:

sewage sludge, river water, foods, biological tissue samples, animal samples, chemicals, chemical compounds, chemical solutions, chemical tracers, paint, fish, and enzymes.

Some other uses of are as follows:
Detecting impurities in industrial and food products,
Tracing the transport and utilization of elements in animal metabolism,
Looking for arsenic in human hair to determine if someone was poisoned,
Checking soil samples of reclaimed dump sites to see if there were any hazardous materials left over from those sites,
Irradiating dinosaur bones to look for iridium to show that a meteor caused their extinction

Advantages of NAA over other chemical analyses

NAA is a nondestructive process of determining the chemical composition of a sample, as the sample is not changed in any manner by chemical manipulation or the introduction of any material prior to irradiation. NAA measures the total amount of an element in a material without regard to chemical or physical form. Samples analyzed can be liquids, solids, suspensions, slurries, or gases and do not have to be put in a solution or vaporized.

NAA has the other following advantages:

Since neutrons activate the nucleus of an atom and do not interact with the electron shell, this method detects the total elemental content, regardless of oxidation state, chemical form or physical location. Neutrons have no charge and will pass through most materials without difficulty. Therefore the middle of the sample becomes just as activated as the outer surface.

The sample is not permanently damaged by NAA, and in the case case of forensic analysis and analysis of rare samples, such as meteorites or archeological finds, the sample can be saved and even subjected to further analysis at a later time. In most other chemical analysis processes, however, the sample may be vaporized, dissolved, burned, melted, or otherwise permanently altered.

NAA is a multi-element analytical technique in that many elements can be analyzed simultaneously in a given sample gamma spectrum without changing or altering the apparatus as is necessary in atomic absorption.

NAA is time efficient for a large number of samples, as many samples can be irradiated at a given time and counted later on a given decay schedule.

Although the sample may become slightly radioactive in NAA, the radiation in the sample decreases with time until it reaches a state similar to which it was before the NAA was performed on it.

NAA is also extremely sensitive to trace elements. The sensitivity obtained by NAA depends on a large number of factors, including neutron flux, the sensitivity of the detector, time irradiated, the neutron cross section of the elements in the sample.

INTRODUCTORY THEORY OF CHROMATOGRAPHY

Introduction

Few methods of chemical analysis are truly specific to a particular analyte. It is often found that the analyte of interest must be separated from the myriad of individual compounds that may be present in a sample. As well as providing the analytical scientist with methods of separation, chromatographic techniques can also provide methods of analysis.

Chromatography involves a sample (or sample extract) being dissolved in a mobile phase (which may be a gas, a liquid or a supercritical fluid). The mobile phase is then forced through an immobile, immiscible stationary phase. The phases are chosen such that components of the sample have differing solubilities in each phase. A component which is quite soluble in the stationary phase will take longer to travel through it than a component which is not very soluble in the stationary phase but very soluble in the mobile phase. As a result of these differences in mobilities, sample components will become separated from each other as they travel through the stationary phase.

Techniques such as H.P.L.C. (High Performance Liquid Chromatography) and G.C. (Gas Chromatography) use columns - narrow tubes packed with stationary phase, through which the mobile phase is forced. The sample is transported through the column by continuous addition of mobile phase. This process is called elution. The average rate at which an analyte moves through the column is determined by the time it spends in the mobile phase.

Distribution of analytes between phases

The distribution of analytes between phases can often be described quite simply. An analyte is in equilibrium between the two phases;

Amobile <===> Astationary


The equilibrium constant, K, is termed the partition coefficient; defined as the molar concentration of analyte in the stationary phase divided by the molar concentration of the analyte in the mobile phase.

The time between sample injection and an analyte peak reaching a detector at the end of the column is termed the retention time (tR ). Each analyte in a sample will have a different retention time. The time taken for the mobile phase to pass through the column is called tM.


A term called the retention factor, k', is often used to describe the migration rate of an analyte on a column. You may also find it called the capacity factor. The retention factor for analyte A is defined as;

k'A = t R - tM / tM


t R and tM are easily obtained from a chromatogram. When an analytes retention factor is less than one, elution is so fast that accurate determination of the retention time is very difficult. High retention factors (greater than 20) mean that elution takes a very long time. Ideally, the retention factor for an analyte is between one and five.

We define a quantity called the selectivity factor, a , which describes the separation of two species (A and B) on the column;

a = k 'B / k 'A


When calculating the selectivity factor, species A elutes faster than species B. The selectivity factor is always greater than one.
Band broadening and column efficiency

To obtain optimal separations, sharp, symmetrical chromatographic peaks must be obtained. This means that band broadening must be limited. It is also beneficial to measure the efficiency of the column.



The Theoretical Plate Model of Chromatography

The plate model supposes that the chromatographic column is contains a large number of separate layers, called theoretical plates. Separate equilibrations of the sample between the stationary and mobile phase occur in these "plates". The analyte moves down the column by transfer of equilibrated mobile phase from one plate to the next.


It is important to remember that the plates do not really exist; they are a figment of the imagination that helps us understand the processes at work in the column.They also serve as a way of measuring column efficiency, either by stating the number of theoretical plates in a column, N (the more plates the better), or by stating the plate height; the Height Equivalent to a Theoretical Plate (the smaller the better).

If the length of the column is L, then the HETP is

HETP = L / N


The number of theoretical plates that a real column possesses can be found by examining a chromatographic peak after elution;




where w1/2 is the peak width at half-height.

As can be seen from this equation, columns behave as if they have different numbers of plates for different solutes in a mixture.



The Rate Theory of Chromatography

A more realistic description of the processes at work inside a column takes account of the time taken for the solute to equilibrate between the stationary and mobile phase (unlike the plate model, which assumes that equilibration is infinitely fast). The resulting band shape of a chromatographic peak is therefore affected by the rate of elution. It is also affected by the different paths available to solute molecules as they travel between particles of stationary phase. If we consider the various mechanisms which contribute to band broadening, we arrive at the Van Deemter equation for plate height;

HETP = A + B / u + C u


where u is the average velocity of the mobile phase. A, B, and C are factors which contribute to band broadening.

A - Eddy diffusion
The mobile phase moves through the column which is packed with stationary phase. Solute molecules will take different paths through the stationary phase at random. This will cause broadening of the solute band, because different paths are of different lengths.

B - Longitudinal diffusion
The concentration of analyte is less at the edges of the band than at the center. Analyte diffuses out from the center to the edges. This causes band broadening. If the velocity of the mobile phase is high then the analyte spends less time on the column, which decreases the effects of longitudinal diffusion.

C - Resistance to mass transfer
The analyte takes a certain amount of time to equilibrate between the stationary and mobile phase. If the velocity of the mobile phase is high, and the analyte has a strong affinity for the stationary phase, then the analyte in the mobile phase will move ahead of the analyte in the stationary phase. The band of analyte is broadened. The higher the velocity of mobile phase, the worse the broadening becomes.

Van Deemter plots
A plot of plate height vs. average linear velocity of mobile phase.


Such plots are of considerable use in determining the optimum mobile phase flow rate.
Resolution

Although the selectivity factor, a, describes the separation of band centres, it does not take into account peak widths. Another measure of how well species have been separated is provided by measurement of the resolution. The resolution of two species, A and B, is defined as


Baseline resolution is achieved when R = 1.5

It is useful to relate the resolution to the number of plates in the column, the selectivity factor and the retention factors of the two solutes;


To obtain high resolution, the three terms must be maximised. An increase in N, the number of theoretical plates, by lengthening the column leads to an increase in retention time and increased band broadening - which may not be desirable. Instead, to increase the number of plates, the height equivalent to a theoretical plate can be reduced by reducing the size of the stationary phase particles.

It is often found that by controlling the capacity factor, k', separations can be greatly improved. This can be achieved by changing the temperature (in Gas Chromatography) or the composition of the mobile phase (in Liquid Chromatography).

The selectivity factor, a, can also be manipulated to improve separations. When a is close to unity, optimising k' and increasing N is not sufficient to give good separation in a reasonable time. In these cases, k' is optimised first, and then a is increased by one of the following procedures:
Changing mobile phase composition
Changing column temperature
Changing composition of stationary phase
Using special chemical effects (such as incorporating a species which complexes with one of the solutes into the stationary phase)

ION CHROMATOGRAPHY-II

ION CHROMATOGRAPHY CONTINUED

The Ion Chromatographic System
The basic components of an ion chromatograph resembles the setup of conventional HPLC systems.

A pump delivers the mobile phase through the chromatographic system. In general,
either single-piston or dual-piston pumps are employed. A pulse-free flow of
the eluant is necessary for employing sensitive UV/Vis and amperometric detectors.
Therefore, pulse dampers are used with single-piston pumps and a sophisticated
electronic circuitry with dual-piston pumps.
The sample is injected into the system via a loop injector. A three-way valve is required, with two ports being connected to the
sample loop. The sample loading is carried out at atmospheric pressure. After
switching the injection valve, the sample is transported to the separator column by
the mobile phase. Typical injection volumes are between 5 µL and 100 µL.
The most important part of the chromatographic system is the separator column.
The choice of a suitable stationary phase and the chromatographic
conditions determine the quality of the analysis. The column tubes
are manufactured from inert material such as Tefzec, epoxy resins, or PEEK
(polyether ether ketone). In general, separation is achieved at room temperature.
Only in very few cases for example for the analysis of long-chain fatty
acids an elevated temperature is required to improve analyte solubility. An
elevated column temperature is also recommended for the analysis of polyamines
in order to improve peak efficiencies.
The analytes are detected and quantified by a detection system. The performance
of any detector is evaluated according to the following criteria:
• Sensitivity
• Linearity
• Resolution (detector cell volume)
• Noise (detection limit)
The most commonly employed detector in ion chromatography is the conductivity
detector, which is used with or without a suppressor system. The main
function of the suppressor system as part of the detection unit is to chemically
reduce the high background conductivity of the electrolytes in the eluant, and to
convert the sample ions into a more conductive form. In addition to conductivity
detectors, UV/Vis, amperometric, and fluorescence detectors are used.
The chromatographic signals can be displayed on a recorder. Quantitative results
are obtained by evaluating peak areas or peak heights, both of which are
proportional to the analyte concentration over a wide range. This was traditionally
performed using digital integrators which are connected directly to the analog
signal output of the detector. Due to low computer prices and lack of GLP/
GLAP conformity, digital integrators are hardly used anymore. Modern detectors
feature an additional parallel interface (e.g., RS-232-C), that enables the connection to a personal computer or a host computer with a suitable chromatography
software. Computers also take over control functions, thus allowing a fully automated
operation of the chromatographic system.
Because corrosive eluants such as diluted acids and bases are often used in
ion chromatography, all parts of the chromatographic system being exposed to
these liquids should be made of inert, metal-free materials. Conventional HPLC
systems with tubings and pump heads made of stainless steel are only partially
suited for ion chromatography, because even stainless steel is eventually corroded
by aggressive eluants. Considerable contamination problems would result,
because metal ions exhibit a high affinity towards the stationary phase of ion
exchangers, leading to a significant loss of separation efficiency. Moreover, metal
parts in the chromatographic fluid path would make the analysis of orthophosphate,
complexing agents, and transition metals more difficult.

Advantages of Ion Chromatography
The determination of ionic species in solution is a classical analytical problem
with a variety of solutions. Whereas in the field of cation analysis both fast and
sensitive analytical methods (AAS, ICP, polarography, and others) have been
available for a long time, the lack of corresponding, highly sensitive methods for
anion analysis is noteworthy. Conventional wet-chemical methods such as titration,
photometry, gravimetry, turbidimetry, and colorimetry are all labor-intensive,
time-consuming, and occasionally troublesome. In contrast, ion chromatography
offers the following advantages:

• Speed
• Sensitivity
• Selectivity
• Simultaneous detection
• Stability of the separator columns

Speed
The time necessary to perform an analysis becomes an increasingly important
aspect, because enhanced manufacturing costs for high quality products and
additional environmental efforts have lead to a significant increase in the number
of samples to be analyzed.
With the introduction of high efficiency separator columns for ion-exchange,
ion-exclusion, and ion-pair chromatography in recent years, the average analysis
time could be reduced to about 10 minutes. Today, a baseline-resolved separation
of the seven most important inorganic anions requires only three minutes.
Therefore, quantitative results are obtained in a fraction of the time previously
required for traditional wet-chemical methods, thus increasing the sample
throughput.

Sensitivity
The introduction of microprocessor technology, in combination with modern
high efficiency stationary phases, makes it a routine task to detect ions in the
medium and lower µg/L concentration range without pre-concentration. The detection
limit for simple inorganic anions and cations is about 10 µg/L based on
an injection volume of 50 µL. The total amount of injected sample lies in the
lower ng range. Even ultrapure water, required for the operation of power plants
or for the production of semiconductors, may be analyzed for its anion and
cation content after pre-concentration with respective concentrator columns.
With these pre-concentration techniques, the detection limit could be lowered to
the ng/L range. However, it should be emphasized that the instrumentation for
measuring such incredibly low amounts is rather sophisticated. In addition, high
demands have to be met in the creation of suitable environmental conditions.
The limiting factor for further lowering the detection limits is the contamination
by ubiquitous chloride and sodium ions.
High sensitivities down to the pmol range are also achieved in carbohydrate
and amino acid analysis by using integrated pulsed amperometric detection.

Selectivity
The selectivity of ion chromatographic methods for analyzing inorganic and organic
anions and cations is ensured by the selection of suitable separation and
detection systems. Regarding conductivity detection, the suppression technique
is of vital importance, because the respective counter ions of the analyte ions as
a potential source of interferences are exchanged against hydronium and hydroxide
ions, respectively. A high degree of selectivity is achieved by using solute specific
detectors such as a UV/Vis detector to analyze nitrite in the presence
of high amounts of chloride. New developments in the field of post-column
derivatization show that specific compound classes such as transition metals,
alkaline-earth metals, polyvalent anions, silicate, etc. can be detected with high
selectivity. Such examples explain why sample preparation for ion chromatographic
analyses usually involves only a simple dilution and filtration of the
sample. This high degree of selectivity facilitates the identification of unknown
sample components.

Simultaneous Detection
A major advantage of ion chromatography especially in contrast to other instrumental
techniques such as photometry and AAS is its ability to simultaneously
detect multiple sample components. Anion and cation profiles may be
obtained within a short time; such profiles provide information about the sample
composition and help to avoid time-consuming tests. However, the ability of ion
chromatographic techniques for simultaneous quantitation is limited by extreme
concentration differences between various sample components. For example, the
major and minor components in a wastewater matrix may only be detected simultaneously
if the concentration ratio is <1000:1. Otherwise, the sample must
be diluted and analyzed in a separate chromatographic run.

Stability of the Separator Columns
The stability of separator columns very much depends on the type of the packing
material being used. In contrast to silica-based separator columns commonly
used in conventional HPLC, resin materials such as polystyrene/divinylbenzene
copolymers prevail as support material in ion chromatography. The high pH
stability of these resins allows the use of strong acids and bases as eluants, which
is a prerequisite for the wide-spread applicability of this method. Strong acids
and bases, on the other hand, can also be used for rinsing procedures. Meanwhile,
most organic polymers are compatible with organic solvents such as
methanol and acetonitrile, which can be used for the removal of organic contaminants.
Hence, polymer-based stationary phases exhibit
a low sensitivity towards complex matrices such as wastewater, foods, or body
fluids, so that a simple dilution of the sample with de-ionized water prior to
filtration is often the only sample preparation procedure.
ADAPTED FROM "Handbook of Ion Chromatography", Joachim Weiss

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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