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