In the past ion chromatography was a very expensive method but today it is much more favourably priced, as this article from metrohm explains
There are many different ways of determining ions qualitatively and quantitatively.
One such technique that is widely used is ion chromatography.
Ion chromatography is one member of the large family of chromatographic methods.
It is used to determine all ions which carry one or two charges.
In the past ion chromatography (IC) was a very expensive method but today it is much more favourably priced, thanks to the quality Compact ICs available from Metrohm.
There are many important fields of application today for ion chromatography such as: 1: the routine investigation of aqueous systems such as drinking water, rivers, effluents and rain water.
2: for the analysis of ions in chemical products, foods, cosmetics, pharmaceuticals etc 3: ultratrace analysis such as in the semi-conductor and power industry.
Ion Chromatography can be used for the analysis of anions, cations, organic acids and amines plus analytes such as carbohyrates.
* Three main modes of Ion Chromatography Columns.
The different modes of chromatography (anion exchange, cation exchange and ion exclusion) simply relate to the different types of columns used to achieve the separation of the ions.
The eluent used depends on the column type and also the mode of detection - however unless stated the following is all based on conductivity detection.
* Ion exchange.
Ion exchange chromatography (IC) is based on a stoichiometric chemical reaction between ions in a solution and the oppositely charges groups functional groups on the column resin.
In the simplest case in cation chromatography these are sulfonic acid groups or carboxylic acid groups (such as maleic acid) and in anion chromatography quaternary ammonium groups.
* Anion exchange.
Anion exchange chromatography forms the largest group of IC methods mainly because there are few alternatives with such simplicity, sensitivity or selectivity - particularly for sulphate.
The two forms are anion exchange with or without suppression and of these two suppressed methods are the most widely used.
Eluents for suppressed chemistries tend to be either carbonate based or hydroxide.
Often, a device called a suppressor is used and is placed between the column and detector as shown above.
When suppression is used the detector is almost certainly conductivity.
The greatest achievment of suppression is to increase the sensitivity of the anion, however at the same time the background conductivity of the eluent is greatly reduced.
The same suppressor units can also be used to increase the sensitivity of organic acids.
The suppressor used in anion chromatography is simply a cation exchanger and its job is to remove cations and replace them with an H+.
So a sodium carbonate eluent (~800uS) would be converted to carbonic acid (~18uS) by the suppressor and the analyte, for example NaCl (~126uS without suppression) would become HCl (~426uS with suppression).
The ways in which this can be done are varied but the two common ways are as follows: 1: The Metrohm MSM (Metrohm Suppressor Module) contains 3 separate suppressor units.
At any one time, one will be in-line with the eluent and conductivity detector, one will be in-line with dilute sulphuric acid (replacing the removed cations with H+) and the third is washed with water.
The benefits of this technique are lack of baseline noise and ruggedness as this MSM unit has a warranty for ten years.
2: The Metrohm Dual Suppressor is a continuous suppression device which removes the cations and replaces them with H+ (which is provided by electrolysis of water) so with a carbonate eluent it forms carbonic acid.
The Dual Suppressor then reduces the conductivity of the carbonic acid (~18uS) by removing it to leave water (~1uS).
As the concentration of the eluent increases throughout the gradient, the baseline rise is a result of the increasing conductivity of the eluents suppression product.
The importance of this is that carbonate eluents can now be used for gradient elution of anions which means more versatility, no system peak, less corrosive eluents and no gases required.
There is a variety of cation columns available, however the modern ones contain carboxylic acid functional groups.
A large number of applications for silica-gel-based ion exchangers exist.
These columns allow simultaneous separation of alkali metals and alkaline earths plus the separation of transitional metal and heavy metal ions is also possible.
Small amines can also be analysed using cation exchange columns.
The eluents used for non-suppressed cation exchange are weak acids with a complexing agent such as dipicolinic acid, the concentration of which can effect the elution of calcium and heavy metals such as iron, zinc and cobalt.
Cations become less sensitive when suppressed and so are analysed with direct conductivity detection which also allows heavy metals to be analysed as shown above.
* Ion exclusion.
Ion exclusion chromatography (IEC) is mainly used for the separation of weak acids or bases.
The greatest importance of IEC is for the analysis of weak acids such as carboxylic acids, carbohydrates, phenols or amino acids.
For a more detailed explanation of the theory of ion chromatography and detection see the Metrohm Monograph 'Practical Ion Chromatography' available free of charge from Metrohm UK.
Other modes of Detection: * Amperometric detection (see 791 IC-VA Detector and 817 Bioscan). In principle voltammetric detectors can be used for all compounds which have functional groups which are easily reduced or oxidized.
The amperometric detector is the most important version.
Amperometry is very sensitive.
Apart from a few cations (Fe3+, Co2+) it is chiefly anions such as nitrite, nitrate, thiosulfate as well as halogens and pseudohalogens which can be determined in the ion analysis sector.
The most important applications lie, however, in the analysis of sugars by anion chromatography and in clinical analysis.
* Photometric detection (see UV-Vis Spectrophotometer and Post Column Reactor).
Because of its extremely wide range of application photometric or UV/VIS detection is the most important detection method used in HPLC, as many organic molecules contain chromophore groups, or can have one introduced or added, which are able to absorb in the UV or VIS spectrum.
In the field of inorganic ion analysis UV/VIS detection plays a smaller role.
While of the simple anions only analytes such as nitrate, bromide or iodide absorb, important analytes such as fluoride, sulfate or phosphate can only be measured indirectly.
Many cations do not absorb at all, but multivalent and transitional metals in particular can be converted in a post-column derivatization with chelate formers such as 4-(2-pyridylazo)-resorcinol (PAR) or Tiron to form colored complexes.
Redox-active analytes such as bromate and other oxohalide ions can be analyzed by UV/VIS detection after undergoing a post-column reaction with an electrochemically active indicator.
* Coupling techniques.
So-called coupling techniques represent the link-up of a chromatography system with an independent analytical method, usually.
In particular, powerful atomic spectrometric detectors are used in ion chromatography (IC).
Examples are atomic emission and mass spectrometry with inductively coupled plasma (IC-ICP-AES, MS); as a result of their element specificity and sensitivity these provide excellent performance data.
This is the reason why, despite their relatively high costs, such systems are used for the analysis of species and in the ultratrace analysis of elements.
