Ion Chromatography is a form of liquid chromatography. It is used to determine anions and cations in aqueous samples. Ion exchange resins packed into columns are used as the stationary phase. The liquid or eluant which is pumped through this resin is also known as the mobile phase. The sample ions momentarily adhere to the stationary phase before they are washed off again. This interaction of the sample ions between these two phases will vary according to the species. It is this retention characteristic that allows us to differentiate between them.
A typical Ion Chromatograph consists of: Eluant, Pump, Injection valve, Guard column, Analytical column, Suppressor, Conductivity Detector*, Computer/Software. (*other types of detector can be used)
A pump is used to push the mobile phase from a reservoir through the guard and analytical columns. The guard column is a shortened version of the analytical column; it reduces the likelihood of getting contaminants on to the more expensive analytical column. A sample is loaded on to a sample loop by the autosampler and introduced into the mobile phase using an injection valve. The sample ions contained in the mobile phase then flow into the columns where they interact with the ion exchange sites in the resin. If the column is an anion column, the resin will consist of millions of positively-charged sites. The anions in the sample will be temporarily attracted to these sites. Eventually, the mobile phase will ‘compete’ with the resin and elute the anion off the stationary phase and into the mobile phase once more. If the column is for cations, then the resin in the columns will consist of negatively-charged sites. The same principle applies.
The stream of ions passes into a suppressor. The suppressor is controlled electronically and reduces the background conductivity of the mobile phase by using what is effectively a neutralisation reaction. The conductivity of the ions is thus increased, relative to the background.
Finally, as the ions pass through the conductivity detector, a peak is produced which is integrated by the computer software. In general, the area under the peak is measured and compared to a series of peaks from calibration standards. Peak area is proportional to ion concentration.
Equipment available: Dionex ICS5000, Dionex BioLC Uses: ICS5000: Determination of a wide range of inorganic anions and organic acids in aqueous samples.
This is a state-of-the-art dual channel Ion Chromatography system. One channel is a capillary system which has a very low flow rate of around 15µL/minute. This means that the pump can be left running continuously and is therefore inherently stable. The IonPac AS11-HC Hydroxide-Selective Anion-Exchange capillary column can be used to determine a huge range of inorganic and organic anions in one run.
Examples of the chromatograms that the IonPac AS11-HC Hydroxide-Selective Anion-Exchange capillary column can produce.
The second channel incorporates a microbore Dionex AS18 column. This is used to determine common inorganic anions and certain organic species in a very short run time.
Examples of the chromatograms that microbore Dionex AS18 column can produce. Determination of inorganic anions and organic acids in aqueous samples.
BioLC: Determination of organic acids, ammonium, alkali metals and alkaline earths in aqueous samples.
This system contains two very different columns and accompanying chemistries.
One column is a Dionex ICE AS1 which is used to determine a large number of organic acids, including Iso-Saccharinic Acid (ISA).
Full capabilities of Dionex ICE AS1 column.
The other column is a Dionex CS12A which is used to determine Group I and II elements and ammonium.
Full capabilities of Dionex CS12A column.
Detection limits: Variable, but typically around 0.05mg/L for most analytes.
- The samples must not be acidified. The presence of any HNO3, HCl, etc. will swamp any ions of interest and also cause big shifts in their retention times.
- Samples must be filtered down to 0.2µm.
- Samples which contain heavy metals, or elements such as silver, will require special pre-treatment.
Sample volume requirements:
A minimum volume of around 0.5mL of filtered sample is sufficient for most applications.
All enquiries regarding Ion Chromatography please contact Alastair Bewsher
Important information for all users: A fully completed job request form and CoSHH risk assessment form must accompany all batches of samples.
Failure to comply with this will mean that the samples do not get analysed.