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Ion Chromatography

Introduction

Ion chromatography is a form of liquid chromatography that uses ion-exchange resins to separate atomic or molecular ions based on their interaction with the resin. Its greatest utility is for analysis of anions for which there are no other rapid analytical methods. It is also commonly used for cations and biochemical species such as amino acids and proteins. Most ion-exchange separations are done with pumps and metal columns, see the HPLC document for typical instrumentation. Simple LC columns are used to separate one ion from others in ultra-trace analytical applications.


Ion-Exchange Resins

The column packings for ion chromatography consist of ion-exchange resins bonded to inert polymeric particles (typically 10 Ám diameter). For cation separation the cation-exchange resin is usually a sulfonic or carboxylic acid, and for anion separation the anion-exchange resin is usually a quaternary ammonium group. For cation-exchange with a sulfonic acid group the reaction is:

-SO3- H+(s) + Mx+(aq) <--> -SO3- Mx+(s) + H+(aq)

where Mx+ is a cation of charge x, (s) indicates the solid or stationary phase, and (aq) indicates the aqueous or mobile phase. The equilibrium constant for this reaction is:

          [-SO3- Mx+]s [H+]aq
   Keq = --------------------
          [-SO3- H+]s [Mx+]aq

Different cations have different values of Keq and are therefore retained on the column for different lengths of time. The time at which a given cation elutes from the column can be controlled by adjusting the pH ([H+]aq). Most ion-chromatography instruments use two mobile phase reservoirs containing buffers of different pH, and a programmable pump that can change the pH of the mobile phase during the separation.


Ion Detection

Ions in solution can be detected by measuring the conductivity of the solution. In ion chromatography, the mobile phase contains ions that create a background conductivity, making it difficult to measure the conductivity due only to the analyte ions as they exit the column. This problem can be greatly reduced by selectively removing the mobile phase ions after the analytical column and before the detector. This is done by converting the mobile phase ions to a neutral form or removing them with an eluent suppressor, which consists of an ion-exchange column or membrane. For cation analysis, the mobile phase is often HCl or HNO3, which can be neutralized by an eluent suppressor that supplies OH-. The Cl- or NO3- is either retained or removed by the suppressor column or membrane. The same principle holds for anion analysis. The mobile phase is often NaOH or NaHCO3, and the eluent suppressor supplies H+ to neutralize the anion and retain or remove the Na+.

Molecular ions, such as proteins, that have absorption bands in the ultraviolet or visible spectral region can be detected by absorption spectroscopy.


Related Topics

Liquid Chromatography (LC)

High-Performance Liquid Chromatography (HPLC)

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