Ion Chromatography (IC)

Ion Exchange Chromatography (IEC)

In a Nutshell

Take Home Point:

Ion concentrations in solutions

What It Provides:

Chromatogram (intensity versus retention time) showing the separation of ions within a sample. The intensity is proportional to the amount of each component within the sample allowing the technique to be quantitative with proper calibration. Thus, ion mixtures are separated, identified and quantified.

Brief Description:

Separation of Ions In Solutions

Ion Chromatography (IC) is a specialized chromatography technique for the separation and detection of ions. It essentially is a type of HPLC that utilizes specialized eluents, columns and detectors that are best suited for the analsis of ions as well as charged biomolecules such as large proteins, small nucleotides and amino acids. In IC a liquid sample is injected into a flow of pressurized liquid (i.e. the eluent). The eluent carries the sample down a column packed with a stationary phase consisting of resin (e.g. polystyrene) containing a covalently bound charged group. As the sample passes through the column, the various charged analytes comprising the sample reversibly adsorb onto the stationary phase. The greater the charge, the more tightly bound the species. The eluent contains ions itself that eventually dislodge the analytes, with the weaker bound species desorbing first forming the route for separation to occur. The eluent/ion mixture most often flows through a suppressor where an ion exchange process occurs with the eluent and analyte ions, thereby reducing the conductivity of the eluent but increasing that of the analyte ions. This reduces noise and increases sensitivity. The eluent/analyte then flows to a detector, most often a conductivity detector.

What is Detected:

Ions: e.g. Na+, Ca , SO4 , Organic Acids, Amines

Charged Biomolecules: e.g. amino acids, proteins, nucleic acids

Detection Limits:

~1-10ppb

Information Depth:

Not Applicable

Applications:

Quality Control of deionized water systems used in semiconductor processing, aerospace industry, pharmaceutical and medical device manufacturing and other industries where high purity water is critical.
Testing of municipal water
Testing of water for environmental contamination
Testing of electroplating baths
Regulatory testing for foods and environmental testing
Testing of raw materials, intermediates, products, waste streams in the pharmaceutical industry
Separation of proteins in biochemical research and development

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Greater Detail

Overview

Ion Chromatography (IC) is a subset of HPLC. Specifically, IC is a chromatography technique used to separate, identify and determine concentrations of ions in solutions based on their charge. It can be used for the analysis of inorganic anions, cations, transition metals and low molecular weight acids and bases. It can also be used for charged biomolecules such as large proteins, small nucleotides and amino acids. Additionally, solid samples can be extracted to determine contaminant species on surfaces (species must be ionic/charged once in solution).

IC analysis is divided into two methods:

1. Cation Exchange Chromatography: Separation/Detection of positively charged species

2. Anion Exchange Chromatography: Separation/Detection of negatively charged species

In basic IC analysis a liquid sample is injected into a flow of pressurized liquid (i.e. the eluent or mobile phase). The eluent choice will differ depending on whether Cation or Anion Exchange Chromatography is being performed, the species being analyzed and the detector used, but typically contains a high level of salt ions. The eluent/analytes proceed down a column packed with a stationary phase consisting of resin (e.g. polystyrene, cellulose) containing a covalently bound charged group. For Cation Exchange Chromatography the group will be negative in charge (e.g. carboxylic acid) while in Anion Exchange Chromatography the group will be positive in charge (e.g. quaternary ammonium group). As the sample passes through the column, the various charged analytes reversibly adsorb onto the stationary phase. The greater the charge, the more tightly bound the species. The eluent consists of ions that compete for the charged adsorption sites on the stationary phase and will eventually dislodge the analytes, with the weaker bound species desorbing first forming the route for separation to occur.

The analytes exit the column within the eluent flow and pass through a suppressor where the conductivity of the highly conductive eluent is reduced while that of the analyte ions is increased by removing the eluent and sample counterions and replacing them with regenerant ions (from a separate flow of liquid called the regenerant) forming species that are less and more conductive, respectively. The suppression process both decreases background and signal to noise levels, resulting in greatly increased sensitivities of the analytes. Generally, there are two common types of suppressors: Chemically and Electrically Regenerated Suppressors, both of which involve passing ions between the eluent and a regenerant through an ion exchange membrane. (Note that there are some applications which do not utilize the suppressor.)

The eluent then caries the analyte to the detector. The most commonly used is the conductivity detector which measures the change in conductivity of the eluent as analyte ions pass through it. Other detectors such as the Amperometric Detector are also sometimes used. The time it takes for a compound to traverse the IC system is termed the 'Retention Time'. The retention times and peak areas for the analyte ions are compared against those from standard solutions for identification and quantification, respectively.

Charged biomolecules (e.g. proteins) are also commonly analyzed by IC. These species can have a positive, negative or zero charge depending on the eluent pH. For these applications the eluent pH and/or salt concentration is gradually changed to selectively desorb the bound analytes.

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