Ion Chromatography (IC)

Ion Chromatography (IC)

Ion chromatography (IC) utilizes a column packed with an insoluble, charged matrix containing exchangeable cations and anions that separates ionic analytes from the mobile phase, using ion-exchange principles. The ion-exchange matrix forms the stationary phase for chromatographic separation and must be selected based on analysis requirements. The ion-exchange matrix is usually in the form of beads, with covalently bound molecular groups having positive or negative charge. Exchangeable counter-ions, suitably selected for the target analyte, are electrostatically held to the charged molecular groups. For example, Polystyrene-divinylbenzene copolymer resins are commonly used for the matrix, while typical bound ionic groups can be Sulfonate groups or Quaternary Amines. Sulfonate groups are negatively charged when in water, hence a positively charged exchangeable counter-ion, such as Sodium (Na⁺) is electrostatically bound to each Sulfonate group, for overall neutrality. This formulation is called a Cation exchange resin, since the exchangeable counter ion, Na+, is a Cation. Conversely, in the case of the Quaternary Amine group, the exchangeable counterion is an anion and this is an anion exchange resin.

Ion Chromatography can be of anion-exchange or cation-exchange types. Cation-exchange chromatography is used when the analyte molecule is also a cation (positively charged). Conversely, anion-exchange chromatography uses exchangeable anions on the stationary phase to remove negatively charged analyte molecules (anions) from the mobile phase by ion- exchange. 

In case of solutions that contain both anionic and cationic analytes, anion and cation exchange must be separately performed. Thus, in an anion-exchanger column, the anionic analyte molecules will be retained within the column, as they exchange places with the anionic counter-ions in the stationary phase and the cationic analyte molecules will exit the column with the mobile phase. Similarly, cationic analytes will be retained within a cation exchange column. The next step, after separating the respective anions and cations from the mobile phase is to elute them out using a specially formulated eluent solution, which will desorb the anionic and cationic analytes from the stationary phase. The strength of electrostatic bonding between analyte ion and the ionic stationary phase determines the sequence of elution. The analytes that form weaker bonds will be eluted first. The eluted molecules can be suitably detected and a chromatogram obtained. Various types of detectors are available, such as conductivity (CD), electrochemical (ECD), variable wavelength (VWD), photodiode array (PDA) ion chromatography (IC), or mass spectrometry (MS) detectors.

Ion exchange chromatography works for any charged organic or inorganic species dissolved in the mobile phase. Examples of inorganic ions are anionic groups like Chloride, Nitrate, and Sulfate which are separated by Anion exchange resins. Similarly, cations of Lithium, Sodium and Potassium are separated using cation exchange resins. Proteins and Amino acids display pH dependent ionization behavior in water and can be separated by Ion Exchange Chromatography.

Common Uses of Ion Chromatography (IC)

• Purification or isolation of proteins, enzymes and Amino acids
• Water analysis
• Determination of sugar and salt content in foods

Advantages of Ion Chromatography (IC)

• Predictable separation and Elution based on nature of ionic groups
• Applicable to organics and inorganics that can be ionized solution

 Limitations of Ion Chromatography (IC)

• Only charged molecules in solution can be separated

 Industrial Applications of Ion Chromatography

• Pharmaceuticals,
• Foods and beverages,
• Environmental monitoring
• Biotechnology