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Application Overview

Ion chromatography is used for the analysis of common anions (such as chloride, nitrite, nitrate, sulfate, etc.) and common cations (such as sodium, ammonium, potassium, etc.) in aqueous samples. It is also commonly used for larger charged molecules such as amino acids and proteins, as well as for carbohydrates.

While there are some alternatives to ion chromatography for the analysis of cations (such as Atomic Absorption Spectroscopy, ICP-MS, etc.), ion chromatography is required for the analysis of anions for which there are no other rapid analytical methods. It is only technique that can provide quantitative analysis of anions at the ppb level.

Ion chromatography is used in the environmental field, the power industry, the chemical industry, the pharmaceutical industry, the food and beverage industry, etc. Ultra-trace ion chromatography is used in fields such as the semiconductor industry.

Principle
Ion chromatography is based on the ionic (Coulombic) interaction of analyte ions with a stationary phase displaying oppositely charged functional groups. The retention of the analyte is related to its affinity to the ion exchanger inside the column.

Ion chromatography includes several methods of separation: ion exchange (the method most commonly referred to), ion exclusion and ion pairing.

Columns
Ion chromatography columns are usually made of functionalized grafted copolymer beads (polystyrene divinylbenzene, ethylvinylbenzene divinylbenzene, etc.) or grafted silica beads.

For anion analysis, the functional group is usually a strong quarternary ammonium group or weaker anion exchangers based on tertiary amine groups. For cation analysis, there is a wide variety of cation exchangers available, such as carboxylates, sulfonates, phosphonates, crown ethers, etc.

Mobile Phases
The mobile phases used in ion chromatography are usually aqueous (typically: dilute acids, alkalis or salt solutions). Eluents for anions include carboxylic acids (aromatic or aliphatic), sulfonic acids and potassium hydroxide. Eluents for cations include inorganic acids (nitric acid…)

Small amounts of water-miscible organic solvents may sometimes be used as eluent modifiers.

Detection Methods
Conductivity detection is the detection method most commonly used in ion chromatography. Other detection methods include spectroscopic detection (UV or fluorescence), potentiometric detection, and electrochemical detection (voltametry, amperometry, coulometry).

Ion Suppression
Suppressors are often used in order to reduce the background conductivity due to the ions present in the mobile phase (eluent), and to improve the conductivity of the ions being tested. This improves the sensitivity of the analytical method, and the linearity of the calibration curves.

Sample Preparation
Aqueous solutions, which may require filtration, dilution, and/or cleaning to remove interferences, are required for analysis. Solid samples are extracted with water to remove ions from the sample surface. Organic liquids may also be extracted with water to obtain an aqueous solution of ions for analysis.

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