Ion Chromatography for Nitrite Determination in Water: Principles and Practice

Nitrite (NO₂⁻) is a key indicator of water quality. It forms through the incomplete oxidation of ammonia and can accumulate in surface water, groundwater, and distribution systems. Because nitrite is toxic and can react with amines to form carcinogenic nitrosamines, its accurate determination is essential for drinking water safety, wastewater monitoring, and environmental assessment.

Ion chromatography (IC) has become the method of choice for nitrite analysis. It offers simultaneous determination of multiple anions, high sensitivity, and minimal sample preparation. This article outlines the general procedure for measuring nitrite in water samples by IC.

Principle

IC separates ions based on their interaction with an ion-exchange resin. For anion analysis, a positively charged stationary phase retains anions according to their affinity. Nitrite elutes at a characteristic retention time, typically between fluoride/chloride and nitrate, depending on the column and eluent used. The separated nitrite is then quantified, most commonly by suppressed conductivity detection. In this approach, the eluent's conductivity is chemically suppressed, allowing the analyte signal to be measured against a low background.

Sample Preparation

For relatively clean water samples such as drinking water or surface water, preparation is straightforward. The sample is filtered through a 0.45 μm membrane filter to remove particulate matter and injected directly into the chromatograph. If the sample contains high concentrations of chloride or sulfate, additional steps may be needed to prevent interference. For samples preserved with acid, the method can tolerate chloride levels up to 150 ppm and sulfuric acid concentrations as high as 0.5%. 

In more complex matrices, such as seawater or industrial wastewater, online sample preparation techniques—including chloride removal and sulfate precipitation—can be integrated into the IC system to enable direct analysis without manual pretreatment.

Chromatographic Separation

A typical IC setup for nitrite determination uses an anion-exchange column with a guard column. The choice of column depends on the sample matrix and the presence of co-eluting ions. For drinking water analysis, columns such as the IonPac AS9 have been widely used with a carbonate/bicarbonate eluent at a flow rate of about 2.0 mL/min. 

For samples containing high chloride, high-capacity columns—such as the IonPac AS19—improve resolution between nitrite and chloride, which is critical because the large chloride peak can otherwise mask the small nitrite peak. Eluent generation systems that produce potassium hydroxide gradients online offer a convenient alternative to manually prepared eluents, providing consistent separation across different sample types.