Determining Nitrite by Ion Chromatography

Nitrite is an intermediate in the nitrogen cycle and a common pollutant in water, originating from microbial decomposition of organic nitrogen, fertiliser runoff, and industrial discharges. Its presence is of particular concern because it can cause methemoglobinemia in infants and react with amines to form carcinogenic nitrosamines. 

Accurate measurement of nitrite is therefore essential for drinking water safety and environmental monitoring. Among the available analytical techniques, ion chromatography stands out for its simplicity, high sensitivity, excellent selectivity, and ability to determine multiple anions simultaneously in a single run.

Principle of the Method

Ion chromatography separates anions based on their differing affinities for an ion-exchange resin. A water sample is injected into the chromatograph and carried by an eluent through a column packed with anion-exchange material. Nitrite ions, along with other anions such as chloride, nitrate, and sulfate, migrate through the column at different rates. Nitrite elutes at a characteristic retention time that distinguishes it from other species. 

After separation, the effluent passes through a suppressor, which reduces the background conductivity of the eluent and enhances the signal of the analyte ions. A conductivity detector measures the electrical signal, producing a chromatogram where nitrite appears as a distinct peak. The identity of the peak is confirmed by its retention time, and the concentration is determined from the peak area or height using a calibration curve prepared from standard solutions.

Sample Collection and Preparation

Proper sample handling is crucial for reliable results. Collect samples in clean polyethylene or glass bottles and analyse them as soon as possible, as nitrite can be transformed by microbial activity. If immediate analysis is not feasible, store samples at 4°C and complete the determination within 48 hours. For clean water samples, simple filtration through a 0.22 or 0.45 µm membrane is sufficient to remove suspended particles. 

For more complex matrices such as wastewater, additional pretreatment may be required. Solid-phase extraction can eliminate organic interferences, and in samples with very high chloride content, appropriate measures should be taken to avoid baseline disturbance. After filtration, the sample is transferred to autosampler vials and injected into the chromatograph.

Chromatographic Conditions

The choice of column, eluent, and operating parameters affects separation quality. Common anion-exchange columns include the IonPac AS19, AS14A, AS17, and Metrosep A Supp 5-150, typically used with a matching guard column. 

The eluent, which drives the separation, can be potassium hydroxide, sodium carbonate-bicarbonate mixtures, or sodium hydroxide, applied either isocratically or with a gradient program to improve resolution. A flow rate of around 1.0 mL/min and injection volumes of 25 or 40 µL are typical. Suppressed conductivity detection is the preferred mode for its sensitivity and stable baseline.