Determination of Nitrite, Nitrate, and Bromide in Seawater by UV/Vis Detection after Suppression

Importance of the Topic

Accurate measurement of nitrite, nitrate, and bromide at trace levels in seawater is critical for monitoring water quality, assessing pollution, and understanding eutrophication processes. The complex ionic composition of seawater and low analyte concentrations demand highly sensitive and selective analytical techniques.

Objectives and Study Overview

This study aims to evaluate an ion chromatography method coupling electrolytic suppression with dual detection—suppressed conductivity and UV/Vis—for simultaneous quantification of nitrite, nitrate, and bromide in seawater. Key performance metrics including linearity, limits of detection, and precision are assessed using external standards and natural seawater samples.

Methodology

Seawater samples from the North Sea were diluted 1:20 (v/v) with ultrapure water and filtered through 0.45 μm PET syringe filters. Chromatographic separation employed isocratic elution with a carbonate/bicarbonate mobile phase at 0.8 mL/min. A 150 mm × 4.6 mm, 3.5 μm Shim-pack IC-SA4 column was maintained at 50 °C, and 50 μL injections were performed.

Instrumentation

• HPLC system: Shimadzu IC with CDD-10AVP suppressed conductivity detector and SPD-20A UV/Vis detector
• Suppression: Electrolytically regenerated suppression chamber
• Column: Shim-pack IC-SA4, 150 mm × 4.6 mm, 3.5 μm
• Mobile phase: 1.7 mmol/L sodium carbonate / 5.0 mmol/L sodium bicarbonate
• Detection wavelength: 218 nm

Results and Discussion

• Calibration curves for nitrite, nitrate, and bromide were linear between 0.1 and 5 ppm with R² ≥ 0.9999 for both detectors
• Limits of detection were determined using ASTM noise calculation: UV/Vis detection yielded lower LODs for nitrite (0.9 ppb) and nitrate (1.4 ppb) compared to conductivity (2.9 ppb and 4.8 ppb), while conductivity provided a lower LOD for bromide (5.3 ppb vs. 12.8 ppb for UV/Vis)
• Precision for retention time (RSD ≤ 0.07%) and peak area (RSD ≤ 0.51%) was excellent across seven anions
• Analysis of real seawater samples confirmed detectable levels of bromide (~65 ppm) and nitrate, with reliable peak identification supported by standard spiking experiments

Benefits and Practical Applications

• Dual detection enhances selectivity and sensitivity, allowing complementary qualitative and quantitative analysis in complex matrices
• The method features high precision, low detection limits, and straightforward sample preparation, suitable for routine environmental monitoring
• Applicable to quality control, pollution studies, and marine research where accurate trace-level determination is required

Future Trends and Applications

Ongoing developments may include coupling suppressed IC with mass spectrometry for further sensitivity and specificity, miniaturization of systems for on-site and online monitoring, and adoption of greener eluent systems to reduce environmental impact.

Conclusion

The combination of electrolytic suppression with UV/Vis and conductivity detection on a Shimadzu IC platform offers a robust, sensitive, and precise approach for simultaneous determination of nitrite, nitrate, and bromide in seawater, meeting stringent environmental analysis requirements.

References

1. Ito, K., et al., Analytical and Bioanalytical Chemistry, 404(8), 2513-2517.