Determination of trace anions in borated waters by ion chromatography using a large-volume direct injection

Significance of the Topic

In pressurized water reactors, boric acid and lithium hydroxide are added to the coolant to control reactivity and pH. Trace levels of corrosive anions such as fluoride, chloride, nitrate, phosphate and sulfate must be monitored in borated waters at sub-µg/L concentrations to prevent stress corrosion cracking of stainless-steel components.

Objectives and Study Overview

The study aimed to develop a simple, robust ion chromatography (IC) method for simultaneous determination of trace F⁻, Cl⁻, NO₃⁻, PO₄³⁻ and SO₄²⁻ in lithium-containing borated waters. A large-volume direct injection approach was combined with inline generation of a tetraborate/potassium hydroxide eluent on a Thermo Scientific Dionex ICS-6000 system.

Methodology

A 50 mM boric acid eluent was titrated online with electrolytically generated KOH (EGC 500 cartridge). A gradient of 4–30 mM KOH in constant 50 mM borate was used to separate weakly and strongly retained anions. Samples (1000 µL) were injected directly without concentrator columns. Calibration standards ranged from 0.3 to 10 µg/L.

Used Instrumentation

• Thermo Scientific Dionex ICS-6000 HPIC system (DP pump, EG module with high-pressure degasser, low-temp detector/chromatography module, CD conductivity detector)
• Thermo Scientific EGC 500 KOH eluent generator cartridge
• Dionex IonPac AS14 analytical (2 × 250 mm) and AG14 guard (2 × 50 mm) columns
• Dionex IonPac ATC-HC 500 borate trap column
• Dionex AERS 500e 2 mm external-water suppressor
• AS-AP autosampler in push mode with 5 mL syringe for large-volume injections

Main Results and Discussion

Typical background conductance was 2.6–2.75 µS/cm with noise <1.5 nS. Calibration curves were linear (r²>0.997). Method detection limits (3×S/N) were 0.03 µg/L for fluoride, 0.10 µg/L for nitrate, 0.20 µg/L for sulfate and 0.35 µg/L for phosphate. Chloride blank contamination gave an apparent MDL of 0.11 µg/L. Recovery tests in simulated borated water (2000 mg/L B, 4 mg/L Li) spiked at 0.3 and 1 µg/L yielded 86–116%. Retention time precision (n=7) was ≤0.11% RSD; peak area RSD was ≤5%. The borate matrix peak elutes early and does not interfere with target analytes.

Advantages and Practical Applications

• Inline eluent generation ensures high-purity, carbonate-free tetraborate with stable baselines and eliminates manual reagent preparation.
• Large-volume direct injection removes need for concentrator columns and simplifies the setup.
• Applicable to monitoring trace anions in nuclear power plant coolant for QA/QC and corrosion prevention.

Future Trends and Possibilities

Further automation of sample handling and integration with online monitoring systems could increase throughput. Miniaturized inline pretreatment and coupling to mass spectrometry may expand applications to complex matrices. Advances in suppressor technology and eluent generation could further lower detection limits and simplify maintenance.

Conclusion

A robust IC method using large-volume direct injection and inline generated tetraborate eluent was validated for determination of trace anions in lithium-containing borated waters. The approach delivers low detection limits, high precision, accurate recovery and streamlined operation for nuclear plant water monitoring.

Reference

1. Aggrawal M., Rohrer J. Determination of trace anions in borated waters by ion chromatography using a large-volume direct injection. Thermo Fisher Scientific Application Update 73866, 2020.