Assay of Nitrite and Determination of Nitrate Impurity in Sodium Nitrite Using a Reagent-Free Ion Chromatography System

Significance of the Topic

The determination of nitrite and trace nitrate impurity in sodium nitrite is critical for pharmaceutical quality control, especially in products used for acute cyanide poisoning treatment. Traditional titrimetric assays are time-consuming and involve hazardous reagents. Modernizing pharmacopeial monographs with chromatographic approaches improves safety, throughput, and data quality.

Objectives and Study Overview

This study aimed to develop and validate a reagent-free ion chromatography (RFIC) method with suppressed conductivity detection for simultaneous quantification of nitrite and nitrate in sodium nitrite. The method supports proposed revisions to USP monographs by using electrolytically generated carbonate/bicarbonate eluents and continuous suppressor regeneration.

Methodology and Instrumentation

A Thermo Scientific Dionex ICS-2100 RFIC system was employed, featuring an EGC 500 K2CO3 eluent generator, EPM 500 pH modifier, and AERS 500 suppressed conductivity module. Separation was achieved on a Dionex IonPac AS12A analytical column (4×250 mm) with AG12A guard (4×50 mm) using a 2.7 mM K2CO3/0.3 mM KHCO3 eluent at 1.5 mL/min and ambient temperature. Detection was in recycle suppressor mode at 22 mA. Method validation followed USP <1225> and ICH Q2 guidelines, covering linearity, LOD/LOQ, accuracy, precision, and robustness.

Main Results and Discussion

• Resolution and Peak Shape: Nitrite and nitrate eluted at relative retention times of 1.0 and ~1.9, respectively; resolution >11 and asymmetry <1.2 under all conditions.
• Linearity: Peak area versus concentration was linear (r²=0.9999) over 20–120 mg/L for nitrite and 0.365–7.30 mg/L for nitrate.
• Sensitivity: LODs of 43.6 µg/L (nitrite) and 97.3 µg/L (nitrate); LOQs of 145 µg/L and 324 µg/L, respectively, meeting USP impurity thresholds.
• Accuracy and Precision: Recovery for nitrite (94–103%) and nitrate (93–101%) across spiked levels; assay precision RSD <0.08% for nitrite and <1.7% for nitrate.
• Robustness: Minor variations (±10% flow rate, ±10% eluent strength, ±6 °C column temperature) produced <4% changes in retention times and maintained resolution >11 on two column lots.

Benefits and Practical Applications

The RFIC approach eliminates manual eluent preparation and hazardous reagents, reduces analysis time to under 18 min per injection, and improves inter-laboratory reproducibility. It supports single-injection dual-analyte assays, aligning with modern pharmacopeial requirements and QC workflows in pharmaceutical manufacturing.

Future Trends and Opportunities

Advances may include on-line IC monitoring of production streams, expanded profiling of related anions in complex matrices, miniaturized systems for point-of‐care testing, and integration with data-analysis algorithms for predictive quality control. Further monograph updates could incorporate gradient separations or multi-dimensional chromatography for enhanced specificity.

Conclusion

The validated reagent-free IC method meets USP and ICH performance criteria for simultaneous nitrite assay and nitrate impurity determination in sodium nitrite. Its simplicity, safety, robustness, and throughput advantages make it a strong candidate for adoption in revised pharmacopeial standard methods.

References

• USP in Process Revision: Sodium Nitrite, Pharmacopeia Forum 40(5) (2015).
• USP in Process Revision: Sodium Nitrite Injection, Pharmacopeia Forum 40(5) (2015).
• USP General Chapter <1225> Validation of Compendial Methods, USP 34 (2011), pp. 778–786.
• ICH Guideline Q2A: Validation of Analytical Procedures – Definition and Terminology (CPMP/ICH/281/95), Geneva (1995).
• ICH Guideline Q2B: Validation of Analytical Procedures – Methodology (CPMP/ICH/281/95), Geneva (1996).