A Headspace GC-FID Method for Nitrite Determination in Pharmaceutical Excipients

We developed a headspace gas chromatography method with flame ionization detection (HSGC-FID) for trace nitrite determination in pharmaceutical excipients. The approach reacts nitrite with sodium cyclamate to generate cyclohexene, enabling sensitive detection by flame ionization detector (FID) on a nonpolar G27 column (e.g., Rtx-5). Key method performance characteristics including sensitivity, accuracy, and specificity were established. The limit of quantitation is 0.2 ng/mL, corresponding to 10 ng/g for common excipients with a sample concentration of 20 mg/mL and 2 ng/g for low-nitrite excipients such as microcrystalline cellulose and dibasic calcium phosphate with a sample concentration of 100 mg/mL. Achieving this sensitivity required a novel procedure to minimize nitrite contamination in the blank, a common challenge in nitrite analysis. For excipients with significant matrix effects (e.g., magnesium stearate, sodium lauryl sulfate, and HPMCAS), cyclohexane is used as an internal standard to achieve the desired accuracy. To address the limited selectivity of FID, an orthogonal confirmation method employing a polar G16 column (e.g., DB-Wax) can be used to resolve interferences from residual solvents on a G27 column. The method was validated in accordance with ICH Q2(R2) and applied to more than 20 commonly used excipients. The range is 10 ng/g to 5000 ng/g by successfully meeting acceptance criteria for linearity ( R ≥ 0.99), spiked recovery (100 ± 30%), and precision (% RSD ≤ 10%). Given its high sensitivity on common HSGC-FID instrumentation, the method is readily transferable to QC laboratories for routine nitrite testing in pharmaceutical excipients.