Solid Phase Extraction Methodology for Robust Isotope Analysis of Atmospheric Ammonium

The stable nitrogen isotope composition (δ¹⁵N) of atmospheric ammonia (NH₃) and ammonium (NH₄⁺) has emerged as a potent tool for improving our understanding of the atmospheric burden of reduced nitrogen. However, current chemical oxidation methodologies commonly utilized for characterizing δ¹⁵N values of NH₄⁺ samples have been found to lead to low precision for low concentration (i.e., < 5 μmol L^-1) samples and often suffer from matrix interferences. Here, we present an analytical methodology to extract and concentrate NH₄⁺ from samples through use of a sample pretreatment step using a solid phase extraction technique involving cation exchange resins. Laboratory control tests indicated that 0.4 g of cation exchange resin (Biorad AG-50W) and 10 mL of 4 M sodium chloride extraction solution enabled the complete capture and removal of NH₄⁺. Using this sample pretreatment methodology, we obtained accurate and precise δ¹⁵N values for NH₄⁺ reference materials and an in-house quality control sample at concentrations as low as 1.0 μM. Additionally, the sample pretreatment methodology was evaluated using atmospheric aerosol samples previously measured for δ¹⁵N-NH₄⁺ (from Changdao Island, China), which indicated an excellent δ¹⁵N-NH₄⁺ match between sample pretreatment and no treatment (y = (0.98 ± 0.05)x + (0.11 ± 0.6), R² = 0.99). Further, this methodology successfully extracted NH₄⁺ from aerosol samples and separated it from present matrix effects (samples collected from Oahu, Hawaii; pooled standard deviation δ¹⁵N-NH₄⁺ = ± 0.5‰,n = 16 paired samples) that without pretreatment originally failed to quantitatively oxidize to nitrite for subsequent δ¹⁵N isotope analysis. Thus, we recommend applying this sample pretreatment step for all environmental NH₄⁺ samples to ensure accurate and precise δ¹⁵N measurement.