Solid Phase Extraction Methodology for Robust Isotope Analysis of Atmospheric Ammonium.

The stable nitrogen isotope composition (δ ¹⁵ N) of atmospheric ammonia (NH ₃ ) and ammonium (NH ₄ ⁺ ) 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 δ ¹⁵ N values of NH ₄ ⁺ 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 NH ₄ ⁺ 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 NH ₄ ⁺ . Using this sample pretreatment methodology, we obtained accurate and precise δ ¹⁵ N values for NH ₄ ⁺ 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 δ ¹⁵ N-NH ₄ ⁺ (from Changdao Island, China), which indicated an excellent δ ¹⁵ N-NH ₄ ⁺ match between sample pretreatment and no treatment ( y = (0.98 ± 0.05) x + (0.11 ± 0.6), R ² = 0.99). Further, this methodology successfully extracted NH ₄ ⁺ from aerosol samples and separated it from present matrix effects (samples collected from Oahu, Hawaii; pooled standard deviation δ ¹⁵ N-NH ₄ ⁺ = ± 0.5‰, n = 16 paired samples) that without pretreatment originally failed to quantitatively oxidize to nitrite for subsequent δ ¹⁵ N isotope analysis. Thus, we recommend applying this sample pretreatment step for all environmental NH ₄ ⁺ samples to ensure accurate and precise δ ¹⁵ N measurement.
Competing Interests: The authors declare no competing financial interest.
(© 2024 The Authors. Published by American Chemical Society.)