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.