Solid Phase Extraction Methodology for Robust Isotope Analysis of Atmospheric Ammonium

The stable nitrogen isotope composition (delta N-15) 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 delta N-15 values of NH4+ samples have been found to lead to low precision for low concentration (i.e., < 5 mu mol 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 delta N-15 values for NH4+ reference materials and an in-house quality control sample at concentrations as low as 1.0 mu M. Additionally, the sample pretreatment methodology was evaluated using atmospheric aerosol samples previously measured for delta N-15-NH4+ (from Changdao Island, China), which indicated an excellent delta N-15-NH4+ 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 NH4+ from aerosol samples and separated it from present matrix effects (samples collected from Oahu, Hawaii; pooled standard deviation delta N-15-NH4+ = +/- 0.5 parts per thousand,n = 16 paired samples) that without pretreatment originally failed to quantitatively oxidize to nitrite for subsequent delta N-15 isotope analysis. Thus, we recommend applying this sample pretreatment step for all environmental NH4+ samples to ensure accurate and precise delta N-15 measurement.