A simple new method for the determination of ammonium isotopes by gas chromatography-mass spectrometry

Abstract

We report a new method for the determination of naturally abundant ¹⁵N (concentration of ¹⁵N) in ammonium (NH₄⁺) in water samples, including drinking water, surface water, groundwater, and seawater, by gas chromatography-mass spectrometry (GC-MS) based on the isotopic analysis of hexamethylenetetramine. The resulting product was obtained by the reaction between ammonium ions and formaldehyde under the given conditions (pH = 6.0, 40 °C, 30 min). Then, the species was extracted (with chloroform) and concentrated at pH = 8.5 and measured by GC-MS. The results showed that the relative standard deviation (RSD) of the δ¹⁵N measurements was less than 10% and the recoveries were in the range of 71–118% (n = 5). The limit of detection was 20 μmol l⁻¹. Compared to the previous method, the present method is simple and highly suitable to be used as a routine method for the ¹⁵N/¹⁴N analysis of NH₄⁺.