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

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