Stable Isotope Analysis

Nitrogen pollution in aquatic ecosystems is an urgent environmental issue in coastal waters around the world. Monitoring ¹⁵N concentration levels has become an increasingly common method of distinguishing between land derived and natural sources of nutrients. Waste-water nitrate has higher concentrations of ¹⁵N than occur naturally and the bacteria present in sewage systems favourably consume ¹⁴N resulting in an even higher ratio of ¹⁵N:¹⁴N in the water reaching the aquifer. The inorganic nitrogen is usually found in the form of ¹⁵NH₁₄⁺ and in recent decades, increased nitrogen input from fertilisers has caused environmental problems including coastal eutrophication, seasonal hypoxia and harmful algae blooms. Traditional methods to determine nitrogen isotope ratios of dissolved ammonium are generally time consuming whilst also requiring large samples and costly equipment.

Figure 5: A typical experimental procedure used to perform MIMS analysis of 15N:14N ratios.

MIMS provides an efficient and cost-effective alternative to traditional methods. By oxidizing the dissolved ¹⁵NH4+ using hypobromite iodine to form nitrogen gas, MIMS can be used to determine the concentrations of ²⁸N2, ²⁹N₂ and 30N₂ present in the sample, Figure 5 depicts this procedure. This method requires only small sample volumes of water or sediment slurries and is rapid, convenient, accurate, and precise over a range of salinities and ¹⁵N:¹⁴N ratios, as shown in Figure 6. The effectiveness of this technique makes it a great method to analyse nitrogen transformation in agricultural systems and surrounding waters including lakes, rivers and coastal regions.

Figure 6 (a & b): Relationships of the known ¹⁵NH4⁺ concentrations under diff erent salinity conditions and the known ¹⁴NH4⁺/¹⁵NH4⁺ ratios with measured signal intensities of ²⁸N₂, ²⁹N₂, ³⁰N₂ and total ¹⁵N₂ at a NH₄Cl concentration of 100 μmol L−1.