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Stable Isotope Analysis

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Advanced Techniques in Assessing Nitrogen Pollution Through Stable Isotope Analysis

The HPR-40 DSA Mass Spectrometer is an exceptional tool for analysing nitrogen transformations in agricultural systems and surrounding waters, such as lakes, rivers, and coastal regions.

Overview

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.

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.

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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.

Further Reading

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Membrane inlet mass spectrometry method (REOX/MIMS) to measure 15N-nitrate in isotope-enrichment experiments 1.84 MB 81 downloads

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