Date of Award


Document Type

Honors Thesis (Open Access)


Colby College. Environmental Studies Program


Denise Bruesewitz

Second Advisor

D. Whitney King

Third Advisor

Gail Carlson


Nitrogen (15N) isotope tracer studies are an invaluable tool for understanding the rate of N transformations in the environment. A mechanistic understanding of N cycling is critical to management of excess N availability in freshwater and marine ecosystems. Conventional methods for measuring 15N:14N of dissolved inorganic nitrogen species are time consuming and require large sample volumes. Here, we present a technique for measuring 15N:14N in ammonium (NH4+) using electrospray ionization mass spectrometry (ESI-MS). NH4+ present in small volumes of sample (3 ml) is complexed with phenol via the Berthelot reaction, producing an indophenol complex (mass 198.05 or 199.05 amu) to increase the molar mass of the N species for mass spectrometry. Excess reagents are removed and the indophenol product concentrated using automated C-18 solid phase extraction. The extracts are run through a diode array UV-Vis detector to measure total NH4+ concentrations and then passed to the ESI-MS to obtain the 15N:14N of the complex. The low N concentration detection limits for ammonium (0.8 μM) coupled with quantitative isotope abundance determination allows for convenient, rapid characterization of N transformations in aquatic environments.


Nitrogen, Isotope analysis, Isotopic tracer, Electrospray ionization, Time-of-flight mass spectrometry