Measurement of geologic nitrogen using mass spectrometry, colorimetry, and a newly adapted fluorometry technique
Date
2017
Authors
Johnson, Benjamin W.
Drage, Natashia
Spence, Jody
Hanson, Nova
El-Sabaawi, Rana
Goldblatt, Colin
Journal Title
Journal ISSN
Volume Title
Publisher
Solid Earth
Abstract
Long viewed as a mostly noble, atmospheric species, recent work demonstrates that nitrogen in fact cycles throughout the Earth system, including the atmosphere, biosphere, oceans, and solid Earth. Despite this new-found behaviour, more thorough investigation of N in geologic materials is limited due to its low concentration (one to tens of parts per million) and difficulty in analysis. In addition, N can exist in multiple species (NO3-, NH4+, N-2, organic N), and determining which species is actually quantified can be difficult. In rocks and minerals, NH4+ is the most stable form of N over geologic timescales. As such, techniques designed to measure NII4+ can be particularly useful.
We measured a number of geochemical rock standards using three different techniques: elemental analyzer (EA) mass spectrometry, colorimetry, and fluorometry. The fluorometry approach is a novel adaptation of a technique commonly used in biologic science, applied herein to geologic NH4+. Briefly, NH4+ can be quantified by HF dissolution, neutralization, addition of a fluorescing reagent, and analysis on a standard fluorometer. We reproduce published values for several rock standards (BCR-2, BHVO-2, and G-2), especially if an additional distillation step is performed. While it is difficult to assess the quality of each method, due to lack of international geologic N standards, fluorometry appears better suited to analyzing mineral- bound NH4+ than EA mass spectrometry and is a simpler, quicker alternative to colorimetry.
To demonstrate a potential application of fluorometry, we calculated a continental crust N budget based on new measurements. We used glacial tills as a proxy for upper crust and analyzed several poorly constrained rock types (volcanics, mid- crustal xenoliths) to determine that the continental crust contains similar to 2 x 10(18) kg N. This estimate is consistent with recent budget estimates and shows that fluorometry is appropriate for large-scale questions where high sample throughput is helpful.
Lastly, we report the first f N-15 values of six rock standards: BCR-2 (1 : 05 +/- 0 : 4%), BHVO-2 (0 : 3 +/- 0 : 2%), G2 (1 : 23 +/- 1 : 32%), LKSD-4 (3 : 59 +/- 0 : 1%), Till-4 (6 : 33 +/- 0 : 1%), and SY-4 (2 : 13 +/- 0 : 5%). The need for international geologic N standards is crucial for further investigation of the Earth system N cycle, and we suggest that existing rock standards may be suited to this need.
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Citation
Johnson, B.W.; Drage, N.; Spence, J.; Hanson, N.; El-Sabaawi, R.; & Goldblatt, C. (2017). Measurement of geologic nitrogen using mass spectrometry, colorimetry, and a newly adapted fluorometry technique. Solid Earth, 8, 307-318. https://doi.org/10.5194/se-8-307-2017