Measurement of geologic nitrogen using mass spectrometry, colorimetry, and a newly adapted fluorometry technique

• Published in: Solid earth (Göttingen) 2017-03, Vol.8 (2), p.307-318
• Main Authors: Johnson, Benjamin W, Drage, Natashia, Spence, Jody, Hanson, Nova, El-Sabaawi, Rana, Goldblatt, Colin
• Format: Article
• Language: English
• Subjects: Adaptation; Analysis; Atmosphere; Biosphere; Colorimetry; Continental crust; Crusts; Distillation; Distilling; Earth; Earth atmosphere; Earth science; Fluorimetry; Fluorometers; Fluorometry; Fourier transforms; Geology; Glacial till; Mass spectrometry; Mass spectroscopy; Measurement; Methods; Minerals; Neutralization; Nitrogen; Nitrogen cycle; Oceans; Phenols; Quality assessment; Reagents; Rocks; Scientific imaging; Species; Spectroscopy
• ISSN: 1869-9529; 1869-9510; 1869-9529
• DOI: 10.5194/se-8-307-2017

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 (NO sub(3) super(-), NH sub(4) super(+), N sub(2), organic N), and determining which species is actually quantified can be difficult. In rocks and minerals, NH sub(4) super(+) is the most stable form of N over geologic timescales. As such, techniques designed to measure NH sub(4) super(+) 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 NH sub(4) super(+). Briefly, NH sub(4) super(+) 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 NH sub(4) super(+) 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 -2x10 super(18)kgN. 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 [delta] super(15) N values of six rock standards: BCR-2 (1.05+ or -0.4%), BHVO-2 (-0.3+ or -0.2%), G-2 (1.23+ or -1.32%), LKSD-4 (3.59+ or -0.1%), Till-4 (6.33+ or -0.1%), and SY-4 (2.13+ or -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 standa