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

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⁺, N₂, 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 NH₄⁺ 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  ∼  2  ×  10¹⁸ 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 δ¹⁵N values of six rock standards: BCR-2 (1. 05  ±  0. 4 ‰), BHVO-2 (−0. 3  ±  0. 2 ‰), G-2 (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.