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 (NO3ĝ', 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 ĝ1/4 ĝ€†2ĝ€† × ĝ€†1018ĝ€kgĝ€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 d 15N values of six rock standards: BCR-2 (1. 05ĝ€† ± ĝ€†0. 4ĝ€†‰), BHVO-2 (ĝ'0. 3ĝ€† ± ĝ€†0. 2ĝ€†‰), G-2 (1. 23ĝ€† ± ĝ€†1. 32ĝ€†‰), LKSD-4 (3. 59ĝ€† ± ĝ€†0. 1ĝ€†‰), Till-4 (6. 33ĝ€† ± ĝ€†0. 1ĝ€†‰), and SY-4 (2. 13ĝ€† ± ĝ€†0. 5ĝ€†‰). 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.