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

Abstract

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.

Nitrogen Mass Fraction and Isotope Determinations in Geological Reference Materials Using Sealed‐Tube Combustion Coupled with Continuous‐Flow Isotope‐Ratio Mass Spectrometry

“…In contrast, by the EA‐IRMS method, δ 15 N signatures of BCR‐2 and BHVO‐2 are found to be 1.05 ± 0.4‰ and −0.3 ± 0.2‰, respectively (Johnson et al . ) (Figure ; Table ), which are lighter than those obtained through the sealed‐tube combustion method. There is thus an obvious and large discrepancy between sealed‐tube combustion and EA‐IRMS.…”

“…For two USGS RMs (BCR‐2 and BHVO‐2), the sealed‐tube combustion technique yielded higher N mass fractions than those from the EA‐IRMS method, and the δ 15 N values were also more positive than results reported by Johnson et al . (). It is thus evident that the EA‐IRMS method is unsuitable for determining nitrogen isotopes of silicates, due to the incomplete release of nitrogen and the high blank background.…”

“…The nitrogen mass fractions of the two samples have previously been widely measured by multiple methods, but the results provided have been inconsistent (Johnson et al . ).…”

“…However, due to the low nitrogen content of most crustal rocks (Johnson et al . , Johnson and Goldblatt ), these present an analytical challenge in acquiring N mass fraction and isotope data.…”

“…Many techniques have been employed to determine the N mass fraction (Johnson et al . ), while δ 15 N analysis mainly relies on mass spectrometry using a gas‐source isotope‐ratio mass spectrometer (IRMS) or a static mass spectrometer. More techniques have been developed in association with the former instrument than the latter, because it is more widely installed.…”