Analyses of ammonium in geologic samples: comparison of indophenol-blue and fluorometric methods

Abstract:

Nitrogen in geologic materials is a tracer and record of biologic activity. Analysis can be difficult, however, as concentrations are often low (~1s to 10s ppm). The most stable form for N to be preserved in rocks and minerals is as NH₄⁺, which is derived from organic matter and substitutes into mineral lattices for K⁺. Thus, measuring NH₄⁺ content serves as a good proxy for total N content.

Fluorometry (FL) is the standard technique for aqueous samples, and has a number of advantages over older indophenol-blue based, colorimetric (IBC) techniques (Hall, 1993). These include lower sample processing time, safer reagents, more stable reactions, and greater precision. In this study, I adapt a fluorometry technique (Holmes et al., 1999) for use in analyzing NH₄⁺ concentration in geologic materials.

Samples and standards were dissolved in hydrofluoric acid, neutralized with potassium hydroxide, then analyzed either with FL or IBC. As part of IBC, a distillation step was carried out to concentrate NH₄⁺, and both techniques were used after this step to as well. Initial results show promise, as reproducible, ppm-level concentrations are demonstrated in several different rock types of various ages: Neoproterozoic siliciclastic sediments, a Jurassic granodiorite, a Cretaceous serpentinite, and Neoproterozoic carbonates. Concentrations are comparable between the two methods, though curiously fluorometry indicates NH₄⁺ concentrations about 22% lower. Distillation appears effective in strengthening the signal for FL, though it may not be necessary for accurate results.

There are several factors affecting the quality of FL. Sample pH appears to be the most important. At pH>9, NH₃ is stable, and can easily escape a solution. Ensuring samples remain below pH 9 should prove key. Additional tests are being carried out to improve sample recovery, lessen time of HF dissolution, and improve accuracy.

This new application of a standard technique should prove useful not only in determining NH₄⁺ content in geologic samples, but can also serve as a first step in determining a strategy for further analysis. It appears to accurately measure concentrations down to a few ppm, and has low associated error (~5%).