Measurement of bromide ion used as a solute-transport monitor via epithermal neutron activation analysis

In soil science (ca. 1970), bromide ion (Br-) in various forms (e.g., KBr, NaBr, SrBr2) was introduced as a non-reactive stable tracer in solute transport studies normally moving freely with the flux of water without substantial chemical or physical interactions with the soil. Typically, Br- is extracted from soil and quantified using either a bromide selective electrode (sensitivity is ∼10 μg/ml) or by high-performance liquid chromatography (sensitivity is ∼0.010 μg/ml). Where the sensitivity is adequate, the selective conductivity method, which is simple, affordable and fast, is preferred. More recently (ca. 1990), workers have reported that 20% of Br- tracers, at low groundwater pH, may be adsorbed by iron oxides and kaolinite when present in the alluvial aquifer. We investigated the use of Epithermal Neutron Activation Analysis (ENAA) as a means of measuring Br- directly in soil samples without an extraction. ENAA was chosen because of its high theoretical advantage factor over aluminum (i.e. ∼20), the principal interfering soil constituent, calculated for the 79Br(n,γ)80Br reaction compared to 27Al(n,γ)28Al. Br- was measured (sensitivity is ∼0.050 μg/g) in one gram soil samples from a 5 s irradiation (φepi = 2.5 x 1012 n x cm-2 x s-1) using a BN capsule. (author)