Negative thermal ionisation mass spectrometry: a new approach to boron isotope geochemistry

The negative thermal ionisation mass spectrometry of boron, a powerful method for the determination of boron isotopic composition (¹¹B/¹⁰B) and concentration in geological materials, is described.

BO₂⁻ ions are formed in a single-filament ion source. With the addition of La(NO₃)₃ as an activator, a 100- to 1000-fold increase in sensitivity as compared to the classical sodium or cesium metaborate method is achieved. With this high sensitivity, as little as 0.1 ng B (0.1 ppb B with 1 g sample size) is needed for analysis. The precision is typically∼ ± 0.2%.

Boron concentrations are determined with a standard isotope dilution technique using a¹⁰B-enriched spike. Precisions are in the range of±0.01% (> 100ppm B)to± 4% (< 0.1ppm B).

For aqueous solutions, water- and HCl-soluble salts and minerals (e.g., borates, brines, seawater, groundwater, carbonates) no chemical treatment is required. The untreated solutions are directly loaded onto the filament.

The method has successfully been tested on a variety of hydrothermal waters and borate minerals from Larderello (Italy), Boron and the Death Valley (California, U.S.A.), and on seawater, tourmalines and carbonatites from different origins. Preliminary results on these investigations are presented.