Abstract

The use of electrospray mass spectrometry (ES-MS) of tetrafluoroborate solutions is proposed for the determination of the boron isotopic ratio (¹⁰B/¹¹B). When a mixture of methanol and water is used as the solvent and a neutral to alkaline medium is kept, spectra obtained at mild electrospray conditions show free-of-interference peaks with base-line resolution in the region of m/z 86 and 87, which corresponds to ¹⁰BF₄⁻ and ¹¹BF₄⁻. The study was carried out on a quadrupolar mass spectrometer Platform II (Micromass, UK). To minimize the memory effect in the ion source, the original pepper pot counter electrode was substituted by a cross-flow counter electrode. This geometry reduces the memory effect, but a cleaning process based on mannitol injection assures a suitable background level. Occasional corona discharge corrupts the numerical results, thus median and median of absolute deviations were used as central tendency and dispersion estimators, respectively, because spikes and outliers do not affect them. The influences of the source and detector voltages and the gas flows over the isotopic ratio were investigated. No systematic effect was observed, but there are mass discrimination and drift that were corrected by successive injections of a solution of boric acid NIST 951 converted to sodium tetrafluoroborate. Common ions such as K⁺, Na⁺, Mg²⁺, Ca²⁺, SO₄²⁻, Br⁻ and Cl⁻ do not cause significant isobaric interference and isotopic fractionation. The isotopic ratio precision obtained for 5 min injection of 100 µmol L⁻¹ boron solution at 10 µL min⁻¹ was typically 0.4‰, which is comparable to those from inductively coupled plasma (ICP-MS) and thermal ionization mass spectrometry (TIMS).