A technique for precise boron isotope ratio measurements with a high detection power has been developed by negative thermal ionization mass spectrometry (NTIMS). Relative standard deviations in the range of 0.03-0.3% have been obtained for the determination of the 11B/10B isotope ratio using nanogram amounts of boron. Ba(OH)2 has been applied as ionization promoter for the formation of negative thermal ions. By adding MgCl2 better reproducibilities of the measurement have been achieved. A possible interference of BO2- ions at mass number 42 by CNO- could be excluded by the sample preparation technique used. Contrary to other NTI techniques no dependence of the measured isotope ratio on the boron amount used has been observed. Anthropogenic and natural saline influences in ground water have been successfully identified by boron isotope ratio determinations with this NTIMS method, due to the different isotopic composition of boron in natural and anthropogenic substances. In sewage, the boron isotope ratio is substantially influenced by washing powder, which contains low 11B/10B ratios (expressed in δ11B values normalized to the standard reference material NIST SRM 951). In contaminated ground water, low δ11B values are normally correlated with high boron and high chloride concentrations. On the other hand, δ11B shifts to higher values in less contaminated samples. For ground water with saline influences, only the δ11B determination, and not the boron or chloride content, allowed the correct identification of this natural source of contamination.
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