Measurement of boron isotope ratios in groundwater studies

Abstract

Boron is present at low levels in groundwater and rainfall in the UK, ranging between 2 and 200 ng ml^–1. A sensitive technique has been developed using inductively coupled plasma mass spectrometry (ICP-MS) to measure boron isotope ratios at low concentrations with a precision (s_r) of between 0.1 and 0.2%. Samples were evaporated to increase elemental boron concentrations to 200 ng ml^–1 and interfering matrix elements were removed by an adapted cation-exchange separation procedure. The validity of measuring boron isotopic ratios by ICP-MS at this concentration level is discussed in relation the theoretical instrument precision attainable based on counting statistics. Using the developed procedure it is possible to use ICP-MS to establish a reliable database documenting the natural variation in boron isotope compositions in aquifers and rain water in the UK. Preliminary results demonstrate a trend in boron isotopic compositions with increasing boron concentration. The method may be used to evaluate possible levels of boron pollution from anthropogenic inputs into natural aqueous systems.