Determination of boron at trace levels in rocks by inductively coupled plasma spectrometry

Abstract

Most geological samples have boron concentrations below 200 µg g^–1(p.p.m.) and often below 10 µg g^–1. In only a small number of samples do boron levels rise to values in excess of 1000 µg g^–1 or even to percentage levels. The accurate determination of boron at low levels may be of considerable value in establishing the origin of rocks, and in some instances it may be a useful indicator of potential mineralisation targets. The analytical problems encountered in the accurate determination of trace boron concentrations have resulted in the element being one of the least well characterised in geological samples. In addition to the low levels that are sought, there are difficulties with conventional sample dissolution methods. A fusion technique is required, boron often occurs in resistant minerals and traditional sample dissolution techniques (HF/HCIO₄ attack, etc.) cannot be used.

The inductively coupled plasma is an excellent technique for the determination of boron, producing good sensitivity and a general absence of interferences. Determinations at “high” levels (in excess of 50 µg g^–1) are relatively straightforward and fusion of the sample with sodium carbonate or sodium hydroxide can be used. However, at low levels the need to use fusion techniques and the dilution these subsequently require restrict the detection limit. A new sample dissolution procedure, based on fusion with potassium carbonate, is introduced. This is followed by a simple chemical separation technique to remove the major constituents and the bulk of the excess of potassium salts. This effectively reduces the working detection limit to below 2 µg g^–1 of boron in the sample.