Quantitative trace-element analysis of diamond by laser ablation inductively coupled plasma mass spectrometry

Abstract

Laser ablation microprobe ICP-MS has been used to determine quantitatively the trace-element composition of diamond. Experiments with different synthetic multi-element carbon-based standards, various lasers and a range of instrument conditions have shown that a 266 nm UV laser at 10 Hz provided the best sensitivity, and synthetic oil and a doped cellulose proved most suitable as external standards; ¹³C was used as the internal standard. The precision and accuracy of the method, and the homogeneity of the cellulose multi-element standard, were tested by multiple analyses. Artefacts resulting from polyatomic interferences were quantified by analysis of a pure synthetic diamond. Concentrations of 41 elements were determined for two fibrous diamonds from Jwaneng in Botswana (JWA 110 and JWA 115), which have been analysed previously by instrumental neutron-activation analysis (INAA) and proton microprobe (PIXE). A comparison of these three analytical techniques shows that the use of the cellulose standard produces accurate and precise data for most elements. Typical detection limits for the rare earth elements are 5–20 ppb, and for transition elements <500 ppb. Sodium and Fe have higher detection limits (2–3 ppm). The precision (expressed as % rsd) ranges through ∼10% for concentrations between 1–100 ppm, ∼15% for values between 0.1–1 ppm, ∼30% for 0.01–0.1 ppm and ∼25% for values <0.01 ppm, with the accuracy lying in the same range. The trace-element patterns obtained by this technique may be used for the characterisation of diamond in genetic studies. Further analyses are required to test whether reliable identification of the source locality of the diamonds is possible; if so this may have important forensic applications.