Boron speciation in acid digests of metallurgical grade silicon reveals problem for accurate boron quantification by inductively coupled plasma – optical emission spectroscopy

Abstract

The boron (B) content is a major parameter rigorously monitored during production of silicon (Si) for different industrial applications, since it is crucial for defining the electronic properties of the final product. During the validation of a newly acquired inductively coupled plasma-optical emission spectrometer (ICP-OES) in an industrial analytical laboratory a significant bias between results of the established and the new instrument was observed. During investigation of the reasons for the observed bias it was found that B in acidic Si digests, prepared using HNO₃ and HF, is present as at least two different molecular species. One of them is easily hydrolyzed during acid digestion and was identified as boric acid (B(OH)₃) by HPLC-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS). The other B species-referred to as unknown B species- is stable under strongly acidic conditions, sensitive to oxidation by H₂O₂ and cannot be generated during sample digestion from isotopically labelled ¹⁰B(OH)₃. Indications for the unknown B species were found in all investigated metallurgical grade Si samples and identified as the source of significant measurement error in solution nebulisation ICP-OES based B quantifications. Oxidation of the unknown B species with a B/Si stoichiometry of approximately 1/10 with H₂O₂ significantly increased the B concentrations determined by ICP-OES and results agreed well with prompt gamma activation analysis (PGAA) measurements.