AN ASSESSMENT OF DISSOLUTION TECHNIQUES FOR THE ANALYSIS OF GEOLOGICAL SAMPLES BY PLASMA SPECTROMETRY

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and ICP-mass spectrometry (ICP-MS) are being used increasingly for the analysis of a wide range of geological materials. The rapidity of these multi-element techniques results in sample dissolution being the limiting step in sample through-put. This study critically evaluates results obtained from two routinely used sample preparation methods: LiBO2 fusion and open-vessel HF-HClO4 digestion, and compares them with data obtained from microwave-heated, sealed-vessel, acid digestion. Detailed procedures are given for each method. Nine standard reference materials (SRM) were used in the comparison. Selected major and trace elements were determined by ICP-AES to assess the general effectiveness of each dissolution procedure, and 24 trace elements plus the 14 rare-earth elements (REE) were determined by ICP-MS. Trace elements determined by both methods show good agreement. The accuracy and precision of the results were dependent on the dissolution technique and the mineralogy/composition of the material. LiBO2 fusion resulted in complete recovery of major and many minor elements including Cr, Hf and Zr, but the volatile elements Pb, Sb, Sn and Zn were lost. The open acid and the microwave digestions produced similar results and proved suitable for the determination of most elements in most materials. Microwave digestion has the advantage of shorter digestion times and uses smaller volumes of reagents. The accuracy of Zr and Hf data was dependent on the sample type. Cr showed a low recovery by open acid attack and this was only partially improved by microwave digestion. Results indicate that a combination of sample preparation techniques is required if quantitative data are sought for the full range of elements studied here.