NETL Digestion Procedure for Bulk Elemental Analysis of Solids

There are many digestion procedures that can be employed for inductively coupled plasma mass spectrometry (ICP-MS) or inductively coupled plasma optical emission spectrometry (ICP-OES) analysis (bulk elemental analysis) of solid coal by-products.  One method employed by NETL-RIC is described below.  Note that it is also critical for the lab to determine the percent recoveries of the rare earths and other elements from certified reference materials.  This serves as a check on the digestion procedure, as well as the analytical procedure.  Also note that ICP-OES has inferior detection limits for the rare earths as compared to ICP-MS.
In the NETL digestion procedure, solid samples were prepared by mixing pulverized sample with calcined LiBO₂ (lithium metaborate) at a ratio of 1:8.  Prior to fusion, samples were mixed by stirring with a platinum rod in a 99.95% platinum crucible.  The open crucibles were heated to 1100°C for five minutes in a Phoenix microwave muffle furnace (CEM).  After 5 minutes, the melt was removed from the oven and allowed to cool to a homogeneous glass.  The glass was digested in 5% HNO₃ on low heat with stirring.  To ensure that the sample glass was completely digested, the platinum crucibles were rinsed 3x with 5% HNO₃ and the sample was diluted to a final volume of 100 ml.  These samples were further diluted in 2% HNO₃ before analysis to determine the optimal balance between internal standard recovery and limit of detection.  For solid samples containing significant carbon content, samples were dried and ashed in a Leco Thermogravimetric Analyzer (TGA701) for one hour under nitrogen at 107°C for drying and then for five hours under air at 550°C for ashing. 

Reference
"Analysis of Rare Earth Elements in Geologic Samples using Inductively Coupled Plasma Mass Spectrometry"
Tracy Bank, Elliot Roth, Phillip Tinker, Evan Granite, DOE Topical Report, available on the NETL Rare Earth website - http://netl.doe.gov/research/coal/rare-earth-elements/publications