A Protocol for the Determination of the Rare Earth Elements at Picomole Level in Rocks by ICP-MS: Results on Geological Reference Materials USGS PCC-1 and DTS-1

Rare earth element analyses are widely used in geology, environmental science and archaeology. Over the past decade inductively coupled plasma-mass spectrometry has become an important source of rare earth data on geological material. However, ICP-MS analysis of rock samples without pre-concentration can be problematic because of complex sample matrices that can generate significant molecular isobaric interferences on rare earth peaks and which need to be corrected. Such problems are exacerbated for ultramafic rocks because the low levels of rare earth elements demand more concentrated solutions in order to maintain signals above background levels. These high solid loads result in intra-run changes in instrument sensitivity which need to be monitored. Pre-concentration chemistries have been developed in order to avoid high solid loads but these are time-consuming and must offer quantitative recoveries or use a yield tracer. Here, we describe an alternative method for rare earth element analysis by ICP-MS, which involves no pre-concentration and is, therefore, able to deliver data rapidly. Our approach is to apply an external correction procedure, based on the analysis of a reference material closely matched in composition to the unknown samples, which allows correction for both interferences and variations in instrument sensitivity. Testing this method, we obtained accurate rare earth element results for basaltic rocks with a precision of about 2% (1s). We demonstrate that the method is also applicable to ultramafic rocks with abundances at ultra-trace (ng g^?1) level and present data for twelve separate dissolutions of the peridotite USGS PCC-1 and four separate dissolutions of the dunite DTS-1 reference materials. The repeatability of the data is between 3% and 9% (1s).