(705c) Uncertainties and Optimum Detection Modes for Rare Earth Analysis in Coal and Coal Ash Using Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is a very powerful technique that can detect

multiple elements at very low concentrations in a variety of matrixes. The advantages of low detection limits

and multi-element analyses make ICP-MS an indispensable tool for rare earth elements (REEs) analysis in soils,

sands, clays, ashes, coals and other solid and liquid matrixes. However, low concentrations of REEs and

complicated matrixes of coals and coal ashes can induce a significant amount of error in the analysis if the

digestion of the sample is incomplete and detection modes are not optimized. In this study standard reference

materials of ash and coal were analyzed for REEs and other trace elements using ICP-MS with a lithium

metaborate fusion digestion method. ICP-MS methods tested for optimum REE recovery included standard

operating mode as well as advanced cell technologies including kinetic energy discrimination (KED) and

dynamic reaction cell (DRC) mode. Each mode had advantages for certain rare earth elements indicating that

using certain modes for specific REE may provide the most accurate method for analysis. The use of different

modes for specific REEs is a relatively easy way to enhance the accuracy of REE measurements in coal and

coal ash and may be a good technique for other complicated matrixes as well.

The instrument used in this study is a Perkin Elmer Nexion 300D ICPMS. The Nexion is designed with

an internal cell that allows the user to apply gases which either chemically or physically react with a sample to

minimize interferences with the analyte(s) of interest. In this study, KED mode uses ultra-high purity helium

gas to physically collide with ions in a sample and minimize the concentration of interferents caused by high

total dissolved solids. The result is fewer total ions reaching the detector, but the effect of removal is much

greater for large diameter ions such as Na+ and Cl-. Alternatively, DRC mode using ammonia removes

molecular ions, such as 35Cl16O-, that may overlap with metals of interest. For each complicated sample matrix,

the exact conditions which optimize interference removal and maximize the signal for analytes of interest need

to be experimentally determined.