Analysis of Rare Earth Elements in Rock and Mineral Samples by ICP-MS and LA-ICP-MS

The group of the rare earth elements (REEs) serves as valuable indicator of numerous geological processes such as magma formation or fluid–rock interaction. The decay systems of the radioactive REE isotopes La, Sm and Lu are used for geochronometric dating of a range of events, starting from first steps of planetary formation to younger steps of geodynamic development. Thus, the abundance of all REEs occurring in a large range of concentrations as well as precise isotope ratios must be analysed in different geomaterials. The inductively coupled plasma (ICP) ion source and various types of mass spectrometers (MS) represent the basis to fulfil the analytical requirements of geoscientific studies. Today, ICP-quadrupole MS and ICP-sector field MS (SFMS) with a single detector or multiple ion collection (MC-ICP-MS) are standard instruments for REE analyses in the geosciences. Due to the need for in situ analysis, laser ablation (LA)-ICP-MS has become an important trace element microprobe technique, which is widely applied for determination of REE concentrations and isotope compositions in geoscientific laboratories. The quality of concentration analysis or isotope ratio determination of REEs by ICP-MS and LA-ICP-MS is affected by many parameters. Most significant are interferences caused by polyatomic oxide and hydroxide ion species formed in the plasma as well as fractionation effects leading to non-stoichiometric behaviour during element determination or to biased isotope ratio measurements. Laser-induced fractionation and isobaric interferences have to be considered as additional effects for LA-ICP-MS. As analyte elements and matrix are unseparated, mineral standards matching the matrix of samples are a prerequisite for accurate and precise REE concentration and isotope ratio determination. Application of fs lasers instead of the more common ns lasers in LA-ICP-MS systems turns out to be a significant step to reduce laser-induced fractionation and to overcome effects of sample matrices.