Isotope Dilution Mass Spectrometry - A Primary Method of Measurement and Its Role for RM Certification

This article describes the application of isotope dilution mass spectrometry (IDMS) to the field of reference material (RM) characterisation focusing on the approach, which is applied by the IDMS group at BAM. Emphasis is placed on IDMS measurements of highest analytical quality. Basic principles as well as the equation system are being recalled. Different calibration strategies, such as single, double or triple IDMS, are critically reviewed and the achievable uncertainties are discussed. Differences in the application of thermal ionization mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICPMS) are discussed as well as differences between different types of mass spectrometers such as single collector versus multi-collector or quadrupole versus magnetic sector instruments. Possible sources of errors and bias are mentioned and correction models introduced and applied within the past years are discussed. Several examples for RM characterisations in the field of elemental analysis are shown, each demonstrating excellent analytical quality. In general it can be stated that IDMS is the most important reference method for elemental analysis, offering highest accuracy and precision or smallest measurement uncertainties, when properly applied. Thus IDMS represents by far the best suited reference met hod for RM characterisation. Due to its universal applicability IDMS offers sufficient potential to follow future needs in analytical chemistry as well as in the RM sector.