Isotope Dilution Mass Spectrometry - A Primary Method of Measurement and Its Role for RM Certification 135 Isotope Dilution Mass Spectrometry - A Primary Method of Measurement and Its Role for RM Certification JOCHEN VOGL and WOLFGANG PRITZKOW BAM Federal Institute for Materials Research and Testing Unter den Eichen 87, 12205 Berlin, Germany e-mail: jochen.vogl@bam.de [Received: 15.04.2009 ; Revised: 31.03.2010 ; Accepted: 27.07.2010] Abstract 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 method 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. © Metrology Society of India, All rights reserved 2010. 1. Introduction The origin of the isotope dilution principle is not known exactly. Different assumptions can be found in the literature, whereof most of them see the origin in zoology. Etienne Roth [1] notes that the isotope dilution principle first was used to count the population of rare birds on islands. Unfortunately he did not give a reference for this. Michael Berglund found a clear reference that the isotope dilution principle has been applied in zoology in 1933 [2]. The basic principle, however, has been invented by MAPAN - Journal of Metrology Society of India, Vol. 25, No. 3, 2010; pp. 135-164 ORIGINAL ARTICLE Hevesy and Paneth already in 1913 for determining the solubility of lead sulphide in water by using a radioactive lead isotope [3]. This kind of application nowadays is referred to as radiochemical isotope dilution analysis. In this case the sample with an unknown quantity of an inactive analyte element is being mixed with a known quantity of a radioactive isotope of the same element, the so-called spike. A subsample has to be isolated and the specific activity has to be determined, which includes the quantification of the isolated analyte element. This method is most suitable for the analysis of elements being hard to quantitatively isolate, for the analysis