Since the early 1950s, the number of international measurement standards for anchoring stable isotope delta scales has mushroomed from 3 to more than 30, expanding to more than 25 chemical elements. With the development of new instrumentation, along with new and improved measurement procedures for studying naturally occurring isotopic abundance variations in natural and technical samples, the number of internationally distributed, secondary isotopic reference materials with a specified delta value has blossomed in the last six decades to more than 150 materials. More than half of these isotopic reference materials were produced for isotope-delta measurements of seven elements: H, Li, B, C, N, O, and S. The number of isotopic reference materials for other, heavier elements has grown considerably over the last decade. Nevertheless, even primary international measurement standards for isotope-delta measurements are still needed for some elements, including Mg, Fe, Te, Sb, Mo, and Ge. It is recommended that authors publish the delta values of internationally distributed, secondary isotopic reference materials that were used for anchoring their measurement results to the respective primary stable isotope scale.

Precise and accurate isotope ratio measurements are an important task in many applications such as isotope dilution mass spectrometry, bioavailability studies or the determination of isotope variations in geological and cosmic samples. There is much more interest in ICP-MS for isotope ratio determinations at present compared with GDMS, which is preferred for the direct measurement of the isotopic composition of metallic solid samples. Spectroscopic interferences and a limited abundance sensitivity can influence the accuracy of isotope ratio determinations by GDMS and ICP-MS. In addition, in ICP-MS the space charge effect always influences the accuracy and a nozzle separation effect may also contribute to the total mass bias. Using a quadrupole ICP-MS the mass discrimination per mass unit can be >10% for elements with mass numbers <10, about 1–5% for mass numbers in the range of 20–120 and only <1% for heavier elements. Mass discrimination is strongly dependent on the potential of the different lenses of the ion optics and on the nebulizer gas flow. Even in magnetic sector field ICP-MS instruments a distinct mass discrimination is observed. Different procedures such as calibration by substances of consistent natural isotopic composition of the same or a neighbouring element and by isotopic standard reference materials, respectively, in combination with various mathematical functions, linear and exponential ones, are used for mass bias correction. In addition to the ion counting statistics, stability of the ion current is one of the most important topics which influences the precision of isotope ratio determinations. Magnetic sector field instruments, producing flat topped peak shapes, coupled with a multi-collector system for simultaneous measurement of different isotopes achieve the best relative standard deviation in the range of typically 0.005–0.02%, which is only comparable with precisions obtained by TIMS. However, ICP-TOFMS also has the potential for similar results. Typical relative standard deviations for other types of plasma source mass spectrometers for isotope ratio determinations are as follows: GDMS 0.1–1, quadrupole ICP-MS 0.1–0.5, and high resolution ICP-MS 0.05–0.2%.

Four methods of pulse counting detector dead time calculation were evaluated on two quadrupole ICP-MS instruments, using the criteria of ease of application and accuracy (i.e., reliable measurement uncertainty). Three of the methods were based on measurements of the ²⁰⁴Pb/²⁰⁸Pb isotope ratio in the NIST Standard Reference Material NBS-982, while the fourth method was based on the comparison of pulse counting signals versus the corresponding analog signals across a range of signal intensities. The uncertainty in the dead time result derived from each method was evaluated and found to vary considerably, with some approaches underestimating this parameter, while others yielded large uncertainties. Overall, a method based on studies of the ²⁰⁴Pb/²⁰⁸Pb isotope ratio measured for different Pb concentration solutions, recalculated using different dead time settings (herein referred to as Method 2), was found to be the most suitable in terms of the specified criteria, particularly with respect to its uncertainty evaluation.

For the determination of elemental species, which normally exist at low concentrations in the environment, coupling of liquid chromatographic systems with inductively coupled plasma mass spectrometry (ICP-MS) is a powerful method with respect to detection limit. However, accurate results are still a problem in trace analyses but can be obtained by the application of isotope dilution mass spectrometry (IDMS). The developed LC–ICP-IDMS system consists of a high-performance liquid chromatography pump, a sample injection valve, a separation column (different types of chromatographic separation systems, e.g., ion or size-exclusion chromatography, were used depending on the separation problem), a non-destructive detector (e.g. a UV detector) for simultaneous determination of organic substances, and an element-specific ICP mass spectrometer. Isotope dilution is carried out by adding an isotopically enriched species-specific spike solution to the sample prior to the separation step in the case of the determination of well-defined species, or by continuous on-line introduction of a species-unspecific spike solution in the case of species with unknown composition and structure. The species-specific spiking method is demonstrated for the determination of iodide and iodate in mineral water using an ion chromatographic separation column. For example, iodate concentrations in the range of 0.5–20 ng ml^–1 could be determined with relative standard deviations of about 2%. The species-unspecific spiking mode is used to determined heavy metal complexes with humic substances at a level of about 1 ng ml^–1 as well as organo-iodine species in the concentration range 0.7–1.4 ng ml^–1 in natural water systems. The accuracy of speciation could be verified by comparing the total element concentration with the sum of the different elemental species.

This article reviews the application of isotope dilution mass spectrometry (IDMS) to the field of reference material (RM) characterisation within the past 15 years, focusing on elemental and elemental species analysis. Emphasis is placed on liquid and solid RMs and IDMS measurements of the highest analytical quality. Basic principles, as well as the equation system, are being revisited. 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 (ICP-MS) 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 and inter-laboratory comparisons for elemental as well as elemental species analysis are shown, each demonstrating excellent analytical quality. In general, it can be stated that IDMS is the most important reference method for elemental and elemental species analysis, offering the highest accuracy and precision or smallest measurement uncertainties, when properly applied. Thus, IDMS represents by far the best suited reference method for RM characterisation. Owing to its universal applicability IDMS offers sufficient potential to follow future needs in analytical chemistry as well as in the RM sector.