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The meaning of calibration in general and in the field of isotopic measurements in particular is described, stressing the fundamental difference between calibration as an aim and tools to achieve calibration. The role of proper uncertainty budgeting as a prerequisite for establishing a calibrated measurement is explained. It leads to the recommendation that the quality of the uncertainty statement should be heavily weighed when ranking or judging calibrated isotopic measurement results evaluated by the Commission on Atomic Weight and Isotopic Abundances.
Two synthetic isotope mixtures for the calibration of sulfur isotope amount ratio measurements were gravimetrically prepared from high purity Ag2S materials enriched in 32S, 33S, and 34S. The mixtures were made so as to closely resemble the (natural) isotopic composition of the materials to be calibrated. This allowed a totally independent evaluation, on the same samples, of the relative combined uncertainty of: (a) the procedure to perform direct measurements of the amount of substance ratios of gas isotopes in the redetermination of the Avogadro constant and (b) the gravimetric preparation procedure. The result of both procedures, mass spectrometry and gravimetry, agree to a relative uncertainty of 3 × 10-4 for sulfur amount ratio measurements of the major abundant isotopes. Thus it seems that a direct measurement of isotopic gas mixtures (e.g. of natural isotopic composition) is now possible for sulfurand probably also for other gaseous isotopeswithout necessarily having to rely on calibration by means of values provided by measurements of gravimetrically prepared isotope mixtures. However, synthetic mixtures may be needed for validation and verification purposes, in particular for quality assurance.
New mathematical models with associated equations for isotope dilution mass spectrometry (IDMS)
(2004)
Vector models which progressively lead to a general model for isotope dilution mass spectrometry (IDMS) are presented for the case of two lsquomonitor isotopesrsquo and one blend involved. They enable one to find the boundary conditions for performing IDMS, and cover the cases of highly enriched isotopes, radioactive isotopes and ratios that are given with different denominator. The models identify the key measurements in their simplest form as well as the conditions which minimise the measurement effort and in some cases the propagated measurement uncertainties. The equations are discussed and compared with other published IDMS equations. Combined with discussion on fundamental aspects of IDMS, this results in an even more lsquogeneralrsquo but also more complex IDMS equation.
Isotope amount ratios of krypton were measured on subsamples from one large batch of high purity krypton separated from the atmosphere. Synthetic mixtures of enriched krypton isotopes were used to calibrate the measurements with small uncertainties. The result is a primary isotopic gas standard (PIGS) IRMM-2030 with certified values for isotope ratios, isotopic composition, and molar mass of krypton with small combined uncertainties uc, evaluated according to the ISO/BIPM Guide (GUM). It is commercially available from IRMM-Geel or from MESSER (Duisburg, D). The certified krypton isotope amount ratios in the PIGS IRMM-2030 are as follows: n(78Kr)/n(84Kr)=0.006 232 5(55), n(80Kr)/n(84Kr)=0.040 107(17), n(82Kr)/n(84Kr)=0.203 43(12), n(83Kr)/n(84Kr)=0.201 79(11), and n(86Kr)/n(84Kr)=0.303 205(59) with expanded uncertainty U=kuc and coverage factor k=2. The molar mass of Kr in this sample is M(Kr)=83.798 02(16) g/mol. These values are in good agreement with published measurements of atmospheric krypton but have smaller combined uncertainties and are calibrated by means of synthetic isotope mixtures. The values of the PIGS are traceable to the SI. Measurements of isotope amount ratios of krypton in other samples can be linked to SI using this PIGS.
Problems with understanding, explaining and communication of the present definition of primary method of measurement are described and amendments put forward for discussion. The conclusion is drawn that in many cases more attention should be given to the measurement result and its uncertainty statement, rather than to a method. Some cases are discussed where methods might have a fundamental characteristic that other methods do not have, a condition for the epitheton primary'.
Calibrated values have been obtained for sulfur isotope abundance ratios of sulfur isotope reference materials distributed by the IAEA (Vienna). For the calibration of the measurements, a set of synthetic isotope mixtures were prepared gravimetrically from high purity Ag2S materials enriched in32S, 33S, and 34S. All materials were converted into SF6 gas and subsequently, their sulfur isotope ratios were measured on the SF5+ species using a special gas source mass spectrometer equipped with a molecular flow inlet system (IRMMs Avogadro II amount comparator). Values for the 32S/34S abundance ratios are 22.650 4(20), 22.142 4(20), and 23.393 3(17) for IAEA-S-1, IAEA-S-2, and IAEA-S-3, respectively. The calculated 32S/34S abundance ratio for V-CDT is 22.643 6(20), which is very close to the calibrated ratio obtained by Ding et al. (1999). In this way, the zero point of the VCDT scale is anchored firmly to the international system of units SI. The 32S/33S abundance ratios are 126.942(47), 125.473(55), 129.072(32), and 126.948(47) for IAEA-S-1, IAEA-S-2, IAEA-S-3, and V-CDT, respectively. In this way, the linearity of the V-CDT scale is improved over this range. The values of the sulfur molar mass for IAEA-S-1 and V-CDT were calculated to be 32.063 877(56) and 32.063 911(56), respectively, the values with the smallest combined uncertainty ever reported for the sulfur molar masses (atomic weights).
Comparative performance study of ICP mass spectrometers by means of U "isotopic measurements"
(2000)
The performance of four commercially available ICPMS instruments of three different types was compared by means of uranium "isotopic measurements". Examined were two quadrupole sector (different generation, different manufacturer), one single detector double focusing magnetic sector and one multiple collector double focusing magnetic sector instruments. The same samples of the IRMM-072 series were used under routine conditions to measure the 233U/235U and the 233U/238U ratios which, in these samples, vary over almost three orders of magnitude from ~ 1 to ~ 2 · 10-3. Within expanded (k = 2) uncertainties, good agreement was observed between the certified values and the data internally corrected for mass-discrimination effects. The magnitude of the evaluated uncertainties was different for each type of instrument. With the multiple collector instrument, expanded uncertainties varied from - 0.04% to- 0.24% for the 233U/235U ratio, and from - 0.08% to - 0.27% for the 233U/238U ratio. They were ~ 1 to 5 times larger with the single detector magnetic sector instrument, and ~ 10 to 25 times larger with both quadrupole sector instruments. With the multiple collector instrument, repeatability of the measurements seemed to be limited by the difficulty of correcting properly for instrumental background, whereas with the single detector magnetic sector instrument the counting statistics was the only limitation (on smallest ratios). Apparent mass-discrimination was clearly found to be larger but more reproducible (and hence easier to correct for) in the case of magnetic sector instruments than for both quadrupole sector instruments. If space charge effects were the main source of mass-discrimination for all instruments, these results are in contradiction with the hypothesis of the size of mass-discrimination decreasing with the acceleration voltage. With the single detector magnetic sector instrument in particular (when operated by changing the ion energy only), our results pointed at more than only one major source of mass-discrimination, with variable size depending on the ratios measured.