Isobaric dilution analysis as a calibration tool for long lived radionuclides in ICP-MS

A novel quantification method named isobaric dilution analysis (IBDA) is introduced for internal calibration using inductively coupled plasma mass spectrometry (ICP-MS). Unlike isotope dilution analysis (IDA), where a sample to be analysed for a target analyte element is spiked with an isotopically enriched solution of the same element, IBDA uses the fact that conventional mass analysers cannot distinguish between two isobaric isotopes of different elements. Therefore, in IBDA, the sample with the element of interest is spiked with a solution of a different element, which shares at least one isobaric isotope and shows similar chemical properties resulting in a similar response. This method offers a less expensive alternative to conventional IDA especially for long-lived radionuclides of elements for which a spiking element that fulfils the above mentioned requirements can be provided. In addition, IBDA offers the advantage of making certain metastable, sometimes monoisotopic elements accessible to internal calibration using a strategy analogous to IDA. One of the elements accessible by the new calibration strategy is technetium (Tc), which suffers from a lack of standards due to safety requirements associated with its radioactivity. In this work, the principle of IBDA is first demonstrated for a certified gadolinium standard, which was diluted with a dysprosium spike exhibiting an isobaric isotope with m/z 160. The results obtained by IBDA were compared with those obtained by a conventional IDA. The concept of IBDA was subsequently used for the determination of 99Tc in a contrast agent. Since no certified Tc standards are available, the response of 99Tc was interpolated allowing accurate determinations with an uncertainty of 1%.