Isobaric dilution analysis: a new calibration strategy for long-lived radionuclides

Calibration strategies that are able to correct for interferences, sensitivity drift and other problems in instrumental analysis are highly welcomed and are the first choice in order to create accurate results. Isotope dilution analysis (IDA) is such an internal calibration strategy that can produce accurate results even for the analysis of complex sample matrices and is therefore called a "primary method". The method is based on the measurement of an isotope ratio for the target analyte and therefore requires the existence of at least two isotopes that can be measured without spectral interference and the availability of standards with known isotope ratio differing from the natural isotope distribution. Unfortunately therefore the method cannot be used for all monoisotopic elements and neither for those for which isotope standards cannot be easily produced.

The new calibration strategy:
Researchers from the University of Münster have recently introduced a new calibration strategy that shares some features of IDA but is not restricted to elements having two stable isotopes. The new strategy called "Isobaric dilution analysis" (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.

In their publication that appeared in the March issue of the Journal of Trace Elements in Medicine and Biology, the authors demonstrated the method first for an element, for which IDA is possible in order to show the capabilities of IBDA in comparison to IDA. The authors used a certified gadolinium standard, which was diluted with a dysprosium spike exhibiting an isobaric isotope with m/z 160 (see Figure 1). The accuracy obtained by the method relies on the similarity of the two elements with respect to their ICP-MS response and chemical behaviour or the correction of possible differences.

The two different methods obtained nearly identical results: 10.04 ± 0.02 (IDA) and 10.03 ± 0.02 mg/L (IBDA) for the Gd standard certified at 10.00 ± 0.06 mg/L.

The authors then demonstrated the real value of the new method by showing that the method can create accurate results for the quantitation of Tc, which suffers from a lack of standards due to safety requirements associated with its radioactivity. Using IBDA for the determination of Tc by using Ru as a spiking element, the ⁹⁹Tc concentration in radiotracers used for scintigraphies could be determined precisely and accurately.  

The authors further mention an additional advantage that should be explored in the future: the technique can be used as a post-column dilution technique, simplifying speciation analysis of Tc, flow injection analysis and online extraction analysis.

July 12, 2020: Isotope dilution analysis using ICP-MS detection for trace element speciation