Interference-free determination of sub ng kg levels of long-lived Zr in the presence of high concentrations (μg kg) of Mo and Nb using ICP-MS/MS.

Long-lived high abundance radionuclides are of increasing interest with regard to decommissioning of nuclear sites and longer term nuclear waste storage and disposal. In many cases, no routine technique is available for their measurement in nuclear waste and low-level (ng kg) environmental samples. Recent advances in ICP-MS technology offer attractive features for the selective and sensitive determination of a wide range of long-lived radionuclides. In this work, inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS)-based methodology, suitable for accurate routine determinations of Zr at very low (ng kg) levels in the presence of high levels (μg kg) of the isobaric interferents Nb and Mo (often present in nuclear waste samples), is reported for the first time. Additionally, a novel and systematic strategy for method development based on the use of non-radioactive isotopes is proposed. It relies on gas-phase chemical reactions for different molecular ion formation to achieve isobaric interference removal. Using cell gas mixtures of NH/He/H or H/O, and suitable mass shifts, the signal from the Nb and Mo isobaric interferences on Zr were suppressed by up to 5 orders of magnitude. The achieved limit of detection for Zr was 1.3 × 10 Bq g (equivalent to 0.14 ng kg). The sample analysis time is 2 min, which represents a significant improvement in terms of sample throughput, compared to liquid scintillation counting methods. The method described here can be used for routine measurements of Zr at environmentally relevant levels. It can also be combined with radiometric techniques for use towards the standardisation of Zr measurements. [ABSTRACT FROM AUTHOR]