Interference-free determination of sub ng kg.sup.-1 levels of long-lived .sup.93Zr in the presence of high concentrations of .sup.93Mo and .sup.93Nb 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.sup.-1) 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 .sup.93Zr at very low (ng kg.sup.-1) levels in the presence of high levels ([mu]g kg.sup.-1) of the isobaric interferents .sup.93Nb and .sup.93Mo (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.sub.3/He/H.sub.2 or H.sub.2/O.sub.2, and suitable mass shifts, the signal from the .sup.93Nb and .sup.93Mo isobaric interferences on .sup.93Zr were suppressed by up to 5 orders of magnitude. The achieved limit of detection for .sup.93Zr was 1.3 x 10.sup.-5 Bq g.sup.-1 (equivalent to 0.14 ng kg.sup.-1). 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 .sup.93Zr at environmentally relevant levels. It can also be combined with radiometric techniques for use towards the standardisation of .sup.93Zr measurements.