Assessing the capabilities of the ICP-MS/MS for radionuclide analysis

Zacharauskas, Zilvinas (2022) Assessing the capabilities of the ICP-MS/MS for radionuclide analysis. University of Southampton, Doctoral Thesis, 161pp.

Record type: Thesis (Doctoral)

Abstract

The development of robust and routine methods for radionuclide analysis using ICP-MS/MS systems has been investigated for application to nuclear decommissioning and waste characterisation, as well as applications to nuclear forensics and emergency response scenarios. The advances of the Agilent 8800, ICP-MS/MS system have increased the range of potentially measurable radionuclides. Overcoming interferences through the use of the tandem mass spectrometers and the collision/ reaction cell, allowed for a faster throughput than traditional radiometric techniques. The study utilised the ICP-MS/MS in conjunction with robust chemical separation steps both offline and online to achieve the most complete interference removal and apply measurement to real world scenarios and matrixes, with specific interest in nuclear waste assays. An initial assessment of non-active ground water, stainless steel, air filter and non-active concrete samples were processed and analysed to demonstrate the potential sensitivities achievable by ICP-MS/MS for direct digest solutions, with high matrix concentration. Low instrumental backgrounds were achievable for many of the actinides, including ²³⁵U, ²³⁶U, ²³⁷Np, ²³⁹Pu, ²⁴¹Am as well as ⁵⁹Ni, ⁹³Zr,⁹⁹Tc, ¹²⁶Sn, ¹³⁵Cs and ¹⁵¹Sm when operating under no gas, MS/MS demonstrates the potential for emergency response analysis, where minimal sample handling and preparation would reduce sample turn around, and improve response times. The technique also offers a means for characterisation and screening of decommissioning waste and samples from a range of different matrix types including ground waters, steels, air filters and concretes, without the need for extensive separation and purification. A focused assessment was carried out on concrete samples, with stable concrete direct digest analysis demonstrating that the isotopes of ²³⁴U, ²³⁵U, ²³⁶U, ²³⁸U, ²³⁹Pu, ²⁴⁰Pu and²⁴¹Am could be measured well below their out-of-scope limits as demonstrated by the low background signal measured. The analysis of variable digest procedures utilising variable acid digests showed that in most cases acid attack in the form of concentrated HNO₃, HCl or a combination of (aqua regia) was not sufficient in liberating the majority of the target analytes from the concrete matrixes investigated. Complete sample digest (LiBO₂ fusion followed by HNO₃digest) was utilised on active concretes to demonstrate the need for complete sample digest to ensure all the actinides and specifically the potentially insoluble Pu species, such as PuO₂, would also be incorporated into the solution. The use of collision gases was shown to not have any benefit for actinide analysis and instead increased the uncertainty on the measurements due to signal dampening and reduction in overall signal sensitivity. The use of MS/MS mode, however, was able to reduce the impact from the bulk matrix loading, potential polyatomic interference formation within the cell and tailing from abundant neighbouring radionuclides. The use of no gas mode ensured maximum signal sensitivity for the target radionuclides. Method development was extended to ¹²⁹I to further demonstrate the capabilities of the ICP-MS/MS. Iodine-129 is a long-lived radionuclide formed most commonly from anthropogenic activity via nuclear fission, with the majority originating from reprocessing facilities and decommissioning of nuclear sites. The long half-life is theoretically suited to measurement using inductively coupled plasma mass spectrometry (ICP-MS), which offers a rapid alternative to decay counting and AMS techniques. However, previous measurement has been limited by the multiple interferences on m/z = 129. Recent availability of tandem ICP-MS/MS has demonstrated a means to overcoming these interferences using a combination of a collision-reaction cell and tandem mass spectrometer setup. This study builds on previous work with additional improvements in sensitivity (~60%increase) through matrix modification prior to sample introduction and demonstrates the importance of selecting an appropriate internal standard. The method was tested on various decommissioning wastes including silo liquors and slurries, ion exchange resins and graphite, with good agreement between ICP-MS/MS and liquid scintillation counting was observed. A detection limit of1.05×10^-4 Bq g^-1 (0.016 ng g-1 ) was achieved for ¹²⁹I, which is two orders of magnitude below the out-of-scope limit of 0.01 Bq g ^-1 (1.53 ng g^-1 ). The results put ICP-MS/MS in a position where routine out-of-scope measurement of¹²⁹I is now a possibility, which is beneficial for rapid assessment of materials at decommissioning nuclear sites, and for long term waste storage and disposal with potential development into environmental monitoring. Having assessed the instrument for direct digest analysis and method development of a specific radionuclide, the instrument was investigated for its capabilities in assessing isotope ratios which traditionally required extensive off line chemical separation and purification prior to analysis by multi-collector ICP-MS. The ICP-MS/MS offers the capabilities of doing direct digest analysis of samples while still being able to accurately measure isotope ratios. Lead isotope ratios were selected as a representative stable element system which is well understood for the anthropogenic source terms with results compared against MC-ICP-MS data. The technique was able to achieve measurement uncertainties <0.2% for the single quadrupole configuration and <0.6 %for ²⁰⁶Pb/²⁰⁴Pb and ~0.25 % for ²⁰⁶Pb/²⁰⁷Pb under MS/MS mode, self-aspirated and using NIST SRM 981 Pb standard sample bracketing. The use of MS/MS was shown to increase uncertainty however, for radiometric isotopic systems where interfering ions associated with isobars and neighbouring isotope tailing in to the m/z = ±1 and ±2, as well as polyatomic interferences, utilising the MS/MS, mass filtering capabilities of Q1 it is possible to reduce this impact and potentially allow for isotope ratios to be measured with results comparable to those achievable by MC-ICP-MS without the need for extensive sample preparation.