Optimising ICP-MS performance for the measurement of very low levels of radioisotopes in food and environmental matrices

Abstract

The development of novel analytical methods for the determination of radioelements in diverse environmental samples using state-of-the-art ICPTQMS system started in the early 2010’s with the introduction of the Agilent 8800 ICP-MS/MS system in 2012. A similar instrument, developed by Thermofisher (iCAP TQ ICP-MS) was used here for the first time for measuring a selection of radioelements in diverse marine matrices. This system is uniquely equipped with a 90o cylindrical ion lens named as Right Angular Positive Ion Deflection (‘RAPID’) ion optics that provides ion transmission across the entire mass range. By way of independent case studies, several instrumental parameters were tested, in particular the plasma condition, lens voltages and gas flow rates as well as the use of an additional desolvation system, manufactured by Teledyne Cetac Technologies - Aridus 3) and connected upstream to the ICAP TQMS system. First, these parameters were optimised for measuring low levels of 90Sr in milk and seawater samples. A significant reduction in dihydrate formation from the natural content of strontium ( 88Sr1H 1H – peak tail) was observed after successfully removing the main isobaric interferences (i.e., 90Y, 90Zr) using a recent chromatographic Sr-resin manufactured by Triskem International. To achieve an even lower limit of detection, samples were first introduced in the Aridus 3 desolvation system, using tested and optimised He, N2 gas flow rates. A good agreement was observed between the results obtained from ICPTQMS analysis and liquid scintillation counting. This novel radioanalytical method was found to be suitable for food and environmental samples presenting a 90Sr concentration of at least 50 Bq L-1 (equivalent to 0.01 pg g-1 approximately) and therefore for monitoring 90Sr more efficiently following an emergency incident or as part of ongoing nuclear decommissioning operations. Instrumental optimisation tests were also undertaken for measuring 129I in seawater and seaweed materials. The two European nuclear fuel reprocessing facilities, Sellafield and La Hague, are known to be the largest sources of iodine-129 is known to be mainly discharged from in the UK 3 marine environment. The main challenge associated with its measurement by mass is related to the presence of 129Xe impurities in the argon plasma gas and the tailing effect from stable 127I. To remove this isobaric interference, different O2 gas flow rates were tested in the collision/reaction cell of the ICPTQMS instrument. Once optimised parameters were retained, this technique was found to be approaching the performance that can be achieved via traditional LSC. A detection limit of 1.53×10-5 Bq g-1 (equivalent to 2.34 pg g-1 ) was achieved for 129I, which would be suitable for environmental monitoring purposes. Finally, the performance of the instrument was also investigated for direct analysis of 226Ra in seawater samples and solid certified reference materials provided by the IAEA (two marine sediments and one NORM waste). The same operating parameters were evaluated. A chromatographic separation, an anion exchange resin and Sr-resin, were also used prior to the ICPTQMS to attempt the removal of potential interferences prior to analysis. Although the sensitivity for measuring 226Ra was improved by using the desolvating system – Aridus 3. The retained chromatographic separative approach could not remove the main polyatomic interference composed of 138Ba and 88Sr, two chemical analogues of elemental radium. Further chemical work would then be required to validate the use of this analytical tool for measuring low levels of 226Ra in diverse marine matrices.

More information

Identifiers

ORCID for Phillip Warwick: orcid.org/0000-0001-8774-5125

ORCID for Andy Cundy: orcid.org/0000-0003-4368-2569

Catalogue record

Export record