The Determination of 226Ra in Nontypical Soil Samples by ICP-MS

Nov 01, 2011

This article describes a way to measure 226Ra using inductively coupled plasma–mass spectrometry (ICP-MS) rather than the conventional method of gamma spectroscopy, taking into account an undocumented interference (207Pb19F) that is caused by the requirement to use hydrofluoric acid during sample preparation. The unusually high Pb levels observed in a number of soil samples caused significant interferences at the very low concentrations of 226Ra that were measured. The expected 208Pb18O interference was insignificant under optimized instrument conditions.

As part of a remediation project, soils were analyzed for a list of elements of interest, plus a number of specific isotopes. Inductively coupled plasma–mass spectrometry (ICP-MS) has seen increasing use for the determination of long-lived isotopes in preference to traditional counting techniques because of various efficiencies. In this study, 230 Th and 226Ra were of specific interest. Typically, 226Ra has been determined using gamma spectroscopy. However, determination by gamma spectroscopy requires equilibration of the sample before final determination can be made, and full equilibration can require up to 30 days. Despite this, the initial phases of the remediation work did not allow for such a long delay period, hence ICP-MS was considered as a more rapid evaluation tool. Without this equilibration, there is danger of producing a significant low bias if gamma counting is used, which is a major factor if a reduction in equilibration times is considered. In matters of health and safety, it is always prudent to choose a method with a potential positive bias rather than one with a negative bias. Hence, it was decided that an ICP-MS method should be developed to speed up the determination of 226Ra in soil samples instead of simply reducing the equilibration period before gamma counting.

It became apparent during the analysis of these samples that they were not typical environmental soils, but more like mine tailings because of the mining and smelter activities over the last several decades in the area they were taken from. Besides having concentrations of many elements that exceeded environmental regulations in Ontario, the lead content in the soils caused an interference on the 226Ra determination by ICP-MS. The known Pb oxide (208 Pb18 O) interference on 226Ra was minimized by optimizing instrument conditions so as to have low oxide formation (<3%). The overall impact was therefore found to be insignificant in all of the samples examined.

However, an undocumented interference was caused by the requirement to use hydrofluoric acid (HF) in the sample preparation to get a complete digestion. The interference caused by 207 Pb19F, although much greater than that due to oxide formation, is not significant for normal soils. The unusually high Pb levels observed in a number of soil samples caused significant interferences at the very low concentrations of 226Ra being measured. This problem led to the present study.