Precise measurement of 228Ra/226Ra ratios and Ra concentrations in seawater samples by multi-collector ICP mass spectrometry

Abstract

We have developed a method for the precise measurement of ²²⁸Ra/²²⁶Ra ratios and ²²⁸Ra and ²²⁶Ra concentrations in seawater by multi-collector ICP mass spectrometry (MC-ICP-MS). The measurement of Ra isotopes has typically relied on decay counting techniques with large volume seawater samples. Adapting pre-concentration approaches from such previous studies, our new protocol pumps 50 to 150 L of seawater through Mn-fibre cartridges to pre-concentrate Ra. The Mn-fibres are ashed and then leached with 6 N HCl to remove Ra which is then precipitated with SrSO₄. The SrSO₄ is converted to SrCO₃ for dissolution, before purification of Ra using two ion-exchange columns—one with AG50-X8 resin and the other with Sr-spec resin. The resulting purified Ra aliquot from the Mn-fibre is used to assess the ²²⁸Ra/²²⁶Ra ratio of seawater. A separate 120 to 500 mL sample of the same seawater is also collected and used to assess ²²⁶Ra concentrations by MC-ICP-MS, by applying isotope dilution with a ²²⁸Ra spike. Seawater ²²⁸Ra concentrations are calculated by multiplying ²²⁸Ra/²²⁶Ra ratios from the Mn-fibre by ²²⁶Ra concentrations from the isotope dilution measurement. Measurements of ²²⁸Ra/²²⁶Ra ratios on an in-house Ra standard demonstrate an internal precision of 1.7% (2 S.E.) and an external repeatability of 2% (2 S.D.). Similar external repeatability is demonstrated for four 100 litre seawater samples from Southampton Docks. The accuracy of the technique, relative to commonly used decay-counting approaches, has been demonstrated for seawater ²²⁸Ra and ²²⁶Ra concentrations using samples collected as part of the GEOTRACES intercalibration effort. Using comparable volumes of seawater to those for existing analytical techniques, MC-ICP-MS provides a lower detection limit (∼0.1 fg ²²⁸Ra), higher precision (2%) and a shorter counting time (20 minutes) for measurement of ²²⁸Ra and ²²⁶Ra concentrations (although it requires a longer pretreatment time for Ra purification). This technique will allow study of ²²⁸Ra in the ocean interior where concentrations are low and will extend the powerful use of this isotope to understand mixing and trace element fluxes in the oceans.