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.