High-precision measurements of seawater Pb isotope compositions by double spike thermal ionization mass spectrometry

Abstract

A new method for the determination of seawater Pb isotope compositions and concentrations was developed, which combines and optimizes previously published protocols for the separation and isotopic analysis of this element. For isotopic analysis, the procedure involves initial separation of Pb from 1 to 2L of seawater by co-precipitation with Mg hydroxide and further purification by a two stage anion exchange procedure. The Pb isotope measurements are subsequently carried out by thermal ionization mass spectrometry using a (207)Pb-(204)Pb double spike for correction of instrumental mass fractionation. These methods are associated with a total procedural Pb blank of 28±21 pg (1sd) and typical Pb recoveries of 40-60%. The Pb concentrations are determined by isotope dilution (ID) on 50 mL of seawater, using a simplified version of above methods. Analyses of multiple aliquots of six seawater samples yield a reproducibility of about ±1 to ±10% (1sd) for Pb concentrations of between 7 and 50 pmol/kg, where precision was primarily limited by the uncertainty of the blank correction (12±4 pg; 1sd). For the Pb isotope analyses, typical reproducibilities (±2sd) of 700-1500 ppm and 1000-2000 ppm were achieved for (207)Pb/(206)Pb, (208)Pb/(206)Pb and (206)Pb/(204)Pb, (207)Pb/(204)Pb, (208)Pb/(204)Pb, respectively. These results are superior to literature data that were obtained using plasma source mass spectrometry and they are at least a factor of five more precise for ratios involving the minor (204)Pb isotope. Both Pb concentration and isotope data, furthermore, show good agreement with published results for two seawater intercomparison samples of the GEOTRACES program. Finally, the new methods were applied to a seawater depth profile from the eastern South Atlantic. Both Pb contents and isotope compositions display a smooth evolution with depth, and no obvious outliers. Compared to previous Pb isotope data for seawater, the (206)Pb/(204)Pb ratios are well correlated with (207)Pb/(206)Pb, underlining the significant improvement achieved in the measurement of the minor (204)Pb isotope.

GGR Biennial Critical Review: Analytical Developments Since 2014

“…Calcium has been measured using a 48 Ca double spike, with measurement repeatability of ± 0.03‰ and measurement reproducibility of ± 0.13‰ reported for d 44/42 Ca measurements on 10-16 lg Ca by Lehn and Jacobson (2015), and with similar results obtained for about 5 lg Ca loads by He et al (2017). Lead has been measured using a 207 Pb double spike, with typical measurement repeatability on the 207 Pb/ 206 Pb ratio of 60 ppm and 40 ppm (2SE) achieved for 2 ng and 10 ng loads of Pb, respectively (Paul et al 2015). Osmium has been measured using a 188 Os double spike, with typical measurement repeatability of ± 0.01-0.02‰ and long-term measurement reproducibility of ± 0.03‰ attained on d 190/188 Os for 2.3-45 ng Os loads; a measurement repeatability of < 40 ppm and measurement reproducibility of 100-300 ppm can be simultaneously achieved for 187 Os/ 188 Os and 186 Os/ 188 Os ratios (Nanne et al 2017).…”

“…Lead has been measured using a 207 Pb‐ 204 Pb double spike, with typical measurement repeatability on the 207 Pb/ 206 Pb ratio of 60 ppm and 40 ppm (2SE) achieved for 2 ng and 10 ng loads of Pb, respectively (Paul et al . ). Osmium has been measured using a 188 Os‐ 190 Os double spike, with typical measurement repeatability of ± 0.01–0.02‰ and long‐term measurement reproducibility of ± 0.03‰ attained on δ 190/188 Os for 2.3–45 ng Os loads; a measurement repeatability of < 40 ppm and measurement reproducibility of 100–300 ppm can be simultaneously achieved for 187 Os/ 188 Os and 186 Os/ 188 Os ratios (Nanne et al .…”

Return of naturally sourced Pb to Atlantic surface waters

“…The Pb concentrations and isotope compositions of the unfiltered seawater samples were determined using thermal ionization mass spectrometry (TIMS) following previously described analytical techniques 66 at Imperial College London. Briefly, Pb concentrations were determined on 50 ml of seawater by isotope dilution.…”

Tracing the Agulhas leakage with lead isotopes

“…The black arrow indicates that the Pb isotope composition for fuel and major South African cities extends beyond the lower left corner of the diagram (see text for further details). The 2 SD error bars are estimated from the long‐term “external” reproducibility obtained in analyses of four in‐house seawater reference samples [ Paul et al ., ]. References for each field: surface seawater from Indian Ocean [ Lee et al ., ], surface seawater from SW Atlantic [ Alleman et al ., ], fuel, and Johannesburg City [ Monna et al ., ], Cape Town City [ Bollhöfer and Rosman , , ], coal [ Monna et al ., ; Díaz‐Somoano et al ., ], and desert dust [ Bollhöfer and Rosman , ; Kamber et al ., ; Vallelonga et al ., ].…”

“…Such a broad range is unlikely to account for the well-defined linear trend described by the coastal seawater samples, in particular, because the effects of such dust contributions are mitigated by the low solubility of Pb from silicate phases in seawater [Erel and Patterson, 1994]. The 2 SD error bars are estimated from the long-term "external" reproducibility obtained in analyses of four in-house seawater reference samples [Paul et al, 2015]. References for each field: surface seawater from Indian Ocean [Lee et al, 2015], surface seawater from SW Atlantic [Alleman et al, 2001b], fuel, and Johannesburg City [Monna et al, 2006], Cape Town City Rosman, 2000, 2002], coal [Monna et al, 2006;Díaz-Somoano et al, 2009], and desert dust [Bollhöfer and Rosman, 2000;Kamber et al, 2010;Vallelonga et al, 2010].…”