Anal. Chem. 1998, 70, 4100-4103
Low Blank Isotope Ratio Measurements of Rhenium, Osmium, and Platinum Using Tantalum Filaments with Negative Thermal Ionization Mass Spectrometry Ke´iko Hattori,* Douglas P. Menagh, and T. James S. Cole
Department of Earth Sciences, University of Ottawa, Ottawa, K1N 6N5, Canada
Platinum is most commonly used as a filament for Re and Os isotopic measurements, but it contains impurities of Re and Os. Tantalum is low in platinum group elements (PGE) and in Re, but it is not used for negative thermal ionization mass spectrometry because of high electron emission and high reactivity with O2. High thermal electron emission from Ta distorts the preoptimized ion source optics. In addition, Ta consumes O2, leaving little for samples, but O2 is essential for isotopic ratio measurements of PGE and Re as they are measured as negatively charged oxides, such as OsO3- and PtO2-. These problems are solved by prebaking a filament to remove tantalum oxides before sample loading, keeping relatively high filament temperatures and high O2 pressures (PO2) during the sample run, and lowering the potential difference between the filament and the drawout plate. At PO2 of ∼1 × 10-5 Torr in the source, strong (>10 V) stable (>6 h) peaks of ReO4-, OsO3-, and PtO2are obtained at 750 °C for Re, 850 °C for Pt, and over 900 °C for Os. Accurate isotopic ratio measurements of Re, Os, and Pt at picogram levels are possible using Ta filaments. Rhenium and Os isotope ratios have been measured using Pt filaments.1,2 Malleable Pt has several advantages as a filament, such as a relatively high melting point (1772 °C) and a high work function for thermal electron emission. Platinum, however, commonly contains significant impurities of Re and of other platinum group elements (PGE). Small samples may require subtraction of the blank contribution from the filament.3,4 These impurities are not evenly distributed in Pt, which makes precise blank corrections difficult.5 Proposed procedures for reducing * Corresponding author: (fax) 613-562-5192; (e-mail)
[email protected]. (1) Vo ¨lkening, J.; Walczyk, T.; Heumann, K. Int. J. Mass Spectrom. Ion Processses 1991, 105, 147-159. (2) Creaser, R. A.; Papanastassiou, D. A.; Wasserburg, G. J. Geochim. Cosmochim. Acta 1991, 55, 397-401. (3) Anbar, A. D.; Creaser, R. A.; Papanastassiou, D. A.; Wasserburg, G. J. Geochim. Cosmochim. Acta 1992, 56, 4099-4103. (4) Yin, Q. Z.; Jagoutz, E.; Verkhovskiy, B.; Wa¨nke, H. Geochim. Cosmochim. Acta 1993, 57, 4119-4128. (5) Yin, Q. Z. N-TIMS Technique for the Re-Os and Ru isotopic systems and its application to selected geochemical and cosmochemical problems. Ph.D. Thesis, University of Mainz, 1995.
4100 Analytical Chemistry, Vol. 70, No. 19, October 1, 1998
the blanks include prebaking and acid leaching,6 but the impurities are locked in Pt crystal structures and cannot be eliminated by the treatment. To overcome the blank problem, Walczyk et al.7 proposed coating a Ni filament with V2O5 for Re isotope measurements. Nickel contains a high level of Re impurities, but the V2O5 coating substantially reduced Re blanks. The coating technique, however, is not suitable for Os isotope measurements. Small samples of Os need to be reduced to metallic form to obtain stable ion currents and Os would be oxidized to volatile OsO4 (mp ) 41 °C) by reacting with V2O5 during sample loading and heating the filament for the reduction. Tantalum has been widely used for Sr and Nd isotopic measurements in positive thermal ionization mass spectrometry (P-TIMS) because of its high melting point, 2996 °C, and high electron work function. The chemical properties of Ta and its natural occurrences are significantly different from those of Re and PGE, thus making Ta essentially free of these impurities. The use of Ta, however, has not been reported in N-TIMS probably because of its high reactivity with O2. Tantalum is easily oxidized, consuming O2. Oxygen is essential for isotopic ratio measurements of PGE because their isotopic ratios are determined from negatively charged oxides, such as OsO3- and PtO2-. Therefore, to compensate for O2 consumption by Ta filaments, the O2 supply requires adjustment. Here, we report that a Ta filament can be successfully used for the isotopic ratio measurements of Re, Os, and Pt with negligible blank contributions. Mass Spectrometric Procedure. All experiments were performed using a single filament assembly in a modified NBS mass spectrometer having 12 in. radius and 68° deflection. The source has an O2 supply system consisting of a stainless steel capillary and a bellows with a capacity of 500 mL containing 5 Torr-liter of O2. The magnet coils were rewound with rectangular Cu tube that has low resistance (total 0.06 Ω) and provided a fast settling (
