HR: 12:05h
AN: V51K-08    [PDF]
TI: High Mass Resolution Plasma Mass Spectrometry of Cr Isotopes
AU: * Holmden, C
EM: chris.holmden@usask.ca
AU: Eglington, B M
EM: bruce.eglington@usask.ca
AB: Natural mass dependent fractionation of Cr isotopes (masses 50, 52, 53 and 54) is associated with redox reactions (Ellis et al., 2002). In reduced aqueous solutions, chromium (VI) is converted to insoluble chromium (III), which is accompanied by a 3 permil isotope effect. Thus the isotopes of Cr may have wide applicability in the earth sciences as a tracer of redox conditions in the solid earth, oceans and atmosphere over time. Our approach for measuring Cr isotopes utilizes the resolving power of the Finnigan Neptune operated in high mass resolution mode. Using a small diameter, high resolution entrance slit and wide diameter collector slits, flat-topped peak shoulders representing the un-interfered Cr isotopes can be located during a mass scan on the low mass side of composite, interfered peaks. Setting the collectors and magnet field to measure on these peak shoulders, near the high mass edge of the Faraday collector, clips the interfering ion beam preventing it from entering the collector. The largest plasma-generated interference is 40Ar14N on mass 54, comprising about 18 percent of the ion bean intensity in a 1 ppm Cr solution. Molecular interferences on the other isotopes of Cr are less than 0.1 percent, and are negligible in $>$10ppm Cr solutions. All experiments utilize the Finnigan stable introduction system and a PFA microflow nebulizer. Preliminary results show that about 4 permil of the ArN interference on mass 54 remains despite the peak shoulder measurement. This may be due to peak tailing from the ArN interference, which is a composite peak with an additional interference on the low mass side that is part of the HNO3 blank, and may accentuate tailing of the 54Cr peak. The 4 permil effect is easily removed by performing a blank subtraction. Normalized to a 50/52 ratio of 0.051859, we obtained ratios of 0.113452 and 0.028223 on 53/52 and 54/52, respectively, in 3 to 10 ppm solutions. Results thus far show that a reproducibility of better than +/-50 ppm (2 standard deviations) on the 53/52 ratio is easily achieved from Cr solutions of 3 to 10 ppm. To measure natural Cr isotope variations in nature requires sample-standard-bracketing or double spike addition to correct for instrumental mass fractionation. Experiments with a 50-54 double spike are currently underway, and these results will also be reported. Ellis et al., Science, V. 295, p. 2060
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting