FURTHER INVESTIGATIONS ON OPTIMIZED TAIL CORRECTION AND HIGH-PRECISION MEASUREMENT OF URANIUM ISOTOPIC RATIOS USING MULTI-COLLECTOR ICP-MS

Abstract

In the present paper, we further examine the optimum conditions for rapid, precise and accurate determination of 234U/238U ratios in geological materials by multiple collector (magnetic-sector) inductively coupled plasma mass spectrometry (MC-ICP-MS). In our experiments, isotopic measurements were performed on a Micromass IsoProbe(TM) instrument, using Faraday collectors in static mode only. Unlike the ion counting system coupled with an energy filter, this technique eliminates the difficulty of proper calibration of the Daly/Faraday gain ratio. On the other hand, since our Micromass instrument has a poor abundance sensitivity (the proportion of the 238U ion beam measured at mass 237 is ~25 ppm), the major issue to be addressed is the tail correction. For this purpose, we have developed a tail correction method slightly modified from Thirlwall [J. Anal. At. Spectrom. 16 (2001) 1121]. This method is based on correction of the actual tail contribution under each peak, as assessed by the tail shape measurements on mono-isotopic ion beams, instead of the usual half-mass zeroes baseline estimation. Our approach enabled us to correct for the large offset that can be observed on isotopic data when tail correction is done by means of linear interpolation between half-mass zeroes, and showed that this latter tail correction method results in nearly 3% underestimation of 234U/238U ratios on the GEOTOP IsoProbe(TM) for material at secular equilibrium.A 236U-233U double spike was employed to correct for mass discrimination bias. Using an Aridus(TM) micro-concentric, desolvating nebulizer sample introducing system, a minimum of 200 ng of sample-U was consumed to carry out a precise 234U/238U analysis, thereby allowing a 234U signal of ~4-5 mV to be monitored for 50 measurement cycles of 5 s each. This time-consuming, 10-min procedure allowed us to obtain an external reproducibility of 0.8%% (2σ) for isotopic measurements of the NBL-112a standard solution. Replicate measurements of this reference material yielded a mean δ234U value of -36.42+/-0.80%% (2σ, n=19), which is highly consistent with values reported by other laboratories. The total reproducibility, including both chemical separation and spectrometric measurement, was assessed using geological samples (a coral and a carbonate rock); the long-term reproducibility obtained was about 1.3%% (2σ).