Isotope Ratio Analysis of NBS U010 Uranium Standard Using External NBS U500 Mass Fractionation Correction 

Introduction  

The NBS Uranium Standard U010 has been analysed in the STATIC Faraday multi-collection mode. This standard has a large range in isotope abundance and the accuracy and precision of an analysis are a good test of the linearity and noise levels of the Faraday cups.  

Because Uranium does not have a stable isotope ratio, any mass fractionation correction must use an external mass calibration. This is achieved by analysing a different standard of known isotopic composition under exactly the same analytical conditions. An ideal standard would have similar isotopic abundances to the unknown sample, but in order to challenge the Phoenix TIMS we have chosen to use NBS U500 which has a 235U/238U of 0.99969, two orders of magnitude larger than that of U010. 

Table 1. Eight replicate measurements of NBS U500 to determine the instrumental mass bias
External Fractionation corrected
Table 2. Replicate measurements of NBS U010 standard

Experimental 

Eight separate samples of two micrograms of U500 were analysed on triple Rhenium filaments. The results are shown in Table 1. The analyses were made with an ion beam of 3 volts for the 238U, and four blocks of 20, 10 second integrations.  

The mean 235U/238U is 1.0023, which provides a mass bias of 0.088% per mass unit. 

 A further set of measurements of the U010 standard were made. 2 micrograms of Uranium was analysed at a 238U intensity of 6 volts, such that the total ion intensity was similar to the NBS U500 measurements. 235U ion signals were 60mV, and the minor 234U and 236U less than 0.4mV. The results are shown in Table 2 and Figures 1 to 3 overleaf.  

Results  

The reproducibility of the 235U/238U is 0.04%. The minor isotopes 236U and 238U have measurement precisions of better than 0.5% and can be reproduced to 1.4% for the 234U/238U and 0.7% for the 236U/238U. 

 Following fractionation correction using the mass bias determined from U500, the 235U/238U ratio is 0.010139 which is within 0.01% of the certified value. This confirms that using the Phoenix TIMS, accurate fractionation correction can be applied even when the chosen isotopic standard has a composition that is 2 orders of magnitude different from that of the sample. 

Summary

Reproducibility of <0.05%1RSD can be achieved with NBS U010, and an accuracy of 0.01% using an external fractionation correction with NBS U500 

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