Mass Fractionation behaviour of U isotopes during Thermal Ionization, the potential for mass fractionation correction using double spike

Introduction  

Until recently it was assumed that the terrestrial ²³⁵U/²³⁸U was 137.88. However high precision ²³⁵U/²³⁸U measurements using ICP multi collector mass spectrometers1 as well as double spiked TIMS have shown small variations in ²³⁵U/²³⁸U at the 1- 2 per mil level. In uranium ores these variations are possibly due to changes in environmental redux conditions2 . Variations in terrestrial minerals of >5 per mil have also been reported³ , the reasons for these variations is not known.  

Whilst TIMS can detect these small variations in ²³⁵U/²³⁸U a very high quality correction of instrumental mass fractionation is required. Many elements e.g. Sr and Nd possess stable isotopes of known ratio which can be monitored during TIMS analysis and used to correct the radiogenic ratio for the effects of mass fractionation. Unfortunately U (as well as Pb) does not possess a stable isotope pair and mass fractionation must be corrected using an external correction factor, by totally evaporating the sample^4,6,7, or by using a double spike method^2,5.  

In the case of total evaporation, small residual mass biases can still exist at the 0.01% level. Analytical reproducibilty is ~0.02 to 0.03% 1RSD (0.2 to 0.3 per mil), which is too large to resolve terrestrial variations.  

The alternative is to use a double “spike” of known isotopic composition to correct for mass fractionation. For example a ²³³U/²³⁶U double spike has been shown to correct ²³⁵U/²³⁸U to a precision of 0.02% 2RSD⁵ .  

This work uses ²³⁴U/²³⁸U to fractionation correct ²³⁵U/²³⁸U in SRM U500 in a series of measurements on a Phoenix Thermal Ionization mass spectrometer. This is the reverse of the traditional method where the major isotopes ²³⁵U/²³⁸U are used to fractionation correct ²³⁴U/²³⁸U and ²³⁶U/²³⁸U⁶ . 

Method  

2 micrograms of SRM U500 was loaded on one side filament of a triple rhenium filament assembly. Following ion focussing an ion beam of 2e^-11 A was maintained during the analysis. Data was collected in the static mode in ten blocks of twenty, 10 second integrations. The sample was not analysed to exhaustion.  

Results  

1. Mass Fractionation during analysis and fractionation correction using ²³⁴U/²³⁸U.  

The results for one analysis are shown in Table 1. During the analysis the ²³⁵U/²³⁸U ratio reduces as would be expected (see also Figure 1). The ratio approximates the NBS value of 0.9997 by the 5th block. The precision of each block also improves to between 10 and 20ppm 1rse.  

The ²³⁴U/²³⁸U data also shows a reduction in ratio after the first 2 blocks of data. The precision of each block of data is approximately 0.015%. This precision is entirely consistent with counting statistics. For example the ²³⁴U ion beam is 2 x 10^-13 A (20mV). The noise associated with a 0.015% error (0.06% 1RSD) on a 20mV ion beam is: 2×10^-13 x 0.0006 = 1.2 x 10^-16 A, which is the equivalent to the noise level of 1e¹¹ ohm resistor for a 10 second integration.  

We can use the ²³⁴U/²³⁸U ratio to fractionation correct the ²³⁵U/²³⁸U. This is shown in Table 1 (²³⁵U/²³⁸U N column) and Figure 1 (pink line). A value of 0.010425 was taken as the reference ratio⁶ , and using an exponential correction a mean ratio of 0.99973 +/- 0.016% 1RSD is obtained for the 10 blocks of data. This is identical to the certified value. The precision is 4 times better than the uncorrected ²³⁵U/²³⁸U (1.00013+/- 0.06%). The precision of each block of the normalized data is worse than the raw ratio, because the largest source of error is the precision of the ²³⁴U/²³⁸U ratio, i.e. the counting statistics on the ²³⁴U measurement.

2. Robustness of this method.  

Table 2 shows the results of 16 analyses of U500 using the same analysis protocol. For each analysis the data are presented as the mean of each 200 ratio analysis and the errors are the percentage standard error. For the 16 different filaments the mean ²³⁵U/²³⁸U without fractionation correction is 1.00016 +/- 0.03%. This is higher than the certified value of 0.9997. The data is comparable to total evaporation data on 500ng loads which gave a ratio of 1.00014 +/-0.02%⁷  

Figure 2, is a plot of ²³⁵U/²³⁸U and ²³⁴U/²³⁸U expressed as a ppm variation in the isotope ratio with the first measurement as the reference. A mass dependent fractionation would define a linear array with a slope of 1.33. The data define a linear array with a slope of 1.4 which is within error of this value, consequently ²³⁴U/²³⁸U can accurately correct the ²³⁵U/²³⁸U, despite the two orders of magnitude difference in isotope abundance between ²³⁴Uand ²³⁸U. The fractionation corrected ²³⁵U/²³⁸U is 0.999586+/-0.009% 1RSD. The ratio is slightly lower than the certified value but is more precise by an order of magnitude than the certification (0.1%). An improvement in precision by a factor of 3 over the uncorrected data is achieved, and an accuracy relative to the certificate of 0.01%.  

Figure 3 shows what happens to the ²³⁵U/²³⁸U with different values of the normalizing ratio. There is a linear relationship between ²³⁵U/²³⁸U N and the ²³⁴U/²³⁸U ratio chosen as the normalizing ratio. In this work a ²³⁴U/²³⁸U of 0.010425 (Richter et. al. 2011) yields ²³⁵U/²³⁸N of 0.999584+/-0.00018 2RSD. Using the certified value of ²³⁴U/²³⁸U 0.010422 would result in a ²³⁵U/²³⁸U of 0.99937 which is not within error of the certified value. The data would therefore support the ²³⁴U/²³⁸U of Richter et. al 2011 as being correct, though a ratio of 0.0104265 would yield the precise value of the CRM ²³⁵U/²³⁸U of 0.9997  

3. Comparison with isotopic composition determined with double spike  

Condon et. al 2010⁵ used IRMM 3636 (²³³U-²³⁶U) double spike to measure ²³⁵U/²³⁸U in CRM U500 from two different laboratories and two different TIMS instruments. The ²³⁵U/²³⁸U was 1.00022+/-0.00016 2RSD. Our uncorrected values of 1.00016+/-0.00060 2RSD is identical within error to this, though both are within error of the CRM (+/-0.1%).  

There appears to be a significant difference between the double spiked value of 1.00022+/-0.00016 and our value of 0.99958+/-0.00018.  

One possible reason for this could be that peak tailing from ²³⁵U on our measurements could result in an over correction of the ²³⁴U, (lower ²³⁴U/²³⁸U). This can be rejected as the abundance sensitivity at -1amu was <2ppm of the major isotope. For a 2e^-11 A ion beam the uncorrected peak tail of ²³⁵U on ²³⁴U would be 4e^-17 A which is less than the Faraday noise for a 10 second integration. A lower measured ²³⁴U/²³⁸U would result in a normalized ²³⁵U/²³⁸U which was higher than 0.99958.  

An interference under the ²³⁴U or uncorrected peak tail, would result in a higher measured ²³⁴U/²³⁸U, producing a larger correction and hence a lower ²³⁵U/²³⁸U. This can be rejected because the measurements were made over several filaments and Figure 2 shows a significant correlation without outliers.  

Our data are consistent with those of Richter et.al. 2011. We therefore do not understand at the moment why the double spiked data is different from our data. 

Conclusions  

1. Internal mass fractionation correction of ²³⁵U/²³⁸U in NBS U500 using ²³⁴U/²³⁸U=0.010425 improves the precision of ²³⁵U/²³⁸U by a factor of 3 to <0.01% 1RSD, despite the large ²³⁴U/²³⁸U 
2. The ²³⁵U/²³⁸U = 0.999586 is within 0.01% of the certificate value of 0.99968. 

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