IRMM184 Uranium Standard on Phoenix TIMS

Introduction

IRMM 184 has an extremely low level of ²³⁶U (0.12ppm relative to ²³⁸U) and a high quality analysis of this minor isotope requires the use of an ion counting detector equipped with a retarding filter.

Application Brief G20111 describes a multi-step analysis routine combining Faraday and Daly ioncounting to measure ²³⁴U/²³⁸U and in this note we extend the analysis to include the ²³⁶U isotope.

The measurements described in this note were obtained on a different spectrometer to that in G20111 highlighting the high degree of consistency between Phoenix instruments.

High Precision and Accuracy

To obtain high precision and accuracy the relative gain between the Daly and the Faradays must be measured extremely precisely. The most convenient method of achieving this is to use a multi-sequence routine where ²³⁵U is measured on both Faraday and Daly detectors permitting the Faraday/Daly gain to be accurately measured in real time (Table1).

Table 1. Analytical sequence

In the first sequence ²³⁴U is measured on the Daly while ²³⁵U and ²³⁸U are measured simultaneously on Faraday collectors. Sequence 2 is initiated with a peak jump which places mass ²³⁵U on the Daly with ²³⁸U on the H3 Faraday. Once Sequence 1 and 2 are complete the Faraday/Daly gain can be determined by dividing the ²³⁵U/²³⁸U measured in sequence1 (F/F) by that in sequence 2 (F/D). The excellent linearity of the Daly means that the gain calculated for ²³⁵U/²³⁸U can also be used to correct ²³⁴U/²³⁸U and ²³⁶U/²³⁸U even though the ratios are significantly different.

Finally, a fractionation correction is calculated by comparing ²³⁵U/²³⁸U from sequence 1 with the accepted value. This is then used to fractionation correct the already gain corrected ²³⁴U/²³⁸U and ²³⁶U/²³⁸U ratios.

Figure 1. Mass scan showing Faraday/Daly coincidence.

Abundance Sensitivity

A scan showing ²³⁴U ,²³⁵U and ²³⁶U on the ion-counting Daly (with WARP filter) is shown in Figure 1. The Daly trace is off-scale for ²³⁴U and ²³⁵U to allow visualization of the very small ²³⁶U signal. The other 3 traces show ²³⁸U on the H2, H3 and H4 Faradays.

Note the lack of any tailing from ²³⁸U on the high mass side of the ²³⁶U peak. With the WARP filter activated the abundance sensitivity measured at m/z 237 with respect to ²³⁸U is approximately 5e-9. At ²³⁶U the ²³⁸U contribution is not measurable on the Daly and so must be less than 1e-10 of the ²³⁸U intensity.

IRMM184 analyses

Single microgram loads of IRMM184 were loaded onto one side filament of a triple Re filament assembly. The routine shown in Table 1 was used with each sequence taking 10 seconds to give a total of 30 seconds per ratio measurement. A total of 200 ratio measurements were taken on each bead to give a total time taken per analysis including filament warm up and focussing of around 3 hours.

The results are shown in Table 2

Table 2. Data from 11 consecutive beads loaded with 1µg of IRMM184

F/D Gain Correction

The RSD of the Faraday/Daly gains measured across all 11 beads is 0.59%. However, the mean of the ‘within run’ gains is an order of magnitude better at 0.04%. This difference is most likely due to the varying position of the sample on each filament leading to differences in focus conditions and so a change in F/D gain. Once an individual bead is focussed and running the gain is very stable. This highlights the importance of using the within-run gain cancelling method as it automatically calculates the gain for each filament rather than assuming that a value calculated on one filament is equally applicable to others.

Results

The mean value for the gain and fractionation corrected ²³⁴U/²³⁸U is 0.00005309 +/- 0.09%. This compares well with 0.00005310+/-0.09% which was obtained on a second Phoenix instrument and matches closely with the certified value. The ²³⁶U/²³⁸U is 1.20e-7 +/- 3.0%. For a sample with this ratio, a ²³⁸U ion beam of 2.2e-11A would result in 236U count rate of around 10cps. The theoretical error on this small ion beam calculated from counting statistics is around 3% which closely matches the RSD recorded here. This is a powerful demonstration of the stability of the Phoenix ion-counting Daly.

Summary

A multi-static analysis routine which combines Faraday and ion counting Daly detector produces sub per mil ²³⁴U/²³⁸U data by online correction of the Faraday/Daly gain. The ²³⁶U can be accurately measured with a WARP filter removing the peak tail from the ²³⁸U

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Determination of 234U and 236U in IRMM184 Uranium Standard on the Phoenix Thermal Ionization Mass Spectrometer