1. Estimating the Formation Rates of Heavy Metal Polyatomic Species in Plutonium Analyses with an HR-MC-ICP-MS with a Desolvating Nebulizer
Alex Mitroshkov, Khris Olsen, and May-Lin Thomas
Overview
When analyzing for low concentrations of Pu at the femtogram levels found in samples associated with environmental and nuclear processes, interferences in the analyzed solution from heavy metal (HM) interaction in the plasma may cause a false positive detection of Pu in the samples. Even the most modern, high resolution multicollector ICP-MS (HR-MC-ICP-MS) systems are not capable of separating Pu isotopes from polyatomic interferences (PIs), such as 208PbNOH, 207PbO2, and 204Pb35C at the 239Pu mass range. The formation of PIs were previously investigated using a quadroupole single detector ICP-MS1 and found the modern desolvating nebulizers (e.g. APEX and Aridus II) significantly reduce the formation of PIs, which could interfere with the Pu measurements compared to similar analysis using a wet plasma nebulizer system. In order to assess the impact of heavy metal most likely to affect our measurements for Pu we conducted a study using state-of-the-art instrumentation for Pu measurements.
Objective
The objective of this study is to evaluate the effects of the presence of 5d and 6p heavy metal on the measurement of plutonium isotopes in solution samples associated with environmental and nuclear processes. Specific tasks were to determine:
The formation rates of PI from 12 heavy metals ranging from Hafnium to Bismuth on 239Pu, 240Pu, 241Pu, 242Pu, and 244Pu isotopes
Determine the concentration of HMs in selected number of samples associated with actual nuclear processes.
Method
This study implements the use of a HR-MC-ICP-MS equipped, with a desolvating nebulizer. The specifics of the instrumentation and standards used are listed below.
NU Plasma HR Multicollector ICP-MS (NU Instruments, United Kingdom) equipped with 11 Faraday cups and 3 ion counters as detectors,
An Autosampler ASX-112FR and Desolvating Nebulizer Aridus II (both CETAC, Omaha, Nebraska) were used for sample introduction.
The analytical conditions were optimized for Pu analyses.
The Quick Wash attachment was used for quick washout of Pu and heavy metals from the system after every analytical run.
CRM137 plutonium standard was used for system calibrations at a concentration of 2.52 ppt.
244Pu was used as a Instrument Dilution spiking solution at ~2.5 pg/mL.
Heavy metal solutions of Pb, Bi, Hf, W, Hg, Au, Re, Pt, Ta, Os, Ir, and Tl at 200 ppb in 4% HCl (Ultra Scientific, Rhode Island) were uses for calibration
Because of the low concentration of Pu in the samples, only ion counters were used with the three-cycle method for analyses of Pu and polyatomic species.
There are three steps in Pu isotopic ratio measurement by the HR-MC-ICP-MS:
Calibrate the instrument by measurements of background or “zeroes”
Calibrate ion counter gains and mass bias factor (MBiasF)
Conduct sample analysis
Each sample is divided into two fractions; non-tracered and tracered (with 244Pu added). Each fraction was analyzed separately.
The first sample analyzed in the analytical sequence was the non-tracered sample followed by the sample containing the 244Pu spike.
Results
The tables below present results for the formation rates of interfering species for the 12 heavy metal
Conclusions
Our experiments demonstrate:
The implementation of desolvating nebulizer coupled with the HR-MC-ICP- MS significantly reduces the formation of heavy metals molecular ions of which could interfere with the measurements of plutonium isotopic ratios. Although some of the data derived from this research are close to previously reported data, especially for Pb[1, 2, 3], in general they are significantly lower (by 2 to 3 orders of magnitude) for most elements, especially for Hg.
Concentrations of heavy metals measured in most samples from nuclear facilities were far from being able to significantly affect the Pu isotopic ratios measurements.
The measured formation rates of PIs allow for estimating their contribution to the Pu results and potentially for making corrections on those results, once the heavy metal concentrations have been determined. Such an approach was used before[1].
The separation procedure used for the removal of heavy metals from the Pu fraction was effective at removing potential interferences which could effect the 239Pu and 240Pu values. However, there still could be effects on the measurement of 241Pu and 242Pu.
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PIs Caused by the Presence of Rhenium, Calculated for Selected Samples
Comparison of Formation Rates
In [1] the rate for PbO2 is given for 208Pb16O2, which is an obvious misprint. The ion 239 is produced by 207Pb16O2.
In [2] the data are taken from the chart with multiple points.
239, Formation Rate
199Hg40Ar
191Ir16O3
207Pb16O2
This research
1.07E-08
8.90E-08
1.69E-09
Pointurier at al [1]
6.00E-06
3.10E-06
2.20E-08
Pointurier at al [2]
8.0E E-05
1.0E E-06
1.0E E-07
Magara et al [3]
3.00E-08 
References: [1] Fabien Pointurier, Philippe Hemet, and Amelie Hubert. J. Anal. At. Spectrom., 2008, 23, [2] Fabien Pointurier, Amelie Hubert, Anne-Laure Faure, Philippe Hemet, and Anne-Claire Pottin, J. Anal. At. Spectrom., 2011, 26, [3] Masaaki Magara, S. Ichimura, M. Takahasi, S. Kurosawa, Y. Saito, F. Esaka, K. Yasuda,
S. Sakurai, K. Watanabe, and S. Usuada. “Study on Interference of Polyatomic Ions in Isotope Ratio Measurement of Actinides by ICP-MS”, poster presented at Winter Conference on Plasma Spectrochemistry, Fort Lauderdale, Florida, January 5-10, 2004.
NU Plasma HR Multicollector ICP-MS
Reproduced with permission from NU Instruments, United Kingdom
For more information, contact:
Alexander Mitroshkov
Pacific Northwest National Laboratory, P.O. Box 999, MS-IN K3-62, Richland, WA
(509) ,
PNNL-SA-#####