1. Development of a Uranium Isotope Ratio Analysis Method for Biological Samples using Multi-Collector
Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) for Nuclear Forensics
Lance A. Schell1,2, John D. Brockman 2, J. David Robertson 1,2, Jim M. Guthrie 2
1 Department of Chemistry, University of Missouri, Columbia, Missouri
2 University of Missouri Research Reactor Center, Columbia, Missouri
ABSTRACT PerkinElmer 300X NexION Quadrupole ICP-MS
Monitoring the production of weapons-grade uranium in nations Spare hair and nail samples were digested using an Ethos Total concentration and isotope ratio measurements were made with a PerkinElmer 300X NexION quadrupole
suspected of pursuing nuclear weapons is a national security Plus microwave digestion station. NCS DC73347 Hair SRM inductively coupled plasma mass spectrometer (Q-ICP-MS) equipped with a FAST SC 4 DX sample introduction system
interest. Nuclear weapons inspections, such as those done by the was used as an additional hair sample for method using 4 mL of diluted samples and a 5-point calibration series with linearities of 0.999 or higher. In this instrument, ion
International Atomic Energy Association (IAEA), rely on the verification and instrument comparison. These materials measurements are made sequentially by a standing quadrupole, which contains alternating positive and negative
measurement of isotope ratios in environmental and atmospheric were digested with 1 mL concentrated HNO3 and 1 mL charges used to separate ions. Q-ICP-MS requires the least amount of sample preparation and is reliable for total
samples. A bioassay of personnel involved in enrichment or peroxide at 140° C for 10 minutes, followed by 25 minutes concentration measurements, but isotope ratios are sensitive to fluctuations in many different facets of the instrument
reprocessing would complement currently-used tests in a nuclear at 190° C. Samples were then diluted to 10 mL using ultra- including plasma flow and sample introduction system. There is no correction for ion gains/mass bias because all
inspection. Currently, urinalysis is used to monitor occupational pure water with a resistivity of 18.2 MΩ. measurements are performed using only one detector. Negligible fluctuation was seen in thallium internal standard.
exposure to uranium and other actinides in US laboratories. We
Figure 2. PerkinElmer 300X NexION • Samples were corrected for background noise using subtraction of counts for blanks and zero-point standards.
propose to assess hair and nail as a biomonitor of occupational
exposure to enriched uranium. Hair and nails are attractive Nu Plasma II Multi-Collector ICP-MS
because the concentration of uranium is at least an order of
magnitude higher than urine. Urine samples are also harder to Isotope ratio analysis was also performed using a Nu Plasma II multi-collector inductively coupled plasma
collect, store, and transport than hair and nail samples. mass spectrometer (MC-ICP-MS). As opposed to Q-ICP-MS, the ion beam is passed through an electrostatic
Finally, uranium in hair and nail is thought to represent long-term analyzer (ESA), which uses an electric field to ensure only ions with the same kinetic energy enter the
exposure to uranium over a time period of 3-12 months. An detection block. Once in the detection block, the beam is split in the plane of the detectors and ions are
effective bioassay for inclusion in a weapons inspection must read simultaneously (in coincidence). Coincident peaks are shown in Figure 1. MC-ICP-MS requires much
reflect exposure to occupational sources of enriched uranium. In more analysis time (around 10 minutes per sample) and extensive expertise for instrument setup and
this project, we developed the analytical methods to measure tuning, but a higher precision is achieved since the same beam is used throughout analysis.
uranium isotope ratios in biological samples using a PerkinElmer • Samples were corrected for mass fractionation using an exponential model described by Marechal et al.5
300X NexION Inductively Coupled Plasma Mass Spectrometer (Q- This corrects for different tendencies of isotopes to stray from the beam due to varying velocities.
ICP-MS) and a Nu Plasma II Multi-Collector Inductively Coupled • Ion counter gain corrections were applied by dividing the natural ratio by the measured ratio in a 10 ppb
Plasma Mass Spectrometer (MC-ICP-MS). The precision of the Figure 1. Standard Output of Nu Plasma II Showing standard to correct for differences encountered when using different detectors for analysis. Figure 3. Nu Plasma II MC-ICP-MS
isotope ratio measurement and the concentration of uranium in Coincident Peaks for Uranium Isotope Analysis
the hair and nail samples will be used to calculate the mass of
20% enriched uranium required to significantly deviate from the RESULTS AND DISCUSSION CONCLUSIONS
natural uranium isotope ratio. • Successfully developed a method for uranium isotope ratio analysis
Table 1. Comparison of Results for Q-ICP-MS and MC-ICP-MS using multi-collector inductively coupled plasma mass spectrometry
INTRODUCTION • Reported minimum amount of 235U necessary to distinguish
NexION 300X Q-ICP-MS Nu Plasma II MC-ICP-MS
The International Atomic Energy Association (IAEA) and other between enriched and natural uranium exposure in a bioassay
Individual and Amount U Amount U • Exposure to 0.998 and 0.31 picograms of uranium would be able
watchdog agencies measure uranium isotope ratios and monitor
Matrix Concentration 238U/235U Standard 235U/238U Standard
fission products in environmental and atmospheric samples as Required for Required for to significantly deviate from the natural ratio of 137.881 and
(ng/g) Deviation Deviation would be detectable by Q-ICP-MS and MC-ICP-MS, respectively.
part of a nuclear weapons inspection. Uranium (U) is a naturally- Deviation (pg) Deviation (pg)
occurring element present as many isotopes, including 238U • MC-ICP-MS was shown to have at least an order of magnitude
Individual 1 11.1 1.24 x 102 2.3 x 101 3.1 x 101 1.4 x 102 2.4 1.4
(99.274% by mass), 235U (0.72% by mass), and 234U (0.0057% by greater precision, making it preferable for isotope ratio analysis.
Nails
mass). Urinalysis assays assessing uranium exposure are well- Individual 2 7.2 1.33 x 102 5.9 x 101 3.4 x 102 1.4 x 102 1.3 4.8 x 10-1
developed1, but are sensitive to recent exposure and are difficult
REFERENCES
Individual 3 10.1 1.30 x 102 6.1 x 101 5.9 x 103 1.4 x 102 2.0 5.5
to collect and store. Hair and nail samples may be stored at room 1 Karpas, Z. Health Phys. 2001, 81, 460.
temperature and have been used in previous studies as Individual 1 24.3 1.41 x 102 2.5 x 101 6.0 x 100 1.4 x 102 2.5 x 102 N/A 2 Bjorkman, L.; Lundekvam, B. F.; Lægreid, T.; Bertelsen, B. I.;
Hair
biomonitors for other trace elements.2,3 MC-ICP-MS has been Individual 2 80.4 1.37 x 102 3.9 x 101 2.4 x 100 1.4 x 102 2.1 x 10-1 7.0 x 10-1 Morild, I.; Lilleng, P.; Lind, B.; Palm, B.; Vahter, M. Environmental
shown previously to be useful for 238U/234U ratio analysis.4 Health: A Global Access Science Source 2007, 6.
These matrices are a metabolically-stable endpoint and can be SRM 67.3 1.34 x 102 1.3 x 101 9.98 x 10-1 1.4 x 102 8.9 x 10-1 3.1 x 10-1 3 Brockman, J. D.; Guthrie, J. M.; Morris, J. S.; Davis, J.; Madsen, R.;
used in long-term situations far beyond the time of initial Measured total concentrations and isotopic ratios for Q-ICP-MS and MC-ICP-MS are provided in Table 1. The Q-ICP-MS measured 238U/235U ratio Robertson, J. D. J. Radioanal. Nucl. Chem. 2009, 279, 405.
exposure. Many techniques have proven useful for uranium RSDs ranged from 18 to 47. The cause of the low precision on this measurement is a result of a low signal-to-noise ratio for 235U as well as the
4 Karpas, Z.; Lorber, A.; Sela, H.; Paz-Tal, O.; Hagag, Y.; Kurttio, P.;
isotope analysis such as thermal ionization mass spectrometry previously mentioned fluctuations in the instrument. The ratio from the Nu Plasma II, measured in coincidence, had a precision at least a Salonen, L. Health Phys. 2005, 89, 315.
(TI-MS) which is very accurate, but requires extensive sample magnitude higher, with RSDs ranging from 0.14 to 1.77. Plots of the measured ratio as a percentage of the natural 238U/235U ratio for Q-ICP-MS and
5 Reference for Marechal…
preparation. Inductively coupled plasma mass spectrometry (ICP- MC-ICP-MS are provided in Figures 4 and 5, respectively. Exposure to 0.998 picograms of uranium would cause a significant shift in the natural
MS) is commercially-available and offers reduced standard uranium isotope ratio that could be detecting using Q-ICP-MS, while use of MC-ICP-MS only requires 0.31 picograms (p < 0.05). A concentration of
preparation times. Multi-collector ICP-MS offers a further uranium was not reported for hair of individual 1 due to negative constituents in the 95% confidence interval. While counts for the sample were
advantage in that it provides resolutions similar to TI-MS in above the limit of detection for Q-ICP-MS analysis, the reported
ACKNOWLEDGEMENTS
addition to shorter preparation times. In this study, these two Figure 4. Q-ICP-MS Ratio Analysis Figure 5. MC-ICP-MS Ratio Analysis
result is limited by the low signal-to-noise ratio of 235U. Analysis 1.10E+00 I would like to thank the University of Missouri Research Reactor
instruments are compared for use in U isotope ratio analysis. 1.60E+00
via MC-ICP-MS was problematic since the overall concentration
1.40E+00
Center for lab space and access to the ICP-MS instruments used in
of uranium in the sample was very low and a peak center could this study, Jim Guthrie for his assistance with the instrumentation
Measured / Natural 238U/235U Ratio
OBJECTIVES
1.05E+00
Measured/natural 238U/U235 ratio
1.20E+00
1.00E+00
not be found as part of the analysis procedure. This low and method development, and finally, Dr. J. David Robertson and Dr.
• Development of an analytical method for measurement of 238U / 8.00E-01 concentration of uranium could simply be a factor of the 1.00E+00
John D. Brockman for their substantial continued
235U ratio in nail and hair samples using multi-collector ICP-MS 6.00E-01 original sample size (0.03 g) compared to other original sample support, mentorship, and assistance throughout my research
• Determine and report amount of 235U necessary to distinguish 4.00E-01 sizes used in this study (0.1 – 0.2 g), which results in a very high 9.50E-01
endeavors.
between enriched and natural uranium exposure in a bioassay 2.00E-01 dilution factor. A future study would need certified SRM for
• Compare use of PerkinElmer 300X NexION and Nu Plasma II 0.00E+00
uranium to experimentally show deviation from the natural
Individual 1 Individual 2 Individual 3 Individual 1 Individual 2 SRM Hair
9.00E-01
Individual 1 Individual 2 Individual 3 Individual 1 Individual 2 SRM Hair This project was made possible through a grant from the University
Multi-Collector ICP Mass Spectrometers for isotopic ratio analysis
Nail Nail Nail
ratio before inclusion in routine nuclear forensic investigations.
Hair
Individual and Matrix
Hair Nail Nail Nail Hair
Individual and Matrix
Hair
of Missouri College of Arts and Science Undergraduate Research
Mentorship Program (COAS URMP).