Integrating Mass Spectrometry Techniques into Radionuclide Metrology

Summary

NIST is working to strengthen its expertise in activity measurements with additional measurement techniques. As many of its calibration and reference material customers use mass spectrometry, we are working to integrate activity (becquerel) with mass-based (mole) measurements. Furthermore, mass spectrometry techniques can precisely determine impurities in samples (e.g., radiopharmaceuticals), quantify trace radionuclide compositions, allow faster and better determinations of medium-and-long-lived nuclides, quantify the isotope composition of stable nuclides to be used as tracers for radionuclides, and aid certification of radionuclide reference materials.

Description

A core mission of the Radioactivity group—radionuclide metrology—is to develop and apply methods to precisely determine the activity (Bq) or massic activity (Bq/g) of a sample. We use primary activity measurement methods to ensure traceability to the SI [1]. Activity (A) can be expressed as a function of a nuclide’s half-life (t_1/2) and the number of atoms (N) of the nuclide through the relationship:

A = λN, where $$λ=\frac{ln (2)}{t_{1/2}}$$

 

(1)

N, the number of atoms, can be determined precisely using gravimetry of a known substance (e.g., a pure metal, dilution of a certified reference material, etc.) as a tracer. Activity can then be calculated using well-known half-lives.

Mass spectrometry allows the following to be determined in a sample:

• Isotopic Ratios/Isotope Amount Ratios

  e.g., n(²³⁵U)/n(²³⁸U)

  $$(\frac{n_a}{n_b})$$

• Isotopic Composition/Isotope Amount Fraction

  e.g., n(²³⁵U)/n(U)

  $$(\frac{n_a}{∑_i n_1})$$

• amount content (mol kg^-1)

Credit: BIPM

Furthermore, although isotope ratios require calibration to a standard, isotope dilution (ID) is considered a primary method and is directly traceable through gravimetry to the kg [2, 3]. In sum, this mass spectrometry program complements activity counting techniques because it offers higher sensitivity for long-lived radionuclides that are harder to observe by activity measurements in trace amounts.

The decay equation (1) allows the determination of the half-life of radionuclides. By leveraging precise activity measurements (the specialty of the Radioactivity Group) with precise atom counting (N) (the specialty of mass spectrometry), we can improve the half-life data for industrially- and medically relevant radionuclides. Improving known fundamental parameters of a nuclide improves that nuclide’s utility.

Current Projects

• ²³⁹Pu isotope ratio and amount content using ID for a reissue of SRM 4330
• Impurity assessment of ²²⁵Ac radiopharmaceuticals
• ID Calibration of SRM 4226 ⁶³Ni solution

Credit: NIST

Get Involved

Input is appreciated. Contact us and let us know what services or reference materials you need, attend a meeting such as Council on Ionizing Radiation Measurements & Standards (CIRMS: add your ideas/suggestions to its needs report), Radiobioassay & Radiochemical Measurements Conference (RRMC), or the International Conference on Methods and Applications of Radioanalytical Chemistry (MARC).

Interested collaborators should contact Mark Tyra (mark.tyra [at] nist.gov (mark[dot]tyra[at]nist[dot]gov)). Students looking to get involved can apply for the Summer Undergraduate Research Fellowship (SURF) or to the Summer Highschool Intern Program (SHIP).

Bibliography

1. Collé, R., Radionuclidic standardization by primary methods: An overview. Journal of Radioanalytical and Nuclear Chemistry, 2009. 280(2): p. 265-273.
2. Yu, L.L., J.D. Fassett, and W.F. Guthrie, Detection Limit of Isotope Dilution Mass Spectrometry. Analytical Chemistry, 2002. 74(15): p. 3887-3891.
3. Vogl, J. and W. Pritzkow, Isotope dilution mass spectrometry—a primary method of measurement and its role for RM certification. Mapan, 2010. 25(3): p. 135-164.