Quantification of ²³⁷Np is required for environmental monitoring, waste characterisation, and nuclear forensics. Isotope dilution mass spectrometry (IDMS) is an established measurement method that the Radiochemistry community uses for assay of trace-level analytes (< 1 μg). IDMS requires a sample and an isotopic spike of known molality to be weighed with a high degree of precision, mixed, equilibrated, and then measured. The preferred isotope dilution tracer for ²³⁷Np quantification is ²³⁶Np. However, there are no commercially available ²³⁶Np or other appropriate isotope tracers of suitable purity and traceability, which is a chronic issue for the Radioanalytical Chemistry community [1].

This presentation outlines alternative approaches and corresponding recommendations for measurement of ²³⁷Np where a suitable ²³⁶Np tracer is not available (Figure 1). Specifically, the production and calibration of ²³⁹Np for chemical yielding, chemical yielding measurements by gamma spectrometry, separation of ²³⁷Np from sample matrix, and a comparison of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) measurement techniques that do not require ²³⁶Np. These techniques include external calibration and IDMS using ²³³U as a tracer.

Method validation data from NIST Standard Reference Material 4341a and NPL Neptunium-237 Standard Solution R21-04-2020020437-1/A200005 are presented that demonstrate measurement accuracy at the sub-picogram level (< 1 × 10^-12 g) with estimated relative uncertainties of \(\sim \) 3 % (k = 1). Finally, the application of these measurements is discussed in the context of recent plutonium radiochronometry work (²³⁷Np/²⁴¹Pu atom ratio) from our laboratory [2], lower-level environmental samples (Standard Reference Material NIST 4350B, Columbia River Sediment), and a recent blind inter-laboratory comparison exercise.

[1] S.M. Jerome, K. Carney, R. Essex, M.E. Fassbender, S. Goldberg, M. Kinlaw, S.P. LaMont, D. Mackney, J.J. Morrison, F.M. Nortier Reference materials for neptunium determination, 2017, 126, 44-48.

[2] M.A. Higginson, J.A. Dunne, C.R.D. Gilligan, S.T.J. Cross, S. Dunn, Z. Varga, Application of plutonium radiochronometry to the analysis of plutonium age dating reference materials, 2022, 331, 3591-3600.