Determination of low-level plutonium in seawater by sector field inductively coupled plasma mass spectrometry: method...

Sources of plutonium isotopes to the marine environment are well defined, both spatially and temporally which makes plutonium (Pu) a potential tracer for oceanic processes. This paper presents the optimisation and validation of an analytical procedure for ultra-trace determination of Pu isotopes (²³⁹Pu and ²⁴⁰Pu) in seawater based on the external calibration and sector field inductively coupled plasma mass spectrometry (SF ICP-MS) determination. Additionally, method for Pu isotope ratio (²⁴⁰Pu/²³⁹Pu) in marine samples is also discussed. A combination of two-step anion exchange (AG1-X8) and one-step extraction chromatography (TEVA) was very efficient resulting in uranium (U) decontamination factor of 5 × 10⁶-1 × 10⁸. A full validation approach in line with ISO 17025 standard and Eurachem guidelines was followed. With this in mind, blanks, recovery (87 ± 8 %, k = 2), within-laboratory repeatability (5.6 %), limits of detection (0.12 and 0.08 fg mL^-1 for ²³⁹Pu and ²⁴⁰Pu, respectively) and expanded uncertainty (13 %, k = 2) were systematically assessed. The procedure was applied for the determination of ²³⁹Pu and Pu in seawater sample coming from Mediterranean Sea. Obtained results were in good agreement with results obtained with alpha spectrometry, applied on the same seawater sample. Pu/²³⁹Pu atom ratio in seawater sample from the Mediterranean Sea was also determined. The precision and accuracy of ²⁴⁰Pu/²³⁹Pu isotopic ratio analysis were carefully examined using NBS-947 isotopic standard. ²⁴⁰Pu/²³⁹Pu ratio was found to be 0.187 ± 0.006 and is in agreement with accepted ratios for the global fallout of Pu.