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.
Keywords: Isotopic ratio; Method validation; Plutonium; SF ICP-MS; Seawater; Traceability; Uncertainty.