Capabilities of high resolution ICP-OES for plutonium isotopic analysis

The potential of a commercial high resolution (HR-)ICP-OES instrument for the reliable determination of the plutonium (Pu) isotopes 238Pu, 239Pu, 240Pu, 241Pu, and 242Pu in nuclear samples was investigated. Using a low flow, high efficiency, desolvating nebulizer enhanced the sensitivity of the employed HR-ICP-OES set-up several-fold, decreasing the Pu concentrations required for analysis to < 1 mg/kg, especially those of the highly radioactive 238Pu. To achieve both optimum optical resolution and highest sensitivity the width of the available apertures of the spectrometer was fine-tuned, yielding unsurpassed values at 20 μm and 15 μm for the entrance slit and exit slit, respectively. Appropriate Pu emission wavelengths for potential spectroscopic Pu isotopic studies were identified recording the emission spectra of mono-isotope solutions of 238Pu, 239Pu, and 242Pu, respectively. HR-ICP-OES spectra of 238Pu are reported for the first time, allowing not only the identification of the accurate position of the peak maxima of the individual Pu isotopes, but also the assessment of their peak width, peak shape and potential hyperfine splitting of emission signals. The largest isotope shift was observed for the Pu emission wavelength regions centered around λ = 402.154 nm, λ = 439.645 nm, and λ = 437.990 nm. The distance between 238Pu and 242Pu HR-ICP-OES signals expanded to 23 pm, 18 pm, and 17 pm for these Pu emission wavelengths. Well separated 238Pu or 239Pu and 242Pu emission signals were obtained at almost all investigated Pu emission wavelengths. In practice, however, peak deconvolution of the spectra is necessary because typical samples contain predominantly 239Pu and 240Pu whose emission signals overlap with each other, as demonstrated on selected samples originating from the institute's research projects related to the nuclear fuel cycle.