Age of individual uranium-plutonium (U/Pu) mixed particles with various U/Pu atomic ratios (1-70) were determined by inductively coupled plasma mass spectrometry. Micron-sized particles were prepared from U and Pu certified reference materials. The Pu reference was stored for 4-6 years since the last purification (July 14, 2008). The Pu purification age was obtained from the (241)Am/(241)Pu ratio which was calculated from the product of three measured ratios of Pu and Am isotopes in the eluted fractions. These ratios were measured by a high-resolution inductively coupled plasma mass spectrometer equipped with a desolvation system. Femto-gram to pico-gram quantities of Am, U, and Pu in a sample solution were sequentially separated on a small anion-exchange column. The (241)Am/(241)Pu ratio was accurately determined by spiking pure (243)Am into the sample solution. The average determined age for the particles for the five independent U/Pu ratios was in good agreement with the expected age with high accuracy (difference age 0.27 years) and high precision (standard deviation 0.44 years). The described analytical technique can serve as an effective tool for nuclear safeguards and environmental radiochemistry. Figure Young (4-6 y) Pu purification age of individual U/Pu mixed micron-sized reference particles for the five independent U/Pu ratios (1-70) were determined with 0.27±0.44 y difference from the expected age. Sub pico-gram quantities of Am, U and Pu were sequentially separated a small column, and their isotope ratios were accurately measured using an ICP-MS by applying the (243)Am spiking technique to the analysis and correcting the impurity and the contaminations.