This work reports the determination of fetogram plutonium using isotope dilution ICP-MS. The interference of protonated uranium at m/z of 239, with a determined UH^+/U^+ ratio of 4.6×10^-^5, was removed by using tri-n-octylamine extraction chromatography. The separation factor of 10^4 was achieved for uranium and the recovery of plutonium was greater than 75%. By applying isotope dilution, fetogram range of plutonium was measured using a high-resolution ICP- MS with a sample volume of less than 1mL. The limit of detection of ^2^3^9Pu is 4.5×10^-^1^5 g/mL for standard solution, and 16.3×10^- ^1^5g/mL for real sample with matrix. The concentration of ^2^3^9Pu in region atmosphere was determined at a level of (4.8±2.9)×10^- ^1^7g/m^3.

Key words: EXTRACTION, PLUTONIUM, TRIOCTYLAMINE