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To accurately determine ultra-trace Pu isotopes in small environmental samples, we explored ICP-MS/MS in NH 3 –He mode, and investigated mechanism of 238U interference removal and measurement sensitivity improvement for plutonium isotopes. The interference of uranium and uranium hydrides was effectively eliminated using 0.4 mL/min NH 3 as reaction gas by shifting them to U(NH m) n + and UH(NH m) n +. The overall interference of uranium was reduced to <2.4 × 10−7, while remaining excellent 239Pu sensitivity (13,900 Mcps/(mg/L)) mainly due to ion focusing effect of Pu by helium gas. On this basis, the purification of plutonium using a single AG1- × 4 column was proved to be sufficient for accurate determination of plutonium isotopes by the developed detection method, and the detection limits for the method were estimated to be 0.16 fg (0.4 μBq) for 239Pu, 0.046 fg (0.4 μBq) for 240Pu and 0.039 fg (0.15 mBq) for 241Pu. The method was validated by analyzing plutonium isotopes in certificated reference materials and reported environmental samples of only 1–2 g. The analytical results of ultra-trace Pu isotopes in small amounts (∼1 g) of lake sediments obtained by the developed method were successfully applied to sediment dating. [Display omitted] • The overall interference of uranium was reduced to <2.4 × 10−7 by mass-shift mode. • Ion focusing effect of Pu by He gas and KED resulted in excellent Pu sensitivity. • Single AG1- × 4 column is sufficient for accurate measurement of Pu in small samples. • Low detection limits of 0.4 μBq for both 239Pu and 240Pu and 0.15 mBq for 241Pu. • Measured results of ultra-trace Pu isotopes succeed in sediment dating application. [ABSTRACT FROM AUTHOR]