A technique used to isolate particles from an environmental sample by removing particles from the sample, spreading them on a fission track detector material, irradiating the detector with thermal neutrons and etching the fission tracks to identify the location of particles containing fissile isotopes (e.g. 239Pu or 235U). This method can be combined with thermal ionization mass spectrometry (TIMS) to provide the isotopic composition of uranium and plutonium in individual particles.