For many years, Lawrence Livermore National Laboratory has collected monthly air-particulate filter samples from a variety of environmental monitoring stations on and off site. Historically, the concentration and isotopic composition of uranium collected on these filters were determined by isotope dilution using a ²³³U spike and thermal ionization mass spectrometry (TIMS). For samples containing as little as 10 ng of uranium, ICP-MS is now used to make these measurements to the required level of precision, 5% in the measured 235/238 and 233/238. Unless particular care is taken to control bias in the mass filter, variable mass bias limits accuracy to a few percent. Measurements of the minor isotopes 236 (if present) and 234 are also possible and provide useful information for identifying the source of the uranium. The advantage of ICP-MS is in rapid analysis, ≈12 min of instrument time per sample.