A portable detection apparatus includes a fluid inlet to acquire a stream of fluid, a fluid outlet and a fluid flowpath therebetween. A pump circulates the fluid through the fluid flowpath. A gamma spectrometer and a mercury analyzer engage the fluid flowpath to analyze and detect radiation emitted by the fluid. A filter trap is in the fluid flowpath downstream from the gamma spectrometer and the mercury analyzer. The filter trap includes a valve assembly and at least a first and second filter for collecting gaseous constituents from the fluid. Each filter is removably connected to the first valve assembly. The valve assembly has a first configuration, in which the first filter is fluidly connected to the fluid flowpath and the second filter is fluidly isolated from the fluid flowpath, and a second configuration, in which the second filter is fluidly connected to the fluid flowpath and the first filter is fluidly isolated from the fluid flowpath.