text: Passive samplers were first developed in the early 1970s to monitor concentrations of airborne contaminants industrial workers might be exposed to, but by the 1990s researchers had developed and utilized passive samplers to monitor contaminants in the aqueous environment. The first type of passive sampler made for use in the aqueous environment was the semipermeable membrane device (SPMD). SPMDs could be used to determine time-weighted average concentrations of hydrophobic organic contaminants, but until the early 2000s passive sampling devices for metal contaminants had not yet emerged.
  Metals in the environment can speciate into different forms. Most metals dissolved in the aqueous environment are present as any of several ionic, complex-ion, and organically bound states. For most toxic metals, bioavailability is greatest for labile metals in their free ionic state. Recognizing the potential usefulness of a passive sampling device that could be used to measure trace amounts of bioavailable toxic metals, researchers at the United States Geological Survey (USGS) and University of Missouri began development on a counterpart to SPMDs that could be used to sample for labile metals.