Passive sampling is an environmental monitoring technique involving the use of a collecting medium, such as a man-made device or biological organism, to accumulate chemical pollutants in the environment over time. This is in contrast to grab sampling, which involves taking a sample directly from the media of interest at one point in time. In passive sampling, average chemical concentrations are calculated over a device's deployment time, which avoids the need to visit a sampling site multiple times to collect multiple representative samples. Currently, passive samplers have been developed and deployed to detect toxic metals, pesticides, pharmaceuticals, radionuclides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and other organic compounds in water, while some passive samplers can detect hazardous substances in the air.
Theory and application
The underlying principle of passive sampling is the flow of contaminant molecules or ions from the sampling medium (air or water) onto a collecting medium (the passive sampler), due to Fick's first law of diffusion and, depending on the passive sampler, a greater binding affinity of contaminants with the collecting medium as compared to the sampling medium. As a result, contaminants concentrate on the collecting medium over time until they reach equilibrium with the surrounding medium. The use of passive sampling provides time-averaged concentrations of contaminants over the sampler's deployment period.
It is important to distinguish passive sampling from active sampling, which has the same underlying principle but employs moving parts, such as pumps, to force the sampling medium onto a collecting medium. Passive sampling solely relies on molecular diffusion and the sorbing or binding of contaminants to agents in the samplers, which is why passive sampling is also called diffusive sampling.
Passive sampling is also distinct from grab sampling, which is the collection of an air, water, or soil sample to analyze directly for contaminants. These samples represent a single point in time and provide information about contaminant concentration at one point in time, unlike passive sampling devices or organisms.
Many different kinds of passive samplers have been developed and have sampled many different contaminants, including: Antibiotics Bisphenols Dioxins Flame retardants Furans Nanoparticles Oxyanions such as phosphates Polybrominated diphenyl ethers (PBDEs) Polycyclic aromatic hydrocarbons (PAHs) Polychlorinated biphenyls (PCBs) Pesticides Pharmaceuticals Radionuclides Toxic metals (Such as copper, zinc, and lead)
Passive sampling in water
Several kinds of passive sampling devices exist for monitoring pollutants present in water. In addition to these devices, organisms, such as mussels, living in the environment also "passively sample" contaminants (bioaccumulation) and can be used to monitor water pollution (biomonitoring).
Chemcatcher
Chemcatcher can passively sample inorganic pollutants (metals) and a wide range of organic pollutants. It is composed of a single-use disc, with or without a membrane, sealed into a plastic support. Types of receiving phases and membranes vary greatly, depending on the target chemicals to be sampled. Time-averaged water concentrations of many chemical pollutants can be determined as long as sampling rate and the water flow rate are known.
Diffusive gradients in thin films (DGT)
Diffusive gradients in thin films (DGT) samplers passively sample ionic trace metals, as well as antibiotics, oxyanions, bisphenols, and nanoparticles in different configurations. They are composed of plastic pistons and caps, with a window that exposes a binding gel, diffusive gel, and filter membrane to the sampling water. They can be used in both freshwater and marine environments, as well as in the water located between freshwater and marine sediment particles, called pore water or interstitial water. Once the mass of accumulated contaminants on the DGT sampler is known, the DGT equation (based on Fick's law) can be used to calculate the time averaged water concentration of contaminants.
Microporous polyethylene tubes (MPT) Microporous polyethylene tubes (MPT) attempt to mitigate the flow-dependency of other kinetic passive samplers such as Chemcatcher and POCIS by introducing a thicker membrane. The diffusive polyethylene layer prevents the thickness of the water-boundary layer (which is affected by flow) from dominating diffusion. The tube is filled with sorbents depending on the chemicals or chemical groups being sampled and has been successfully used to sample glyphosate, AMPA, illicit drugs and pharmaceuticals and personal care products.