Passive sampling

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.[1]

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.[9] 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.[6]

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[10]
• Bisphenols[10]
• Dioxins[5]
• Flame retardants[5]
• Furans[5]
• Nanoparticles[10]
• Oxyanions such as phosphates[10]
• Polybrominated diphenyl ethers (PBDEs)[5]
• Polycyclic aromatic hydrocarbons (PAHs)[5]
• Polychlorinated biphenyls (PCBs)[5]
• Pesticides[5]
• Pharmaceuticals[5]
• Radionuclides[2][3]
• Toxic metals (Such as copper, zinc, and lead)[4][10]

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).[11]

Semipermeable membrane devices (SPMDs)

Main article: Semipermeable membrane devices

Semipermeable membrane devices (SPMDs) passively sample nonpolar organic contaminants with a log octanol-water partition coefficient (K_ow) value greater than 3. Examples of these types of chemicals include polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), chlorinated pesticides, dioxins, and furans. SPMDs are composed of sealed plastic tubing filled with triolein, in which nonpolar organics are very soluble and which serves as a representation of the fatty tissues of aquatic organisms. The tubing is then weaved between metal rods and enclosed in a metal cage. The sampler can be made in varying lengths of tubing for different applications, since sampling rate depends on the surface area of tubing exposed to the water. As long as the amount of water passing over the sampler is known, nonpolar organic contaminant water concentrations can be calculated after extracting contaminants from a SPMD.[5]

A 7.5 centimeter SLMD, filled with a 1:1 mixture of Kelex-100 and oleic acid.

Passive sampling can also be accomplished for contaminants in the air, including airborne particles and hazardous vapors and gases. This can be done with man-made devices or with biomonitoring organisms, such as lichens.[7][14]

Contaminant concentrations from passive sampling reflect average contamination throughout the sampler deployment time, meaning the sample will capture contaminant concentration fluctuations over the whole deployment period. Traditional grab sampling does not do this, since collected samples only represent a single moment in time and multiple grab samples must be taken to observe variation in contaminant concentrations over time.[1] This integrative sampling method can also can result in the detection of chemicals present at such low concentrations that they would be undetected in a grab sample, due to concentration of the chemicals on the sampler over time. As a result, passive sampling has the potential to be a less time-intensive, less expensive and more accurate sampling method than grab sampling.

In addition, passive samplers are often easy to use and deploy, have no pumps or moving parts, and do not require electricity, since they rely on the molecular diffusion of contaminants or binding of contaminants to agents within the samplers, unlike active sampling.[6] They may also be inexpensive and simple to construct, such as SLMDs, which only require sealed plastic tubing and two chemical components.[4]

Passive sampling may also more accurately reflect metal concentrations that are bioavailable to organisms than other sampling methods. For example, the SPMD sampler uses a semipermeable membrane and triolein (a triglyceride), both of which mimic toxicant uptake by organism fatty tissue.[5] However, this depends on the type of passive sampler used, since some samplers, such as peepers, rely solely on molecular diffusion,[12] which may be too simple to be reflective of the complex processes of contaminant uptake in an organism.

Since passive sampling provides information about average contaminant concentrations, all possible concentrations over the sampler deployment time are included in this average value. However, there is no way of finding out the complete range of contaminant concentrations over the deployment time at a single site with only passive sampling.[1] If high and low concentrations of contaminants throughout the sampling period are needed, other sampling methods should be used in conjunction with passive sampling.

Not all passive samplers have universally accurate ways to convert contaminant masses accumulated into water concentrations, which are used in government regulation, such as the United States Environmental Protection Agency Clean Water Act.[15] With some samplers, as with the DGT, this can be done using equations developed for the samplers, but not all samplers have these. Passive sampler deployment time is also limited depending on the sampler's capacity; for example, SLMDs have been deployed for month-long periods, but may reach saturation and stop sampling much sooner if metal concentrations and water flow rates are high enough.[4] However, this issue is avoidable if literature on the relevant passive sampler is examined for background information about sampler capacity and ideal deployment times prior to deployment.