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[[File:Silicon firefighter tag.jpg|thumb|A [[firefighter]] wearing a [[silicone]] passive sampler on an elastic necklace, shaped like a [[dog tag]]]]
'''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 [[pollutant]]s in the environment over time. This is in contrast to [[Environmental monitoring#Grab samples|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.<ref name = Main>{{cite journal |last1=Górecki |first1=Tadeusz |last2=Namieśnik |first2=Jacek |title=Passive sampling |journal=Trends in Analytical Chemistry |date=2002 |volume=21 |issue=4 |pages=276–291 |doi=10.1016/S0165-9936(02)00407-7 }}</ref> Currently, passive samplers have been developed and deployed to detect toxic metals, [[pesticide]]s, [[Drug|pharmaceuticals]], [[radionuclide]]s, [[polycyclic aromatic hydrocarbon]]s (PAHs), [[polychlorinated biphenyl]]s (PCBs), and other organic compounds in water,<ref name = Chem1>{{cite journal |last1=Charriau |first1=Adeline |last2=Lissalde |first2=Sophie |last3=Poulier |first3=Gaëlle |last4=Mazzella |first4=Nicolas |last5=Buzier |first5=Rémy |last6=Guibaud |first6=Gilles |title=Overview of the Chemcatcher® for the passive sampling of various pollutants in aquatic environments Part A: Principles, calibration, preparation and analysis of the sampler |journal=Talanta |date=2016 |volume=148 |
==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 laws of diffusion|Fick's first law of diffusion]] and, depending on the passive sampler, a greater [[Binding constant|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 [[Chemical equilibrium|equilibrium]] with the surrounding medium. The use of passive sampling provides time-averaged concentrations of contaminants over the sampler's deployment period.<ref name = Main />
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.<ref>{{cite web |last1=Salter |first1=Eddie |title=Active versus passive air sampling |url=https://www.envirotech-online.com/article/health-and-safety/10/skc/active-versus-passive-air-sampling-eddie-salter/923 |website=Environmental Technology |publisher=Labmate Online |
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.
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*[[Antibiotic]]s<ref name = DGT />
*[[Bisphenol]]s<ref name = DGT />
*[[
*[[Flame retardant]]s<ref name = POCISSPMD />
*[[Furan]]s<ref name = POCISSPMD />
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==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 [[mussel]]s, living in the environment also "passively sample" contaminants ([[bioaccumulation]]) and can be used to monitor water pollution ([[Aquatic biomonitoring|biomonitoring]]).<ref name="Mussel">{{cite journal |last1=Sericano |first1=JL |last2=Wade |first2=TL |last3=Jackson |first3=TJ |last4=Brooks |first4=JM |last5=Tripp |first5=BW |last6=Farrington |first6=JW |last7=Mee |first7=LD |last8=Readmann |first8=JW |last9=Villeneuve |first9=JP |last10=Goldberg |first10=ED |title=Trace Organic Contamination in the Americas: An Overview of the US National Status & Trends and the International 'Mussel Watch' Programmes |journal=Marine Pollution Bulletin |date=1995 |volume=31 |issue=
===Chemcatcher===
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===Diffusive gradients in thin films (DGT)===
{{Main|Diffusive gradients in thin films}}
Diffusive gradients in thin films (DGT) samplers passively sample [[ion]]ic trace metals, as well as [[antibiotic]]s, [[oxyanion]]s, [[bisphenol]]s, and [[nanoparticle]]s 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
=== 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.<ref>{{Cite journal|last1=Fauvelle|first1=Vincent|last2=Kaserzon|first2=Sarit L.|last3=Montero|first3=Natalia|last4=Lissalde|first4=Sophie|last5=Allan|first5=Ian J.|last6=Mills|first6=Graham|last7=Mazzella|first7=Nicolas|last8=Mueller|first8=Jochen F.|last9=Booij|first9=Kees|date=2017-03-07|title=Dealing with Flow Effects on the Uptake of Polar Compounds by Passive Samplers|journal=Environmental Science & Technology|volume=51|issue=5|pages=2536–2537|doi=10.1021/acs.est.7b00558|pmid=28225255|bibcode=2017EnST...51.2536F|s2cid=206567423 |issn=0013-936X|doi-access=free}}</ref> The diffusive polyethylene layer prevents the thickness of the water-boundary layer (which is affected by flow) from dominating diffusion.<ref name=":0">{{Cite journal|last1=Fauvelle|first1=Vincent|last2=Montero|first2=Natalia|last3=Mueller|first3=Jochen F.|last4=Banks|first4=Andrew|last5=Mazzella|first5=Nicolas|last6=Kaserzon|first6=Sarit L.|date=2017|title=Glyphosate and AMPA passive sampling in freshwater using a microporous polyethylene diffusion sampler|journal=Chemosphere|volume=188|pages=241–248|doi=10.1016/j.chemosphere.2017.08.013|issn=1879-1298|pmid=28886558|bibcode=2017Chmsp.188..241F}}</ref> 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.<ref name=":0" /><ref>{{Cite journal|date=2020-02-20|title=Calibration and validation of a microporous polyethylene passive sampler for quantitative estimation of illicit drug and pharmaceutical and personal care product (PPCP) concentrations in wastewater influent|url=https://www.sciencedirect.com/science/article/abs/pii/S0048969719358863|journal=Science of the Total Environment|language=en|volume=704|article-number=135891|doi=10.1016/j.scitotenv.2019.135891|issn=0048-9697|last1=McKay|first1=Sarah|last2=Tscharke|first2=Ben|last3=Hawker|first3=Darryl|last4=Thompson|first4=Kristie|last5=O'Brien|first5=Jake|last6=Mueller|first6=Jochen F.|last7=Kaserzon|first7=Sarit|pmid=31838300|bibcode=2020ScTEn.70435891M|s2cid=209386153 |url-access=subscription|hdl=10072/396186|hdl-access=free}}</ref>
===Peepers===
Peepers are passive [[diffusion]] samplers used for metals in freshwater and marine [[sediment]] pore water, so they can be used to find areas that may have metal-contaminated sediments. Peepers are plastic vessels filled with clean water and covered in a [[Dialysis (biochemistry)|dialysis membrane]], which allows metals in sediment pore water to enter the water inside the peeper.<ref name="Peeper1">{{cite journal |last1=Serbst |first1=JR |last2=Burgess |first2=RM |last3=Kuhn |first3=A |last4=Edwards |first4=PA |last5=Cantwell |first5=MG |title=Precision of Dialysis (Peeper) Sampling of Cadmium in Marine Sediment Interstitial Water |journal=Archives of Environmental Contamination and Toxicology |date=2003 |volume=45 |issue=3 |pages=297–305 |pmid=14674581 |doi=10.1007/s00244-003-0114-5|bibcode=2003ArECT..45..297S |s2cid=23462490 }}</ref> They are usually placed deep enough into sediment to be in an [[Anoxic waters|anoxic]] environment, in which metals will be soluble enough to sample.<ref name = Missouri /> If the peepers are deployed long enough so the sediment pore water and contained peeper water reach equilibrium, they can accurately provide metal concentrations in sampled sediment pore water.<ref name = Peeper1 />
===Polar organic chemical integrative sampler (POCIS)===
{{Main|Polar organic chemical integrative sampler}}
Polar organic chemical integrative samplers (POCIS) sample [[Chemical polarity|polar]] organic contaminants with a log [[octanol-water
===Semipermeable membrane devices (SPMDs)===
{{Main|Semipermeable membrane devices}}
Semipermeable membrane devices (SPMDs) passively sample [[Chemical polarity|nonpolar]] organic contaminants with a log octanol-water partition coefficient (K<sub>ow</sub>) value greater than 3. Examples of these types of chemicals include [[polycyclic aromatic
[[File:Stabilized liquid membrane device (SLMD).jpg|thumb|75px |A 7.5 centimeter SLMD, filled with a 1:1 mixture of Kelex-100 and oleic acid.]]
===Stabilized liquid membrane devices (SLMDs)===
{{main|Stabilized liquid membrane devices}}
Stabilized liquid membrane devices (SLMDs) passively sample [[ion]]ic metals in freshwater. They are made of [[low-density polyethylene]] plastic tubing sections that are sealed on both ends and filled with an equal mixture of [[oleic acid]] and metal [[Chelation|chelating agent]]. They work by interacting with calcium and magnesium ions in freshwater, which forms a hydrophobic film on the outside the SLMD plastic membrane in which the chelating agent can bind to metals in the sampling water.<ref name = Brumbaugh /> They have been deployed for up to month-long periods in the field, alone or covered by a plastic tube housing to mediate water flow.<ref name="Missouri">{{cite journal |last1=Brumbaugh |first1=William G |last2=May |first2=Thomas W |last3=Besser |first3=John M |last4=Allert |first4=Ann L |last5=Schmitt |first5=Christopher J |title=Assessment of Elemental Concentrations in Streams of the New Lead Belt in Southeastern Missouri, 2002–05 |journal=USGS Report |series=Scientific Investigations Report |date=2007 |page=78 |publisher=United States Geological Survey|doi=10.3133/sir20075057 |bibcode=2007usgs.rept...78B }}</ref> Metal weight accumulated by a SLMD over its deployment period can be calculated and divided by the SLMD deployment time to get an average metal weight accumulated per time unit, but currently, no method has been developed to convert this to an average metal concentration. In addition, SLMD sampling rates greatly vary with water flow rate, which plastic housings can be used to control.<ref name = Brumbaugh /><ref name = Missouri />
==Passive sampling in air==
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 [[Lichen#Effects of air pollution|lichens]].<ref name = LichData /><ref name="Lichen">{{cite journal |last1=Garty |first1=J |title=Biomonitoring Atmospheric Heavy Metals with Lichens: Theory and Application |journal=Critical Reviews in Plant Sciences |date=2001 |volume=20 |issue=4 |
===Sorbent tubes===
{{Main|Sorbent tube}}
Sorbent tubes are passive samplers for [[volatile organic compound]]s (VOCs). They are glass tubes filled with adsorbing materials, such as [[charcoal]] or [[silica gel]], which the air to be sampled passes through. The adsorbing materials remove VOCs from the air that passes through them, and the VOCs can be desorbed and analyzed. Air concentrations can be calculated using the amount of air that flowed through the sorbent tube and the amount of contaminants desorbed.<ref name="Tubes">{{cite web |title=Sorbent tubes |url=http://airsamplingsolutions.com/index.php/sorbent-tubes/ |website=Air Sampling Solutions |publisher=Casella |
==Advantages==
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.<ref name = Main /> This integrative sampling method can also
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.<ref name = Sigma /> They may also be inexpensive and simple to construct, such as SLMDs, which only require sealed plastic tubing and two chemical components.<ref name = Brumbaugh />
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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.<ref name = Main /> 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]].<ref name="EPA">{{cite web |title=National Recommended Water Quality Criteria - Aquatic Life Criteria Table |url=https://www.epa.gov/wqc/national-recommended-water-quality-criteria-aquatic-life-criteria-table |publisher=United States Environmental Protection Agency |
==References==
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