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[[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|ionic]] metals in freshwater. They are made of [[Low-density polyethylene|low-density polyethylene]] plastic tubing sections that are sealed on both ends and filled with an equal mixture of [[Oleic acid|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></ref> 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 |date=2007 |accessdate=3 June 2018 |publisher=United States Geological Survey}}</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><ref name = Missouri></ref>
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