USGS: SPMDs:  Passive sampling of organic compounds in water, air, and soil/sediment by SPMDs

  • Summary
  • Analytes
  • Revision
  • Data and Sites
Official Method Name
 Semipermeable Membrane Device
Current Revision
Accessed 2008
Media
 VARIOUS
Instrumentation
 Semipermeable Membrane Device (SPMD)
Method Subcategory
 Organic
Method Source
  USGS
Citation
  The Virtual Fish: SPMD Basics. Web site of the U.S. Geological Survey's Columbia Environmental Research Center. http://wwwaux.cerc.cr.usgs.gov/SPMD/index.htm
Brief Method Summary
 Lipid-containing SPMDs are a passive sampling technology for monitoring and assessing trace levels of hydrophobic organic contaminants. The SPMD is typically constructed from barefoot (no additives) layflat tubing of low-density polyethylene (LDPE). The thin-walled (<100 µm) LDPE tubing used in SPMDs is normally described as nonporous. However, random thermal motions of the polymer chains form transient cavities with maximum diameters of approximately 10 Å. Because these cavities are extremely small and dynamic, hydrophobic solutes are essentially solubilized by the polymer. The cross-sectional diameters of nearly all environmental contaminants are only slightly smaller than the polymeric cavities. Therefore, only dissolved (i.e., readily bioavailable) organic contaminants diffuse through the membrane and are concentrated through time. The sequestration media consist of both the thin film/plug of a large molecular weight (> 600 daltons) neutral lipid such as triolein and the LDPE membrane. Contaminant residues concentrated in SPMDs are simultaneously recovered and separated from the lipid in intact SPMDs (after carefully cleaning exterior surface of the membrane) by dialysis in an organic solvent.

SPMDs accomplish three tasks simultaneously:
  • Mimics the bioconcentration of organic contaminants in fatty tissues of organisms
  • Provides a highly reproducible passive in situ sampler for monitoring contaminant levels, which is largely unaffected by many environmental stressors that affect biomonitoring organisms
  • Enables in situ concentration of trace organic contaminant mixtures for toxicity assessments and toxicity identification evaluation (TIE)
Scope and Application
 SPMDs can be used to sample air (vapor phase), water (surface and groundwater), and sediment-soil (pore-water and soil vapor phase). They have been used in environments from the tropics to the Arctic and Antarctic, and in highly turbulent systems to stagnant backwater areas. In general, water quality parameters do not affect SPMD sampling of solutes, but in some cases (e.g., pH and DOC) may affect the amount of chemical available for uptake.

Current applications of SPMDs include: a) determination of pollutant sources and relative levels at different locations, b) estimation of ambient solute or vapor phase time weighted average concentrations, c) in situ biomimetic concentration of ambient bioavailable chemicals for bioassay and immunoassay, d) estimation of organism exposure or bioconcentration potential, e) analytical enrichment of contaminant residues, and f) use in toxicity identification evaluation procedures.
Applicable Concentration Range
Interferences
 SPMD sampling rates rise with water temperature increases and fall when air temperature increases. Also, pH affects the amount of ionizable compounds (eg., chlorinated phenols) sampled, and the magnitude of this effect is proportional to the increase or decrease in the neutral species.

Biofouling may impede but does not stop the uptake of chemicals by SPMDs. Laboratory studies have shown that the uptake of some compounds by heavily fouled SPMDs is reduced by as much as 69 %.

Streamflow rates have only small effects on the sampling rate of SPMDs because transport across the membrane is the rate-limiting step. Larger flows and turbulence can have an effect but can be minimized by designing deployment apparatuses with the capacity to baffle flow. Permeability/performance reference compounds (PRCs) are available and can be used to adjust SPMD-derived estimates of ambient concentrations to reflect site-specific environmental conditions of an exposure.
Quality Control Requirements
 The exact level of QC required is determined during the development of the experimental design phase of a project and is dependent on project goals. At a minimum, QC samples should address: deployment, retrieval, storage, processing, fractionation, enrichment, and analysis. QC samples should represent 20 to 50-percent of the sample set and include SPMD-fabrication blanks, SPMD-process blanks, reagent blanks, field-blank SPMDs, permeability reference compound samples, SPMD spikes, and procedural spikes.
Sample Handling
  • Typically, SPMDs are stored and shipped in clean gas-tight metal cans of various sizes
  • Metal containment structures (storage cans and deployment devices) must be free of cutting oils or other potential interferences
  • Minimize use of plastic components, except Teflon and some types of PVC, due to the possible presence of leachable organic residues
  • The structural design of the deployment device should minimize abrasion of the membrane even in turbulent environments while baffling the very high flow/turbulence of some media
  • Current velocity/turbulence is also a concern in terms of tethering, especially during floods
  • If a loop design (SPMD) is used, the two sides should not make contact
  • If water turbidity is low, then a shading structure may be required for analytes such as PAHs that undergo photolysis (caution: estimated photolysis half-lives of PAHs in direct sunlight range from 0.1 h to 5 h)
  • For sampling PAHs from air, the deployment structure must reduce ambient sunlight levels to near zero
  • Unless permeability/performance reference compounds (PRCs) are used (see description of PRCs in subsequent section), the flow and temperature regime of exposure sites should be similar to facilitate inter-site comparisons
  • Because vandalism is always a potential problem in the field, the deployment structure should be amenable to hiding
  • Deployment structures are commercially available
Maximum Holding Time
Relative Cost
 Greater than $400
Sample Preparation Methods
WHO WE ARE
  • NWQMC
  • USGS
  • EPA
LEGAL