Passive sampler devices

Systematic attempts have been made to develop passive sampling systems that accumulate chemicals, and from which reliable exposure concentrations can be calculated. The passive samplers used in such systems are usually designed either as kinetic samplers or as equilibrium samplers.  

The concept of the equilibrium sampler is analogous to that of the octanol-water equilibrium partition coefficient (fQ,w) used since the 1970s to predict the potential for persistent nonpolar contaminants to concentrate in aquatic organisms [71]. The use of equilibrium-t) e passive samplers in the aquatic environment depends on the development of a sampler-water partition coefficient (fCs ) defined as the ratio of sampler to water concentration of the compound of interest at thermod)mamic equilibrium. The other key parameter determining the utility of an equilibrium-type passive sampler is the time taken to reach an approximate equilibrium condition. A range of approaches applied in developing equilibrium-t)q)e passive samplers include polyethylene or silicon sheets of various volume to surface area ratio [72] and solid-phase microextraction techniques [73]. 

Examples of Passive Samplers for Trace Aquatic Pollutants 

Chemicals of Interest for Sampling Purposes Sampler Types Comments 

Nonpolar organic pollutants, for Semipermeable A relatively large surface area of nonpolar 

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In the following sections we highlight only selected works that have contributed toward the further development of passive samplers for SVOCs and/or HOCs. The literature related to the development and use of passive samplers for monitoring gases or VOCs in occupational environments is large. However, these publications are discussed only briefly, because lipid-containing semipermeable membrane devices (SPMDs) are primarily designed for SVOCs. 

Shoeib and Harner (2002) and Wania et al. (2003) separately developed large capacity passive samplers for integrafively monitoring the atmospheric transport of HOCs. The sorbents used in these devices act as an infinite sink for HOC vapors, and have been used earlier to actively sample large volumes of air and... 

Table 1.1 compares key aspects and performance characteristics of selected passive samplers, including the triolein-containing SPMD. Of the eight devices examined, only a few appear to have overlapping functions. Clearly, no one device can provide the desired data for all exposure scenarios. 

If the aim of an investigator is to determine equilibrium concentrations in samplers, then the residence time (tm) is a logical parameter to compare among samplers. The tm is the mean length of time that a molecule spends in a passive sampling device, where solute exchange follows first-order kinetics. Residence time is given by... 

Ockenden, W.A. Sweetman, A.J. Prest, H.E Steinnes, E. Jones, K.C. 1998b, Toward an understanding of the global atmospheric distribution of persistent organic pollutants The use of semipermeable membrane devices as time-integrated passive samplers. Environ. Sci. Technol. 32 2795-2803. 

Strandberg, B. Wagman, N. Bergqvist, P.-A. Haglund, R Rappe, C. 1997, Semipermeable membrane devices as passive samplers to determine organochlorine pollutants in compost Environ. Sci. Technol. 31 2960-2965. 

The authors have been intimately involved in eondueting research to address many aspects of environmental contaminants for about three decades. Historically, samples of environmental matrices, particularly water and air have been collected at narrow windows of time (i.e., minutes or several hours) which are not representative of the exposure experienced by organisms. Consequently, we initiated the development of what would ultimately be the semipermeable membrane device (SPMD). The SPMD has subsequently proven to be an effective passive sampler for a wide range of hydrophobic contaminants in multiple media. To date, there are more than 180 peer reviewed publications in the open scientific literature, where SPMDs are used for a variety of applications. Some of these publications are critical of the use of passive samplers for certain applications. However, constructive criticism has greatly aided in defining information gaps and limitations of the passive sampling approach. 

Passive personal samplers for NO2 and NO are an efficient collection device for these toxic gases and are accurate to - 10% for determining individual employee exposures to NO2 and/or NO during an entire workshift. It is speculated that other toxic gases, such as SO2 or CO, could also be collected by passive samplers, given that suitable adsorbents or absorbents could be identified and incorporated into the system. 

A number of models has been developed to improve the understanding of the kinetics of analyte transfer to passive samplers.9,12,19,37 These models are essential for understanding how the amount of analyte accumulated in a device relates to its concentration in the sampled aquatic environment as well as for the design and evaluation of laboratory calibration experiments. Models differ in the number of phases and simplifying assumptions that are taken into account, for example, the... 

The substance-specific kinetic constants, kx and k2, and partition coefficient Ksw (see Equations 3.1 and 3.2) can be determined in two ways. In theory, kinetic parameters characterizing the uptake of analytes can be estimated using semiempirical correlations employing mass transfer coefficients, physicochemical properties (mainly diffusivities and permeabilities in various media), and hydro-dynamic parameters.38 39 However, because of the complexity of the flow of water around passive sampling devices (usually nonstreamlined objects) during field exposures, it is difficult to estimate uptake parameters from first principles. In most cases, laboratory experiments are needed for the calibration of both equilibrium and kinetic samplers. 

Besides the advantages that passive sampling may offer, it is important to recognize that in many cases these devices measure a different fraction of contaminants than that defined for the checking of EQS compliance within the WFD. This becomes especially important when monitoring very hydrophobic chemicals (log Kow > 4), where a large fraction of the total amount present in a spot water sample is bound to colloids and particles. In contrast, most passive samplers used for monitoring hydrophobic compounds (e.g., SPMD, Chemcatcher, and silicone materials) measure only the truly dissolved fraction of these chemicals. 

Esteve-Turrillas, F.A., A. Pastor, V. Yusa, and M. De La Guardia. 2007. Using semi-permeable membrane devices as passive samplers. Trends Anal. Chem. 26 703-712. 

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