Ongoing Laboratory Performance Study on Chemical Analysis of Hydrophobic and Hydrophilic Compounds in Three Aquatic Passive Samplers.

The quality of chemical analysis is an important aspect of passive sampling-based environmental assessments. The present study reports on a proficiency testing program for the chemical analysis of hydrophobic organic compounds in silicone and low-density polyethylene (LDPE) passive samplers and hydrophilic compounds in polar organic chemical integrative samplers. The median between-laboratory coefficients of variation (CVs) of hydrophobic compound concentrations in the polymer phase were 33% (silicone) and 38% (LDPE), similar to the CVs obtained in four earlier rounds of this program. The median CV over all rounds was 32%. Much higher variabilities were observed for hydrophilic compound concentrations in the sorbent: 50% for the untransformed data and a factor of 1.6 after log transformation. Limiting the data to the best performing laboratories did not result in less variability. Data quality for hydrophilic compounds was only weakly related to the use of structurally identical internal standards and was unrelated to the choice of extraction solvent and extraction time. Standard deviations of the aqueous concentration estimates for hydrophobic compound sampling by the best performing laboratories were 0.21 log units for silicone and 0.27 log units for LDPE (factors of 1.6 to 1.9). The implications are that proficiency testing programs may give more realistic estimates of uncertainties in chemical analysis than within-laboratory quality control programs and that these high uncertainties should be taken into account in environmental assessments.

Determining the Mass Transfer Coefficient of the Water Boundary Layer at the Surface of Aquatic Integrative Passive Samplers.

Passive sampling devices (PSDs) offer key benefits for monitoring chemical water quality, but the uptake process of PSDs for hydrophilic compounds still needs to be better understood. Determining mass transfer coefficients of the water boundary layer (kw) during calibration experiments and in situ monitoring would contribute toward achieving this; it allows for combining calibration data obtained at different temperature and hydrodynamic conditions and facilitate the translation of laboratory-derived calibration data to field exposure. This study compared two kw measurement methods applied to extraction disk housings (Chemcatcher), namely, alabaster dissolution and dissipation of performance reference compounds (PRCs) from silicone. Alabaster- and PRC-based kw were measured at four flow velocities (5-40 cm s-1) and two temperatures (11 and 20 °C) in a channel system. Data were compared using a relationship based on Sherwood, Reynolds, and Schmidt numbers. Good agreement was observed between data obtained at both temperatures, and for the two methods. Data were well explained by a model for mass transfer to a flat plate under laminar flow. It was slightly adapted to provide a semi-empirical model accounting for the effects of housing design on hydrodynamics. The use of PRC-spiked silicone to obtain in situ integrative kw for Chemcatcher-type PSDs is also discussed.

Passive Sampler Exchange Kinetics in Large and Small Water Volumes Under Mixed Rate Control by Sorbent and Water Boundary Layer.

Exchange kinetics of organic compounds between passive samplers and water can be partly or completely controlled by transport in the sorbent. In such cases diffusion models are needed. A model is discussed that is based on a series of cosines (space) and exponentials (time). The model applies to mixed rate control by sorbent and water boundary layer under conditions of fixed aqueous concentrations (open systems, infinite water volumes, in situ sampling) and fixed amounts (closed systems, finite water volumes, ex situ sampling). Details on the implementation of the model in computational software and spreadsheet programs are discussed, including numerical accuracy. Key parameters are Biot number (ratio of internal/external transfer resistance) and sorbent/water phase ratio. Small Biot numbers are always indicative of rate control by the water boundary layer, but for large Biot numbers this may still be the case over short time scales. Application to environmental monitoring of nonpolar compounds showed that diffusion models are rarely needed for sampling with commonly used single-phase polymers. For determining sorption coefficients in batch incubations, the model demonstrated a profound effect of sorbent/water phase ratio on time to equilibrium. Application of the model to sampling of polar organic compounds by extraction disks with or without a membrane showed that moderate to major sorbent-controlled kinetics is likely to occur. This implies that the use of sampling rate models for such samplers needs to be reconsidered. Environ Toxicol Chem 2021;40:1241-1254. © 2021 SETAC.

Kinetic Passive Sampling: In Situ Calibration Using the Contaminant Mass Measured in Parallel Samplers with Different Thicknesses.

The use of single-phase passive samplers is a common method for sampling bioavailable concentrations of hydrophobic aquatic pollutants. Often such samplers are used in the kinetic stage, and in situ calibration is necessary. Most commonly, exchange kinetics are derived from the release rates of performance reference compounds (PRCs). In this study, a complementary calibration approach was developed, in which measuring the contaminant mass ratio (CMR) from two samplers with different thicknesses allows the dissolved concentrations to be determined. This new CMR calibration was tested (1) in a laboratory experiment with defined and constant concentrations and (2) in the field, at a storm water retention site. Silicone passive samplers with different thicknesses were used to sample a range of dissolved polycyclic aromatic hydrocarbons. In the laboratory study, the concentrations derived from the CMR calibration were compared with those from water extraction and passive dosing and differences below a factor 2 were found. In the field study, CMR-derived concentrations were compared to those from PRC calibration. Here, differences ranged by only a factor 1 to 3 between both methods. These findings indicate that the CMR calibration can be applied as a stand-alone or complementary calibration method for kinetic passive sampling.

POCIS Calibration for Organic Compound Sampling in Small Headwater Streams.

Field-based atrazine sampling rates (Rs ) obtained by the polar organic chemical integrative sampler (POCIS) method were measured in 9 headwater streams over 3 yr covering 5 to 6 exposure periods of 2 to 3 wk/site/yr. Rates were best in line with the model Rs = 148 mL/d, with a standard deviation of 0.17 log units (factor 1.5). The POCIS canisters reduced mass transfer coefficients of the water boundary layer by a factor of 2 as measured by alabaster dissolution rates. A mechanistic model that accounts for flow and temperature effects yielded a fair estimate of the effective exchange surface area (12.5 ± 0.8 cm2 ). This model could only be tested for higher flow velocities because of uncertainties associated with the measurement of flow velocities <1 cm/s. Pictures of sorbent distributions in POCIS devices showed that the effective exchange surface area varied with time during the exposures. Error analysis indicated that sorbent distributions and chemical analysis were minor error sources. Our main conclusion is that an atrazine sampling rate of 148 mL/d yielded consistent results for all 3 yr across 9 headwater streams. Environ Toxicol Chem 2020;39:1334-1342. © 2020 SETAC.

Passive samplers to quantify micropollutants in sewer overflows: accumulation behaviour and field validation for short pollution events.

Contaminants in sewer overflows can contribute to exceedances of environmental quality standards, thus the quantification of contaminants during rainfall events is of relevance. However, monitoring is challenged by i) high spatiotemporal variability of contaminants in events of hard-to-predict durations, and ii) a large number of remote sites, which would imply enormous efforts with traditional sampling equipment. Therefore, we evaluate the applicability of passive samplers (Empore styrene-divinylbenzene reverse phase sulfonated (SDB-RPS)) to monitor a set of 13 polar organic contaminants. We present calibration experiments at high temporal resolution to assess the rate limiting accumulation mechanisms for short events (<36 h), report parameters for typical sewer conditions and compare passive samplers with composite water samples in a field study (three locations, total 10 events). With sampling rates of 0.35-3.5 L/d for 1 h reference time, our calibration results indicate a high sensitivity of passive samplers to sample short, highly variable sewer overflows. The contaminant uptake kinetic shows a fast initial accumulation, which is not well represented with the typical first-order model. Our results indicate that mass transfer to passive samplers is either controlled by the water boundary layer and the sorbent, or by the sorbent alone. Overall, passive sampler concentration estimates are within a factor 0.4 to 3.1 in comparison to composite water samples in the field study. We conclude that passive samplers are a promising approach to monitor a large number of discharge sites although it cannot replace traditional stormwater quality sampling in some cases (e.g. exact load estimates, high temporal resolution). Passive samplers facilitate identifying and prioritizing locations that may require more detailed investigations.

Effects of hydrodynamic conditions and temperature on polar organic chemical integrative sampling rates.

The effects of changing hydrodynamic conditions and changing temperatures on polar organic chemical integrative sampler (POCIS) sampling rates (Rs ) were investigated for 12 crop protection chemicals. Exposure concentration was held constant in each laboratory experiment, and flow velocities were calculated from measured mass transfer coefficients of the water boundary layer near the surface of POCIS devices. At a given temperature Rs generally increased by a factor of 2 to 5 between a stagnant condition and higher flow velocities (6-21 cm/s), but Rs for most compounds was essentially constant between the higher flow velocities. When temperature was varied between 8 and 39 °C for a given flow condition, Rs increased linearly. In general, Rs increased by a factor of 2 to 4 and 2 to 8 over this temperature range under flow and stagnant conditions, respectively. An Arrhenius model was used to describe the dependence of POCIS sampling rates on temperature. Adjustments of Rs for temperature did not fully explain observed differences between time-weighted average concentrations of atrazine determined from POCIS and from composite water sampling in a field setting, suggesting that the effects of other competing factors still need to be evaluated. Environ Toxicol Chem 2018;37:2331-2339. © 2018 SETAC.

Review of atrazine sampling by polar organic chemical integrative samplers and Chemcatcher.

A key success factor for the performance of passive samplers is the proper calibration of sampling rates. Sampling rates for a wide range of polar organic compounds are available for Chemcatchers and polar organic chemical integrative samplers (POCIS), but the mechanistic models that are needed to understand the effects of exposure conditions on sampling rates need improvement. Literature data on atrazine sampling rates by these samplers were reviewed with the aim of assessing what can be learned from literature reports of this well-studied compound and identifying knowledge gaps related to the effects of flow and temperature. The flow dependency of sampling rates could be described by a mass transfer resistance model with 1 (POCIS) or 2 (Chemcatcher) adjustable parameters. Literature data were insufficient to evaluate the temperature effect on the sampling rates. An evaluation of reported sampler configurations showed that standardization of sampler design can be improved: for POCIS with respect to surface area and sorbent mass, and for Chemcatcher with respect to housing design. Several reports on atrazine sampling could not be used because the experimental setups were insufficiently described with respect to flow conditions. Recommendations are made for standardization of sampler layout and documentation of flow conditions in calibration studies. Environ Toxicol Chem 2018;37:1786-1798. © 2018 SETAC.

Aquatic Global Passive Sampling (AQUA-GAPS) Revisited: First Steps toward a Network of Networks for Monitoring Organic Contaminants in the Aquatic Environment.

Organic contaminants, in particular persistent organic pollutants (POPs), adversely affect water quality and aquatic food webs across the globe. As of now, there is no globally consistent information available on concentrations of dissolved POPs in water bodies. The advance of passive sampling techniques has made it possible to establish a global monitoring program for these compounds in the waters of the world, which we call the Aquatic Global Passive Sampling (AQUA-GAPS) network. A recent expert meeting discussed the background, motivations, and strategic approaches of AQUA-GAPS, and its implementation as a network of networks for monitoring organic contaminants (e.g., POPs and others contaminants of concern). Initially, AQUA-GAPS will demonstrate its operating principle via two proof-of-concept studies focused on the detection of legacy and emerging POPs in freshwater and coastal marine sites using both polyethylene and silicone passive samplers. AQUA-GAPS is set up as a decentralized network, which is open to other participants from around the world to participate in deployments and to initiate new studies. In particular, participants are sought to initiate deployments and studies investigating the presence of legacy and emerging POPs in Africa, Central, and South America.

A method to account for the effect of hydrodynamics on polar organic compound uptake by passive samplers.

Mass transfer coefficients of the water boundary layer (kw ) were measured using alabaster dissolution kinetics in a diffusion cell that was operated at stirring rates between 90 min-1 and 600 min-1 , aiming to provide a more robust characterization of the effect of hydrodynamics on the uptake of polar compounds by passive samplers, as compared with characterizations in terms of stirring rates and water flow velocities. The measured kw helped to quantitatively understand calcium sulfate transport through a poly(ethersulfone) membrane and 2 water boundary layers (at both sides of the membrane). Alabaster-based kw value were used to understand atrazine transport in the diffusion cell, allowing the conclusion that atrazine transport in the membrane is via the pore space, rather than via the polymer matrix. The merits of measuring alabaster dissolution rates for passive sampler calibration and application in the field are discussed. The authors propose that passive sampler calibrations be carried out under controlled kw conditions, rather than under controlled stirring rates or flow velocities. This would facilitate the interpretation of passive sampler calibration studies and the translation of laboratory-based water sampling rates to flow conditions that apply in the field. Environ Toxicol Chem 2017;36:1517-1524. © 2016 SETAC.

Laboratory performance study for passive sampling of nonpolar chemicals in water.

Two laboratory performance studies with 21 and 11 participants were carried out for passive sampling of nonpolar chemicals in water, using silicone samplers that were deployed for 7 wk and 13 wk at 2 river sites in the Netherlands. Target analytes were polychlorinated biphenyls, polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenyl ethers (PBDEs), hexachlorobutadiene, hexachlorobenzene, and a number of performance reference compounds (PRCs). Calculation of aqueous concentrations based on prescribed input values and a prescribed uptake model was also included. Between-laboratory coefficients of variation (CVs) in the analysis of target compounds were approximately 20% at concentrations of 100 ng g-1 and approximately 100% at concentrations of 0.01 ng g-1 , which was similar to previous results for the analysis of biota samples. The analysis of PRCs yielded water sampling rates with a between-laboratory CV of 18% to 30%. The sampling rate model showed a nearly perfect match with the consensus values of retained PRCs. The implications of the present study for future interlaboratory exercises are discussed. Environ Toxicol Chem 2017;36:1156-1161. © 2016 SETAC.

Passive Sampling in Regulatory Chemical Monitoring of Nonpolar Organic Compounds in the Aquatic Environment.

We reviewed compliance monitoring requirements in the European Union, the United States, and the Oslo-Paris Convention for the protection of the marine environment of the North-East Atlantic, and evaluated if these are met by passive sampling methods for nonpolar compounds. The strengths and shortcomings of passive sampling are assessed for water, sediments, and biota. Passive water sampling is a suitable technique for measuring concentrations of freely dissolved compounds. This method yields results that are incompatible with the EU's quality standard definition in terms of total concentrations in water, but this definition has little scientific basis. Insufficient quality control is a present weakness of passive sampling in water. Laboratory performance studies and the development of standardized methods are needed to improve data quality and to encourage the use of passive sampling by commercial laboratories and monitoring agencies. Successful prediction of bioaccumulation based on passive sampling is well documented for organisms at the lower trophic levels, but requires more research for higher levels. Despite the existence of several knowledge gaps, passive sampling presently is the best available technology for chemical monitoring of nonpolar organic compounds. Key issues to be addressed by scientists and environmental managers are outlined.

Passive samplers of hydrophobic organic chemicals reach equilibrium faster in the laboratory than in the field.

The use of passive sampling methods for monitoring hydrophobic organic chemicals frequently requires the determination of equilibration times and partition coefficients in the laboratory. These experiments are often carried out by exposing passive samplers in a finite water volume, and errors are easily made when the obtained results are applied to the field, where water volumes are essentially infinite. The effect of water volume on the equilibration rate constant is discussed, using a mechanistic model. Application of this model to two literature reports illustrates that aqueous concentrations in the field may be underestimated by a factor of 10 or more, when the water volume effect is neglected. Finally, it is shown that the concept of "sorption capacity" (sampler mass times partition coefficient) allows for a more intuitive understanding of the passive sampling process in small and large water volumes, which may reduce the risk of laboratory-field extrapolation errors.

Passive sampling of nonpolar contaminants at three deep-ocean sites.

Concentrations of polychlorinated biphenyls, polyaromatic hydrocarbons, hexachlorobenzene, and DDE were determined by passive sampling (semipermeable membrane devices) with exposure times of 1-1.5 years at 0.1-5 km depth in the Irminger Sea, the Canary Basin (both North Atlantic Ocean), and the Mozambique Channel (Indian Ocean). The dissipation of performance reference compounds revealed a pronounced effect of hydrostatic pressure on the sampler-water partition coefficients. Concentrations in the Irminger Sea were uniform over the entire water column (0.1-3 km). At the Canary Basin site, concentrations were 2-25 times lower near the bottom (5 km) than at 1.4 km. Concentrations in the Mozambique Channel (0.6-2.5 km) were lower than at the other two locations, and showed a near-bottom maximum. The data suggest that advection of surface waters down to a depth of about 1 km is an important mechanism of contaminant transport into the deep ocean.

Passive sampling of perfluorinated chemicals in water: in-situ calibration.

Perfluorinated chemicals (PFCs) have been recognised as environmental pollutants that require monitoring. A modified polar organic chemical integrative sampler (POCIS) is able to quantify aqueous PFCs. However, with varying external water velocity, PFC sampling rates (Rs) may change, affecting accuracy of derived water concentrations. To facilitate field deployment of this sampler, two methods of in-situ calibration were investigated: performance reference compounds (PRCs) and passive flow monitors (PFMs). Increased Rs's (by factors of 1.2-1.9) with PFM loss rate (g d(-1)) were observed for some PFCs. Results indicate PFMs can be used to correct PFC specific Rs's for more reliable estimates of environmental concentrations with a precision of about 0.01 L d(-1). Empirical models presented provide an improved means for aquatic monitoring of PFCs. The PRC approach was unsuccessful, confirming concern as to its applicability with such samplers.

Passive sampling of perfluorinated chemicals in water: flow rate effects on chemical uptake.

A recently developed modified polar organic chemical integrative sampler (POCIS) provides a means for monitoring perfluorinated chemicals (PFCs) in water. However, changes in external flow rates may alter POCIS sampling behaviour and consequently affect estimated water concentrations of analytes. In this work, uptake kinetics of selected PFCs, over 15 days, were investigated. A flow-through channel system was employed with spiked river water at flow rates between 0.02 and 0.34 m s(-1). PFC sampling rates (Rs) (0.09-0.29 L d(-1) depending on analyte and flow rate) increased from the lowest to highest flow rate employed for some PFCs (MW ≤ 464) but not for others (MW ≥ 500). Rs's for some of these smaller PFCs were increasingly less sensitive to flow rate as this increased within the range investigated. This device shows promise as a sampling tool to support monitoring efforts for PFCs in a range of flow rate conditions.

Multi-ratio equilibrium passive sampling method to estimate accessible and pore water concentrations of polycyclic aromatic hydrocarbons and polychlorinated biphenyls in sediment.

The freely dissolved concentration (C(w,0)) in the pore water and the accessible (releasable) concentration in the sediment (C(as,0)) are important parameters for risk assessment. These parameters were determined by equilibrating contaminated sediments and passive samplers using largely differing sampler­sediment ratios. This method is based on the principle that incubations at low sampler/sediment ratios yield the concentration in the pore water (minor depletion of the sediment phase) and incubations at high sampler/sediment ratios yield the accessible concentration in the sediment (maximum depletion of the sediment phase). It is shown that equilibration was faster in dense suspensions and at high sampler/sediment ratios when compared to low sampler/sediment ratios. An equilibrium distribution model was used to estimate C(w,0) and C(as,0) by nonlinear least-squares regression. The method was evaluated for three sediments (harbor, estuarine, marine). Accessible concentrations of 13 PAHs were 2 (low K(ow)) to 10 (high K(ow)) times lower than the total concentrations (three sediments). By contrast, the accessible concentrations of 15 PCBs were about 1.2 times lower than the total concentrations and displayed no trend with K(ow) (one sediment). Implications for risk assessment and considerations for application of multi-ratio equilibrium passive sampling with other sediments are discussed.

Use of passive sampling devices for monitoring and compliance checking of POP concentrations in water.

BACKGROUND: The state of the art of passive water sampling of (nonpolar) organic contaminants is presented. Its suitability for regulatory monitoring is discussed, with an emphasis on the information yielded by passive sampling devices (PSDs), their relevance and associated uncertainties. Almost all persistent organic pollutants (POPs) targeted by the Stockholm Convention are nonpolar or weakly polar, hydrophobic substances, making them ideal targets for sampling in water using PSDs. Widely used nonpolar PSDs include semi-permeable membrane devices, low-density polyethylene and silicone rubber. RESULTS AND DISCUSSION: The inter-laboratory variation of equilibrium partition constants between PSD and water is mostly 0.2-0.5 log units, depending on the exact matrix used. The sampling rate of PSDs is best determined by using performance reference compounds during field deployment. The major advantage of PSDs over alternative matrices applicable in trend monitoring (e.g. sediments or biota) is that the various sources of variance including analytical variance and natural environmental variance can be much better controlled, which in turn results in a reduction of the number of analysed samples required to obtain results with comparable statistical power. CONCLUSION: Compliance checking with regulatory limits and analysis of temporal and spatial contaminant trends are two possible fields of application. In contrast to the established use of nonpolar PSDs, polar samplers are insufficiently understood, but research is in progress to develop PSDs for the quantitative assessment of polar waterborne contaminants. In summary, PSD-based monitoring is a mature technique for the measurement of aqueous concentrations of apolar POPs, with a well-defined accuracy and precision.