An international intercomparison exercise on passive samplers (DGT) for monitoring metals in marine waters under a regulatory context

https://doi.org/10.1016/j.scitotenv.2022.157499Get rights and content

Highlights

  • ILC European exercise focused on the use of DGT technique in the WFD framework.

  • Participation of nine expert laboratories of the Interreg MONITOOL consortium

  • All performance steps during processing and analysis of DGT samples were compared.

  • Cd, Ni and Pb reproducible concentrations were obtained by most laboratories.

  • DGT analysis can be performed acceptably by laboratories with relevant experience.

Abstract

In order to move forward in the acceptance of a novel contaminant monitoring technique (Diffusive Gradients in Thin-films: DGT) for assessment of marine water bodies, sensu the WFD, an Inter-Laboratories Comparison (ILC) exercise (nine Europeans laboratories) was organized in the framework of the Interreg Atlantic Area MONITOOL project, which focused on the use of the DGT technique for the measurement of WFD priority metals (Cd, Ni and Pb).

Reproducible results were obtained for each metal by several laboratories, supporting the assertion that DGT analysis can be performed satisfactorily by laboratories experienced in measuring metals at trace levels in marine environments, even if they have limited practice in DGT analysis.

According to the Z-score analysis, among the 9 participating laboratories, 3 had 100 % of satisfactory results for Cd, Ni, and Pb, 3 had >80 % satisfactory results and 2 had about 60 % satisfactory results.

This work highlights the need to clearly describe the DGT method in order to control sources of contamination during analytical steps, in particular the resin gel retrieval and the elution steps.

Such international intercomparison exercise is an important step to develop the laboratory network involved in DGT analysis and contributes to the improvement of data quality.

Graphical abstract

Participating laboratories Z-Scores for Cd, Ni and Pb (X-axis: participating laboratories anonymously represented by a number)

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Introduction

The European Water Framework Directive (WFD; 2000/60/EC) aims to achieve a good ecological and chemical status for all European Union water bodies, including transitional/estuarine and coastal waters. The assessment of the chemical status of a water body is usually based on the collection of spot water samples and the comparison of the concentrations of a list of priority substances, defined at European level, to the existing Environmental Quality Standards (EQS; Directive 2013/39/EU). Regarding metals, annual average and maximum allowable concentration EQS (AA-EQS and MAC-EQS) are expressed as the mean or maximum dissolved (i.e., 0.45 μm filtered water) concentration, respectively, measured in 12-monthly spot water samples. This approach presents several shortcomings, such as (i) the risk of contamination during the handling and pre-treatment (i.e., filtration, preconcentration, extraction) of water samples before analysis, notably when working at trace levels, such as in marine waters, and (ii) the lack of representativeness of “one-off” samples, especially in highly dynamic systems like transitional and coastal waters (Vrana et al., 2005), leading to the potential under/overestimation of real concentrations (Twiss and Moffett, 2002; Dunn et al., 2003; Vrana et al., 2005; Allan et al., 2006a, Allan et al., 2006b).

Passive sampling technique has been proposed as a good solution to address these problems and to obtain reliable time-integrated concentrations for the chemical assessment of water bodies (Vrana et al., 2005). This technique consists of the deployment of devices containing a resin/polymer that presents a high affinity towards the compounds of interest. Passive samplers (PS), while in the kinetic phase, accumulate contaminants continuously during the deployment time, and enable the measurement of time weight averaged concentrations (TWA) of contaminants. This technique enables the in situ measurement of solutes and reduces the potential contamination of the samples, while lowering the limit of quantification (LOQ). Moreover, the PS technique is based on the diffusion of contaminants and the accumulation of the most labile species or forms, which is considered a better proxy of the potential bioavailable concentration. Among PS, Diffusive Gradients in Thin-films (DGT; Davison and Zhang, 1994) is the commonly used PS for the measurement of metals and has been applied in a wide variety of aqueous environments (Zhang and Davison, 1995; Davison et al., 2000; Twiss and Moffett, 2002; Dunn et al., 2003; Forsberg et al., 2006; Lafabrie et al., 2007; Schintu et al., 2008, Schintu et al., 2010; Gonzalez et al., 2013, Gonzalez et al., 2015, Gonzalez et al., 2015; Montero et al., 2012; Marras et al., 2020; Rodrigo Sanz et al., 2021).

Trace metal concentrations in the marine environment have been monitored for years, often determined on grab samples after filtration at 0.45 μm (Kremling and Hydes, 1988; Fileman et al., 1991; Cotté-Krief et al., 2002; Boutier et al., 2000; Lagerström et al., 2013; Gao et al., 2019; Bersuder et al., 2021; Caetano et al., 2022). In this sense, the directive establishes that the analytical methods used for compliance should present a maximum uncertainty of measurement of 50 % at the level of relevant EQS and a LOQ equal or below a value of 30 % of the relevant EQS. However, at low concentrations (i.e., at EQS level), any error introduced during the sampling and processing of water samples (e.g., filtration, preconcentration), especially when working at trace levels such as in marine waters, will represent a significant proportion of the chemical originally present, introducing a high uncertainty in the process. This is also true for DGTs, but the use of laboratory and field blanks, enables to reduce the uncertainty associated to the potential contamination of the samples. In this sense, measured DGT concentrations can be safely used when the DGT blanks account for <10 % of the concentration found in deployed DGTs (e.g., Buzier et al., 2014; Marras et al., 2020; Rodríguez et al., 2021). At higher blank concentrations, subtraction could be considered if the contamination among samples is reproducible (Dabrin et al., 2016).

However, apart from the importance of reducing the uncertainty associated to the potential contamination of the samples, the application of PS techniques in a regulatory context, for the assessment of the chemical status of water bodies (in the case of DGTs, assessing the most labile forms of trace metals), is conditional on obtaining valid and internationally comparable analytical results that provide relevant information to policy makers. It is therefore essential to ensure the reliability of measurements carried out by the different laboratories involved in the implementation of the WFD. Keeping this objective in mind, in 2011, an international standard guidance on passive sampling in surface waters (ISO 5667-23: 2011, reviewed in 2016) was released, specifying procedures for the determination of the concentration of metals and organic compounds. However, this standard procedure provided little information on the analytical criteria (handling, methodology) for the DGT technique. Thus, Inter-Laboratory Comparison (ILC) exercises are essential to assess laboratories' capability in processing and analysing DGT passive samplers. Accordingly, metallic compounds were measured, using both DGT and Chemcatcher type PSs, in an intercomparison exercise in 2010; the only European level ILC to evaluate both PS types (Miège et al., 2012; Dabrin et al., 2016). In that ILC exercise, participants deployed their own passive sampling devices at two sites (continental and coastal) and used their own methodology (i.e., DGT conditioning, field deployment and retrieval), analytical process (analyte extraction and analysis) and TWA concentration calculation. Data were interpreted in the frame of the technical validation of the method (NF T90-120). However, it was beyond the scope of that ILC (intending Dabrin et al. (2016) ILC) to investigate which steps within the handling and analytical procedures introduced these biases.

Therefore, in order to move forward in the acceptance of the DGT technique for the chemical status assessment of marine water bodies, sensu the WFD, this ILC exercise was organized in the framework of the Interreg Atlantic Area MONITOOL project (EAPA_565/2016). DGTs, due to their integrative capacity, could be used for compliance checking with AA-EQS, but are less suitable for comparison with MAC-EQS, because peak concentrations will be integrated as accumulated mass in the sampler, but the timing and magnitude of the peak cannot be specified (Smedes et al., 2010). Currently, among the four metals classified as priority substances, only Cd, Ni and Pb have an AA-EQS which could be compared to the data obtained by DGTs (i.e., for Hg only MAC-EQS and Biota-EQS are available), and thus, this ILC exercise will focus on these metals (Belzunce-Segarra et al., 2019).

Nine expert laboratories of the MONITOOL consortium participated in this ILC exercise, which aimed to identify the critical handling (resin gel retrieval and elution) and analytical steps when working with DGT samplers, to establish recommendations to prevent misleading results. The ILC experimental design consisted of the deployment by Ifremer of DGT samplers (all from the same manufacturing batch and acquired from DGT® Research Ltd., Lancaster, UK) at a marine site (Lazaret Bay, France) and the subsequent delivery to participants of these DGTs and DGT components at various stages of handling and analysis, to enable a “step-by-step” investigation of where biases are introduced. Specifically, the following were sent to participants of: (i) exposed and fully intact DGT samplers (not opened), (ii) binding resin gels of exposed DGTs samplers, already peeled off and placed in a dry elution tube, (iii) binding resin gels of exposed DGT samplers, already peeled off and placed in an elution tube with acid and (iv) laboratory blank DGT samplers (not opened) from the same batch as the exposed DGT samplers.

The specific objectives were: i) to test the performance of laboratories when analysing DGTs for trace metals; ii) to investigate the influence of the analytical steps in the results uncertainty and ultimately; iii) to propose standardized practices to improve the reliability of the results. This work also provides recommendations on the critical steps in the treatment process of DGT samplers.

Section snippets

DGT principle and description

DGT passive samplers (DGT Research Ltd., UK) were firstly described by Davison and Zhang (1994) for the measurement of trace metals in natural waters, and nowadays, their use has been extended to sediments and soils. Detailed description of the DGT principles and use can be found elsewhere (www.dgtresearch.com; Zhang and Davison, 2001). Briefly, the DGT devices (for cationic metals) used in this study are composed of a ABS (Acrylonitrile Butadiene Styrene) plastic holder presenting a 2 cm

Exposed DGTs

The measured mass in exposed DGT devices, provided by each laboratory, is presented in Fig. 1. Means and standard deviations of DGT replicates are presented for each step of the analytical process (eluate, resin gel, not opened exposed DGT). The number of replicates per analysis varied from 3 to 6. The number in each data marker indicates the number of replicates for that data point. The LOQ of DGTs has been reported to be 0.1 μg/L for Ni, Cd and Pb (Caetano et al., 2022). The LOQ and LOD are

Conclusions and recommendations

This inter-comparison exercise (ILC) aimed to assess the performance of nine participating laboratories when analysing DGTs for trace metals and to identify the steps increasing the biases of the obtained results, in order to propose standardized practices towards the use of DGTs in a regulatory context. Most of the participating laboratories (80 %) had no prior experience in processing DGT devices for analysis, but they were experienced in measuring traces metals in water. Therefore, this ILC

Funding source

The MONITOOL project was co-funded with €1,92 million provided by Interreg Atlantic Area Transnational Cooperation Programme 2014–2020.

CRediT authorship contribution statement

Jean-Louis Gonzalez: Validation, Conceptualisation, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft, Visualisation, Funding acquisition. Isabelle Amouroux: Validation, Conceptualisation, Investigation, Writing – review & editing, Funding acquistion. Stephane Guesdon: Validation, Conceptualisation, Formal analysis, Investigation, Data curation, Writing – review & editing, Visualisation. Florence Menet-Nedelec: Validation, Conceptualisation, Investigation,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by MONITOOL project, co-financed by the European Regional Development Fund through the Interreg Atlantic Area Programme (n° contract: EAPA_565/2016).

The Scottish Government - Marine Scotland (United Kingdom), Foras na Mara - Marine Environment and Food Safety Services (Ireland), Consejo Insular de Aguas de Gran Canaria (Spain), Viceconsejería Medioambiente del Gobierno de Canarias (Spain), Scottish Environment Protection Agency (United Kingdom), Environmental Protection

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