Elsevier

Analytica Chimica Acta

Volume 618, Issue 2, 23 June 2008, Pages 157-167
Analytica Chimica Acta

Calibration and use of the Chemcatcher® passive sampler for monitoring organotin compounds in water

https://doi.org/10.1016/j.aca.2008.04.052Get rights and content

Abstract

An integrative passive sampler (Chemcatcher®) consisting of a 47 mm C18 Empore™ disk as the receiving phase overlaid with a thin cellulose acetate diffusion membrane was developed and calibrated for the measurement of time-weighted average water concentrations of organotin compounds [monobutyltin (MBT), dibutyltin (DBT), tributlytin (TBT) and triphenyltin (TPhT)] in water. The effect of water temperature and turbulence on the uptake rate of these analytes was evaluated in the laboratory using a flow-through tank. Uptake was linear over a 14-day period being in the range: MBT (3–23 mL day−1), DBT (40–200 mL day−1), TBT (30–200 mL day−1) and TPhT (30–190 mL day−1) for all the different conditions tested. These sampling rates were high enough to permit the use of the Chemcatcher® to monitor levels of organotin compounds typically found in polluted aquatic environments. Using gas chromatography (GC) with either ICP-MS or flame photometric detection, limits of detection for the device (14-day deployment) for the different organotin compounds in water were in the range of 0.2–7.5 ng L−1, and once accumulated in the receiving phase the compounds were stable over prolonged periods. Due to anisotropic exchange kinetics, performance reference compounds could not be used with this passive sampling system to compensate for changes in sampling rate due to variations in water temperature, turbulence and biofouling of the surface of the diffusion membrane during field deployments. The performance of the Chemcatcher® was evaluated alongside spot water sampling in Alicante Habour, Spain which is known to contain elevated levels of organotin compounds. The samplers provided time-weighted average concentrations of the bioavailable fractions of the tin compounds where environmental concentrations fluctuated markedly in time.

Introduction

The organotin compounds, monobutyltin (MBT), dibutyltin (DBT), tributyltin (TBT) and triphenyltin (TPhT) are considered among the most hazardous compounds of anthropogenic origin that have been introduced into the aquatic environment. These compounds exhibit toxicity towards a variety of waterborne organisms at the ng L−1 level [1]. Anti-fouling paints are one of the most important sources of TBT in the aquatic environment, with other inputs being derived from the use of certain bactericides and pesticides that contain this compound. MBT and DBT can be formed as environmental degradation products of TBT and are also used as heat stabilizers in PVC. MBT, DBT and TBT are classified as persistent environmental pollutants; they degrade slowly in water but once accumulated in sediments they are stable. TPhT shows biocidal effects and is applied as a contact fungicide throughout the world to treat a variety of crops. Organotin compounds are included in the list of priority pollutants of the US Environmental Protection Agency (US-EPA) and the European Commission. In 1999, the US-EPA [2] recommended a maximum of 10 ng L−1 of TBT (as a cation) in seawater and 63 ng L−1 in freshwater, although recently it has established the ambient aquatic life water quality criteria for TBT (as cation) in which the criterion to protect saltwater aquatic life from chronic effects is 7.4 ng L−1 and 420 ng L−1 from acute toxic effects [3]. Meanwhile the European Environmental Quality Standard (EQS) of TBT for all types of waters covered by WFD is 0.2 ng L−1 for annual average concentration and maximum allowable concentration of 1.5 ng L−1 in unfiltered water samples [4]. Organotin compounds are bio-accumulable and relatively high levels of these compounds can be found in fatty tissues of exposed biota [5]. Measurement of the concentration of these pollutants in the water column is therefore important for assessing their potential long-term biological impact.

As the total concentration of organotin compounds in contaminated water is typically at the ng L−1 level, it is usually necessary to use large volumes of water and a pre-concentration step prior to analysis in order to achieve the required detection limit. A number of different pre-concentration and extraction methods has been used for the analysis of organotin compounds in water including: liquid–liquid extraction [6], supercritical fluid extraction [7], solid-phase extraction (SPE) [8] and solid-phase microextraction (SPME) [9]. Extracts are usually derivatized with sodium tetra-ethylborate (NaBEt4) and analysed by highly sensitive and selective instrumental techniques such as gas chromatography (GC) with flame photometric (FPD) [6] or inductively coupled plasma mass spectrometric (ICP-MS) [10] detection.

Both bio-monitoring (measurement of the accumulation of pollutants in tissues of living organisms) [11], [12], [13] or the collection of bottle, grab or spot samples of water [14] can be used to monitor levels of organotin compounds in the aquatic environment. The latter technique, however, provides information of the concentration of pollutants only at the time and point of sampling. Where levels fluctuate over short periods (e.g. tidal cycles, input of effluents) within a water body it is desirable to monitor over a longer time interval in order to obtain information on the time-weighted average (TWA) water concentration using for example the passive sampling devices.

The use of passive sampling techniques as an alternative strategy for monitoring water quality has been gaining considerable interest in recent years [15], [16]. Passive sampling devices measure the freely dissolved (and usually bio-available) fraction of compounds in water and have a number of advantages over the use of spot sampling and bio-monitoring methods [17]. All passive sampling devices use a receiving phase with a high affinity for the analytes of interest. This phase is separated from the external aqueous environment by a diffusion membrane. Pollutants in the water are sequestered by the receiving phase and accumulated. If the sampling rate of an analyte is known (usually determined using a flow-through calibration tank) the TWA concentration of a pollutant in the water column can be calculated. Devices can be deployed for short (days) or long (months) periods, are relatively low-cost, and can be used in a range of environments including sites that have limited security and/or are remote with little or no infrastructure.

A number of different devices are available for monitoring a wide range of priority pollutants including both organic and inorganic compounds; these have been recently reviewed [18]. Semi-permeable membrane devices (SPMD) are designed to sequester non-polar (log Kow > 3.0) organic compounds, such as PAHs, PCBs and selected pesticides from water [19]. They have also been used to monitor concentration gradients of organotin compounds in seawater [10], [12]. The Polar Organic Chemical Integrative Sampler (POCIS) [20] can be used for monitoring more polar (log Kow  3.0) hydrophilic organic compounds. For inorganic compounds the most commonly used sampler is the Diffusion Gradients in Thin films (DGT) [21] device and this has been used to measure TWA concentrations of most (e.g. Cd, Cu, Ni, Pb and Zn) heavy metal pollutants. Other designs include the Stabilized Liquid Membrane Device (SLMD) for sequestering toxic metal ions [22], and the Ecoscope for the simultaneous screening of both non-polar organics and metals [23]. In most cases the calibration data relating the amount of pollutant measured in the receiving phase back to their TWA concentrations are not available. Therefore, in some field applications data are reported only in terms of amounts of a chemical found in the device rather than the estimated TWA water concentration. Although this may be valuable information when investigating overall water quality, these data cannot be used for regulatory purposes such as required by the European Union's Water Framework Directive [4].

The Chemcatcher® passive sampler can be used to measure the TWA concentration of both organic [24], [25] and heavy metal [26] pollutants in a range of aquatic environments. A common PTFE sampler body is used and selectivity and accumulation rates are regulated by the choice of the diffusion-limiting membrane and the receiving phase material employed in the different configurations of the device. Unlike other passive samplers currently available, the Chemcatcher® uses a solid, bound, receiving phase in the form of a 47 mm Empore™ disk. Recently a new configuration for the measurement of organotin compounds and inorganic mercury was developed and preliminary results have been reported [27]. For organotin compounds a combination of a C18 Empore™ disk as the receiving disk overlaid with a thin cellulose acetate (CA) diffusion limiting membrane was used. For the organotin compounds tested a linear uptake was obtained over the 14-day test period.

The aim of this study was investigate the effects of water temperature and turbulence on the sampling rate of organotin compounds and their stability once sequestered by the Chemcatcher®. The sampler was tested at a marine harbour site and the TWA concentrations obtained for the various organotin compounds were compared with those obtained with spot water sampling.

Section snippets

Theory of passive sampling

A number of reviews of the principles governing the uptake of an analyte by different designs of passive sampling device, including the Chemcatcher®, have been published [15], [25], [28]. The uptake kinetics of a chemical into a device can be described as an exponential approach to a maximum, and this can be divided into three stages: approximately linear, curvilinear and finally steady state. During initial deployment the accumulation rate is approximately linear, and during this period the

Chemicals and reagents

All solvents and reagents were of analytical grade or better purity. Ultrapure Milli-Q water (Millipore, OH, USA) was used throughout. All volumetric glassware had glass stoppers and was cleaned overnight with 10% HNO3 and rinsed several times with water before use. Tributyltin chloride (97%), dibutyltin dichloride (95%), monobutyltin trichloride (95%), triphenyltin chloride (95%) and tripropyltin chloride (95%) were from Alfa Aesar (Karlsruhe, Germany). Tripropyltin was commonly used as a

Uptake and off-load rates of organotin compounds in the tank studies

To determine the TWA concentration of organotin compounds in water it is necessary to know the compound-specific sampling rate (Rs expressed as mL day−1) for the prevailing environmental conditions. For most non-polar compounds sampling rates are affected by water temperature, turbulence and degree of biofouling on the surface of the diffusion membrane [15]. The uptake is affected by two diffusion barriers: the unstirred water boundary layer and the diffusion membrane. In more turbulent

Conclusions

A new version of the Chemcatcher® passive sampler that uses a C18 chromatographic phase as receiving disk overlaid with a CA diffusion-limiting membrane has been developed and a calibration database established to enable its use as an integrative sampling tool to monitor selected organotin compounds in water. The sampler is simple to use and deploy and compared with most other designs of passive sampler, the receiving phase material is easy to analyse using existing laboratory protocols. The

Acknowledgement

We acknowledge financial support of the European Commission (Contract EVK1-CT-2002-00119; http://www.port.ac.uk/research/stamps/) for this work.

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