New perspectives in the use of semipermeable membrane devices as passive samplers

Available online at www.sciencedirect.com

Talanta 74 (2008) 443–457

Review

New perspectives in the use of semipermeable membrane devices as passive samplers Francesc A. Esteve-Turrillas, Vicent Yus`a, Agust´ın Pastor ∗ , Miguel de la Guardia Analytical Chemistry Department, University of Valencia, Edifici Jeroni Mu˜noz, 50th Dr. Moliner, 46100 Burjassot, Valencia, Spain Received 8 March 2007; received in revised form 4 June 2007; accepted 15 June 2007 Available online 22 June 2007

Abstract This review shows the state of the art, from 2000 to nowadays, of the use of semipermeable membrane devices (SPMDs) for monitoring persistent organic pollutants in both, air and aquatic environments. Since their first use in 1990, SPMDs have been employed for many environmental purposes, like air and water pollution monitoring. We have focussed the study in three subjects: (i) novel compounds accumulated by SPMDs, (ii) modifications of SPMDs to improve their specific uptake properties and (iii) alternatives in sample pre-treatment for the determination of pollutants accumulated in SPMDs. © 2007 Elsevier B.V. All rights reserved. Keywords: SPMD; Semipermeable membrane devices; Triolein; Passive sampler; Water; Air

Contents 1. 2. 3.

4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scientometric evaluation of the literature on SPMDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evolution of passive samplers based on SPMDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Generic configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Solvent-free low density polyethylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Solvent-containing passive samplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Ionic liquid-containing semipermeable membrane devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Triolein-containing cellulose acetate low density polyethylene devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applicability of SPMDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Water samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Air sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SPMD handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analytical procedures for sample processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. External cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Pollutant extraction procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

444 444 445 445 446 446 446 448 448 448 448 449 449 449 449

Abbreviations: ASE, accelerated solvent extraction; BTEX, benzene, toluene, ethyl benzene and xylene; CA, cellulose acetate; o-CPh, o-chlorophenol; p-CDB, p-dichlorobenzene; ECD, electron capture detector; FD, fluorescence detector; FID, flame ionization detector; GC, gas chromatography; GFF, glass fibre filter; GPC, gel permeation chromatography; HCB, hexachlorobenzene; HPLC, high-performance liquid chromatography; IL-SPMD, ionic liquid-containing semipermeable membrane devices; Kow , octanol–water coefficient; LDPE, low-density polyethylene; MAE, microwave-assisted extraction; MC, musk compounds; MS, mass spectrometry; MS–MS, tandem mass spectrometry; OCP, organochlorinated pollutants; PAH, polycyclic aromatic hydrocarbon; PBDE, polybrominated diphenyl ethers; PCB, polychlorinated biphenyl; PCDD, polychlorinated dibenzo-p-dioxin; PCDF, polychlorinated dibenzo-p-furan; PCN, polychlorinated naphthalenes; POP, persistent organic pollutant; PRC, performance reference compounds; PUF, polyurethane foam; QA, quality assurance; QC, quality control; SEC, size exclusion chromatography; SPMD, semipermeable membrane devices; TCAPE, triolein-containing cellulose acetate low density polyethylene; TECAM, triolein-embedded cellulose acetate membrane; TOF, time of flight; TRIMPS, trimethylpentane-containing passive sampler; UAE, ultrasound assisted extraction; UV, ultraviolet ∗ Corresponding author. Tel.: +34 96 354 44 54; fax: +34 96 354 48 38. E-mail address: [email protected] (A. Pastor). 0039-9140/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2007.06.019

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6.2.1. Dialysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Accelerated solvent extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3. Microwave-assisted extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.4. Ultrasonic extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.5. Head-space direct determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Clean-up of extracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1. Gel permeation chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2. Adsorption columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Methods for pollutant determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1. Bioassays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2. Chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Estimation of air/water pollutant concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Biofouling effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Comparison of SPMDs with standard samplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1. SPMD against biomonitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. SPMD against active samplers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Semipermeable membrane devices (SPMDs), introduced by Huckins et al. [1], consist of an additive free lay-flat tube, made of low-density polyethylene (LDPE), filled with triolein, spreading it into the tube surface and then heat-sealed at each end. The low interfacial tension causes an intimate contact between the triolein and the LDPE. Passive sampling of pollutants by using SPMDs has been widely used for screening and source identifications of a variety of non-polar and moderately polar organic contaminants from aqueous and air environments. SPMDs mimic the absorption of compounds through cell membranes and also can be used to evaluate the bioconcentration factor in aquatic animals. Organic pollutants present in water can be dissolved or adsorbed to particles, such as humic acids or sediments; being the adsorbed compounds less available to animals. So, SPMDs are often used as an indicator of the bioavailability of hydrophobic contaminants in the environment [2]. SPMDs have been successfully tested as passive samplers for several kinds of persistent organic pollutant (POPs), such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzo-p-furans (PCDFs) and several organochlorinated pesticides (OCPs). The sampling rate of different series of chemicals depends on their physical–chemical properties, being the octanol–water coefficient (Kow ) the most important. However, other environmental variables also affect the contaminant uptake, including temperature, turbulence, flow rate and biofouling [3]. The use of SPMDs have several advantages compared to other sampling methods: (i) can be deployed for extended time periods to integrate long-term data; (ii) only bioavailable compounds are sampled; (iii) are easy to use and less expensive than active samplers; and (iv) are more reproducible than live biota samplers, avoiding drawbacks related to migration, mortality, metabolism

449 450 450 451 451 452 452 452 452 453 453 453 454 454 454 455 456 456 456

or selective depuration of contaminants. As well as a passive sampler, the use of SPMDs provides selective analyte isolation and especially pre-concentration [4]. So, the selectivity and sensitivity of pollutants determination methods can be considerably improved. 2. Scientometric evaluation of the literature on SPMDs An extensive review of SPMDs used for monitoring pollutants in various media has been published by Petty et al. in 2000 showing a summary of considerations for their effective and efficient uses [3]. Later Lu et al. in 2002 reviewed conventional applications of SPMDs in aquatic environments that emphasizes in their use in chemical monitoring and in their comparison to biomonitoring organisms [5]. Additionally, in a revision of Stuer-Lauridsen for passive accumulation devices a special attention has been made for the use of SPMD samplers [6]. Finally, two books are commercially available related to the use of SPMDs as samplers of organic chemicals in the environment [7,8]. The scientific references found in the literature related to different applications of SPMDs have been growing significantly in the latest years (see Fig. 1). From this figure, it can be seen that after a period of time of 5 years, till 1995 in which the number of published studies remain practically constant the literature about this topic increases at a rate of 13.6 papers per year till 2000. From this date till 2006, a two times increased speed of 28.6 papers year−1 was reached, indicating that nowadays SPMDs are well accepted by the scientific community and start to be applied to solve as much as possible environmental control processes. The classification of published papers by the journals of publication is shown in Fig. 2. Most of them have been published in environmental journals (Environmental Sciences with a 49% and Environmental Engineering with 17% of total published studies) while a minority has been published in basic or applied analytical journals (only a 13%) [9].

F.A. Esteve-Turrillas et al. / Talanta 74 (2008) 443–457

445

Fig. 1. Evolution of the accumulated number of published papers on the use of SPMDs. Note: references compiled in ISI Web of Knowledge for the period 1945–2006 [9].

The aforementioned results show that most of the studies carried out involving SPMDs are field studies, and only a few research groups have been worked on new applications or methodological improvements of the use of these devices. Three are the main reasons for the tremendous increase of the literature about SPMDs in the new century: (i) the considerable modifications in the SPMD standard devices, (ii) the search for alternatives to long and tedious dialysis procedures classically used for sample processing after retention of analytes in SPMDs and (iii) the studies on novel compounds to be sampled with SPMDs. 3. Evolution of passive samplers based on SPMDs As indicated before, the initial configuration of SPMDs proposed by Huckins included a LDPE tube filled with triolein, but this generic configuration has been replaced, in some cases by the use of solvent free low density polyethylene samplers or LDPE tube filled with isooctane, ion liquid or triolein containing cellulose acetate.

Fig. 3. Scheme of a generic SPMD device.

3.1. Generic configuration LDPE tubing, 70–90 ␮m wall thickness is described as nonporous material, but random thermal motions of the polymer chains form transient cavities with maximum diameters of ˚ Because of the small size and dynamic approximately 10 A. character of these cavities, hydrophobic solutes are essentially dissolved by the polymer. This solute size limitation excludes large molecules as well as those that are adsorbed on colloids or humic acids. Only truly dissolved (bioavailable) and non-ionised contaminants diffuse through the LDPE membrane and can be separated by the sampler. A scheme of a conventional SPMD is shown in Fig. 3. The advantages in the use of triolein (1,2,3-tris-cis-9octadecenoyl glycerol), a major non-polar lipid found in aquatic organisms, in front of other lipids are: (i) the easy availability as a high purity synthetic product, (ii) the low melting point (close to 0 ◦ C) and (iii) the large capacity to dis-

Fig. 2. Classification of published papers on SPMDs by journal of publication. Note: references compiled as indicated in Fig. 1.

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solve non-polar compounds. Furthermore, there is a close correlation between triolein–water partition coefficients and octanol–water ones, being the latter easy-known for whatever compound. Usually, the commercially available SPMD consists of a piece of 106 cm long and 2.54 cm wide that contain 1 mL pure triolein inside [3]. The device weights approximately 4.5 g, is about 20% triolein and the surface area–triolein volume ratio is about 460 cm2 /mL. However, some studies have been carried out with mini-SPMD of 10 cm long, filled with 0.1 mL triolein [10,11] or 30 cm long, filled with 0.3 mL triolein [12]. Any SPMD length is considered to be a “standard” SPMD if the surface area–triolein volume ratio is maintained at 460 cm2 /mL. 3.2. Solvent-free low density polyethylene “Is triolein indispensable in the use of SPMD?” It is one of the most frequently questions about the use of SPMD. Thus, some authors have studied the pollutant uptake behaviour using only LDPE strips without triolein [12–16]. In these studies LDPE layflat tubing is cut twice along the side edges in order to obtain a single layered strip, being it used in the same way than generic SPMDs. Difficulties in the spike of LDPE were solved by Booij et al., which incorporated deuterated PAHs and PCBs on the polymer by soaking it in aqueous/methanolic solutions [13]. The effect of temperature on PAH sampling rates in air has been evaluated using both SPMDs and LDPE strips under controlled flow conditions [12]. No significant differences were observed on sampling rates, but LDPE samplers reached equilibrium faster because of their smaller sorption capacity. Moreover, LDPE water partition coefficients were larger at low temperatures, while for SPMD remained invariable. A comparison of PAHs uptake in LDPE strips, SPMDs and pink salmon eggs was also performed in aqueous environments, being observed that accumulations of PAHs were highly similar in strips than those observed in SPMDs [16]. The uptake of 12 priority PAHs was evaluated for LDPE strips at two different thicknesses (100 and 200 ␮m), but with the same cross-section area [15]. In this study, when equilibrium is approached, the PAH amount accumulated and the time required to reach it, is proportional to LDPE thickness. So, authors suggest the use of an unique sampler composed with two LDPE strips, in the thinner one the smaller PAHs reach equilibrium and in the thicker one the larger PAHs are in the linear uptake, being both behaviours considered by standard SPMD models. Finally, it must be emphasized the importance of the absence of triolein in LDPE samplers, because it considerably decreases the cost of each device. 3.3. Solvent-containing passive samplers A simple modification of SPMDs was introduced by Leonard et al. [17], by using 2,2,4-trimethylpentane (isooctane) instead of triolein. These devices were so called trimethylpentanecontaining passive samplers (TRIMPS). The change of the lipid provides two great advantages as passive sampler: (i) the absence

of periphytic growth or biofouling, that can affect the sampling rates of absorbed pollutants; and (ii) the simplification of the sample processing, being not required any clean-up step after the dialysis of retained analytes. However, the use of TRIMPS could release trimethylpentane to the surrounding water. The release of solvents is only influenced by the deployment time and not by the river flow or water temperature [17]. The kinetics of uptake for non-polar pesticides (log Kow >3.5) is linearly proportional to the ratio of chemical activity of the pesticide between water and trimethylpentane [18]. Thus, TRIMPS have been successfully applied for the sampling of endosulfan, chlorpyrifos-ethyl, profenofos and sulprofos [17] and of tributylphosphate, endosulfan sulfate and chlorpyrifos-ethyl [19] in river waters. A study of accumulation of PAH and petroleum biomarker compounds (22 steranes and 31 hopanes) in SPMD, LDPE strips and TRIMPS samplers, performed by Luellen et al., shows that the LDPE sampling rates are 30% higher than the other two designs [20]. In spite of it, the three passive samplers tested are effective for discriminate petroleum sources. A binary solvent mixture of 1-dodecanol and isooctane (3:2) was used by Hyne et al., being effective in the measurement of polar pesticides from waters [18]. Other devices tested have been LDPE membrane bags of 7.5 cm length filled with 100 ␮L dimethyl sulfoxide. These devices, employed by Roger et al., were used for toxicity assays of toxic industrial chemicals in air. When the deployment time is reached, SPMD is cut and dimethyl sulfoxide analyzed directly through rapid toxicity assays [21]. 3.4. Ionic liquid-containing semipermeable membrane devices Ionic liquids are organic salts, which are liquids at ambient temperatures. Their exclusive properties such as nonvolatility, nonflammability, and excellent chemical and thermal stability have made them an environmentally attractive alternative to conventional organic solvents. One of the most special properties for ionic liquids is their high polarity, thus the replacing of triolein for an ionic liquid in SPMD samplers was recently suggested for Zhao et al. [22]. Eighteen centimetres long segments layflat LDPE tubing were filled with 0.5 mL ionic liquid, forming a ionic liquidcontaining semipermeable membrane devices (IL-SPMD). The ionic liquid employed was 1-butyl-3-methylimidazolium hexafluorophosphate ([C4MIM][PF6]) and the effective length was 15 cm. IL-SPMDs were used for monitoring five PAHs in water. PAHs in the ionic liquid were directly analyzed by HPLC, without the dialysis plus clean-up procedures. A comparative study was carried out in the Yellow River in China; both IL-SPMDs and conventional triolein containing SPMDs were deployed for 5 days in several sites and PAH residues were measured. Ionic liquid devices provided lower amounts of PAHs, these differences are explained by authors because of the sample processing in the case of IL-SPMDs only concerns the ionic liquid measurement and involves the whole LDPE membrane with triolein in classical devices.

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447

Table 1 Deployment conditions of SPMDs for sampling novel compounds Type

Compounds

Matrix and location

Degradation of Nonylphenol, octylphenol, nonylphenol alkylphenol ethoxylate monoethoxylate, nonylphenol surfactants diethoxylate Organotin tributyltin, dibutyltin, monobutyltin

Sewage plant, Great Lakes (USA)

Polycyclic Musk

Triclosan

Galaxolide, tonalide, celestolide, phantolide, traseolide, musk xylene, musk ketone triclosan (T), methyl triclosan (mT)

Seawater—Oslo fjord (Norway) Sewage Moldau river (Czech Republic)

Water—Greifensee lake (Switzerland) Water—Air Basin river (UK) Petroleum biomarkers Sterane, hopane Water UV filters Benzophenone-3, 4-methylbenzylidene Water—Zurich lake camphor, ethylhexyl methoxy cinnamate, (Switzerland), octocrylene, butyl Water–Greifensee lake methoxydibenzoylmethane (Switzerland) Nitrated PAH 1-Nitronafthalene, 2-nitronaphtalene, Air—(Austria, Czech 2-nitrofluorene, 9-nitroanthracene, Republic, Poland, 3-nitrofluoranthene, 1-nitropirene Slovakia and Sweden) Toxic industrial chemicals Diketene, Phosphorusoxychloride, Contaminated air acrolein, trichloroacetyl chloride, methanesulfonyl chloride, stilbene, 1-octanethiol, sulfuryl chloride, formaldehyde, allylamine, methyl chloroformate, chloroacetone, methyl chlorsilane, diisopropylfluorophosphate, Methylhydrazine, acetone cyanohydrin, 1,2-dibromoethane Polychlorinated Tri-PCN, Tetra-PCN, Penta-PCN, Air naphthalenes (PCN) Hexa-PCN PCN-54, PCN-67, PCN-73, PCN-75 Sewage treatment plant—Carraixet (Spain) BTEX Benzene, toluene, ethylbenzene, xylenes Vehicle compartments (VC) (Spain) Petrol station air (PSA) (Spain) Herbicides Diuron, simazine, atrazine, hexazinone, Great Barrier Reef fluometuron (Australia) Pesticides Atrazine, chlorpyrifos, chlorothalonil Estuarine ecosystems (FL, USA) Pyrethroid insecticides Allethrin, prallethrin, tetramethrin, Contaminated indoor air bifenthrin, phenothrin, ␭-cyhalothrin, (Spain) permethrin, cyfluthrin, cypermethrin, flucythrinate, esfenvalerate, fluvalinate, deltamethrin, piperonyl butoxide Pesticides Propamocarb, propoxur, carbosulfan, Irrigation water, Albufera diazinon, pirimicarb, ethiofencarb, Lake (Spain) chlorpyrifos-methyl, metribuzin, metalaxyl, terbutryn, malathion, chlorpyrifos, fenthion, pendimethalin, allethrin, oxadiazon, buprofezin, oxyfluorfen, diclofop-methyl, propargite, carbofuran, bifenthrin, tetramethrin, fenoxycarb, phenothrin, ␭-cyhalothrin, permethrin, flucythrinate, fluvalinate, esfenvalerate Polar pesticides Diuron, atrazine, metolachlor, fipronil, Irrigation water, molinate Murrumbidgee (Australia)

Deployment time (days) Concentration found –

References

–

[26]

0.4–10 ng/L

[27,28]

–

[29–31]

21–42

70–1300 ng/L (T) 1–11 ng/L (mT)

[32–34]

29 24–48

– 1–35 ng/L

[10] [35,36]

21

0.001–4 ng/day

[37]

1

–

[21]

2–450

0.17–32.5 pg/m3

[38]

–

[39]

0.6–160 ␮g/m3 (VC)

[40]

15–30 –

20 1

0.04–58 mg/m3 (PSA) 8–11

0.06–2.5 ng/L

[41]

4

2.7–48 ng/L

[42]

2

–

[43]

30

4–1450 ng/SPMD

[44]

22

0.0001–32.9 ␮g/L

[18]

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3.5. Triolein-containing cellulose acetate low density polyethylene devices Xu et al. have studied the use of triolein-embedded cellulose acetate membrane (TECAM) as passive sampling devices; they were produced by embedding triolein drops in the matrix of cellulose acetate polymers [23]. Results from both laboratory and field experiments indicate that TECAM can quickly and efficiently accumulate hydrophobic OCPs from water. The uptake rate of 16 PAHs in aquatic environments shows that these pollutants required shorter times to achieve equilibrium in TECAMs than in SPMDs [24]. A combination between SPMD and TECAM is proposed by Liao et al. The device consists of a thin film of neutral lipid triolein, enclosed in thin-walled tubing made of composite cellulose acetate membrane (CA) supported by linear LDPE. The whole device, so-called TCAPE, is formed with a LDPE support (25 ␮m thickness), an external slide of CA (10 ␮m thickness) and filled with 0.5 mL triolein. TCAPE keep the standard surface area–triolein volume ratio of 450 cm2 /mL. In a comparative study, TCAPE accumulates hydrophobic OCPs in water more quickly and efficiently than standard SPMDs; where in only 20 h the uptake equilibrium is reached [25]. 4. Applicability of SPMDs 4.1. Water samples Uptake of compounds from the environment depends upon their chemical and physical properties, such as molecular weight, water solubility, Kow and of course also upon media conditions (temperature, flow rate and biofouling). In general, any neutral and hydrophobic compound with a log Kow higher than three can be significantly concentrated by SPMDs, although compounds with log Kow values bigger than one could be also retained. Ionic species and very polar organic compounds are not concentrated in SPMDs. Petty et al. show, in their review in 2000, a list of environmental contaminants that have been effectively sampled in different environments by SPMDs, such as; PAHs, polychlorinated biphenyls (PCBs), organochlorinated pollutants (OCPs), polychlorinated dibenzop-dioxin (PCDDs), polychlorinated dibenzo-p-furan (PCDFs), alkylated phenols, organophosphate and pyrethroid insecticides, neutral organometallic compounds, polybrominated diphenyl ethers (PBDEs) and some heterocyclic aromatic compounds [3]. Nowadays, a large number of compounds, summarized in Table 1, have been added to this list. Novel compounds have been sampled in different sites, such as: butyltin derivates in seawater [27,28]; petroleum biomarkers in water [10]; polycyclic musk xylene, musk ketone and their amino metabolites in water from river and from sewage plants [29–31]; triclosan [32–34] and UV filters [35,36] in lake waters; or polychlorinated naphthalenes (PCNs) in sewage plants [39]. Uptake of pesticide compounds using SPMD has been widely used in several studies, mainly for chlorinated pesticides. Recently, a few categories of other pesticides have been studied such as: the pesticides atrazine, chlorpyrifos, chlorothalonil

Fig. 4. Recovery of different pesticide families from 2 L water spiked with 100 ng of each studied compound by using SPMDs after different deployment times, from 2 to 6 days.

in estuarine ecosystems [42]; the herbicides diuron, simazine, atrazine, hexazinone and flumeturon in seawater [41], or the polar pesticides diuron, atrazine, metolachlor, fipronil, and molinate in irrigation waters [18]. The uptake of pesticides from water was evaluated by Esteve-Turrillas et al. using 37 neutral pesticides with different properties, with log Kow ranging from 0.08 to 8.00 and water solubility from 5 ␮g/L to 7 g/L [44]. This study classifies the SPMD pesticide uptake by families, such as chlorinated, organophosphorus, pyrethroid, carbamates and others. Fig. 4 shows a bar graphic with the mean uptake of pesticides, using SPMD, from spiked water after 2, 4 and 6 deployment days. The graphic shows that pyrethroid compounds are the most easily retained, due mainly to their high log Kow values followed by organophosphorus and chlorinated compounds. Carbamates were weakly absorbed. 4.2. Air sampling SPMDs were developed in a first attempt for water sampling of organic pollutants. In fact, most of the published papers are focussed in water field studies. Even solid matrices have been also sampled by using SPMDs, for the determination of organochlorined pollutants in contaminated lake soils or in indoor household compost [45,46] In 1993, Petty et al. proposed the use of SPMDs for air sampling purposes [3]. However, only few papers have studied the uptake of pollutants, mainly for PCBs, PAHs and OCPs sampling [47,48]. Nowadays, there is an increasing interest on the use of SPMDs for air sampling of novel compounds and the comparison with other well-established samplers. New compounds sampled in air are PBDEs [49,50], pyrethroid insecticides and pyperonyl butoxide [43], nitrated PAHs [37], toxic industrial chemicals [21], PCNs [38], or benzene, toluene, ethylbenzene and xylenes (BTEX) [40]. Several comparative studies were performed for the use of SPMD in front of active samplers [51,52], passive samplers [38] or biological samplers, such as semiaquatic plants [53] or pine needles [54].

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The effect of different wind-speeds on the calibration of SPMD uptakes of PAHs and PCBs from air, using performance compounds reference was evaluated by Soderstrom and Bergqvist [55]. SPMDs have been also used to sample the air houses with an extensive burning of domestic wood. Strandberg et al. found PAHs and PCBs in these sites, being the PAHs levels comparables to urban contaminated sites, while PCBs levels were slightly lower [56].

In a recent study, the behaviour of SPMDs was assessed for the uptake of neutral pesticides from waters as a function of water temperature (10, 18 and 25 ◦ C), ion strength (0, 0.1, 1 and 5 g/L of NaCl), pH (4, 7 and 9 with HCl and NaOH), presence of solvents (0.1, 0.5 and 1 mL acetone) and presence of organic matter (1 g/L of both glucose and starch). Authors concluded that there is no big variation in the pesticide uptake from water at different salinity, pH or organic matter composition, but temperature has a strong influence on the absorption rates, increasing at high temperatures [44].

5. SPMD handling

6. Analytical procedures for sample processing

Different protective apparatus are usually employed during the sampling with SPMDs, being preferred the use of stainless steel surfaces in front of plastic components, to minimize the possible release of organic compounds. Hardware employed must maximize the effective area of the sampler. For the sampling of photosensible compounds, SPMD must to shade from sun with a special cover. In addition, hardware employed must to be moored and hidden in order to avoid losses and any kind of vandalism or sabotage. The deployment time for sampling with SPMDs can varied from days to months depending on the expected concentration levels and the kind of contaminants. Deployment times of 10–30 days are typically employed for POPs uptake in water sampling, but in the case of air the deployment time is of the order of 24 h. SPMD uptake of all types of chemicals can be intensively affected by environmental variables, such as temperature, flowrate, water composition, level of biofouling and others. It would be important to fill a detailed documentation for each SPMD sampling that includes as much data as possible, including field conditions, visual evaluation of biofouling and an estimation of flow-rates. Differences in absorbed amounts of PCBs and PAHs generally cannot be attributed to changes in temperature, except when very large geographic scales or summer/winter comparisons are involved [12]. Although in other studies it has been reported that uptake rates of PAHs, PCDDs, PCDFs and PCBs decreased slowly with temperature [57,58]. In a laboratory study, Cicenaite et al. estimated the temperature influence on the SPMD-air coefficient partition for naphthalene, o-chlorophenol and p-dichlorobenzene. Temperatures tested were −16, −4, 22 and 40 ◦ C, being the time required to reach equilibrium inversely proportional to air temperature [59]. The water flow velocity has a considerably influence on the sampling rate, with values ranging from 4 to 200 L day−1 for chlorobenzenes, PCBs and PAHs performed in water at several velocities, as it has been shown by Booij et al. [12]. Others studies have confirm this behaviour, showing the significant differences in the release kinetic of PAHs at different flow rates [60]. Sampling rates for PAHs were evaluated under different water conditions, concluding that salinity does not affects the uptake, but there is a slight effect in the solubility of these compounds in salted water [57].

Extraction of analytes retained in SPMDs is commonly made by dialysis with organic solvents, with the exception of specific applications such as PAHs sampling using IL-SPMDs, where ionic liquid is directly measured by HPLC [22]. Although SPMDs are generally easier to be analyzed than other environmental matrices, such as sediments, tissues, etc., analysis of contaminants retained in SPMDs can be affected by the presence of polyethylene waxes, oleic acid and methyl oleate evolved from the sampling device [7]. 6.1. External cleaning A general recommendation, by Petty et al., for a right external cleaning of a SPMD after deployment in water involves: removal of the superficial biofouling and debris using hexane, brushing and immersion in dilute acid; and rinsing of the SPMD surface with water, acetone and isopropanol [3]. When SPMDs are used as air samplers, the surface is not affected by biofouling and thus the external cleaning can be avoided or carried out rinsing with water and organic solvents. 6.2. Pollutant extraction procedures Dialysis and modern extraction alternatives, based on accelerated solvent extraction (ASE), microwave-assisted extraction (MAE) and ultrasound assisted extraction (UAE) have been employed to extract pollutants retained on SPMDs, additionally than the direct determination of volatile compounds through head space gas chromatography. 6.2.1. Dialysis A general view of the dialysis procedures used in the literature, for POPs extraction is summarised in Table 2. Hexane is employed as solvent is most of cases. As it can be seen, standard methods are time-consuming, from 24 to 72 h, and require the use of high volumes of solvent, from 100 to 900 mL. The main drawbacks of dialysis procedures are high extraction times and excessive solvent consumption that increase considerably the cost of analysis. Because of that nowadays a high effort has been do in the use of environmentally friendly procedures that minimizes solvent consumption and waste generation, based on the use of ASE, MAE, USE or head-space (HS) direct volatilization of the analytes (see Table 3). On the other hand, Fig. 5 shows

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Table 2 Dialysis procedures employed in the literature for the recovery of pollutants retained in SPMDs Sample

Contaminant

Solvent

Volume (mL)

Time (h)

References

Air Air Air Air/water Sewage Water Water Water Water Water Water Water

PCBs PAHs Naph, o-CPh, p-CDB PCBs, HCBs PCBs, PAHs, OCPs, PAHs PCDDs, PCDFs PCBs, OCPs PAHs PCDDs, PCDFs, PCBs PAHs PAHs, OCPs, PCBs

Hexane Hexane Hexane Hexane-DCM (80:20) Cyclohexane Hexane Hexane Hexane Cyclopentane Hexane Hexane Hexane

250 300 100 120 100 150 900 250 – 400 150 750

48 48 24 48 48 48 48 48 24 48 48 72

[61] [62] [59] [63] [64] [57] [65] [66] [67] [45] [4] [68]

Abbreviations: o-CPh, o-chlorophenol; p-CDB, p-dichlorobenzene; HCB, hexachlorobenzene; Naph, naphthalene; OCP, organochlorinated pollutants; PAH, polycyclic aromatic hydrocarbon; PCB, polychlorinated biphenyl; PCDD, polychlorinated dibenzo-p-dioxin; PCDF, polychlorinated dibenzo-p-furan.

a flow chart that can help us to select the appropriate extraction procedure depending on the analyte selected. 6.2.2. Accelerated solvent extraction This technique, also known as pressurized fluid extraction or pressurized liquid extraction uses an organic solvent or a combination of solvents at high pressure and temperature. The extraction under these conditions provides increased solubility, better desorption and enhanced diffusion of POPs from the matrix. Thus the extraction time can be considerably reduced. In 2004, Wenzel et al. suggest the first reported procedure as a good alternative to dialysis extraction of POPs for SPMDs. The so-called “rapid dialysis procedure” employs an accelerated solvent device to extract OCPs, PCBs and PAHS [11]. The procedure is suitable for two SPMD sizes: 10 and 91 cm length, using for both 33 mL cells, but it is necessary the use of an stain-

less steel mesh that ensures the optimal, reproducible fixing of the SPMD in the cell and to avoid contact with the inner walls of the cell. The effect of several instrumental parameters such as: extraction pressure, temperature and time, solvent composition, and number of cycles were evaluated on spiked SPMDs. The optimal parameters were an adjusted extraction pressure of 3.45 MPa, a temperature of 50 ◦ C, extraction time of 10 min, n-hexane/acetone 9:1 as solvent and four extraction cycles. The average recovery values found varied from 88 to 100% for OCPs, to about 100% for all the PCBs studied and from 89 to 123% for PAHs [11]. 6.2.3. Microwave-assisted extraction MAE has attracted growing interest, as it allows rapid extraction of solutes from solid matrices by employing microwave

Table 3 Comparison of extraction conditions used in recent publications for the recovery of preconcentrated POPs in SPMDs Parameter

Dialysis

MAE

ASE

Sonication

Head space

Studied compounds

POPs

OCPs, PCBs, PAHs

Air/water Hexane/cyclohexane

MCs, PCBs, PBDEs, OCPs Water Hexane

BTEX

Samples Solvents

Solvent consumption Sample processing time Clean-up required Automation Serial extraction Main advantages

140 mL 40 min Yes Yes 36 reactors Rapid Low solvent consumption

300 mL 60 min Yes No Yes Rapid Easy; no instrumentation required

Main drawbacks

100–900 mL 6–48 h Yes No Yes Easy No instrumentation required; few triolein coextraction Very slow

PCBs, PAHs, OCPs, PBDEs, PCNs, pesticides Air/water Hexane:water (10:1, v/v); Hexane:acetone (1:1, v/v); Toluene 60–120 mL 5–60 min Yes Yes 12 reactors Rapid Low solvent consumption

Analyte unstability; risk of SPMD collapse; high matrix coextraction

Investment Equipments used

Low –

High sample manipulation; easy contamination; high matrix coextraction Low Ultrasonic water bath

References

[3,7]

Analyte unstability; a internal mesh is required; high matrix coextraction High Dionex ASE 200; Dionex ASE 300 [11]

None 20 min No Yes 32 vials Rapid Direct; no sample treatment; no solvent consumption Only for very volatile compounds; SPMD collapse

Low; moderate Domestic oven; Milestone ETHOS SEL [39,43,44,69]

Air Hexane:acetone (10:1, v/v)

[31]

Air None

Moderate Thermo AS2000 [40]

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mixture (1:1, v/v) [39]. The effects of time, volume of solvent and temperature on the yield of the extraction of PBDEs and PCNs were evaluated using a full factorial design. The optimized extraction conditions were 60 mL of solvent, a temperature of 85 ◦ C and two extraction cycles of 1 min each one (with a total time of 5 min). Recoveries obtained varied between 72 and 91% for PBDEs and between 96 and 103% for PCNs. Esteve-Turrillas et al. used MAE for the extraction of pyrethroid insecticides from SPMDs [43]. The maximum temperature resisted by SPMDs, depends on the solvent employed. High polarity solvents; such as acetonitrile or ethyl acetate, permit treatments at 100 ◦ C in front of the low temperatures (till 70 ◦ C) resisted in the presence of apolar solvents as hexane or toluene. Mixtures of toluene and hexane with acetone allow extraction temperatures of 90 ◦ C. Treatments with high temperatures than those indicated dissolve and collapse the polyethylene membrane and do not allow the compound determination due to the high amount of plastic residues coextracted. The best recoveries were obtained for toluene with values ranging from 69 ± 3 to 113 ± 4% and for hexane:acetone with recovery data from 61 ± 8 to 103 ± 7%.

Fig. 5. Flowchart to select an appropriate analytical procedure for POPs determination in SPMDs.

energy as a source of heat, with extraction efficiency values comparable to that of classical techniques. The partitioning of analytes from the sample matrix to the extractant solvent depends upon the temperature and the nature of the solvent. As compared with classical heating systems, microwave radiation provides a fast and selective heating of polar compounds, leading to a very short extraction time. Yus`a et al. proposed the use of MAE for the extraction of POPs from SPMDs. First studies were carried out using domestic microwave ovens, were the only modifiable parameters were the maximum power exit and the extraction time. OCPs, PCBs and PAHs were extracted by using several mixtures of solvents, as hexane–acetone (1:1, v/v), hexane–water (10:1, v/v), toluene–water (10:1, v/v) and isooctane–acetonitrile, (1:1, v/v), being found that hexane–water provided the highest recoveries for all compounds tested, being the results comparables with those found by the dialysis procedure. The solvent volume employed was 33 mL, enough to cover up the whole SPMD inside the reactor. Finally, three cycles of 3 min each one and a total volume of 90 mL hexane were required for MAE [69]. Experiments carried out by using a laboratory Milestone ETHOS SEL microwave oven, which allows a better manage of the process with the accurate control of the extraction temperature, evidenced that polyhalogenated compounds PBDEs and PCNs can be extracted from SPMDs using a hexane–acetone

6.2.4. Ultrasonic extraction Sonication is a simple extraction technique, in which the sample is immersed in an appropriate solvent inside a vessel and placed in an ultrasound water bath. The efficiency of extraction depends on the polarity of the solvent, the homogeneity of the matrix and the sonication time. ˇ Setkov´ a et al. suggest the use of an ultrasound bath for the extraction of OCPs, PCBs, PBDEs and musk compounds (MCs) retained in SPMDs [31]. The sampler was lengthwise cut using sharp scissors and then extracted three times with 100 mL hexane for 20 min each. A total volume of 300 mL hexane and 1 h was needed for the extraction of each SPMD. The corresponding extract dissolves the whole amount of triolein inside the SPMD and, because of that a clean-up step based in GPC was applied by authors to remove interferent compounds. Recoveries of all the target compounds varied in the range of 89 to 103%. 6.2.5. Head-space direct determination HS is widely applied in the analysis of volatile organic compounds such as flavour, fragrance and many POPs in both environmental and food samples. Esteve-Turrillas et al. introduced the use of HS for the direct determination of BTEX compounds from SPMDs, 10 cm long filled with 0.1 ␮L triolein [40]. Oven temperature and extraction time are the main parameters that affect the extraction by head space. After an optimization of these parameters, using a central composite design, a temperature of 150 ◦ C and an extraction time of 20 min were considered as the optimum. In this study, calibration curves were prepared with spiked SPMDs. The great advantages provided by HS in front of all considered previous extraction alternatives, include the absence of sample handling, the reduction of interferences and the possibilities of a full automation of the method. A total processing time of 20 min for each sample allows a sample throughput of 72

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Fig. 6. GPC chromatogram elution windows (Waters EnvirogelTM 19 mm × 150 mm and 19 mm × 300 mm) of different families of POPs compared with these obtained for undeployed SPMD devices extracted by both, dialysis (· · · ·), two cycles with 125 mL hexane for 24 h, and MAE (—), three cycles with 33 mL hexane:water (10:1, v/v) for 3 min. Elution windows were obtained from literature data [27,39,69,72].

samples per day, being the fastest methodology developed for the determination of volatile compounds retained in SPMDs. 6.3. Clean-up of extracts Compounds coextracted with analytes from SPMDs, are basically triolein, polyethylene oligomers and triolein impurities, as oleic acid and ethyl oleate. The main source of methyl oleate is 95% triolein that contains about 40 mg/mL of it and most is extracted by dialysis; while oleic acid source is a methyl oleate hydrolysation on the exterior side of SPMD which diffuses back [7,70]. If SPMDs are manufactured with 99% triolein instead 95%, the problems related to methyl oleate and oleic acid interferences may be prevented. After the extraction of analytes retained in SPMDs, it is mandatory to remove the aforementioned compounds in order to avoid capillary column degradation and to reduce the background noise on the baseline. Elemental sulphur is also a potential interference, above all when bioassays are employed for toxicity evaluation. Gel permeation chromatography and adsorption columns were employed to do a correct clean-up of extracts. 6.3.1. Gel permeation chromatography This method, sometimes referred as size exclusion chromatography (SEC), is particularly suitable for the clean-up of fatty samples, such as oil, egg or fish muscle. The different compounds injected are separated based on their size. So, when it is used for the treatment of SPMD extracts, the separation takes place principally between the big molecules as polyethylene waxes or triolein, and the small molecules of the analytes. Most studies used S-X3 BioBeads (200–400 mesh) in a range of column sizes and solvents [31,48,71]; and currently also two Waters EnvirogelTM GPC columns (19 mm × 150 mm and 19 mm × 300 mm) coupled in series using dichloromethane as mobile phase [27,39,69] for the clean-up of SPMD extracts.

Fig. 6 shows the elution windows for several compounds and the Envirogel GPC chromatogram of a SPMD extracted by dialysis with two cycles with 125 mL hexane for 24 h, and MAE using three cycles with 33 mL hexane:water (10:1, v/v) for 3 min. GPC not only removes the interfering materials, but also can fractionate the analytes. Some compounds can not be separated from SPMD matrix using GPC (see Fig. 6), and because of that a clean-up based on adsorption columns must be performed. Finally, the great advantages of GPC in front conventional methodologies are that it can be fully automated and it is easily applicable to the isolation of unknown contaminants. 6.3.2. Adsorption columns Alumina, silica and florisil columns in different mesh sizes, levels of activity and column size, either separately or in combination, have been widely used to reduce matrix interferences from different samples. There are many studies that involve the use of these adsorbents, above all in the cases where the GPC elution window of the analytes overlaps with the SPMD matrix, e.g. the use of C18 -alumina cartridges for the clean-up of pyrethroid insecticide residues extracted from SPMDs [43]. Chromatographic fractionations on silica gel, florisil, or alumina can separate methyl oleate from some analytes, but methyl oleate has approximately the same polarity as PAHs and some pesticides. Thus, complete separations cannot always be achieved [70]. In some cases a combination of GPC with adsorption columns were proposed, e.g. S-X3 BioBeads and florisil were employed for PCBs determination [61] and also for musk compounds determination [29]. 6.4. Methods for pollutant determination Bioassays and chromatography procedures are the main systems used for the determination of POPs retained in SPMDs.

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6.4.1. Bioassays In order to avoid instrumental methods of analysis, complex mixtures of chemicals concentrated by SPMDs can be assessed using bioindicator experiments. These methodologies are faster than instrumental methods, but do not provide information about identification and concentration of pollutants. The preconcentrated extracts obtained from SPMDs can subsequently be combined with a variety of bioassay procedures to assess both, the level and the biological effects of water contaminants [73]. Selection of suitable bioassays is an important factor in the use of SPMDs for toxicity and genotoxicity screening of POPs. An extensive study was performed by Sabaliunas et al., using bioassays such as Microtox, Mutatox, Daphnia pulex immobilization assay and the sister chromatid exchange genotoxicity test [74]. Microtox seems to be a compatible assay with SPMDs, while the suitability of the other assays is questionable, due to insufficient sensitivity and complexity. Additionally, Gilli et al. evaluate the toxicity of drinking waters by using bioassays Microtox and Ames mutagenicity test, being also highlighted the usefulness of Microtox toxicity test [75]. Sometimes, dialyzed extracts were highly toxic in Microtox but the toxicity was significantly reduced after clean-up procedures. This toxicity is well correlated with the abundance of hydrophobic elemental sulphur, which may be reduced to sulphides in the bacterial cell, being this compound very toxic in various bioassays [74]. Chec et al. propose a procedure based on direct addition of tester bacteria cultures into SPMDs [76]. A bacterial (Vibrio harveyi) culture was added directly inside a deployed SPMD, which was then re-sealed and deployed. After that, SPMDs were incubated for an indicated time and then luminescence of the internal liquid was measured. The developed procedure provides more sensitive and more rapid results than those based on testing the extracts.

6.4.2. Chromatography The extract obtained after SPMDs treatment can be analyzed by several chromatographic methodologies, depending on the type of analyte and the concentration reached. Gas chromatography (GC) separation and detection with FID or ECD has been the common method of choice for routine analysis. GC coupled to mass spectrometry (MS) is a more reliable technique for the quantization of analytes, because of its improved selectivity, good sensitivity and the availability of deuterated or 13 C labelled compounds. Moreover, the numerous operation modes, as negative chemical ionization (NCI), that enhances the specificity of MS, make it an extremely flexible tool for the analysis of different compounds. Thus, in some studies the concentration of OCPs in SPMDs has been determined by GC–MS–NCI [66]. Tandem mass (MS–MS) acquisition, performed by ion trap or triple quadrupole MS, allows exceptional selective determinations and lower detection limits than MS. Therefore, MS–MS has been used for the determination of several contaminants as OCPS, PCBs, PBDEs, PCNs or some pesticides retained on SPMDs [39,43,69].

453

The use of time of flight (TOF) detectors provides the highest sensitivity, which is really useful in the identification of unknown compounds found in SPMDs. As example, TOF-MS has been employed for the determination of high-molecular-weight PAHs in SPMDs exposed in air and water near to a chemical plant in the Czech Republic [77]. High-performance liquid chromatography (HPLC) can be also used for the quantification of different compounds from SPMDs. UV detectors rarely provides enough sensitivity and selectivity, thus more selective detectors are employed, as fluorescence (FD) or MS ones. PAHs were retained in SPMDs and determined by using HPLC-FD [69,78,79] and also using HPLC-MS [77]. 7. Estimation of air/water pollutant concentrations Data obtained for the concentration of analytes in SPMDs must be treated to establish the corresponding concentration in the media of deployment and to do it mathematical models must be assayed. The uptake of chemicals into passive samplers is initially linear over time, then moves into a curvilinear stage, and finally can approach equilibrium (see Fig. 7). There are mathematical models for both, lineal and equilibrium behaviours. In these models it is always assumed that air/water concentrations are constant. The different compounds studied exhibit a wide range of physical and chemical properties and because of that, for a selected deployment time, the SPMD can be used as equilibrium sampler for some chemicals and as kinetic sampler for others [7]. Mathematical models used to estimate water or air concentrations from amounts found in SPMDs have been discussed by some authors [15,80]. The equilibrium partitioning is described by Eq. (1), where CMedia is the POP concentration in the deployed media (air or water), CSPMD is the POP amount found in SPMD and KSPMD/Media the SPMD-media distribution factor. CMedia =

CSPMD KSPMD/Media

(1)

The linear absorption of a POP is explained by Eq. (2), where NSPMD is the POP amount found in SPMD, Rs is the sampling rate of a selected compound and t is the deployment time of

Fig. 7. Model of variation of POPs concentration in SPMDs as a function of time.

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SPMD. CMedia =

NSPMD RS t

(2)

Some studies calculate Rs values for the most studied POPS, such as PAHS or PCBs under different conditions, as temperature, deployment time, etc. [7]. The use of SPMDs in laboratory studies generally provides good precision between replicates. However, in real media, it is difficult to control parameters as biofouling, turbulence and temperature [81]. The uptake and release of organic contaminants are governed by the same molecular processes. So, it has been suggested that any change in the uptake rate of contaminants to SPMDs should be reflected by a change in the dissipation rate from SPMDs. Spikes of SPMDs with permeability reference compounds (PRCs), usually deuterated or 13 C compounds, are widely used as a quality control tool for the sampling step [82]. The function of PRCs is similar to that of internal standards and they are added to the SPMDs to correct for any changes in uptake rates as a result of unknown environmental factors. 8. Biofouling effect The external surface of LDPE membranes submersed in water eventually become colonised by bacteria and various flora and fauna that may form a biofilm. The composition and thickness of this biofilm depend on the aquatic system and they can be very variable. Biofouling affects the resistance to mass transfer by both reasons: (i) increasing the thickness of the barrier and (ii) blocking any water-filled pores in the LDPE membranes [73]. Thus a decreasing in samples rates can be produced in devices with big amounts of biofouling. Two methodologies have been used to quantify the biofouling effect on sampling rates [83]. The first approach involves biofouling of LDPE membrane in the field, followed by measuring the contaminant uptake rates in the laboratory by using biofouled and non-biofouled SPMDs. The second method is based on the assumption that biofouling has a similar effect on uptake and release of compounds, then PRCs were spiked into the triolein prior to exposure and differences in release rates of PCRs were related to differences in biofouling [84]. Richardson et al. used LDPE membranes previously biofouled at a clean site in coastal waters for periods of 1–4 weeks. Later, triolein was added to the SPMDs and they were exposed to a range of OCP and PAH concentrations in waters under laboratory conditions. Some unfouled devices were also exposed as control samples. Results evidenced that the uptake of contaminants by SPMDs was severely reduced until a 50% using biofouled devices in comparison to unfouled ones. Authors concluded that using PRC inside SPMDs allows a realistic evaluation of POPs concentration in aquatic environments [85]. A number of attempts have been made to reduce the biofouling colonisation of the SPMDs. Thus, Huckins et al. suggested that the addition of organic solvents and small amounts of pesticides may help to reduce biofouling [1]. Some solvent-filled membrane devices (e.g. TRIPMS) are protected from biofoul-

ing by slow release of solvent from the sampler during exposure [73]. Moreover, Luellen et al. do not observed biofouling on SPMDs that were wiped and dipped in a solution of copper sulphate and an herbicide every 2 days [20]. 9. Comparison of SPMDs with standard samplers Biomonitoring and active samplers are the standard methods used to determine short and long term exposition of air and water to toxic substances. In our opinion SPMDs are compatible with the aforementioned systems and offers complementary information. 9.1. SPMD against biomonitoring SPMDs were designed to mimic the parts of animals that cause bioconcentration. Thus, contaminants which are less available to animals would also be less available to these samplers. SPMDs do not need to be fed, they do not eat each other or die from disease, predators, lack of oxygen in the water, or from the contaminants that we are trying to measure. So, the great advantage of SPMDs is that they can be used in almost any environment. Additionally, analyte recovery and enrichment procedures for SPMDs usually require less effort than those for tissue and sediment matrices, due to the perfect knowledge of the components of the sampler. Another advantage is that we can use samplers to back-calculate a water-borne contaminant concentration, by using mathematical models, which were usually unable to do with fish or mussel [2]. Evidently, SPMD is not a perfect model for the uptake of pollutants to fish or mussel, because organisms can depurate some contaminants and pick up contaminants through their diet. Both mussels and SPMDs have proven to be rugged and effective in concentrating trace levels of non-polar organic contaminants from water, providing time-integrated contaminant information. SPMD takes the water-soluble fraction of pollutants, which is probably the most important route of exposure for organisms; while the residues found in mussels represent both the water-soluble and particle-bound fraction. So both techniques give important information about the bioavailability of POPs to marine organisms [86]. The blue mussel Mytilus edulis and the green mussel Perna viridis have been used for multitude of comparative uptake studies instead SPMDs (see Table 4). Several studies show that mussels are more efficient to sequester PAHs from water than SPMDs [87,91,97]. This behaviour is also observed for PCBs and OCPs compounds [92,98]. In a study carried out with blue mussels, the bioaccumulation of PBDEs was calculated as 10 times higher than for PCBs [89]. Additionally, a good correlation was found between SPMDs and mussels for the uptake of organotin compounds in a study carried out in water of the Norwegian fjord [28]. Other studies have been made employing others mussel species, and as an example Mytilus trossulus has been compared with SPMDs for the uptake of PAHs, being these devices more efficient to sequester small PAHs than mussels [95]. The lake mussels Anodonta piscinalis was also employed for the

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455

Table 4 Published papers in recent years related to comparative studies between SPMDs and biosamplers in several environments Biosampler

Compounds evaluated

Deployment site

SPMD deployment time (days)

References

Blue mussel (Mytilus edulis)

PAHs, PCBs, OCPs, Organotin, PBDEs

28–95

[28,86–94]

Blue mussel (Mytilus trossulus) Green mussel (Perna viridis)

PAHs PAHs, OCPs

30 20–30

[95] [96–98]

Lake mussel (Anodonta piscinalis)

OCPs, Pesticides

20

[99,100]

Mussel (Elliptio complanata) Brown trout (Salmo trutta) Rainbow trout (Oncorhychus mykiss) Carp (Cyprinus carpio) Rudd, Tench, Crucian carp, Eel Fish Pink salmon egg Pacific oyster (Crassostrea gigas) Oligochaetes (Lumbriculus variegatus) Oligochaetes (Lumbriculus variegatus) Pine needles

Surfactants PCBs PAHs, OCPs PAHs Musk compounds PAHs PAHs PAHs PAHS, PCBs PAHs PAHs

Fjord water (Norway); North Sea water (Netherlands); North Sea water (Norway); Aluminium reduction plant (Sweden); Dorchester and Duxbury Bays (MA, USA); Corio Bay (Australia) Beach sediment (Alaska, USA) Spiked seawater; Coastal waters (Hong Kong, China) Water laboratory studies; Lake water (Finland) Great Lakes (USA) Contaminated groundwater Water laboratory studies Water – – Water Water laboratory studies Lake H¨oyti¨ainen sediment (Finland) Lake H¨oyti¨ainen sediment (Finland) Air (China)

– 28 21–28 – – – – 20 – 28 90

[26] [101] [102,103] [104] [29] [105] [16] [106] [107] [108] [54]

evaluation of pesticide uptake in a laboratory study, being the uptake rates four to six times higher in SPMDs than in mussels [99]. The accumulation of pollutants in fish muscle has been also compared with SPMDs. Several fishes and shellfishes, such as trout, carp or oyster, have been studied. The rate constants for the uptake of PAHs, PCBs and OCPs by SPMDs and fishes were very similar, with Rs values for SPMDs ranging from 1 to 2.5 times those of the fish [101,102]. Pink salmon eggs were also compared for the uptake of pollutants from aqueous environments, where accumulations of PAH were highly similar to those in SPMDs [105]. In the case of oysters, the concentration factors for PAHs were, from 4 to 20 times lower than those of the same compounds in SPMDs. An exception is the case of PAH compounds with log Kow greater than 5.5, where levels in oysters were higher than in SPMDs [106]. Other organisms have been employed to compare the uptake from sediments instead SPMDs, such as oligochaetes for PCBs and PAHs [107,108]. Pine needles were employed to compare the uptake of PAHs from air instead SPMD sampling [54]. Total concentrations found for these compounds were similar for both samplers, however pine needles preferably accumulate small and high volatile PAHs, while SPMD collect high molecular weight ones. 9.2. SPMD against active samplers Persistent organic pollutants are conventionally monitored in the atmosphere using high volume air samplers, also called active samplers. These systems incorporate a pump together with a filter or a sorbent, such as polyurethane foam (PUF) or Tenax. The main drawbacks of active samplers are their expensive preparation, maintenance and operation, where power supply

is often required. So, SPMDs provide clear advantages to be used for POPs determination in atmosphere. The main disadvantages of SPMD in front of active sampling are that (i) they only sample the bioavailable fraction, not the total amount, (ii) the absorption of hydrophobic compounds by triolein is seriously reduced in waters with high levels of organic matter and the (iii) risk of theft, vandalism or possible loss of devices. The cost of SPMDs has been considered by some authors as a significant inconvenience, but this problem can be overcome by the use of alternatives to triolein filling. Some researchers have employed both SPMDs and an active sampler on the same site, to compare the uptake behaviour of each one. Parallel studies have been carried out with PUF as active sampler of air for the uptake of several compounds as PAHs, PCBs, OCPs, PCNs [38,54,109,110], while other samplers such as XAD-2 resin for air or Tenax-TA and polyethylene tube dialysis for sediments have been used to evaluate the PAH uptake [108,111,112]. Conventional techniques for water sampling, as bailing, lowflow and bailing/filtering, have been compared with the use of SPMD for monitoring PAHs [4]. PAH concentrations in the groundwater evaluated with SPMDs were similar to values obtained by the conventional techniques. Furthermore, a high sensitivity was observed for SPMD sampling, allowing the detection of some PAHs not detected by conventional approaches. Wastewater has been sampled both by liquid–liquid extraction with hexane and by SPMD passive devices for the determination of PCBs and OCPs [113]. The wastewater concentrations estimated from SPMDs were slightly lower than those obtained by liquid–liquid active sampling, but the sampling with SPMDs was able to evaluate the global contamination of hydrophobic compounds at trace concen-

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tration. Authors remark that data obtained for both, active and passive samplers are complementary and both must be employed for the evaluation of the POP contamination of waters. 10. Future trends Research focussed on the development and search for applications of SPMDs in sampling air and water is increasing. Future trends could be in the following areas: (i) novel pollutants, mainly non-polar compounds with low octanol–water coefficient; (ii) increasing of environmentally friendly methodologies for the extraction of pollutants retained in SPMDs, based principally in the reduction of analysis times and minimization of solvent consumption and waste generation; (iii) improvement of calibration models and sampling rates evaluation of studied compounds; (iv) comparative studies to evaluate the sampling of SPMDs in front alternative samplers; and (v) new applications of SPMDs, specially as air sampling or occupational exposure measurements. So, it can be expected a growth of studies on SPMDs in the next years and especially concerning the development of screening methodologies for as much as possible classes of pollutants in both, water and air. Acknowledgements The authors acknowledge the financial support of the Ministerio de Educaci´on y Ciencia (Project CTQ2005-05604, FEDER), Direcci´o General d’Investigaci´o i Transfer`encia Tecnol`ogica de la Generalitat Valenciana (Project ACOMP/2007/131) and Universitat de Valencia (Convocat`oria d’Accions Especials, Project UV-AE-20070213). F.A.E.T. also thanks the grant “V Segles” provided by the Universitat de Val`encia to carry out this study. References [1] J.N. Huckins, M.W. Tubergen, J.A. Lebo, R.W. Gale, T.R. Schwartz, J. Assoc. Off. Anal. Chem. 73 (1990) 290. [2] A. Kot, B. Zabiegala, J. Namiesnik, Trends Anal. Chem. 19 (2000) 446. [3] J.D. Petty, C.E. Orazio, J.N. Huckins, R.W. Gale, J.A. Lebo, J.C. Meadows, K.R. Echols, W.L. Cranor, J. Chromatogr. A 879 (2000) 83. [4] K.E. Gustavson, J.M. Harkin, Environ. Sci. Technol. 34 (2000) 4445. [5] Y.B. Lu, Z.J. Wang, J.N. Huckins, Aquat. Toxicol. 60 (2002) 139. [6] F. Stuer-Lauridsen, Environ. Pollut. 136 (2005) 503. [7] J.N. Huckins, J.D. Petty, H.F. Prest, R.C. Clark, D.A. Alvarez, C.E. Orazio, J.A. Lebo, W.L. Cranor, B.T. Jonson, A Guide For The Use Of Semipermeable Membrane Devices (SPMDs) as Samplers of Waterborne Hydrophobic Organic Contaminants, American Petroleum Institute (2002) Publication Number 4690. [8] J.N. Huckins, J.D. Petty, K. Booij, Monitors of Organic Chemicals in the Environment Semipermeable Membrane Devices, Springer, 2006. [9] Isi Web Of Knowledge, The Thompson Corporation, http://isiwebofknowledge.com. 2000. [10] D.R. Luellen, D. Shea, Chemosphere 53 (2003) 705. [11] K.D. Wenzel, B. Vrana, A. Hubert, G. Schuurmann, Anal. Chem. 76 (2004) 5503. [12] K. Booij, H.E. Hofmans, C.V. Fischer, E.M. Van Weerlee, Environ. Sci. Technol. 37 (2003) 361. [13] K. Booij, F. Smedes, E.M. Van Weerlee, Chemosphere 46 (2002) 1157.

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