Passive Sampler for Dissolved Organic Matter in Freshwater

In a comparative study DOM isolated from Lake Ontario cost ∼$0.30/mg to isolate .... Nathaniel L. Sanders , Ewa Sokol , Richard H. Perry , Guangming...
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Anal. Chem. 2006, 78, 8194-8199

Passive Sampler for Dissolved Organic Matter in Freshwater Environments Buuan Lam and Andre´ J. Simpson*

Department of Chemistry, University of Toronto, Scarborough Campus, Toronto, Ontario, Canada M1C 1A4

A passive sampler for the isolation of dissolved organic matter (DOM) from freshwater environments is described. The sampler consists of a molecular weight selective membrane (1000 kDa) and an anion exchange resin (diethylaminoethylcellulose (DEAE-cellulose)). NMR indicates the samplers isolate DOM that is nearly indistinguishable from that isolated using the batch DEAEcellulose procedure. In a comparative study DOM isolated from Lake Ontario cost ∼$0.30/mg to isolate using the passive samplers while DOM isolated using the traditional batch procedure cost ∼$8-10/mg. The samplers have been shown to be effective in a range of freshwater environments including a large inland lake (Lake Ontario), fast flowing tributary, and wetland. Large amounts (gram quantities of DOM) can be easily isolated by increasing the size or number of samplers deployed. Samplers are easy to construct, negate the need for pressure filtering, and also permit a range of temporal and spatial experiments that would be very difficult or impossible to perform using conventional approaches. For example, DOM can be monitored on a regular basis at numerous different locations, or samplers could be set at different depths in large lakes. Furthermore, they could potentially be deployed into hard to reach environments such as wells, groundwater aquifers, etc., and as they are easy to use, they can be mailed to colleagues or included with expeditions going to difficult to reach places such as the Arctic and Antarctic. Dissolved organic matter (DOM) represents the largest pool of mobile carbon on the Earth and is a fundamental link between the terrestrial and aquatic environments. If aquatic DOM experienced a 1% oxidation in a year, the CO2 released would be greater than that generated annually from the combustion of fossil fuels.1 DOM is a heterogeneous, complex mixture, ubiquitous in the environment, including soil, sediment, rain, and oceans, and represents one of the largest reservoirs of carbon on earth.2,3 This, combined with the natural cycling of DOM from terrestrial to aquatic environments,2 makes DOM a significant contributor to the global carbon cycle,4,5 an important mediator in the physical * To whom correspondence should be addressed. Tel +1 416 287 7547. Fax +1 416 287 7279. E-mail address [email protected]. (1) Hedges, J. I. In In Biogeochemistry of Marine Dissolved Organic Matter; Hansell, D. A., Carlson, C. A., Eds.; Academic Press: New York, 2002; pp 1-33. (2) Hedges, J. I. Mar. Chem. 1992, 39, 67-93. (3) Ogawa, H.; Tanoue, E. J. Oceanogr. 2003, 59, 129-147.

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and chemical interactions that govern the fate and transport of many contaminants, and an integral player in climate change.6 However, despite its importance, many of the basic structural components in DOM have yet to be elucidated, due in part to the difficulty in its isolation. Traditionally, the isolation of DOM has required large sample volumes to overcome the low concentrations in natural waters.7,8 These techniques, however, require extensive on-site practices that are labor-intensive and time-consuming, often requiring chemicals, columns, filters, generators, etc., to be used in the field. Traditionally, these methods often employ nonionic, macroporous resins such as XAD to bind and extract DOM.8-10 However, a large majority of these resins must be maintained at low pH to adequately bind components.9 Since DOM from natural waters is present at near-neutral pH, the use of these resins requires acidification that could potentially alter the chemical composition of the isolated DOM. Diethylaminoethyl-cellulose (DEAE-cellulose), however, has been shown to isolate up to 90% of DOM from freshwater within the pH range of natural waters.11 This characteristic gives DEAE-cellulose the potential to be an effective resin for isolating and concentrating DOM in its natural form. Promising resins alone, however, are not sufficient to effectively enhance the isolation of DOM for analysis. Large sample volumes are still required to overcome the low natural abundance of DOM, and filtering is critical to remove insoluble and biological species. To prevent biological alteration during transport, it is beneficial to at least filter samples, or carry out the entire isolation, in the field. In this study, we introduce the development and application of a passive sampler to effectively isolate and concentrate DOM from freshwater environments in situ. The use of passive samplers in environmental chemistry is far from new. However, the vast majority of samplers thus far have focused on the concentration of contaminants.12-14 The sampler described here, is to the (4) Trumbore, S.; Druffel, E. Role of Nonliving Organic Matter in the Earth’s Carbon Cycle; John Wiley & Sons: Chichester, 1995. (5) Baldock, J. A.; Masiello, C. A.; Gelinas, Y.; Hedges, J. I. Mar. Chem. 2004, 92, 39-64. (6) Keeling, C. D.; Whorf, T. P.; Wahlen, M.; Vanderplicht, J. Nature 1995, 375, 666-670. (7) Leenheer, J. A. Environ. Sci. Technol. 1981, 15, 578-587. (8) Thurman, E. M.; Malcolm, R. L. Environ. Sci. Technol. 1981, 15, 463-466. (9) Aiken, G. R.; Thurman, E. M.; Malcolm, R. L.; Walton, H. F. Anal. Chem. 1979, 51, 1799-1803. (10) Maccarthy, P.; Peterson, M. J.; Malcolm, R. L.; Thurman, E. M. Anal. Chem. 1979, 51, 2041-2043. (11) Miles, C. J.; Tuschall, J. R.; Brezonik, P. L. Anal. Chem. 1983, 55, 410411. 10.1021/ac0608523 CCC: $33.50

© 2006 American Chemical Society Published on Web 10/25/2006

Figure 1. (a) Schematic showing the components of the passive sampler. (1) The 1000-kDa MWCO poly(vinylidene fluoride) membrane. (2) Porous HDPE casing to house sampler unit (designed in-house) to prevent large organisms (fish, etc.) and debris from compromising the membrane. (3) DEAE-cellulose resin. (b) Expanded region showing the resin/membrane/water interface. (4) Dissolved negatively charged DOM enters the membrane and is sorbed onto the resin and concentrated manyfold, (5 + 6) Dissolved neutral or positively charged species (for example, metals in the case of positively charged species) can enter the membrane but are not retained (note the vast majority of DOM is negatively charged). (7 + 8) Large species including particulate organic matter and biological species cannot enter the membrane. The use of the membrane removes the need for filtering.

authors’ knowledge, the first passive sampler designed for the isolation of DOM from the environment and the first to use a sizeselective membrane (to negate the need for filtering and distinguish between biological species and dissolved species) in combination with a selective resin (in this case DEAE-cellulose) to concentrate the negatively charged DOM species. This passive isolation provides a means to collectively isolate, filter, and concentrate DOM in the field, eliminating problems associated with traditional on-site extractions and large sample volumes. Additionally, these samplers have applications in spatial and temporal studies to monitor changes in DOM at various depths (for example, numerous samplers can be deployed at specific depths using a single mooring) and locations (potentially hundreds of samplers could be deployed on the same day by just one research team (the same is true with recovery)). The samplers thus provide an economical and easy means by which researchers can attain a vast array of DOM samples in a short time, critical to understanding its local/global variability, transformations, and reactivity. MATERIALS AND METHODS Sampler: General Design and Preparation. The passive sampler is simple in both concept and design as highlighted in Figure 1. Three components comprise the passive sampler: (1) DEAE-cellulose (Sigma Aldrich, Order No. D6418)), a selective resin that adsorbs negatively charged species at neutral pH, (2) a poly(vinylidene fluoride) (PVDF) porous membrane with a (12) Vrana, B.; Mills, G. A.; Allan, I. J.; Dominiak, E.; Svensson, K.; Knutsson, J.; Morrison, G.; Greenwood, R. TrAC-Trends Anal. Chem. 2005, 24, 845868. (13) Stuer-Lauridsen, F. Environ. Pollut. 2005, 136, 503-524. (14) Namiesnik, J.; Zabiegala, B.; Kot-Wasik, A.; Partyka, M.; Wasik, A. Anal. Bioanal. Chem. 2005, 381, 279-301.

molecular weight cutoff (MWCO) of 1000 kDa (∼0.1 µm, (Spectrapor, Order No. 138525)), and (3) a high-density polyethylene (HDPE) casing with predrilled holes (constructed in-house). Note, the authors have found simple “Nalgene” style HDPE disposable screw cap bottles can be drilled and used as a protective casing if specially designed casings are not available. Prior to use, DEAE-cellulose was precleaned using a cycle of acid, base, and distilled water washings. Specifically, the cleaning regiment consisted of 0.1 M hydrochloric acid, 0.1 M sodium hydroxide, and distilled water washings in between acid/base cleanings. A minimum of 10 full acid-water-base cycles were performed, followed by a minimum of 100 rinses with excess distilled water, and finally freeze-drying. Note, the resin settles fairly quickly (in ∼30 min). Cleaning can be easily carried out by mixing in a tall container (such as a large measuring cylinder) and then carefully decanting the supernatant after settling. The complete process (including cleaning and freeze-drying) will take close to 1 month. The authors advise cleaning a large batch of resin, which can be stored in its freeze-dried form in an airtight sterile container for later use. Cleaned DEAE-cellulose (250 mg) was slurry packed with distilled water into 7-cm-length (24-mm-width) PVDF porous membranes, which were presoaked in 0.1% sodium azide for a minimum of 48 h. Preliminary experiments (not described here) demonstrated that 250 mg/7-cm length of tubing was the most efficient use of both resin and PVDF membrane; this ratio allowed resin mobility within the sampler such that its entire surface area was available for DOM sorption. Packed membranes were placed into the constructed HDPE casings to form the passive samplers. Laboratory Studies. Initial laboratory studies were performed to test the applicability of the samplers for the isolation/ concentration of DOM. Two separate studies were performed. The Analytical Chemistry, Vol. 78, No. 24, December 15, 2006

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first used the International Humic Substances Society (IHSS) Standard Suwannee River natural organic matter, isolated from the Suwannee river by reverse osmosis,15 and the second dissolved organic matter isolated by the XAD resin procedure from Methwold Fen, Norfolk, United Kingdom (see Simpson et al.16 for isolation details and characterization). Samplers (250 mg of resin, in 7 × 24 mm) were suspended (using fishing line) in 10 ppm DOM solutions (no additional salts were added), which were contained in stoppered 1-L bottles and continuously mixed using a magnetic stirrer. The pH of the Suwannee River and Norfolk DOM samples were 7.1 and 6.9, respectively. Each experiment was carried out in triplicate. Samplers were left to equilibrate over a period of 2 weeks, and the concentration of DOM remaining in solution was determined by total organic carbon (TOC) analysis (see below). Field Studies. A field study was carried out at the Lynde Shores Conservation Area (representative of a wetland) close to the mouth of Lynde Creek, Ajax, Ontario. Passive samplers were placed in triplicate in a steel cage and suspended 50 cm below the surface of the water. Samplers were removed from the cage on days 1, 3, 7, 14, and 28, extracted, as described below, and DOM yields determined gravimetrically. In addition, a conventional DEAE-cellulose batch extraction was performed. Briefly 40 L of water was collected, filtered through 0.22-µm Teflon filters (Millipore), batch extracted with DEAE-cellulose,11 and recovered as described below. In addition to Lynde Shores Conservation Area samplers were also placed in highland creek (a fast-flowing tributary), and Lake Ontario (to represent a large freshwater lake). Sample Extraction. Extraction of bound DOM from the passive samplers was performed by cutting and removing the resin from the PVDF membranes. The resin was then placed in 50-mL Teflon centrifuge tubes and extracted using ∼40 mL of 0.1 M sodium hydroxide. The tubes were centrifuged at 10000g for 10 min to pellet the resin, and the supernatant was decanted. The pellet was then resuspended, and the previous steps were repeated four more times, or until the extracting solvent was clear, to ensure complete extraction of DOM from the resin. The extracted DOM was then ion-exchanged using Amberjet 1200H Plus resin (Aldrich) and freeze-dried. The freeze-dried sample was resuspended in deuterated dimethyl sulfoxide (DMSO-d6) or deuterium oxide (D2O (with 2 µL of 40% by weight NaOD added, to ensure complete solubility)) for NMR analysis. Sample Analysis. NMR is a well-established tool for the analysis of dissolved organic matter.17-19 Samples were analyzed using nuclear magnetic resonance (NMR) spectroscopy on a Bruker Avance 500 equipped with a 1H BB-13C 5-mm triple resonance broadband inverse probe. Unless otherwise stated, 1-D solution state 1H NMR experiments were performed with 512 scans, a recycle delay of 3 s, and 32 K time domain points. Solvent suppression was achieved by presaturation utilizing relaxation (15) IHSS, http://www.ihss.gatech.edu/, accessed Aug 2006. (16) Simpson, A. J.; Tseng, L. H.; Simpson, M. J.; Spraul, M.; Braumann, U.; Kingery, W. L.; Kelleher, B. P.; Hayes, M. H. B. Analyst 2004, 129, 12161222. (17) Simpson, A. J. Magn. Reson. Chem. 2002, 40, S72-S82. (18) Kaiser, E.; Simpson, A. J.; Dria, K. J.; Sulzberger, B.; Hatcher, P. G. Environ. Sci. Technol. 2003, 37, 2929-2935. (19) Kim, S.; Simpson, A. J.; Kujawinski, E. B.; Freitas, M. A.; Hatcher, P. G. Org. Geochem. 2003, 34, 1325-1335.

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Figure 2. 1H NMR spectra showing DOM dissolved in D2O/NaOD. (A) Fraction from the Suwannee River reverse osmosis standard collected on a passive sampler after 2-weeks equilibration. (B) Suwannee River reverse osmosis standard dissolved directly in D2O/ NaOD. PURGE20 water suppression was applied to the residual water signal at ∼4.7 ppm.

gradients and echos (PURGE).20 Spectra were apodized by multiplication with an exponential decay corresponding to 1-Hz line broadening in the transformed spectrum and a zero filling factor of 2. Blank 1H NMR were run on completely assembled passive samplers soaked in distilled water, and no background signals from the samplers were detected. TOC analysis was carried out on a Shimadzu TOC-V Series analyzer. Samples were acidified to pH 2 followed by sparging to remove dissolved inorganic carbon. The sample aliquots were then subjected to high-temperature catalytic oxidation to form CO2 and subsequent CO2 concentrations measured with a nondispersive infrared detector.21 RESULTS Initial laboratory experiments demonstrated that 72 ( 5 (Suwannee River) and 89 ( 4% (Norfolk) DOM was removed from the 10 ppm laboratory solutions after 2 weeks of equilibration. Figure 2A shows the NMR spectrum of the Suwannee river DOM isolated on the passive sampler compared to the original sample (Figure 2B). The NMR spectra are generally similar, indicating that the passive samplers give a reasonably representative overview as to DOM components present in the environment. The largest difference in the material concentrated on the passive sampler is the reduction of small molecular weight sugars (indicated by sharp signals in the 3-4 ppm region). Such small components are highly soluble and may be less likely to perma(20) Simpson, A. J.; Brown, S. A. J. Magn. Reson. 2005, 175, 340-346. (21) Wetzel, R. G.; Likens, G. E. Limnological Analyses, 3rd ed.; Springer: New York, 2000.

Figure 3. Field study showing the uptake of DOM on passive samplers from Lynde Shores Conservation Area. Samplers were deployed in triplicate each containing 250 mg of resin. Yields are expressed per individual sampler, and bars indicate the standard deviation between triplicate samplers.

nently bind to the DEAE-cellulose than the high molecular weight multidentate species that are thought to predominate in DOM. It is important to note the relative yield of Suwannee river isolated on the passive sampler (72 ( 5%) is less than that reported by Miles et al.11 (88%) by the DEAE batch procedure. Later in this paper we demonstrate using NMR analyses that the passive sampler and batch DEAE procedures used to isolate DOM from the field are extremely similar. The difference in yields observed here is likely to be, at least in part, a result of the considerable amount of salt that is concentrated alongside the organic compounds by reverse osmosis (the method used to concentrate the IHSS Suwannee River Standard). Negative ions (mainly chloride) will compete for binding sites on the DEAE-cellulose and slow down or prevent the sorption of all DOM components. Salt concentration is just one factor that will affect the rate of DOM uptake in the environment, DOM concentration, and water dynamics, among others may also play key roles. Considering these factors, it is critically important to demonstrate that the passive samplers can isolate DOM in reasonable quantities in the field and that the DOM isolated is the same as that obtained by DEAE in conventional batch extraction, which is now becoming a popular technique for isolating DOM from the environment.22-25 Figure 3 shows the yields of DOM isolated in the field at Lynde Shores Conservation Area, Ajax, Ontario. The yields are expressed in terms of individual samplers each containing 250 mg of resin. The uptake follows the generalized uptake profile expected for a passive sampler device.26 After 1 month. 18 ( 2 mg of DOM had concentrated on each of the three passive samplers, giving a total yield on all three samplers of 54 mg. As a comparison, water was collected from the conservation area midway through the sampling period and the DOM isolated by batch extraction. Figure 4A shows the NMR spectrum of the DOM collected on the passive sampler in (22) Peuravuori, J.; Pihlaja, K.; Valimaki, N. Environ. Int. 1997, 23, 453-464. (23) Raastad, I. A.; Ogner, G. Commun. Soil Sci. Plant Anal. 1997, 28, 13111321. (24) Peuravuori, J.; Ingman, P.; Pihlaja, K.; Koivikko, R. Talanta 2001, 55, 733742. (25) Peuravuori, J.; Monteiro, A.; Eglite, L.; Pihlaja, K. Talanta 2005, 65, 408422. (26) Mayer, P.; Tolls, J.; Hermens, L.; Mackay, D. Environ. Sci. Technol. 2003, 37, 184A-191A.

Figure 4. 1H NMR spectra of DOM isolated from Lynde Shores Conservation Area using (a) passive samplers and (b) traditional isolation, involving filtering and batch isolation on DEAE-cellulose.

comparison with that extracted by the conventional batch procedure (Figure 4B). It is clear that the material is extremely similar, indicating that the passive sampler approach isolates the same components as conventional DEAE-cellulose batch extraction. Slight variations in the NMR profile are expected considering that the water for batch extraction was collected on a single day in the middle of the sampling study, whereas the DOM from the passive samplers represents “the average” DOM that has been collected over the period of the whole month. Table 1 compares yields collected from various sites. From Lake Ontario, 36 mg of DOM/g of DEAE-cellulose was isolated, whereas nearly twice that, 60 mg, was obtained for the Lynde shores conservation area. This is in line with TOC concentrations from the two sites, which were 2.8 and 5.0 ppm, respectively. In addition to the relatively small scale studies described so far, a large-scale study was carried out to ensure “bulk” amounts of DOM could be isolated if required. In this case, 750 mg of resin was packed into 21 cm × 24 mm tubes (such that the ratio of 250 mg of resin/7 cm of membrane was preserved) to create much longer samplers. Sixty of these larger samplers (equivalent to 180 of the smaller samplers) were deployed, and after 2 weeks, 2.79 g of DOM was isolated. This equates to 62 mg of DOM/g of resin used and is consistent with the 60 mg of DOM/g of resin isolated from the same site using the smaller samplers (although sampled a month later, see Table 1). Analytical Chemistry, Vol. 78, No. 24, December 15, 2006

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Table 1. Isolated DOM Yields Obtained with the Passive Sampler from Various Field Locationsa

sample location

number of samplers

sampling time

total yield

DOM yield normalized to 1 g of resin

Lake Ontario, Burlington, Ontario Highland Creek, Scarborough, Ontario Lynd Shores Ajax, Ontario (July, 2006) Lynde Shores, Ajax, Ontario (July-Aug, 2006)b Lynde Shores, Ajax, Ontario (June 2006)

8 12 3 3 180c

2 weeks 2 weeks 2 weeks 1 month 2 weeks

72 mg 140 mg 45 mg 54 mg 2.79 g

36 mg 47 mg 60 mg 72 mg 62 mg

a The number of samplers is based on each sampler containing 250 mg of resin. Lake Ontario represents a large freshwater lake, Highland Creek a fast-flowing tributary, and Lynde Shores a wetland system. b This field study was carried out in triplicate, and the yields of DOM varied by ( 2 mg for the different samplers. All other amounts are cumulative yields from all samplers combined. c These samplers were actually deployed as 60 × 750 mg samplers (equivalent to 180 × 250 mg samplers); the ratio of resin to membrane was kept the same (see text).

DISCUSSION Fundamental Differences: Passive Sampler versus Batch Extraction. It is important to consider that even if samples were isolated from the same location at the “same time” there would still likely be small variances between DOM isolated using a passive sampler approach and those from more traditional procedures. First, with the more traditional isolation, sampling is often carried out over just 1 or 2 days, providing a “snapshot” of DOM in time. Whereas, the samplers are deployed over one or more weeks, providing a more integrative DOM sample over a defined time period. Thus, the DOM isolated on a single day could theoretically be very different from that collected over a larger period, for example, over 2 weeks. One could argue that this is an advantage in that the DOM isolated by the passive sampler approach is more representative of the material present on average and less susceptible to specific daily fluxes. Alternatively, if high temporal resolution is required (for example, DOM samples need to be isolated every few hours), the passive sampler approach would not be suitable. Second, and arguably more important, when using the passive samplers a filtration step (most commonly done under pressure) is not required. It is quite feasible that pressure filtration (which itself is an established method for lysing cells27 and nearly always required for conventional isolations) may in fact rupture cells or small aquatic species (plankton, etc.), which it turn contribute material that inadvertently becomes operationally defined as part of the DOM. While the contribution of such “mechanical forces” is very difficult to assess in the field and across a wide range of environments, the passive sampler approach does provide a significant advantage in that filtration is not required. Cost and Labor. Traditional isolation methods may range from various resin treatments, to ultrafiltration, or simple concentration by lyphilization. Each has their advantages and disadvantages;25 however, nearly all either involve the transportation of large amounts of water to a laboratory or require the transportation of extensive apparatus, including generators, filters, resins, columns, and ultrafiltration apparatus to a field site. In turn, numerous personnel and specialized transport are required. In 2003, the authors carried out a study isolating DOM from Lake Ontario using the DEAE-cellulose method. Approximately 500 L of water was filtered at three different sites (1500 L total). Disposable Millipore 0.22-µm filters were used with peristaltic pumps (generator required), and the filters required replacement (27) Guerlava, P.; Izac, V.; Tholozan, J. L. Curr. Microbiol. 1998, 36, 131-135.

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after ∼25 L of water at a cost of ∼$US 200 each (∼$4000 per site). Large amounts of resin, 1.5 kg per site, were used (∼$1500 per site). In addition, personnel (a team of 5), large-capacity columns, transport, and numerous smaller consumables (silica tubing, fitting, carboys, etc.) placed isolated costs for the three sites in excess of $20 000 and cost per site in the area of $8 000$10 000. Approximately 1 g of DOM was isolated per site, resulting in an isolation cost of $8-10/mg using the conventional approach. In a comparative study of Lake Ontario in 2005 using the passive samplers, each (0.25 g) passive sampler required ∼7 cm of PVDF tubing (1 m ) $33) and 0.25 g of DEAE-cellulose resin (cleaned and freeze-dried cost, $1/g) and a polycarbonate case designed in-house ∼$0.1 each. Thus, each sampler costs ∼$2.70 each. From Lake Ontario, over a 14-day period, 72 mg was collected using eight samplers in total at an approximate cost of $0.3/mg or ∼$21 for the entire sample (∼30 times more costeffective than the conventional approach). As a further example, in another study DOM for Lynde shores conservation area, 54 mg was concentrated on only three samplers left for 1 month during midsummer. This equates to ∼$8.10 for the entire sample or $0.15/mg. Novel Studies. Financial grounds alone may be sufficient to justify using passive samplers for the isolation of DOM from freshwater aquatic environments, but as well as providing a simple and cost-effective alternative, the passive sampler also permits a range of temporal and spatial experiments that would be very difficult or impossible to perform using conventional approaches. For example, DOM can be monitored on a regular basis at numerous different locations, or samplers could be set at different depths in large lakes to monitor how DOM at depth changes with time. They could be deployed into hard to reach environments such as wells or groundwater aquifers, where removal of large amounts of water, needed for conventional approaches, could detrimentally effect the environment or may be difficult to access. Furthermore, as the samplers are low cost and easy to use, they can be mailed to colleagues or included with expeditions going to difficult to reach places such as the Arctic and Antarctic. Applications are not just limited to the aquatic environment, and DOM collected from buried passive samplers in soil/sediment could be used to isolate the mobile fraction of DOM that is carried through the terrestrial environment. Isolating organic components from soil that possess the greatest potential to be leached can help establish models to predict the fate and transport of chemical contaminants associated with this mobile fraction.28

Future and Further Considerations. The advantages of using the passive samplers described here to isolate DOM from a freshwater environment are numerous. However, it is important to discuss some potential drawbacks that may limit the universal application of this method for DOM isolation, at least in the near future. First, and most importantly, the DEAE-cellulose resin is an ion-exchange-based resin. In a salt water environment, the high concentration of Cl- ions will compete for binding sites on the resin. One sampler was deployed in the Irish sea, and while a trace amount of DOM could be recovered, the abundance of salts made subsequent analyses nearly impossible. The authors do not recommend this present design to isolate DOM from salt water. Presently, work is underway to design a resin that would perform better under saline conditions. Another potential concern is biological growth on the membrane surface, this could block the pores in the sampler, and the biological species could contribute exudates that are in turn concentrated by the sampler. With the present design, no biological growth was noted on any of the samplers with the PVDF membranes presoaked in sodium azide. In fact, Figure 4 shows that DOM isolated from the passive sampler is nearly identical to that from batch extraction, indicating during a whole month of “summer sampling” in an algae-rich wetland, there is no evidence of additional biological contributions to the DOM composition (as compared to the DOM isolated by the traditional batch extraction (28) McCarthy, J. F.; Jimenez, B. D. Environ. Sci. Technol. 1985, 19, 10721076.

procedure). However, the “upper time limits” of the present design are not known and will vary between environments. Thus, future studies should be carried out with this in mind, especially if very long exposure periods are planned or the membrane composition or pretreatment is altered. With the present knowledge, the authors recommend the samplers be deployed for a period of 2 weeks, up to 1 month. In summary, the passive samplers described here provide an economical and efficient device for the concentration of DOM from freshwater environments. Not only are they an attractive alternative to traditional concentration approaches, but their easy implementation permits a novel range of experiments that are impossible using large-scale isolation. ACKNOWLEDGMENT We thank colleagues B.P. Kelleher and M. Alaee for help in deploying samplers. C.M. Febria for help in TOC analysis, and Professors Myrna Simpson, William Kingery, Dudley D. Williams, and Dr Emma L. Smith for their scientific and editorial input. We thank the National Science and Engineering Research Council of Canada (NSERC) for providing funding in the form of a discovery grant (A.J.S.).

Received for review May 9, 2006. Accepted September 27, 2006. AC0608523

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