Dissolved silver measurements in seawater

Trends in Analytical Chemistry, Vol. 26, No. 8, 2007

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Dissolved silver measurements in seawater Jose L. Barriada, Alan D. Tappin, E. Hywel Evans, Eric P. Achterberg There is a paucity of data on dissolved silver in the worlds oceans and almost no data for European marine waters. The available data indicate that silver co-varies with silicate in oceanic environments, suggesting a link to biological processes. Nevertheless, silver is a highly toxic element. The main sources of silver for the marine environment derive from anthropogenic inputs, so silver can be used as a tracer for inputs of domestic and industrial pollution. Typical concentrations in seawater samples are very low (pmol/L). These low concentrations, combined with the complexity of the seawater-sample matrix, make the determination of silver in these samples extremely challenging. Developments in sensitive sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) instruments, combined with effective approaches for removal of the seawater matrix, have resulted in powerful analytical methods that can be used to overcome these challenges and help to improve our knowledge on the distribution, effect and fate of silver in the marine environment. This article briefly reviews the analytical techniques used for silver determination in seawater, and describes new trends in analyzing dissolved silver in seawater. ª 2007 Elsevier Ltd. All rights reserved. Keywords: Electrothermal atomic absorption spectrometry; Environmental analysis; ET-AAS; Flow injection; Marine waters; Sector field inductively coupled plasma mass spectrometry; SF-ICP-MS; Silver

Jose L. Barriada Instituto Universitario de Medio Ambiente, Universidad de la Corun˜a, Pazo de Lo´ngora 15179, Oleiros, La Corun˜a, Spain Alan D. Tappin, E. Hywel Evans School of Earth, Ocean and Environmental Science, University of Plymouth, Plymouth PL4 8AA, United Kingdom Eric P. Achterberg* National Oceanography Centre, Southampton, School of Ocean and Earth Science, University of Southampton, European Way, Southampton SO14 32H, United Kingdom

*

Corresponding author. E-mail: [email protected]

0165-9936/$ - see front matter ª 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2007.06.004

1. Introduction Metals occur at very low concentrations (typically nmol/L to pmol/L) in seawater and the complex seawater matrix makes their analysis a challenge [1]. Modern analytical techniques can achieve low limits of detection (LODs) required for the analysis of marine samples. Most instrumental techniques require separation approaches prior to trace-metal analysis in seawater [1], with a subsequent risk of contamination. Ideally, analysis should be performed in situ without the need to transport samples to a laboratory, but this is still uncommon [2], so, for the determination of trace metals in seawater, clean procedures for sample collection, treatment and preservation have to be adopted [1,3]. Organisms require a range of essential trace metals (e.g., Fe, Cu and Zn) for their growth and functioning [4]. However, metals become toxic at enhanced concentrations. Domestic and industrial inputs have greatly increased background metal levels in coastal waters, with negative consequences for ecosystem functioning in some instances [5]. A range of metals (Hg, Cd, Pb, Zn and Cu) are routinely monitored in coastal waters, but dissolved silver has attracted little attention [6]. Previous work has shown that silver is one of the most toxic, readily accumulated trace metals, surpassed by only mercury [7–9]. Furthermore, silver is strongly bioaccumulated by marine invertebrates, phytoplankton and seaweeds [7]. Despite the potential negative effects of dissolved silver on ecosystems, there is a paucity of data on it in the marine environment. Most information on coastal silver biogeochemistry is from the west coast of 809

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Table 1. Summary of dissolved-silver measurements, with locations and techniques Location

Dissolved-silver concentration range

Technique

Ref.

North Atlantic Ocean South Atlantic Ocean San Francisco Bay North-East Pacific Ocean Eastern Atlantic Ocean Southern California Bight North Pacific Ocean (near Japan) North Pacific Ocean Southern Ocean Texas estuaries North-West Atlantic Ocean

0.69–6.9 pM 1.2–31.7 pM 6–243 pM 0.3–22.8 pmol/kg <0.24–9.6 pM 3–307 pM 4.2–46.8 pmol/kg 1.0–87.7 pM 8.93–22.4 pM 5–170 pM 5.9–20.4 pM

Solvent extraction/ET-AAS Solvent extraction/ICP-MS Solvent extraction/ET-AAS Solvent extraction/ET-AAS Solvent extraction/ET-AAS Solvent extraction/ET-AAS Solvent extraction/ICP-MS Solid extraction/ICP-MS Solvent extraction/ET-AAS Solid extraction/ET-AAS FI-SF-ICP-MS

[46] [15] [12] [14] [45] [6] [17] [47] [16] [19] [18]

the USA (San Francisco Bay and San Diego Bay) [6,10– 13], which are urbanized, industrialized estuarine systems. Dissolved silver observed in San Francisco Bay was in the range 6–243 pmol/L [12], and in San Diego Bay was up to 307 pmol/L [6]. Oceanic dissolved-silver concentrations are considerably lower than coastal concentrations, increasing from 0.3 pmol/L in surface waters to 22.8 pmol/L in deep oceanic waters. Open ocean depth profiles for dissolved silver show recycling, with depleted surface-water concentrations and enrichment with depth [14–18]. Oceanic silver profiles resemble those of silicate, suggesting that silver becomes incorporated in the silicon frustules of diatoms [17]. Table 1 shows the main dissolved-silver data reported so far for marine waters. Table 1 indicates the lack of data on dissolved-silver concentrations for coastal and oceanic waters in general, and coastal European waters in particular. Highest dissolvedsilver concentrations have been reported for coastal or estuarine waters in the vicinity of human settlements, with important sources including mining, electronics, photographic and metal-processing industries. Silver therefore provides a good tracer for industrial and domestic sewage discharges into these waters [6,19]. The geochemistry of silver determines its low dissolved concentrations and toxic potential. Silver sorption onto particles is strong [6,10,11,19,20], resulting in enhanced removal in estuarine systems [7,20]. Furthermore, the toxicity of silver depends on not only its total concentration, but also its speciation [8,19,21,22]. Dissolved-silver speciation is expected to change markedly in estuaries, owing to changes in chloride concentration (as a proxy for salinity) [9]. Formation of the neutral chlorocomplex, AgCl0, may increase bioaccumulation of silver. The low polarity of this complex will increase the diffusion of silver across biological membranes [7], so dissolved-silver measurements in near-shore waters and estuaries with intensive human activities should be undertaken in order to assess the effects of this element on the functioning of marine ecosystems. 810

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2. Analytical techniques The determination of silver in marine waters presents several challenges. The main one is concentration, which can be as low as pmol/L (see Table 1). Such trace-metal concentrations require clean procedures for sample collection, treatment and preservation and ultrasensitive analytical techniques [13]. Moreover, the complex seawater matrix generally confounds instrumental techniques, and most approaches for silver determination require additional separation steps. 2.1. Atomic spectroscopy Atomic spectroscopic techniques used for the determination of dissolved silver in seawater typically involve a separation process to eliminate the saline matrix of the seawater sample. Many workers have used a variation of the solvent-extraction process with ammonium 1-pyrrolidine dithiocarbamate/ diethylammonium diethyldithiocarbamate (APDC/DDDC), as described by Bruland [23,24]. Although the solvent-extraction procedure represents a good way to remove matrix interferences, it is time consuming, and large volumes (100–250 mL [24–26]) of sample are required. Furthermore, the multiple extraction steps (including back extraction into nitric acid) increase the risk of contamination. An alternative separation technique involves the use of an extraction column filled with silica-immobilized 8-hydroxyquinoline [19], which requires a sample volume of ca. 200 mL. Following the preconcentration or matrix-removal process, silver is determined using electrothermal atomic absorption spectrometry (ET-AAS). This approach provides excellent LODs (i.e. values averaging about 4 pmol/L – 0.37 pmol/L [11], 1 pmol/L [26] and 8.2 pmol/L [27]). 2.2. Stripping voltammetry There are several examples of voltammetric techniques applied to the determination of silver in aqueous samples [22,28–31]. A main advantage of the electrochemical

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approaches is that there is no separate matrix-removal step required. Hence the methods are relatively straightforward, require low sample volumes (ca. 10 mL) and have a low contamination risk. In order to achieve a stable electrode response, many workers have applied working electrode conditioning prior to the determination of silver [22,32]. This procedure may entail mechanical electrode treatments (e.g., grinding and polishing), chemical treatments with acids or bases, and electrochemical treatments (e.g., repeat deposition and potential stripping cycles) [33]. Furthermore, in order to enhance the sensitivity, modified working electrodes have been used, including the adoption of complexing reagents (e.g., tricresyl phosphate [29], 2-mercaptobenzothiazole [34] and 1,4,7,10tetrathiacyclododecane [31]), which improve chemical interaction of silver with the electrode. The application of special electrode-film coatings has also been reported (e.g., chitosan [35], dicyclohexyl-18-crown-6 in Nafio117 [32] or mercury [30]), and these facilitate silver preconcentration. The preconcentration of silver ions is followed by an anodic stripping step (often after medium exchange [36]) to quantify the silver on the electrode. An important drawback of the voltammetric techniques is the time required for sample analysis. In order to achieve the required pmol/L LOD, electrodeposition times of 30–120 min [29,32] are needed for a single analysis. Furthermore, voltammetric techniques for the determination of silver suffer from interferences arising from other metals present in solution [29] or matrix-complexing agents [22]. Workers have reported successful application of stripping voltammetry for dissolved-silver analysis in seawater [31], with an LOD of 50 pmol/L; nevertheless, low oceanic concentrations and interferences have precluded widespread application of this technique to marine waters. 2.3. Inductively coupled plasma mass spectrometry (ICP-MS) The emergence of ICP-MS instruments has provided us with ultra-sensitive elemental detection techniques. The high sensitivity allows on-line sample preconcentration or matrix-removal approaches using small sample volumes. Recently, workers have reported the application of isotope dilution (ID) ICP-MS for the determination of dissolved silver in seawater following on-line matrix removal [37]. ID-ICP-MS provides excellent accuracy and precision because a ratio rather than an absolute intensity measurement is used to quantify the analyte concentration, providing two interference-free isotopes of a given element are available for analysis. In the case of silver, the ratio Ag107/Ag109 was used [37]. The ruggedness of the on-line removal methodology developed by these workers resulted in highly reproducible silver retention and recovery using a minicolumn (see

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below), and also enabled a simple standard calibration approach to be used instead of the ID approach [37]. This simpler calibration approach does not require the use of isotopic standards, thereby allowing more widespread application of ICP-MS for the determination of silver in seawater. The minicolumn used for the on-line separation of dissolved silver from the saline matrix was filled with a strong anion-exchange resin [37]. This method required small sample volumes (ca. 10–12 mL) and was based on the predominance of negatively-charged silver chlorocomplexes in marine waters [9,11,37]. Subsequent elution of the retained complexes from the column with strong nitric acid, followed by silver analysis in the eluent with an appropriate sensitive technique (e.g., sector field (SF)-ICP-MS), yielded the silver concentration in the sample. The method is focused on matrix removal rather than analyte preconcentration, so a highly sensitive detection technique was required in order to determine pmol/L silver concentrations in seawater. An attractive characteristic of the separation method is in the possibility of incorporating it into a flowinjection (FI) manifold [37]. This approach would result in minimal sample manipulation, thereby reducing the risk of sample contamination. Moreover, automation of an FI system is readily achieved, facilitating enhanced sample throughput [18,37].

3. Flow-injection analysis coupled with SF-ICP-MS The SF-ICP-MS method with minicolumn matrix removal has been applied in our laboratory for the determination of dissolved silver in estuarine, coastal and open ocean waters. The low silver concentrations in these waters require stringent procedures for sampling, sample handling and analysis. 3.1. Sample handling, storage and analysis 3.1.1. Sampling. Sample-collection protocols for dissolved-silver studies in marine waters must follow demanding procedures to avoid contamination. Depth samples were collected using a hydrowire with trace-metal clean, Teflon-lined Go-Flo or Niskin bottles. Surface-water (ca. 0.3 m depth) samples were typically collected directly into trace-metal-clean, low-density polyethylene (LDPE) bottles from the bow of an inflatable boat. Following filtration, samples for dissolved-silver analysis were stored in cleaned LDPE bottles. The bottlecleaning procedure was as follows:  immersion in detergent (1% v/v Deacon-90) for 24 h, followed by a thorough rinse using ultra-high purity (UHP) water; http://www.elsevier.com/locate/trac

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 immersion in 6 mol/L HCl (Aristar Grade) for one week, followed by a UHP rinse; and,  final immersion in 3 mol/L HNO3 (Aristar Grade) for one week [38]. After a final rinse, bottles were filled with UHP water, acidified to pH 2 using sub-boiling distilled HNO3 acid (SB-HNO3) and stored double-bagged prior to use. Although the cleaning process is time-consuming, it ensures that no contamination from the storage containers will affect the dissolved-silver data. 3.1.2. Filtration procedure and preservation. Following sample collection, the seawater was filtered using 0.2-lm pore-size, acid-cleaned, polycarbonate membrane filters, and SB-HNO3 was subsequently added to acidify the samples to pH 2. Although sub-boiling distilled HCl acid (SB-HCl) is commonly used to preserve trace-metal samples following filtration [38], this is not recommended for silver. In our experience, silver contamination in HCl is difficult to remove satisfactorily, even following double distillation of the acid. The contamination was reduced to negligible levels in SB-HNO3, so this acid was used for the acidification process. 3.2. On-line matrix removal Direct analysis of trace metals in marine samples using ICP-MS is not recommended. The salt matrix causes non-spectroscopic interference, and deposition of salt on the nebulizer, cones and lenses results in severe signal suppression. Sample dilution (10–50 times) with UHP water has been applied successfully for trace-metal

analysis in coastal waters with enhanced elemental concentrations [39]. However, this approach would not be feasible for silver because of the low concentrations, so a matrix-removal process is required. In our laboratory, an automated FI system (PrepLab, Fisons Instruments, Elemental Analysis, Winsford, UK) has been utilized. The main part of the system is a dual six-way switching valve, which pneumatically switches between its two positions. All ports and fittings of the dual six-way valve are made of polyetheretherketone (PEEK), an inert material suitable for trace-metal analysis. The combination of the six-way valve with two peristaltic pumps allowed the sample to be pumped through a 1-cm long, 85-lL internal volume minicolumn (Global FIA, Inc., FoxIsland, WA, USA) filled with a slurry of a strong anion-exchange resin (Dowex 1X8, 200–400 mesh, Supelco, Bellefonte, CA, USA). Negatively-charged silver chlorocomplexes present in the marine samples were retained on the anionic resin. A subsequent rinse with UHP water removed any salt remaining in the minicolumn. Finally, the eluent (e.g., SB-HNO3) removed and carried the silver chlorocomplexes to the SF-ICP-MS. Fig. 1 shows a flow diagram of the steps in an analytical run. A major advantage of this approach is that the analysis procedure requires a sample volume of only 12 mL and analysis can be achieved in less than 10 min per sample. Silver quantification by the SF-ICP-MS was undertaken by determination of the 107Ag isotope in single-ion monitoring mode. Table 2 shows the SF-ICP-MS instrument conditions and settings used in our laboratory.

Position B

0.5

UHP water pre-rinse

4

Step process

Silver chlorocomplexes

2

Step time

UHP water minicolumn salt rinse

3

Elapsed time

Silver chlorocomplexes

9.5 minutes

Waste

Waste

Pump

Diluted UHP Water or sample

Pump

UHP Water Eluant

SF-ICP-MS

SF-ICP-MS

Position A Figure 1. Flow diagram for matrix removal and dissolved-silver analysis using flow injection sector field inductively coupled plasma mass spectrometry (FI-SF-ICP-MS).

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Table 2. Optimal settings for SF-ICP-MS instrument (Axiom VG Elemental, Winsford, UK) ICP Forward power (W) Plasma gas (L/min) Auxiliary gas (L/min) Nebulizer gas (L/min) Sample flow (mL/min) Torch Nebulizer Spray chambers

1225 13.5 0.85 0.77 2 Fassel (quartz) Concentric (quartz) Cyclonic + Bead Impact

Interface Sampler Skimmer

Ni Ni

Mass spectrometer Ion masses (m/z) Resolution

107 Ag 400

Data acquisition Single-ion monitoring Dwell time (ms)

110 points 2000

3.3. Sample analysis: the effect of chloride The analysis of dissolved silver is based on the separation of the silver chlorocomplexes formed in solution. These complexes are retained by the anion-exchange resin according to the equilibrium: 



Rz N R3 Cl þ X  ¡Rz N R3 X  þ Cl where X represents the anion retained on the column. Silver retention in the column therefore directly depends on the speciation of the silver chlorocomplexes, which Fig. 2 presents as a function of chloride concentration. The inorganic silver speciation was modeled using MEDUSA thermodynamic speciation software [40], for a total silver concentration of 30 pmol/L and chloride concentrations of 0–0.55 mol/L (i.e. the concentration range that can be found in estuarine-seawater systems). Five different equilibrium conditions are considered: Agþ þ Cl ¡AgClaq Agþ þ 2Cl ¡AgCl 2

K 1 ¼ 103:27 K 2 ¼ 105:27

Agþ þ 3Cl ¡AgCl2 3

K 3 ¼ 105:29

Agþ þ 4Cl ¡AgCl3 4

K 4 ¼ 105:51

Agþ þ Cl ¡AgCl #

K 5 ¼ 109:75

where Ki is the equilibrium constant. In the majority of our seawater samples (salinities ca. 4–35), the single negatively-charged chlorocomplex AgCl 2 is the main species formed. At enhanced chloride concentrations, the concentrations of AgCl2 and 3 AgCl3 increase (Fig. 2a). Since both these complexes are 4 negatively charged, they will be retained on the anionexchange resin. Lack of retention could potentially arise

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at low chloride concentrations. At 0.055 mol/L (see Fig. 2b), neutral chlorocomplex AgCl contributes to ca. 20% of the total silver concentration, and the single negatively-charged complex will be the main species retained by the resin. At lower chloride concentrations, the contribution of the neutral species concentration increases, and, at [Cl]TOT 0.01 mol/L, the AgCl concentration forms ca. 50% of total silver concentration. Consequently, in low-salinity estuarine (salinity <4; ca. 20 mM Cl1) and river waters, the fractions of negatively-charged complexes are expected to be smaller and a lower percentage of silver will be retained on the column by the resin. This effect is illustrated in Fig. 3, which shows elution peaks for silver from samples with different conductivities (as a proxy for contrasting chloride concentrations), but with similar dissolved-silver concentrations. The samples were obtained during a transect from the Atlantic Ocean to the Mero river, in La Corun˜a, NW Spain, and were spiked to achieve similar silver concentrations (75–78 pmol/L). A clear reduction in peak area with a decrease in salinity was observed. However, the excellent reproducibility of silver recovery by the anion-exchange column positioned in the FI system provides high confidence in the silver quantification. Fig. 4 shows a standard-addition plot for a low-salinity (<0.05) sample, indicating that a good linear response is obtained under these conditions. Nevertheless, these examples show that the slope of calibration plots will alter at different chloride concentrations due to the change in the silver chlorocomplex speciation. Moreover, whilst the reduction in peak area negatively influences the signal-to-noise ratio, the instrument sensitivity is high enough to allow analysis of pmol/L silver concentrations in low-salinity samples. 3.4. Sample calibration The influence of chloride on silver retention by the column warrants use of internal standard additions [3] when analyzing samples with differing salinities. External calibration plots, whereby sample concentrations are obtained through interpolation, should be used only for samples collected from relatively high salinity environments, and then only following a thorough sensitivity assessment. The application of internal standard additions for samples with differing salinities results in reduced sample throughput compared with the external addition approach, but provides a much higher confidence in the concentrations measured. 3.5. Influence of organic matter on determination of silver A range of trace metals, including copper and iron, is strongly complexed by organic ligands in seawater. No evidence of silver-organic complexation was observed in samples from the Weddell Sea and San Francisco Bay http://www.elsevier.com/locate/trac

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[Ag+]TOT = 0.03 nM Ag+

1.0

AgCl2 –

Fraction

0.8

0.6 AgCl

0.4 AgCl32 AgCl43

– –

0.2

0.0 0

100

200

300

400

500

–

[Cl ]TOT mM [Ag+]TOT = 0.03 nM Ag+

1.0

AgCl2

–

AgCl32

–

0.8

Fraction

AgCl

0.6

0.4

0.2

0.0 0

10

20

30

40

50

–

[Cl ]TOT mM Figure 2. The distribution of chlorocomplex species as a function of chloride concentration in the range (a) 0–0.55 mol/L; and (b) 0–0.055 mol/L. Concentration of total dissolved silver in both cases was 30 pmol/L.

[41], indicating that silver-chloride complexes dominate dissolved-silver speciation. Nevertheless, recent work has indicated that UV-digestion released silver in samples, which were stored at pH 2 for 1–2 years, resulting in an increase in silver concentrations by 10–70% in estuarine and oceanic samples [42]. The sample analysis for this study was conducted using a similar FI-ICP-MS approach to that in our laboratory. In this study, the additional silver released 814

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using UV digestion correlated with chlorophyll-a concentrations in the estuarine waters of San Francisco Bay [42]. While these observations do not directly provide evidence of silver-organic complexation, they indicate that organic matter interferes with the matrixremoval step of the Dowex resin in the anion-exchange column. Consequently, UV-digestion of seawater samples prior to FI-ICP-MS analysis should be incorporated in the analytical procedures.

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7000 Decrease in salinity

Counts per second

6000 5000 4000 3000 2000 1000 0 0

50

100

150

200

Elution time / s Figure 3. Effect of salinity on the elution peak area for samples with a conductivity of 59.2 mS/cm (ÆÆ–ÆÆ), 36.1 mS/cm (  ÆÆ) and 1.6 mS/cm (——).

100000 90000

y = 1E+15x + 8921.3 2 R = 0.9973

80000 Peak area

70000 60000 50000 40000 30000 20000 10000 0 -2E-11

0

2E-11

4E-11

Silver added / mol·L

6E-11

8E-11

-1

Figure 4. Standard addition plot for a low salinity (<0.05) sample.

3.6. Analysis of certified reference material and coastal water samples The analysis of CASS-2 certified reference seawater material (CRM; salinity 29.2), utilizing internal additions for calibration, yielded a silver concentration of 62.2 ± 2.4 pmol/L. The LOD for the dissolved-silver analysis was calculated from the regression line as 3 times the standard error of the fitting [43] and amounted to 0.5 pmol/L. The concentration of silver observed in CASS-2 in our laboratory was somewhat higher than the values reported by other workers using ID-ICP-MS (48 ± 1 pmol/L) [37] and APDC-DDDC extraction with ET-AAS (48 ± 2 pmol/L) [12]. Furthermore, workers have also reported a range of values for another coastal certified reference seawater (CASS-4). The lowest reported value for CASS-4 was 50.2 ± 0.7 pmol/L [37], whereas the highest value was 78.6 ± 18.2 pmol/L for UV-treated

aliquots [42]. As no certified silver concentration is available for either CASS-2 or CASS-4, it is impossible to assess the correct value. However, it appears that organic matter may interfere with the analysis, so UV treatment of samples is important. The FI-SF-ICP-MS method has been applied to samples from different marine waters. Fig. 5a shows the variation in concentration of dissolved silver with salinity along a transect in Restronguet Creek (Cornwall, UK) during April 2003. Restronguet Creek is situated in a region with numerous abandoned mine workings, and receives metal-rich inputs from disused mines. Extensive ironpyrite and chalcopyrite formations contribute to the mineralogy of the region, so the area is enriched in Fe, Cu, Zn, As, Ag and Sn [44]. Enhanced silver run-off from the disused mines is evident in Fig. 5a, with dissolvedsilver concentrations of up to 190 pmol/L and decreasing at higher salinities due to mixing with cleaner waters from the Fal Estuary. The maximum silver concentrations in Restronguet Creek are comparable to those reported in San Francisco Bay during 1989 (up to 250 pmol/L) [12]. Fig. 5b shows a depth profile of dissolved silver obtained in the vicinity of the River Po plume in the northern Adriatic Sea (Lat. 44N 25.38 0 , Long. 12E 44.16 0 ) during October 2002. Enhanced dissolved-silver concentrations (ca. 28 pmol/L) were observed in the surface waters, coinciding with a minimum in salinity (not presented). This surface maximum can most probably be attributed to silver inputs from the River Po. Dissolved-silver concentrations decreased with depth to ca. 14–20 pmol/L. There is a paucity of silver data for coastal waters; nevertheless, these concentrations were higher than those reported for the adjacent eastern Atlantic Ocean (0.7–10 pmol/L) [45]. http://www.elsevier.com/locate/trac

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Ag (pmol L-1)

200

150

100

50

0 0

5

10

15

20

25

30

35

40

salinity Ag (pmol L-1) 0

10

20

30

0

Depth (m)

5 10 15 20 25 Figure 5. (a) Dissolved silver versus salinity for a transect in Restronguet Creek (Cornwall, U.K.). (b) Depth profile of dissolved silver in the Adriatic Sea (Lat. 44N 25.38 0 , Long. 12E 44.16 0 ).

4. Conclusions and future trends Significant progress has been made in recent years with dissolved-silver analysis in marine waters. A recently developed on-line separation method, coupled with SF-ICP-MS detection, has allowed silver measurements in seawater at pmol/L concentrations, whilst reducing the number of sample-extraction steps, sample manipulation and sample volume, and enhancing sample throughput. For samples with differing salinities, the internal standard-addition method is required for silver quantification because of the variation in silver retention by the anion-exchange column in response to changes in chlorocomplex speciation of silver. Further application of the ID technique is envisaged. Although this approach is more complex, as it requires an isotopically-enriched spike, it is independent of column-extraction efficiency, which presents clear benefits. UV treatment of samples prior to analysis is to be recommended, as it removes interferences from dissolved organic material. These recent analytical developments permit more comprehensive studies into the biogeochemical behavior 816

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of silver in oceanic environments, and its fate and bioavailability in estuarine and coastal waters. In particular, the use of dissolved silver as a tracer for domestic and industrial sewage inputs is facilitated by the high throughput FI-SF-ICP-MS method, as such studies require very frequent spatial sampling.

Acknowledgement This research has been supported by a Marie Curie Fellowship of the European Community programme Energy, Environment and Sustainable Development under contract number EVK3-CT-2001-50004.

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