Novel approach for determination of trace metals bound to suspended solids in surface water samples by inductively coupled plasma sector field mass spectrometry (ICP-SFMS)

Abstract

An approach for rapid determination of ultra-trace concentrations of As, Cd, Cr, Cu, Ni, Pb and Zn in suspended solids present in surface water was developed and validated for three Austrian rivers (Danube, Leitha, Schwechat). Elemental analysis was performed by ICP-SFMS via slurry-type nebulization of non-centrifuged (total elemental concentration) and centrifuged (dissolved elemental concentration) surface water samples. The elemental concentrations in suspended solids (c_ss) were determined, relating the difference of the total elemental concentration and the dissolved elemental concentration to the concentration of suspended solids (gravimetric determination). The quality of the obtained data was evaluated calculating the expanded uncertainty of measurement according to Eurochem/CITAC. The applied analytical procedure yielded values for c_ss associated with low uncertainties for elements with high particle bound fractions (Zn, Pb) and suspended particle concentrations >5 mg L^–1. The novel method provided partition patterns and related partition coefficients (K) which were in agreement with available literature data. The investigated elements could be classified into three groups: (i) elements showing both considerable particle bound fractions (Ni 0%–50%, Cr 22%–33%, Cu 37%–100%) and large fractions of dissolved metals; (ii) elements with a pronounced tendency towards particle binding, such as Pb and Zn, showing particle bound fractions of 56%–100% and 71%–83%; and (iii) elements being predominantly present in the liquid phase (As, Se). The concentrations of Ag and Cd were consistently below the limits of detection for all investigated rivers.

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Introduction

Water protection is a major objective of environmental protection policy. So far, within the European Union (EU), environmental quality standards (EQS) and required analytical quality standards have been regulated by member states individually. With the introduction of the water framework directive 2000/60/EC (WFD) the EU aimed at harmonized, uniform and high quality goals for surface water bodies with respect to organic and inorganic micropollutants.¹ A list of priority substances which comprises mainly persistent organic pollutants and several inorganic pollutants was issued.² During the current implementation process of the WFD, monitoring of alternative entities (suspended solids, sediment and biota) has been discussed due to their importance with respect to transport budgets, distribution pathways in the environment and assessment of the toxicological impact of water pollution.

In recent years there has been great interest in trace metal(loid)s of ecotoxicological relevance, namely As, Cd, Cr, Cu, Hg, Ni and Pb, and the role that suspended matter plays in their distribution and partition in the aquatic environment.^3–9 Several programs for monitoring river-born suspended solids and sediments were conducted in Germany and Austria, especially focusing on the evaluation of established measuring programs and the need for monitoring alternative components.^10,11

For the analysis of trace metals in suspended solids it is common practice to collect and isolate sufficient amounts of suspended solids. Following freeze-drying, suspended matter is prepared for elemental analysisvia acidic digestion.¹² Elemental concentrations are usually determined by atomic spectrometry using either inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES) or atomic absorption spectroscopy (AAS).¹⁰ Collection of suspended solids is commonly performed with sedimentation traps, which require long sampling periods and are therefore prone to significant sample changes due to ageing. Alternative approaches consist in the filtration of large volumes of surface water by continuous flow centrifuges or in-situfiltration.^10,13–15 A common drawback is that the implemented procedures are tedious, time consuming and, more importantly, prone to contamination.¹⁶

Alternatively, some elaborated schemes for investigation of trace metal partition using size fractionation techniques to distinguish between particulate, colloidal and “truly dissolved” metal have been tested.^17–19 So far, no scheme has been suggested for the determination of trace metal concentrations in suspended matter through measurement of native surface water.

Nevertheless, a recent Austrian surface water monitoring program confirmed the necessity of measuring particle bound metal fractions for the assessment of ecotoxicological relevance.²⁰

In the presented study a novel, fast and contamination free sample preparation scheme for measurement of As, Cd, Cr, Cu, Ni, Pb and Zn at ultra-trace levels by ICP-SFMS is presented. The scheme utilizes a combination of centrifugation and slurry-type nebulization for quantification of the particle bound metal fraction of the investigated compounds.

Experimental

Materials and methods

Chemicals. Ultra-pure chemicals were used throughout the study. All quantitative element standards were certified single element ICP standards traceable to NIST for trace analysis, purchased from Merck KGaA (Darmstadt, Germany). For quality assurance two certified reference materials (CRM) were integrated in all ICP-SFMS measurements: SLRS-4 river water reference material obtained from the National Research Council of Canada NRC (Ottawa, Ontario, Canada) and a low level fortified standard TM-27.2 of the National Water Research Institute NWRI (Burlington, Ontario, Canada), respectively.

Prior to use, 65% nitric acid of p.a. grade, purchased from Merck, was double-distilled using a duoPUR quartz sub-boiling-unit produced by MLS Lab Systems GmbH (Leutkirch, Germany). Ultra-pure water was used for the preparation of the standards: following reverse-osmosis and ion exchange with a HQ-5 system supplied by REWA GmbH (Gladenbach, Germany) the water was finally processed with a duoPUR quartz sub-boiling-unit. Water and nitric acid were stored in PFA bottles under clean room conditions until use.

Laboratory materials. All bottles, vials, plugs and pipette tips used within the study were either fabricated from polyethylene (PE), polypropylene (PP), Teflon (PTFE) or perfluoralkoxy-polymer (PFA). The PE- and PP-materials were submitted to a routine cleaning protocol involving incubation in acid baths and rinsing with ultra-pure water. Sample vials and bottles for preparation and storage of standards were made of perfluorated polymer (PFA, PTFE) and pre-cleaned with a CleanDest acid vapour cleaning system obtained from MLS Lab Systems.

Laboratory conditions. Sample preparation and measurements were carried out under clean room conditions (class 100 000 and class 10 000, respectively) with temperature control (20 °C) and overpressure (+5 Pa). Dilution of samples and addition of internal standard (In) were carried out on clean benches (class 100) situated in the clean room class 10 000.

Instrumental. Elemental analysis was carried out on an Element 2 ICP-SFMS (ThermoFisher, Bremen, Germany). Two different sample introduction systems, i.e., a system for slurry-type nebulization (set-up A) using a V-groove nebulizer (Glass Expansion, Melbourne, Australia) with an inner diameter (i.d.) of 145 µm, or an “ultra-pure” self-aspirating set-up (set-up B) for measurement of particle-free samples with a PFA micro-flow nebulizer (Elemental Scientific Inc., Cuming, Omaha, USA) with an i.d. of 45 µm were used during the study. Set-up A consisted of the V-groove nebulizer fed by a peristaltic pump at 0.30 mL min^–1, a PC³ cyclonic quartz chamber (ESI) operated at 4 °C, a quartz injector pipe and torch (Thermo Fisher), aluminium sampler and skimmer cone (Thermo Fisher). For set-up B the self aspirating PFA micro-flow nebulizer (ESI) at a flow of 100 µL min^–1 was combined with a PFA double pass Scott-type spray chamber, a sapphire injector pipe, a quartz torch and platinum sampler and skimmer cones (all parts Thermo Fisher). The following ICP-SFMS operating conditions were applied in this study: for both systems rf power of 1300 W and plasma gas flow of 16 L min^–1 were applied. Set-up A was operated at a sample gas flow of 1.08 L min^–1 and an auxiliary gas flow of 1.00 L min^–1. For measurements with set-up B sample gas and auxiliary gas flows were set to 1.06 L min^–1 and 0.86 L min^–1, respectively.

¹⁰⁹Ag, ¹¹¹Cd, ²⁰²Hg, ²⁰⁸Pb at low resolution (LR), ⁵²Cr, ⁶⁰Ni, ⁶⁵Cu, ⁶⁶Zn at medium resolution (MR) and ⁷⁵As, ⁷⁷Se at high resolution were the selected isotopes for interference-free ICP-SFMS measurement. During all measurements ¹¹⁵In was used as internal standard at all resolutions. Nominal mass resolutions of the Element 2 ICP-SFMS for low resolution (LR), medium resolution (MR) and high resolution (HR) are 350, 4500 and 10 000, respectively.

Sampling sites and sample collection. Surface water samples were collected at three rivers in the eastern part of Austria: Samples were taken from a side arm of the river Danube in the urban area of Vienna, the river Schwechat at Mannswoerth, situated close to industrial facilities of the OMV (Austrian Mineral Oil Company) and the river Leitha in the rural area of Nickelsdorf, close to the border with Hungary.

The collection of surface water samples was performed with a metal-free, custom-fitted device for contamination-free sampling. This device consisted of a polypropylene plate attached to a 1.5 m PP-rod. Round holes were drilled into the plate to contain the necks of 500 mL PE bottles, which were fastened by cut-open PE-bottle screw caps. To assure contamination-free sampling pre-cleaned PE wide necked bottles and caps were used. During sampling and sample transport all bottles and devices were handled wearing PE-gloves. Bottles were transported and stored in sealed plastic bags. The actual sampling sites—except for the urban site in Vienna—were situated off-road to avoid traffic related contaminations.

Sample preparation

The concentration of dissolved metal(loid)s (c_dis) was determined after centrifugation of the respective surface water samples. Centrifugation of 10 mL sample aliquots was performed with a Sigma laboratory centrifuge 2–5 (Sigma Laborzentrifugen GmbH, Osterode am Harz, Germany) in acid-cleaned polypropylene tubes (10 mL) at 3900 rpm for 30 min. The centrifugates were transferred into pre-cleaned PP-tubes with an Eppendorf pipette and stabilized by addition of ultra-pure double-distilled nitric acid (pH < 1.5). Aliquots of the original non-centrifuged surface water samples representing the concentration of total metal(loid) were transferred to pre-cleaned PP-tubes and stabilized by volumetric addition of 10 µL ultra-pure nitric acid per mL of sample (final pH < 1.5).

As the reference method for the determination of the total concentration of metal(loid)via slurry-type nebulization, surface water sample aliquots were submitted to microwave digestion with H₂O₂–HNO₃ in parallel. Sample aliquots of 2 mL plus 500 µL of ultra-pure nitric acid and 200 µL of 30% H₂O₂ solution (ultra-pure, Merck) were filled into acid-steam-cleaned PFA-microwave digestion vessels and closed with pre-cleaned PTFE-caps. The digestion was performed by applying a microwave program employing maximum microwave power of 450 W. After cooling, the digested samples were transferred to PP-vials and filled up to 10 mL with ultra-pure water. Microwave digestion blanks were prepared using 2 mL of ultra-pure water.

For quantification multi-element standards were prepared from Merck ICP-single element standard in PFA bottles and vials through dilution in ultra-pure water and addition of 1% nitric acid. The acid content was matched with the content in the respective samples. Prior to ICP-SFMS measurement, In was added to all samples and standards as internal standard at a final concentration of 1 µg L^–1. TM 27.2 and SLRS 4 were used for calibration quality control. The certified values and the obtained values agreed within measurement uncertainty for both tested CRMs.

Gravimetric determination of x_ss was performed according to established methods.²¹ Sample volumes of 1 L were filtered through glass fibre filters (GF A, Whatman, Brentford, Middlesex, UK) using a polycarbonate filter unit manufactured by Sartorius AG (Göttingen, Germany). The loaded filters were dried at 105 °C in a drying cupboard, applying a fixed drying time of 120 min, and subsequently placed in an evacuated exsiccator for equilibration to room temperature. x_ss was obtained via differential weighing using an MC-5 micro-balance by Sartorius AG (Göttingen, Germany).

Results and discussion

In this study a novel approach for the determination of elemental concentrations in suspended solids via ICP-SFMS was validated and applied. We designed a simple and straightforward sample preparation protocol for assessment of dissolved and total concentration of metals and metalloids, respectively. The proposed methodology exploits the unique possibility of slurry-type nebulization, thereby permitting the determination of total metal(loid) content without digestion. Table 1 details the implemented approach.

The concentration of the dissolved metal(loid) (c_dis) was determined from centrifuged and subsequently acidified river water, while the concentration of total metal(loid) (c_tot) was measured by introduction of untreated acidified river watervia slurry-type nebulization (V-groove nebulizer). Consequently, the particle bound fraction of metal(loid) was calculated as the difference of these two experimental parameters with respect to the concentration of suspended solid (x_ss), which was determined gravimetrically.

Method validation

Analytical figures of merit. Procedural limits of detection (LOD) and limits of quantification (LOQ) were determined for the two introduction systems, i.e., slurry-type nebulization (set-up A) and a PFA-microflow nebulizer (set-up B). The method blanks for the centrifugation procedure were processed using acidified ultra-pure water (1% nitric acid, n = 6). The results are summarized in Table 2. LODs and LOQs were calculated as the quantified 3- and 10-fold standard deviation of the blank signal, respectively. Generally, the obtained LODs were superior for set-up B as self-aspiration results in improved precision. For the slurry-type nebulization in combination with a peristaltic pump, inter-sample precisions in the range of 10%–15% could be achieved. Microflow nebulization in self-aspirating mode exhibited inter-sample precisions of typically <5%. For the self-aspirating set-up B the Hg measurement was compromised by the fact that the microflow nebulizer did not consist of glassware.

Table 1 Analytical strategy for determination of metal concentrations in suspended solids of surface water. Quantification of the particle bound metal(loid) fraction (c_ss) of the investigated compounds in a river water sample was achieved by subtracting the concentration of dissolved metal(loid)s from the total elemental concentration and subsequent division by the gravimetrically determined concentration of suspended solids (x_ss)

Surface water sample
Sample aliquot 1	Sample aliquot 2	Sample aliquot 3
Centrifugation (3900 rpm for 30 min)	Acidification of surface water aliquot (pH < 1.5) with ultra-pure nitric acid	Determination of concentration of suspended solids according to DIN 38409-H2
Acidification of centrifugate (pH < 1.5) with ultra-pure nitric acid	Addition of internal standard
Addition of internal standard	ICP-SFMS with slurry-type nebulization
ICP-SFMS with slurry-type nebulization
Concentration of dissolved metal(loid) (cdis)	Concentration of total metal(loid) (ctot)	Concentration of suspended solids (xss)
Calculation of metal concentration in suspended solids css (in µg g^–1) using ctot, cdis (both in µg L^–1) and xss (in g L^–1) in a differential approach: css = (ctot – cdis)/xss

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Table 2 Procedural limits of detection (LOD) and quantification (LOQ) of ICP-SFMS using slurry-type nebulization (set-up A) and PFA micro-nebulizer (set-up B): LODs and LOQs were calculated as 3 and 10 standard deviations of the blank signal according to DIN 32645 (n = 6). The obtained values are compared with the annual average EQS-values (AA-EQS) suggested by the WFD for inland surface water for priority metals

Element	LODA/µg L^–1	LOQA/µg L^–1	LODB/µg L^–1	LOQB/µg L^–1	AA-EQS/µg L^–1
a For class 1: 40 – < 100 mg L^–1 CaCO3. b The values represent AA-EQS deduced using an added risk approach based on maximum permissible addition (MPA) and assuming a negligible natural background concentration in Austria.
Ag	0.002	0.007	0.0004	0.001	n.a.
As	0.005	0.0167	0.005	0.017	n.a.
Cd	0.05	0.167	0.0003	0.001	≤0.08^a
Cr	0.011	0.037	0.004	0.015	n.a.
Cu	0.023	0.075	0.004	0.013	n.a.
Hg	0.017	0.057	0.014	0.048	0.05
Ni	0.039	0.13	0.004	0.014	1.7^b
Pb	0.003	0.010	0.002	0.007	2.1^b
Se	0.040	0.117	0.053	0.177	n.a.
Zn	0.1	0.133	0.021	0.069	n.a.

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Corresponding experiments addressing filtration by acid cleaned Teflon filters for separation of suspended particles showed a significant increase of blank levels (up to 5–8 times for all investigated elements except Ag). Consequently, centrifugation of samples instead of filtration is the sample preparation method of choice for trace and ultra-trace analysis of dissolved metals and metalloids.

Summarizing for all elements the method LODs obtained with both experimental set-ups, i.e., self-aspirating PFA after centrifugation or slurry nebulization, were below the current EQS and hence suitable for the concentrations to be monitored in Austrian rivers.

Storage of non centrifuged samples. The time interval between sampling and sample preparation was considered as a crucial factor governing the accurate determination of the dissolved metal(loid) concentration (c_dis). As contamination-free centrifugation could only be assured under clean room conditions and not in the field, an optimized period of time between sampling and sample preparation was necessary to restrict the effect of sample ageing. The stability of the native surface water samples was investigated by applying the suggested sample preparation protocol to aliquots of surface water taken from the Danube after storage periods of <24 h, <48 h and <96 h. Prior to sample preparation the native samples were stored at 5 °C.

Fig. 1 summarizes the results of this stability study. It reveals that most of the investigated elements show clear evidence of sample ageing as the dissolved fraction significantly decreased with increasing storage time. Based on these findings a maximum time of 24 h between sampling and centrifugation is proposed for accurate determination of c_dis.

Fig. 1 Stability of metal(loid)s in native surface water (Danube) during storage at 5 °C prior to sample preparation. Data were obtained with ICP-SFMS using ultra-pure set-up B after separation of suspended solids viacentrifugation and subsequent acidification (1% HNO₃).

Moreover, storage of processed samples (i.e., centrifuged and acidified samples representing the dissolved metal(loid) fraction) was investigated with respect to wall adsorption. The samples were stored at 5 °C for 2 weeks in PP tubes showing no significant loss except for Hg. Mercury loss would be expected from plastics, which is why samples for mercury determination are stored in glass and generally preserved on-site, for instance by addition of chromate to the sample which would hamper the analysis of Cr. Hg was therefore excluded from our study and will be investigated separately in future monitorings (see conclusions).

Fractionation of suspended solids from surface water by centrifugation. The separation efficiency of suspended solids from water samples was investigated with a UV photometer at wavelengths 254 nm and 550 nm. The discontinuous centrifugation used in this study was compared with conventional methods such as filtration (0.45 µm) and continuous flow centrifugation by parallel processing of a surface water sample (River Danube). HQ water was used for blank measurement. Sample aliquots were centrifuged at 3900 rpm for 30 min (n = 3), filtered by glass-fibre filters, and treated by conventional centrifugation in continuous flow mode. At 550 nm the absorption of all centrifuged and filtered samples did not significantly differ from the absorption of HQ water. At the second investigated wavelength of 254 nm, which can be used for the determination the sum of organic compounds in surface water, the measured absorption increased in the following order: blank (A = 0.000) < filtrate (glass fibre filter) and discontinuous laboratory centrifugation (both A = 0.028) < continuous centrifugation (A = 0.039). The main part of the residual absorption at 254 nm was explained by the presence of small organic molecules and colloids with sizes below the size cut-off of these separation techniques. In this study, laboratory centrifugation was selected for the determination of c_dis taking into account the efficiency of particle removal and the key advantage of contamination-free sample preparation.

Slurry-type nebulization of unfiltered samples. In this part of our study slurry introduction of river water samples into the ICP was investigated as direct approach for determination of the total metal(loid) content (c_tot).

As a first step, sedimentation of suspended solids in the sample vials was considered as a potential source of error in the analysis of the total metal(loid) concentration by slurry nebulization. An experiment was designed comparing river water sample aliquots, which were either stirred during ICP-SFMS analysis using set-up A or measured without stirring. Lead was selected as the reference element in this experiment, because it was found to be predominantly adsorbed to suspended matter. When the samples were continuously mixed during the ICP-SFMS measurements using slurry-type nebulization a concentration of 0.567 ± 0.025 µg L^–1 (n = 6) was obtained. Without stirring the measured concentration was 0.544 ± 0.123 µg L^–1 (n = 6). Accordingly, the influence of sedimentation was found to be negligible for the investigated river water samples.

For validation of the accuracy of the slurry nebulization procedure, acidified river water samples were measured directly by slurry-type nebulization as well as after microwave assisted digestion. A scheme of the sample preparation for the respective experiments is given in Table 3. Measurement of slurry-type nebulized and acid digested samples did not reveal significant differences in metall(loid) concentrations. The results of the method comparison are given in Table 4. Only in the case of Ni were the values obtained by microwave digestion significantly lower than those obtained by slurry nebulization, which is not understood yet and needs further investigation. The high standard uncertainty of Zn can be explained by a possible contamination during microwave digestion. For Ag and Cd the concentrations in the river water samples ranged below the respective detection limits for both investigated types of sample preparation. However, our findings confirm the suitability of slurry-type nebulization for the determination of the total metal content in river water containing suspended solids.

Table 3 Preparation scheme for samples used for validation of ICP-SFMS measurement of total concentration c_tot

Surface water
Aliquot A	Aliquot B	Aliquot C
Processing of aliquots (10 mL):		Microwave digestion in PFA-inlets:
–Acidification with ultra-pure HNO3 (1% or pH < 1.5)		–2 mL sample
–Storage at room temperature (20 °C) until measurement with ICP-SFMS		–0.5 mL HNO3
		–0.25 mL H2O2
Stirred during measurement	Measured without stirring	Filled up to 10 mL and measured
Unfiltered sample (stirred)	Unfiltered sample (no stirring)	Microwave digestion

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Table 4 Measurement of total element concentrations in surface water: Slurry-type nebulization (set-up A) versus microwave digested samples (set-up B) for the river Danube. (All values represent the average of 6 independently prepared and measured samples.)

Element	Non-centrifuged sample with set-up A/µg L^–1	Microwave digestion with set-up B/µg L^–1
Ag	<LODA	<LODB
As	1.23 ± 0.13	1.10 ± 0.10
Cd	<LODA	<LODB
Cr	0.712 ± 0.059	0.807 ± 0.141
Cu	2.19 ± 0.23	2.05 ± 0.02
Hg	0.022 ± 0.001	0.025 ± 0.035
Ni	1.19 ± 0.16	0.614 ± 0.153
Pb	0.612 ± 0.063	0.569 ± 0.067
Se	0.145 ± 0.001	0.100 ± 0.102
Zn	2.76 ± 0.34	3.35 ± 1.35

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Determination of metal(loid)s in suspended solids of the rivers Danube, Schwechat and Leitha

The distribution of the elements Ag, As, Cd, Cr, Cu, Ni, Pb, Se and Zn between suspended solids and the liquid phase was determined in the rivers Danube, Schwechat and Leitha. The measurements were performed according to the validated analysis scheme as shown in Table 1. Total elemental concentrations (c_tot), concentrations of the dissolved elemental fraction (c_dis) and the concentration of suspended solids (x_ss) were determined.

For determination of c_dis a non-acidified sample aliquot was centrifuged and the centrifugate was acidified prior to ICP-SFMS measurement. c_tot was determined via ICP-SFMS using slurry-type nebulization of an unfiltered, acidified sample aliquot. The values of c_tot and c_dis obtained for the rivers Schwechat, Danube and Leitha are summarized in Fig. 2–4, respectively.

Fig. 2 Investigation of total concentrations (c_tot) and concentration of dissolved metal(loid)s (c_dis) in the river Schwechat (concentration of suspended solids (x_ss) = 4.3 mg L^–1) according to the proposed scheme using slurry-type nebulization.

Fig. 3 Investigation of total concentrations (c_tot) and concentration of dissolved metal(loid)s (c_dis) in a side arm of the river Danube in the urban area of Vienna (x_ss = 14.9 mg L^–1) according to the proposed scheme using slurry-type nebulization.

Fig. 4 Investigation of total concentrations (c_tot) and concentrations of dissolved metal(loid)s (c_dis) in the river Leitha (x_ss = 9.9 mg L^–1) according to the proposed scheme using slurry-type nebulization.

The results were first evaluated with respect to elemental partitioning between the solid and the liquid phase. The investigated elements could be classified into three groups: Group 1 consisted of elements mainly bound to suspended solids (Pb, Zn), the second group (Cr, Cu, Ni) exhibited considerable dissolved and particle bound fractions, while the third group (As, Se) was predominantly present in the liquid phase. Ag and Cd were consistently below the LODs for all three rivers.

The detailed results of elemental fractions bound to suspended solids expressed as percentage of total elemental concentration are given in Table 5.

Table 5 Elemental fractions bound to suspended solids (F_ss) expressed as percentage of total elemental concentrations and obtained concentrations of As, Cu, Cr, Ni, Pb, Zn in suspended solids of three Austrian rivers. The data of c_ss are presented with respective total combined uncertainties U and compared with relevant concentration found in river sediment in fraction below 40 µm (c_sed) at the respective sites

	As	Cr	Cu	Ni	Pb	Zn
River Danube
F ss (%)	31	78	40	49	>97	83
c ss/µg g^–1	25	37	59	41	41	154
U (k = 2)	79%	22%	52%	53%	20%	34%
c sed/µg g^–1	n.a.	50	42	34	53	104
River Schwechat
F ss (%)	<10	40	13	n.a.	64	66
c ss/µg g^–1	n.a.	22	44	n.a.	85	384
U (k = 2)	n.a.	31%	105%	n.a.	15%	59%
c sed/µg g^–1	<5.0	53	64	28	53	200
River Leitha
F ss (%)	<10	34	48	14	56	71
c ss/µg g^–1	n.a.	17	62	21	55	224
U (k = 2)	n.a.	23%	37%	68%	25%	25%
c sed/µg g^–1	13.2	66.2	52.8	33.5	49.2	187

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Our distribution patterns are in accordance with those found in another study on suspended matter in the river Elbe, carried out by Prange and co-workers.¹¹ In this work dissolved fractions were determined from filtered surface water, while fractions bound to suspended matter were determined in microwave digested aliquots of collected suspended matter. Elemental analysis was carried out using ICP-AES and ICP-quadrupole-MS, respectively.

Ag, Cd, Hg, Pb and Zn were found to be associated with suspended solids, while for other elements (Cu, Cr, Ni) large dissolved fractions were reported. These findings correlate with the observations made in our study: For Pb and Zn large particle-bound fractions of 56%–100% and 71%–83%, respectively, were found in all three rivers. For Ni (0%–50%), Cr (22%–33%) and Cu (37%–100%) a wide range of values for bound metal fractions was obtained for the rivers included in this survey. Prange et al. found As predominantly in the dissolved fractions (>70%), which is supported by dissolved fractions >69% during this study in all investigated rivers. Selenium, which in our study was almost exclusively found in dissolved form in all three rivers, was found in very low concentrations in the Elbe in suspended solids and could not be detected in the liquid phase.

The concentration of metal(loid)s in the suspended solids c_ss was calculated according to eqn (1) in Table 1. In Table 5 the values are given with the total combined uncertainties (U) calculated according Eurochem/CITAC with a coverage factor of 2 by error propagation of all experimental parameters.²²

As the differences between the dissolved As concentration and the total As concentration were not significant in the case of the rivers Schwechat and Leitha, no concentration of As in the suspended solids could be given. For the Danube river a concentration of 25 µg g^–1 was found. However the expanded uncertainty was 79% with the low concentration of bound arsenic as a major contribution to the uncertainty. Owing to the low concentration of suspended solids in the river Schwechat (4.3 mg L^–1) the concentration of Cu in suspended solids showed high uncertainty (105%). This finding is further supported by the fact that for Ni also, a metal with significant binding to particulate matter in the other studied rivers, no significant differences between dissolved and total concentrations were found in the samples of the river Schwechat. For Se no significant bound fractions were present in any of the studied rivers. Summarizing, in terms of analytical figures of merit, the applied analytical procedure yields c_ss values associated with low uncertainties for elements with high particle bound fractions and suspended particle concentrations >5 mg L^–1.

During a study (Joint Danube Survey) suspended solids were collected from the Danube at several sites along the stream. Average values for metalloids in suspended solids for As (18.2 µg g^–1), Cr (62.3 µg g^–1), Cu (52.7 µg g^–1), Ni (47.3 µg g^–1), Pb (34.6 µg g^–1) and Zn (166 µg g^–1) were reported.²³ Considering the uncertainties attached, these values agree very well with our findings.

As data on the concentration in suspended matter are only available for the Danube, the results of the other two rivers are discussed with respect to available sediment data.²⁴ At all sampling sites river sediment in the size fraction below 40 µm (c_sed) was monitored during the past 10–15 years on several occasions by the Austrian Federal Ministry of Agriculture, Forestry, Environment and Water Management (BMLFUW). The data from the last campaign at the respective sites were compared with the data obtained within this study. For rivers Schwechat and Leitha river sediment was investigated in 2005, while Data for Danube sediment at the investigated site were comparatively old (1996). As can be readily observed in Table 5 the concentration levels in the suspended solids were in the same range as the concentration levels in the sediment.

Finally the obtained data was used to calculate distribution factors (K) for the metals based on the obtained average values of the concentration of dissolved metalloid c_dis and the obtained concentrations in suspended solids c_ss using eqn (1).

K = c_ss/c_dis (1)

The calculated values were compared with corresponding values used in risk assessment studies during the implementation of the WFD. The literature values for lead and nickel represent approximated values taken from the substance data sheets for the implementation of the WFD or were deduced by Ruedel et al. from reported data obtained during EU wide risk assessment studies.^20,25–27

A comparison of the data (Table 6) reveals that the results obtained in this study agree with values given in the literature, indicating that the developed procedure is well suited for determination of accurate K-values.

Table 6 Distribution coefficients K (L kg^–1) calculated from concentrations of metal(loid) in suspended solids c_ss and concentrations of dissolved metal(loid) c_dis for three Austrian rivers

	Pb	Zn	Cr	Cu	Ni	As
Danube	n.a.	319 000	233 000	44 500	69 900	29 900
Schwechat	412 000	457 000	153 000	33 900	n.a.	n.a.
Leitha	128 000	252 000	51 700	91 600	16 900	n.a.
Literature values^25–27	146 000	110 000	191 000	30 246	8300	10 000

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Conclusion

The main advantages of the proposed method compared with routine methods used in the field are as follows: (1) Sampling consists of contamination-free collection of native surface water. (2) The samples are only subjected to simple operational steps (pipetting, centrifugation). (3) The use of filter media which are often mentioned as possible source of contamination is avoided in the preparation of samples for trace elemental analysis.

The method exhibited limitations for elements with low total concentration and highly dissolved fractions (e.g., As, Se, Ni) with respect to quantification in suspended solids because measurement of significant differences is hampered in these cases. Evidently, the accuracy of quantification is compromised in the case of samples with low concentration of suspended solids (<5 mg L^–1) as well.

Determination of elemental distribution between solid and liquid phases is essential for establishing environmental monitoring standards. In our investigations the studied set of metal(loids) exhibited distinct partition patterns. Indeed, Pb and Zn should, in future, not be monitored as dissolved metals only, due to their high particle bound fraction. Arsenic and selenium, on the other hand, were almost exclusively found in the liquid phase and hence can be quantified in the liquid fraction of surface water. Hg could not be successfully included into this procedure due to its high tendency to adsorb on all available surfaces (wall adsorption). As a matter of fact, Hg and its organometallic compounds are strongly recommended for biota-monitoring in several detailed studies.²⁰

Acknowledgements

Financial support from the Austrian Federal Ministry of Agriculture, Forestry, Environment and Water Management (BMLFUW, Project Title: “Determination of trace metals in the water phase and suspended solids of surface water by ICP-SFMS”) is gratefully acknowledged. Dr Martin Wimmer from the division of surface water, BMLFUW, is gratefully acknowledged for many fruitful discussions.

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Footnote

† Presented at the 2007 European Winter Conference on Plasma Spectrochemistry, Taormina, Italy, February 18–23, 2007.

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