Anal. Chem. 2003, 75, 3573-3580
Performance Study of Diffusive Gradients in Thin Films for 55 Elements Øyvind Aaberg Garmo,*,† Oddvar Røyset,*,‡ Eiliv Steinnes,† and Trond Peder Flaten†
Department of Chemistry, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway, and Norwegian Institute for Water Research (NIVA), N-0411 Oslo, Norway
The technique of diffusive gradients in thin films (DGT) is a fairly new and useful tool for in situ measurements of labile metal ions in water. The applicability of DGTs was investigated by comparing independently determined or estimated diffusion coefficients with DGT effective diffusion coefficients (DDGT) for 55 elements. The DGTs were exposed at a controlled fluid velocity of 0.1 m s-1 and a concentration of 1 ng mL-1 at four pH levels between 4.7 and 6.0, and the DDGT values were determined from the uptake by the sampler. The measured DDGT values for the elements Co, Ni, Cu, Zn, Cd, Pb, Al, Mn, and Ga were close to previously published values with some deviations for Pb and Zn. The uptake of V, Cr, Fe, U, Mo, Ti, Ba, and Sr varied with pH, and there were some experimental problems that require further investigations. A novel set of DDGT values for the lanthanides (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Yb, Lu, Y) was established. The DDGT values for these were about 10-15% lower than for free ions in water and indicate that diffusion coefficients of metal ions in the agarose polyacrylamide diffusive hydrogel are 10-15% lower than in water. The high consistency of the data for the lanthanides establishes these elements as new performance test metals for the DGT sampler. The accumulation of the elements Li, Na, K, Rb, Mg, Ca, B, Tl, P, S, As, Bi, Se, Si, Sn, Sb, Te, Zr, Nb, Hf, Ta, W, Th, and Ag was low (DDGT lower than 10% of theoretical values). A more efficient elution procedure using concentrated nitric acid for the absorbent gel was established, with elution efficiencies between 95 and 100% for most metals. For deployment times of 24 h, detection limits from 0.001 to 1 ng mL-1 were achieved with moderate precautions to prevent contamination. The technique of diffusive gradients in thin films (DGT), developed by Zhang and Davison1 for passive sampling of labile metal ions in aquatic environments, has proved to be useful because of its simplicity and wide applicability. Whereas diffusive passive samplers have become very successful in measurements of gases in air during the last 20 years, the DGT sampler is the first diffusive passive sampler for ions in water that takes full advantage of diffusion theory for calculations of time-averaged * Corresponding author. E-mail:
[email protected]. Tel: 47 22185100. † Norwegian University of Science and Technology. ‡ Norwegian Institute for Water Research. (1) Davison, W.; Zhang, H. Nature 1994, 367, 546-548. 10.1021/ac026374n CCC: $25.00 Published on Web 06/19/2003
© 2003 American Chemical Society
concentrations. Provided that the process of diffusive mass transport is understood and quantified, such as described in some of Davison and Zhang’s early works,2 the sampler gives reasonably accurate values for the time-averaged concentration during exposure. Moreover, since the technique relies on diffusion of ions through a rather narrow-pore hydrogel, the DGT technique is a new speciation tool, permitting measurements of labile metals ions in solution. Several papers describing laboratory testing and field applications of DGT have been published since 1994. Most of the studies have been conducted on the metals Cu, Zn, Cd, Pb, Ni, Co, Mn, and Fe (see Davison et al.3 for overview). Gimpel et al.4 studied the collection of Cd, Co, Mn, Zn, and Cu in the pH range 2-13. They found that Cu could be determined accurately in the pH range 2-10, while the corresponding pH range for Cd, Co, Mn, and Zn was between 5 and 10. The aim of our work was to use the DGT sampler in studies of speciation and biological effects of metals in acid surface waters in Norway in the pH range 4.5-6.0, most typical for such water types. The metals associated with soil acidification (Al, Fe, Mn), the classical heavy metals, and other elements of environmental interest were of high priority. The absorbent of the DGT sampler is an iminodiacetate chelating resin (Chelex) known for binding a large number of divalent and trivalent metal ions selectively in the presence of high concentrations of alkali metals such as in seawater.5,6,7 The binding of metals by iminiodiacetate resins depends on pH. The best conditions for quantitative absorption of the most common divalent/trivalent metal ions are found at pH values above 4 (H+ ions are bound at lower pH), while the upper pH limit ranges from 10 to 13. The optimum conditions depend for individual metals on their aqueous ionic forms in this pH range. For multielement collection by iminodiacetate resins, it is recommended to adjust the pH into a pH window where most elements match with quantitative absorption. The usual approach has been to buffer the pH in the range from 5 to 8 or even to (2) Zhang, H.; Davison, W. Anal. Chem. 1995, 67, 3391-4000. (3) Davison, W.; Fones, G.; Harper, M.; Teasdale, P.; Zhang, H. Dialysis, DET and DGT: In situ diffusive techniques for studying water, sediments and soils. In In situ monitoring of aquatic systems: Chemical analysis and speciation; Buffle, J., Horvai, G., Eds.; John Wiley & Sons: New York, 2000; pp 495-569. (4) Gimpel, J.; Zhang, H.; Hutchinson, W.; Davison, W. Anal. Chim. Acta 2001, 448, 93-103. (5) Vandecasteele, C.; Block, C. B. Modern methods for trace element determination; John Wiley & Sons: New York, 1993; pp 36-53. (6) Samczynski, Z.; Dybczynski, R. J. Chromatogr., A 1997, 157-167. (7) Chelex-100 chelating ion-exchange resin instruction manual; Bio-Rad Laboratories, 1993.
Analytical Chemistry, Vol. 75, No. 14, July 15, 2003 3573
adjust to two separate pH levels to catch elements within different pH-optimum windows.6,7 By sampling with the DGT in situ, it is not possible to manipulate the pH; therefore, the potential for absorption of the individual ions in the solution must be understood at the pH on exposure. The sampling is thus performed under compromise pH conditions where some elements may be outside their optimum pH range. Until now, pH windows for uptake by the DGT sampler have been examined for ∼10 metals.2,4 The DGT sampler has a defined geometry, which facilitates the use of diffusion theory to develop reasonably accurate uptake rates. To do so, information about diffusion coefficients of metal ions in water and in the gel membrane is needed. In this study, an overview of the uptake and binding properties for metal ions of the DGT sampler in the pH range most relevant to acid surface water was desired. A broad laboratory study was performed with the sampler exposed for low concentrations of 55 elements dissolved as nitrates in the pH range 4.7-6. It was necessary to establish this basic knowledge before moving on to more complicated systems such as natural waters. When metal ions are sampled in natural water, it must be considered how the distribution of different species of each metal ion affects the uptake (colloids, hydrolysis, inorganic ligands, precipitations, humic complexes, etc.). The DGT sampler measures the amount of metal collected during the exposure time. To calculate the average concentrations based on the species collected, reliable data for diffusion coefficients of the individual ions are needed. Otherwise, the calculated DGT average concentrations become uncertain and comparisons with alternative speciation methods for validation of measurements by DGT samplers become difficult. The information regarding diffusion coefficients of metal ions examined together with the DGT sampler is limited, and the intention of this work was a first step toward filling this gap. This work covers some general performance characteristics of the sampler necessary for measuring reliable diffusion coefficients such as elution efficiencies and diffusive boundary layer conditions. The diffusion coefficients were calculated and compared with diffusion coefficients for free ions in water where such data were available. This paper presents the most important results from this study performed by Garmo.8 MATERIAL AND METHODS Theoretical Basis: Diffusion. For examination of diffusion theory, see the general literature such as the textbook by Cussler.9 This paper will just refer to the necessary formulas needed for DGT experiments. The principle of diffusive sampling relies on the formation of a steady-state concentration gradient and diffusion-controlled mass transport, which is described by the flux of molecules (dm/dt) over the concentration gradient (dC/dz) by Fick’s first law of diffusion (eq 1). If the sampler absorbent is an
flux: dm/dt ) -D dC/dz
(1)
efficient sink for ions (i.e., the concentration of ions at the absorbent surface approximates zero and no back-diffusion oc(8) Garmo, Ø. Laboratory study of the technique Diffusion Gradients in Thin films for determination of free ions and labile species in water. Cand. Scient. thesis, Norwegian University of Science and Technology, Trondheim, Norway, 2001. (9) Cussler, E. L. Diffusion mass transfer in fluid systems; Cambridge University Press: Cambridge, U.K., 1997; pp 1-200.
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curs), the concentration (Co) of ions outside the sampler may be estimated by integrating eq 1, forming eq 2 for time-integrated
time-integrated uptake: m(t) ) CotD(A/L)
(2)
mass uptake (m(t)). The terms L, D, and A are constants: the diffusion coefficient (D, cm2 s-1 specific to each metal ion), the length (L, cm), and cross sectional area (A, cm2) of the diffusion membrane. Equation 2 may be solved for the time-averaged concentration (Co) as shown in the conventional passive sampler eq 3. The L term in eqs 2 and 3 must be expanded summing up
time-averaged concentration:
Co ) (m/t)(L/DA)
(3)
the total length of the diffusion layer, i.e., the thickness of the gel membrane (G), the filter ( f ), and the diffusive boundary layer (δ) forming eq 4. The DA/L term in eq 2 represents the uptake
time-averaged concentration: Co ) (m/t)((G + f + δ)/DA) (4) rate factor (cm3 s-1). For ions with typical D values of 5 × 10-6 cm2 s-1 uptake rates are in the order of ∼0.5 mL/h or ∼12 mL/ 24 h at 25 °C (using the typical dimensions of the sampler: A ) 3.14 cm2 and L ) G + f + δ ) 1020 µm). The uptake rate is not a sampling rate but the volume of water “diffusively emptied” of ions per time unit. Diffusion coefficients depend on temperature (T ) and viscosity (η), and adjustments of D are done by the Stokes-Einstein equation (eq 5). Since the viscosity of water is temperature
Stokes-Einstein correction of D:
Dt ηt/Tt ) Doηo/To (5)
dependent (eq 6), the exact temperature correction of D requires the combination of eqs 5 and 6.
temperature correction of viscosity: ηo 1.37023(t - 25) + 0.000836(t - 25)2 log ) (6) ηt 109 + t The diffusive boundary layer (DBL) is a water layer in the interface between the membrane surface and the bulk water, where mass transport is dominated by diffusion and not convection. This diffusive boundary layer has a thickness δ that adds to the defined diffusive length of the sampler. The form and thickness of the DBL depends on the shape and dimension of the sampler. In the case of a flat plate, δ can be estimated from fluid mechanics as proposed by Levich.10 Garmo and Naqvi8 showed that a modification of the boundary condition is sufficient to adapt Levich’s analysis to a nonzero concentration at the solid-solution interface. This model (eq 7) was used to estimate the DBL, where 3
diffusive boundary layer:
δ ) 3.3
2
x xνxU D ν
(7)
v is the kinematic viscosity (cm2 s-1), x (cm) is the distance from the leading edge, and U is the flow velocity (cm s-1) in the free
solution whereas D is the diffusion coefficient. The DBL can also be determined experimentally by the so-called reciprocal plot method (eq 8) described by Zhang and Davison.2 δ can be
reciprocal mass plot for DBL:
1 G+f δ ) + m DCoAt DCoAt
(8)
calculated from measurements with DGTs with at least two gel thicknesses, plotting reciprocal mass (1/m) versus (G + f ). Diffusion coefficients may be estimated from limiting equivalent conductance (LEC) data by eq 9, where kB is Boltzman’s constant
LEC estimation of D:
D ) (kBT/|z|)λ
(9)
(J K-1 mol-1), T is temperature (in K), z is the absolute value of the ionic charge, and λ is the limiting equivalent conductance (cm2 mol-1 Ω-1; see Cussler9 for details). High-Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICPMS) Procedures. A Thermo Finnigan Element1 HR-ICPMS instrument was used to determine the element concentrations in the resin gel extracts and the samples from the test solution. The calibration was performed with multielement mixtures delivered by High Purity Standards and Teknolab AS (Spectrascan), all made from high-purity primary element/metal solutions of purity 99.99% or better. Altogether they contained the elements (oxidation state is given in parentheses if described by the manufacturer): Li(I), Na(I), K(I), Rb(I), Mg(II), Ca(II), Sr(II), Ba(II), V(III), Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Cd(II), Al(III), Pb(II), B(III, borate), Ga(III), Tl(I), P(V, phosphate), S(VI, sulfate), As(III), Bi(III), Se(VI), Ti(IV), Si(IV, silicate), Sn(II), Sb(III), Te, Mo(VII, molybdate), Zr(IV), Nb, Hf, Ta, W, U(IV), Th(IV), La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Tb(III), Lu(III), and Y(III). All the stock solutions were prepared in nitric acid solutions free of chloride to avoid common Cl molecular ions in ICPMS. The acidity of calibration solutions was matched with that of the samples (10 mL of concentrated nitric acid/100 mL of the DGT eluates and 1 mL/100 mL of water samples). Five internal standards were used containing Li-6 (95% Li-6), Sc, Ge, In, and Re. A HR-ICPMS analysis program containing 55 elements was designed with resolutions to avoid expected molecular ion problems.8 Generally the matrixes achieved from the extracts of the DGT samplers and the test solutions are so simple that it is not expected to find serious interference problems. The strength of HR-ICPMS is that most of the serious molecular ions problems encountered in classical quadropole ICPMS are removed when appropriate instrument resolutions of 4000 and 10 000 are used. The same 55-element calibration mixtures are used at the Norwegian Institute for Water Research (NIVA) for quadrupole ICPMS procedures and have been thoroughly tested for stability and analytical performance for ∼10 years, and several methods based on these calibration solutions are accredited by Norwegian Accreditation according to EN 17025. Unless otherwise stated, the water used was deionized from a Millipore MilliQ system, and the nitric acid was of high-purity quality (Merck Suprapure 65%). (10) Levich, V. G. Physicochemical hydrodynamics; Prentice Hall Inc.: Englewood Cliffs, NJ, 1962; pp 9-20, 57-60, and 87-91.
Figure 1. Rotor used to test DGTs in the exposure tank.
DGT Equipment. All DGT equipment was purchased from DGT Research Ltd. (Lancaster, U.K.). The regular DGT deployment moldings made of polyethylene were used. Diffusive agarose polyacrylamide (APA) gels were of the regular type (a formula patented by DGT Research, Ltd.). Diffusive and Chelex gels were 0.80 and 0.40 mm, respectively, for standard experiments whereas 0.40-mm diffusive gels were used for diffusive boundary layer experiments. Chelex is the trade name of the iminodiacetate chelating resin produced by Bio-Rad.7 A cellulose nitrate filter (Sartorius, pore size 0.45 µm, thickness 0.12 mm) was placed on top of the diffusive gels in the deployment molding in experiments. To fit the 0.4-mm diffusion gels into the deployment molding, an extra 0.40-mm diffusive gel was placed under the Chelex gel. Detailed descriptions of the sampler are found at DGT Research’s homepage (http://www.dgtresearch.com). Capacity and Exposure Time Considerations of the DGT Sampler. Assuming the thickness of the Chelex layer to be 0.1 mm in a 0.4-mm membrane, the resin bed volume is ∼0.03 mL for a 20-mm-diameter absorbent membrane. The binding capacity of Chelex is ∼0.5 mequiv mL-1 wet resin,7 corresponding to 15 µequiv for a 0.4-mm resin gel, in good agreement with experimental capacity values of 0.56 mg of Cd (12 µequiv) obtained by Davison and Zhang.2 At concentrations of 1 ng mL-1 (0.01-0.1 nmol mL-1, summing up to 0.001-0.01 µequiv for 50 metals) and typical uptake rates of 10 mL/24 h, a theoretical exposure time of 300 days may be estimated, ensuring that the capacity during the exposure experiments is not exceeded. Although the sampler also collects major cations such as Na, K, Ca, and Mg, these have much lower selectivity for the Chelex resin and are normally exchanged during exposure with other divalent/trivalent trace metal ions as discussed by Zhang and Davison.2 DGT Exposure Chamber. A 50-L polyethylene bucket was used as the exposure chamber. Two circular rotor plates made of PVC with a 20-cm diameter was mounted ∼15 cm below the lid of the bucket and connected to a small electric motor. The rim of each rotor plate had 12 holes where DGT deployment units could be fixed (Figure 1). The motor speed was adjusted so the rim of the rotor had a periphery velocity of 0.1 m s-1, to ensure a constant fluid velocity for all DGTs tested. Before experiments, the tank was carefully acid cleaned. Analytical Chemistry, Vol. 75, No. 14, July 15, 2003
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Preparation of Test Solutions. Preparation of test solutions containing 55 elements in the pH range 4.7-6.0 is complicated, especially avoiding precipitation of divalent and trivalent metals with low solubility. Ions such as Fe(III) and Al(III) are problematic because of the low solubility of their hydroxides in this pH range (about 1 ng mL-1 at pH 5 for Fe(OH)3 and 5-10 ng mL-1 at pH 6 for Al(OH)3 based on chemical equilibrium calculations). Having access to the very sensitive HR-ICPMS technique, it is possible to reduce the concentrations to as low as 1 ng mL-1 and still achieve reasonably accurate determinations in the eluates of exposed samplers. Therefore, the multimixture test solutions were prepared at a concentration of 1.0 ng mL-1. An exception was made for Al where the concentrations were 50.0, 10.0, and 5.0 ng mL-1 as data of high quality for this element was of particular interest. The 50-L exposure chamber was filled with ∼45 L of deionized water, and NaNO3 (p.a. Merck) was added to 0.01 M to adjust to constant ionic strength. The 55 metals were added to a calculated concentration of 1.00 ng mL-1 (prepared by the same mixtures as for calibration standards of HR-ICPMS; see above). After mixing, the pH of the test solution was ∼4.0 due to the nitric acid added from the concentrated stock solutions. The pH value was then adjusted to 4.7, 5.0, 5.4, or 6.0 with NaOH (p.a., Merck), the volume adjusted to 50 L, and the solution left to stabilize for 24 h with mixing by the rotor. Control of Test Solutions. At a concentration as low as 1 ng mL-1, surface adsorption, contamination, and possibly precipitation may change the concentrations during experiments. Another consideration is the volume needed to prevent a significant removal of ions by the samplers during exposure. The typical uptake rate of the samplers are around 10-15 mL/24 h, and 12 simultaneously exposed samplers will “diffusively empty” 120200 mL/24 h. Using a 50-L exposure chamber; this is only 1-1.5% during 72 h, so that this volume was considered sufficient. To check all these effects, water samples were collected every 24 h, preserved with nitric acid (1% v/v, i.e., 0.10 mL/10 mL) and the element concentrations determined by HR-ICPMS. Exposure Experiments. The DGT units were exposed in the test solution for 24, 48, and 72 h, respectively, at room temperature of 20 °C. After exposure, the DGT units were dismounted, the Chelex gels were transferred to acid-cleaned 15-mL polypropylene tubes, 1.0 mL of concentrated nitric acid was added, mixed, left for 24 h for elution, and then 9.0 mL of deionized water was added. After mixing, the resulting solution was decanted into a new 15mL polypropylene tube leaving the Chelex gel in the first tube. The removal of the gel is important to prevent back-absorption of metals to the Chelex gel after 1 + 9 dilution of the nitric acid (1.4 M) as nonquantitative elution efficiency has been reported using 1 M nitric acid.2 This mixing and decanting procedure took 5-10 s for each sample. Finally, solution was spiked with internal standards before HR-ICPMS analysis. Laboratory experiments at pH 4.7, 5.0, 5.4, and 5.9 were conducted, with both tests performed twice to check the reproducibility of the tests. The measured concentrations in the extracts were corrected for blank values of unexposed Chelex membranes (see data in Table 1), and the effective diffusion coefficient was calculated by solving eq 3 for D. All the parameters of eq 3 were known (A, G. and f are constants) or measured: the diffusive 3576
Analytical Chemistry, Vol. 75, No. 14, July 15, 2003
boundary layer (δ), the accumulated mass (x), the time (t), and the concentration (Co) of each metal ion in the test solution. The diffusion coefficients achieved at 20 °C were adjusted to 25 °C by eq 5 for comparison with values from the literature. Estimation of the Thickness of the Diffusive Boundary Layer. The DBL experiments were performed with samplers equipped with diffusion membranes of 0.40 and 0.80 mm. The value of DBL (δ) can then be estimated for different elements based on the reciprocal plot method of eq 8. The DBL was also estimated by means of the fluid mechanical model of eq 7, for comparisons with the reciprocal plot method. RESULTS AND DISCUSSION Exposure Chamber. Using the exposure chamber shown in Figure 1, the DGTs were exposed at the rim of the rotor plate, so that all samplers faced an equal fluid velocity calculated by the periphery velocity of the rotor. This system performed well during the laboratory experiments, as relative standard deviations between parallel exposed samplers of 10-20% for many of the most important metals were achieved (see Table 3). Stability of Metal Test Solutions. A decrease in concentration during the 72-h exposure period was observed for some metals but was for most metals below 10%. The elements Sn, Ti, and Fe were the most problematic, and we observed reductions in concentrations of 20 -50% down to 0.5 ng mL-1. For the elements Zn and Cu, some contamination was observed, leading to a concentration of 1.2-1.5 ng mL-1. in the solution. In all cases, the calculation of D values was based on the average measured concentration in the exposure chamber during the exposure period and not the nominal calculated initial concentration of 1.00 ng mL-1. Thus, the calculations were corrected for the changes in concentration observed during the experiments. Determination of Elution Efficiency. Davison and Zhang2 extracted the gels with 1 M nitric acid for six metals and reported relative elution efficiencies around 80% with standard deviations of 2-6%.2 We believed that using concentrated nitric acid would increase the elution efficiency and checked this by eluting exposed Chelex gels twice with concentrated nitric acid. The results in Table 1 show elution efficiencies of 97 and 99% for most metals with standard deviations between 1 and 2%. The elution efficiency for Al, Ni, and Cr were somewhat lower, but still around 94%, and RSD from 1 to 3%. The data in Table 1 are compared with the results achieved by Zhang and Davison2 for Cd, Cu, Zn, Mn, Ni, and Fe using 1 M nitric acid and shows that elution with concentrated nitric acid is both more quantitative and reproducible than with 1 M nitric acid. An exception is the low elution efficiency for Fe found both in this work (80%) and by Zhang and Davison.2 The low elution efficiency for Fe is probably an artifact caused by a contamination problem for this element during the experiment. An approach to achieve complete elution of collected metals is digestion of the complete Chelex membranes by hot concentrated nitric acid. Heating the extract in the polypropylene tubes at 120 °C removed the gel, but not the resin, while digestion in closed microwave bombs (∼170 °C at elevated pressure) removed both the gel and the Chelex resin. However, using microwave bombs for this purpose, we observed considerably higher blank levels than with elution with only nitric acid (Table 1), which agrees with the present experience that PFA/PTFE materials have
Table 1. Summary of Elution Efficiencies Compared with Earlier Results, Blank Values of Unexposed Chelex Membranes, and Calculated Detection Limits (LOD) in Water elution efficiency in 1 M nitric acida (in 1.0-mL eluate) av (%) Ag Ba Cd Co Cu Mn Ni Pb Sr Zn Al Cr Fe Ga Ce Dy Er Eu Gd Ho La Lu Nd Pr Sm Tb Tm Y Yb U V Mo
RSD (%)
83.9
2.7
79.3 81.4 81.6
6.4 2.2 6.9
80.3
5.5
69.7
5.0
elution efficiency in concentrated nitric acidb (in 10-mL eluate)
blank value (N ) 21) (in 10-mL eluate)
LODc (in samples)
av (%)
RSD (%)
av (ng mL-1)
SD(ng mL-1)
3 × SD/24 h (ng mL-1)
98.4 98.8 98.7 98.6 97.7 98.3 93.5 97.4 97.0 96.3 94.7 93.7 80.8 97.3 97.7 97.7 97.7 97.7 97.5 97.7 97.7 97.8 97.7 97.6 97.6 97.6 97.7 97.8 97.7 96.2 97.7 93.9
0.7 0.9 1.1 1.2 1.0 1.1 0.8 0.5 1.9 1.6 2.2 2.5 10.1 1.2 1.5 1.5 1.5 1.4 1.6 1.5 1.4 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.3 1.4 4.0
0.019 0.62 0.0053 0.010 0.097 0.036 0.41 0.10 0.77 0.63 1.32 0.16 1.75 0.0023 0.0022 0.00022 0.00031 0.00096 0.0072 0.00030 0.0040 0.00012 0.00121 0.00054 0.00050 0.00027 0.00010 0.00063 0.00050 0.00076 0.0042 0.020
0.012 0.37 0.0015 0.019 0.026 0.011 0.048 0.016 0.25 0.22 0.19 0.048 0.56 0.00054 0.0018 0.00029 0.00057 0.00032 0.0033 0.00022 0.0041 0.00008 0.00083 0.0011 0.00088 0.00064 0.00006 0.00041 0.00038 0.00054 0.0013 0.0067
0.158 0.90 0.0032 0.037 0.047 0.021 0.087 0.019 0.58 0.58 0.43 0.18 2.6 0.00096 0.0044 0.00062 0.0012 0.00068 0.0051 0.00047 0.010 0.00018 0.0017 0.0022 0.0018 0.0014 0.00013 0.00097 0.00084 0.0015 0.0035 0.057
a These elution efficiencies was reported by Zhang and Davison.2 b This work. c The LOD is calculated by eq 3 based on 3 times the standard deviation of the blanks and an exposure time of 24 h using our DDGT diffusion coefficients of Table 3.
tedious memory effects for elements such as Al, Fe, Mn, Cr, Ni, Cu, and Zn.11 Thus, based on the experience achieved so far, the cold nitric acid elution is the method of choice. There may be room for further improvement of the elution efficiency for those below 96% (Al, Cr, Ni, Fe, and some others). Estimation of the Thickness of the Diffusive Boundary Layer. The DBL was calculated by the reciprocal plot method (eq 8) based on the data from the 0.4- and 0.8-mm membranes. These results were compared to calculations by the fluid mechanical model (eq 7) at a fluid velocity of 0.1 m s-1 and a distance of 1 cm from the leading edge (in the middle of the DGT window). The results in Table 2 shows that both methods give rather similar values of DBL for the lanthanides, Pb, Co, Cu, Al, and Ni, while the values based on Cd and Mn deviate somewhat and are probably too high. Based on the average value 87 and 104 µm by the fluid mechanical and reciprocal methods, respectively, we decided to use a DBL value of 100 µm in the calculations of D. The estimates based on the reciprocal plot method show standard errors ranging from 45 to 80 µm. As the total diffusion length of the system is 1020 µm (G ) 800, f ) 120, and δ ) 100), the uncertainty in the estimate of 40-80 µm contributes with a total error of 4-8% to the D values obtained. The fluid mechanical (11) Røyset, O. Norwegian Institute for Water Research (NIVA), Oslo, Norway, unpublished results, 2003.
model (eq 7) predicts an increase in δ at fluid velocity below 0.1 m s-1, so that the importance of accurate estimates of this parameter increases at lower fluid velocity as also stated by Gimpel et al.4 A detailed discussion of the implications of the different methods of estimating the DBL is beyond the scope of this work. Further work is needed to achieve better understanding of how the DBL develops above the DGT surface especially at lower fluid velocities. Detection Limits. The sensitivity of HR-ICPMS is very good with instrumental detection limits from 0.000 01 to 0.001 ng mL-1 for most metals. The limiting factor to achieve low detection limits for DGTs is the blank values from unexposed Chelex membranes and other contamination sources in the elution procedure. During the laboratory experiments, several unexposed membranes were extracted and analyzed. Table 1 presents the results for average blank values of Chelex extracts, standard deviations of blanks, and estimated detection limits for DGTs normalized to 24-h deployment periods (calculated from the variance of the blank value and eq 3). Detection limits from 0.1 to 1 ng mL-1 were achieved for the elements most commonly associated with contamination (Al, Cu, Zn, Cr, Ba, Sr). For many trace metals, detection limits were in the range 0.01-0.1 ng mL-1. For the lanthanides, where contamination is much less pronounced, our detection limits were around 0.001 ng mL-1. All the detection limits reported in Table 1 were achieved with moderate preAnalytical Chemistry, Vol. 75, No. 14, July 15, 2003
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Table 2. Measurements of DBL Thickness by the Reciprocal Plot Method and Calculated from the Fluid Mechanical Model measured by reciprocal plots (eq 8)
Pb Cd Mn Co Cu Ni Al La Ce Pr Nd Sm Eu Gd Dy Ho Er Tm Yb average std dev maximum minimum
calculated from model (eq 7)
δ (µm)
std error (µm)
δ (µm)
93 152 167 95 100 108 82 96 119 107 116 102 105 83 86 87 91 79 108 104 22 167 79
45 70 76 57 75 74 63 73 75 70 71 71 74 69 78 79 73 74 74 71 8 79 45
99 90 89 89 90 89 75 86 86 86 86 86 85 85 85 85 85 84 85 87 4 99 75
cautions for contamination control. The samplers were mounted and eluted in a class 100 000 clean room, and the Chelex gels were used as delivered from the manufacturer without further cleaning. Olofsson et al.12 used acid-cleaned Chelex resin, made their own Chelex gels, used strict procedures for avoiding contamination, and reported considerably lower detection limits than those reported in the present work for several elements. Different batches of sampler gels as well as changes in procedures may alter the blank levels encountered, and our LOD data of Table 1 is only a guideline of expected levels. The detection limits should therefore be continuously checked during the use of DGT samplers, so that these always are updated according to batch differences or procedure changes. Our detection limit data are at a medium level, and we believe it is possible to reduce the levels for several of these elements by more careful procedures to avoid contamination. Nevertheless, the detection limits in Table 1 are adequate for many applications such as in assessment of reactive Al toward fish species, where toxic effects arise at levels well above 1 ng mL-1. The detection limit is inversely proportional to time. One way to decrease the LOD are using longer exposure periods, a LOD of 0.01 ng mL-1 per 24 h can be lowered to ∼0.0003 ng mL-1 by increasing the exposure time to 30 days (at an uptake rate of 10 mL/24 h). Accumulation of Metal Ions as a Function of Deployment Time. As long as the capacity of the Chelex resin gel of 10-15 µequiv is not exceeded, eq 2 predicts linear uptake of ions with time for a DGT sampler. The amount of metal accumulated at 24, 48, and 72 h was plotted, and the linearity was examined.8 Good linearity up to 72 h for most of the elements listed in Table 3 was (12) Olofsson, R.; Dahlqvist, R.; Rodushkin, I.; Ingri, J. Sector field ICP-MS combined with the technique of DGT for measurement of labile species of 37 metals in natural waters. Poster, European Winter Conference on Plasma Spectrochemistry, Lillehammer, Norway, February 2-7, 2001.
3578 Analytical Chemistry, Vol. 75, No. 14, July 15, 2003
found at the concentration studied (1 ng mL-1). Exceptions were Ba and Sr, where a decrease in uptake was observed above 48 h, whereas Ag, Be, Fe, Mo, and Ti showed some mixed patterns. The decrease in uptake for Ba and Sr is the same as observed by Olofsson et al.12 and Dahlquist et al.;13 the uptake decreased after ∼50 h, caused by the lower selectivity of these elements. For the elements Li, Na, K, Rb, Mg, Ca, B, Tl, P, S, As, Bi, Se, Si, Sn, Sb, Te, Zr, Nb, Hf, Ta, W, and Th, linear uptake with time was not observed in our study, indicating that these metals are not quantitatively collected. Calculations of Diffusion Coefficients. Diffusion coefficients may be estimated with diffusion cells as done by Zhang and Davison14 or calculated from the property limiting equivalent conductance, as done by Li and Gregory15 based on data tabulated by Robinson and Stokes.16 With the diffusion cell experiments, Zhang14 obtained data for diffusion through the APA membrane, so that the restriction to diffusion by the membrane is included in the D values. The limiting equivalent conductance-based diffusion coefficients express free diffusion in water without the restriction imposed by the gel. The diffusion coefficients obtained from this study are a mixed expression of the diffusion rate of ions through the diffusion gel membrane and the binding ability of the receiving Chelex absorbent. The measurements can therefore most precisely be denoted DGT effective diffusion coefficients (DDGT). Thus, for elements for which the Chelex adsorbent has lower selectivity, back-diffusion may be expected and the DDGT value obtained will be lower than what can be calculated or measured from diffusion cells. It will be difficult to establish reliable diffusion coefficients for metals with significant back-diffusion by the approach used in this study. The best way is to compare the DDGT value with those from free diffusion in water, and if the deviation is above 2030%, a significant back-diffusion occurs that prevents quantitative absorption. Nevertheless, provided that both absorption and backdiffusion are linear with time, the sampler may still collect and concentrate ions to some extent. However, then the sampler becomes a qualitative, but not quantitative measurement tool for those ions. We therefore consider exposure tests with DGTs as a good way to achieve diffusion coefficients related to DGT, as it sorts out the elements that are quantitatively collected. The data from diffusion cells or limiting equivalent conductance calculations are important as external references to judge if we have quantitative absorption. Table 3 shows the results achieved in this study. Table 3 also shows values estimated from limiting equivalent conductivity (DLEC) gathered by Li and Gregory15 or based on data from Robinson and Stokes,16 together with the set of diffusion coefficients measured by Zhang and Davison17 by diffusion cells (DDC) with the same type of APA gel as used in our DGT experiments. The DDGT values listed in Table 3 are close to previously published values for Co, Ni, Cu, Cd, Al, Mn, and Ga. For Cd, there is a slight decrease at pH 4.7, probably due to lower selectivity (13) Dahlquist, R.; Ingri, J.; Zhang, H.; Davison, W. Anal. Chim. Acta 2002, 460, 247-256. (14) Zhang, H.; Davison, W. Anal. Chim. Acta 1999, 329-340. (15) Li, Y.; Gregory, S. Geochim. Cosmochim. Acta 2002, 38, 703-714. (16) Robinson, R. A.; Stokes, R. H. Electrolytic solutions; Butterworths Publications Ltd.: London, U.K., 1959.
Table 3. Diffusion Coefficients Obtained from Our DGT Experiments (DDGT) at pH 4.7-5.9 Compared To Values Obtained from Diffusion Cells (DDC) and from Limiting Equivalent Conductance (DLEC) Data DDGT 4.7 (N ) 18) elema Ag(I) Ba(II) Sr(II) Cd(II) Co(II) Cu(II) Mn(II) Ni(II) Pb(II) Zn(II) Cr(III) Fe(III) Al(III) Ga(III) Ce(III) Dy(III) Er(III) Eu(III) Gd(III) Ho(III) La(III) Lu(III) Nd(III) Pr(III) Sm(III) Tb(III) Tm(III) Y(III) Yb(III) U(IV) V(III) Ti(IV) Mo(VII) b
DDGT 5.0 (N ) 16)
DDGT 5.4 (N ) 18)
DDGT 5.9 (N ) 20)
DDGT all 4 tests (N ) 72)
av
RSD (%)
av
RSD (%)
av
RSD (%)
av
RSD (%)
av
RSD (%)
10-6 0.5 3.9 3.3 4.7 5.8 6.6 5.1 6.3 10.3 3.8 4.0 2.6 5.1 5.8 4.5 5.6 5.8 5.8 8.1 5.6 4.5 5.1 5.5 5.9 5.8 5.7 5.6 4.8 5.4 4.8 5.1 2.1 2.8
40 26 31 17 20 22 23 21 21 20 23 69 22 18 16 24 22 22 40 23 16 22 20 21 22 22 23 17 23 20 17 24 30
10-6 0.7 4.3 4.5 5.2 6.0 6.4 5.6 6.3 9.8 4.6 3.6 3.2 5.1 6.2 5.1 5.5 5.5 5.7 6.3 5.7 5.0 5.3 5.7 5.8 5.5 5.5 5.5 4.9 5.3 4.7 4.8 2.0 1.5
35 288 23 12 11 11 12 13 10 18 15 24 11 12 15 12 12 11 16 12 17 12 12 11 12 11 10 14 12 13 15 31 19
10-6 0.8 5.7 5.5 5.4 6.4 6.5 6.2 6.6 10.1 4.6 3.0 2.4 5.2 6.9 4.7 5.6 5.6 5.5 7.4 5.6 4.9 5.1 5.6 5.9 5.6 5.6 5.4 5.1 5.3 4.0 4.3 1.0 0.9
45 37 14 11 15 16 13 17 17 15 24 74 18 16 12 21 20 20 44 23 11 20 19 16 19 20 20 15 21 11 13 40 57
10-6 1.7 4.8 6.1 5.5 5.8 5.5 6.4 6.0 9.4 4.6 1.3 1.4 5.0 6.7 4.2 4.8 5.2 4.6 8.1 4.9 4.0 4.3 4.8 5.4 5.1 5.1 4.8 4.5 4.4 3.4 2.4 0.7 0.2
69 42 28 20 18 167 16 18 17 24 25 61 18 20 21 20 20 19 19 19 19 20 19 21 20 21 19 19 20 19 26 27 31
10-6 0.92 4.70 4.85 5.36 5.99 6.25 5.82 6.29 9.91 4.38 3.00 2.38 5.10 6.38 4.64 5.38 5.52 5.40 7.49 5.45 4.58 4.97 5.40 5.75 5.50 5.46 5.34 4.82 5.11 4.22 4.16 1.30 1.37
45 37 14 11 15 16 13 17 17 15 24 74 18 16 12 21 20 20 44 20 11 20 19 16 19 20 20 15 21 11 13 53 57
DLECb
DDCc
10-6 16.6 L 8.48 L 9.45 L 7.17 L 6.99 L 7.33 L 6.88 L 6.79 L 9.00 L 7.15 L 5.94 L 6.07 R 5.59 L
10-6 14.1 6.09 5.94 6.23 5.85 5.77 8.03 6.08 5.05 6.11 4.75
6.19 L 5.82 L 5.85 L 6.02 R 5.97 R 5.88 R 6.17 6.16 R 6.16 L 6.18 L 6.08 L 5.80 L 5.50 L 5.82 L 4.26 L 9.91 L
a Most probable oxidation state of ionic species. All data are adjusted to 25 °C by Stokes-Einstein and expressed as exponentials of 10-6. Values marked R are from Robinson and Stokes,14 while those marked L are from Li and Gregory.13 c Zhang, unpublished.15
below pH 5, as the pH window for quantitative absorption of Cd is from 5 to 10.4 For Pb, our results are 10-20% higher than DC and LEC data, while our Zn result is 20% low. Further investigations are needed to sort out whether this difference for Pb and Zn is real or only an experimental artifact. It is very promising that the DDGT values for Al are consistent with those achieved by Zhang and Davison17 and Li and Gregory.15 Labile reactive Al is the most important metal in acid surface waters in Norway for assessments of toxic effect toward fish, and we have recently gotten results showing that DGT labile Al is a good predictor for gill uptake of Al in salmonids in acid surface water.18 The DDGT values for the lanthanides (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Lu, Y) are very consistent, and ∼1015% lower than DLEC-based diffusion coefficients in water (with the exception of Gd, which probably is too high). The diffusion coefficients for all the lanthanides are also very similar ranging from 5.5 × 10-6 to 6.2 × 10-6. Our DDGT values are 10-15% lower than DLEC for most of the lanthanides. This indicates that diffusion coefficients for these ions in the APA hydrogel are 10-15% lower than in water. A reduction of this magnitude have also been suggested by Alfaro-De la Torre et al.19 based on observations (17) Zhang, H.; Davison, W.; Teasdale, P. Multi-element determination of metal species in natural water using DGT with ICP-MS detection, in preparation, 2003. (18) Røyset, O. Passive sampling of metal ions in water by DGTs, Nordic Conference on Plasma Spectrochemistry, Loen, Norway, June 2-5, 2002.
for Cd. Their explanation is that the APA hydrogel imposes a resistance to ionic movement of this order of magnitude. Recently, Gimpel et al.20 have reported similar observations. For Ba and Sr, the D values increase with pH from ∼40% of LEC at pH 4.7 to ∼60-70% at pH 5.9. This may be due to limited selectivity of the Chelex absorbent, as the uptake is reduced by competition from H+-ions at lower pH. For U, V, Mo, and Ti, the DDGT values decreases from pH 4.7 to 5.9. Our DDGT values for U from 4.7 to 5.4 are close to the DLEC value obtained from Robinson and Stokes.16 The value decreases ∼30% from pH 4.7 to 5.9. No comparable data are available for V, but as V has about the same ionic size as Mn, a diffusion coefficient of ∼6 × 10-6 may be reasonable. Thus, the DDGT value of 5.1 × 10-6 at pH 4.7 indicates that V is nearly quantitatively absorbed at this pH, but the absorption decreases at higher pH. For Mo and Ti, the DDGT values decrease more significantly from 4.7 to 5.9. The DDGT value of Mo at pH 4.7 is ∼30% of the DLEC value. For Ti, the value is probably also too low. The decrease in DDGT values for V, Mo, and Ti when pH increases is probably caused by changes in hydrolysis of these ions, making these ions less vulnerable for absorption by the Chelex resin. (19) Alfaro-De la Torre, M. C.; Beaulieu, P. Y.; Tessier, A. Anal. Chim. Acta 2000, 418, 53-68. (20) Gimpel, J.; Zhang, H.; Davison, W.; Edwards, A. C. Environ. Sci. Technol. 2003, 37, 138-146.
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Figure 2. Overview of uptake properties of the DGT sampler for metal ions tested in the range 4.7-6.
The DDGT values for Cr and Fe at pH 4.7 and 5.0 are ∼50% of those from DLEC and DDC and decrease at pH 5.9. Fe and Cr also have complicated aqueous chemistry in this pH range forming hydroxy/oxy species with charge and stability highly dependent on pH, so that it is not easy to draw clear conclusions about the behavior with respect to uptake with the Chelex. More thorough investigations are needed to produce better documentation for the DDGT values for both Fe and Cr, as well as for the elements Ti, V, Mo, and U. The elements Li, Na, K, Rb, Mg, Ca, B, Tl, P, S, As, Bi, Se, Si, Sn, Sb, Te, Zr, Nb, Hf, Ta, W, Th, and Ag showed low uptake. Our DDGT-values were in the range 1 × 10-7 to 1 × 10-8, which is less than 10% of calculated values of D from limiting equivalent conductance data by Robinson and Stokes.14 The reason is low to moderate selectivity for the group I and II cations (Li, Na, K, Rb, Mg, Ca). The anionic elements are excluded by the Chelex; this is the case for B, P, S, As, Bi, Se, Si, Sb, and Te. The low uptake for Sn, Zr, Nb, Hf, Ta, W, and Th is probably due to the fact that those elements form hydrolysis products (hydroxy/oxy ions) that are not collected by Chelex in this pH range. Both Tl and Ag show low uptake, probably because they are monovalent cations with low selectivity for Chelex. Chelex have been reported to have high selectivity for Ag,7 so further investigations are necessary before we can draw final conclusions about the uptake of this metal. Summary of General Patterns. Figure 2 sums up the general patterns achieved. These patterns agree very well with the known selectivity of the Chelex resin, as summarized by Vandercasteele.5 The general pattern showed in Figure 2 is restricted to our study of the pH range 4.7-5.9. Since the selectivity of Chelex toward individual metal ions is pH dependent, the general patterns may become different at higher pH. Although the performance of the DGT sampler has been studied earlier in the pH range 5-13 for 5-10 metals,2,4 we need more thorough investigations to conclude about general patterns at higher pH, at least for the elements not studied earlier. This information is necessary if we want to study the performance of DGTs in more alkaline water media such as seawater. CONCLUSIONS This study presents the most comprehensive set of diffusion coefficients achieved to date employing DGT technology. Our results give further insights into the performance of the DGT sampler for metal ions in water in the pH range 4.7-5.9. Of the (21) Røyset, O. Norwegian Institute for Water Research, Oslo, Norway, unpublished results, 2003.
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55 elements tested, we get DDGT values close to those reported earlier for 24 elements, which indicate quantitative absorption by the Chelex for these metals (Pb, Zn, Co, Ni, Cu, Cd, Al, Mn, Ga, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Lu, Y). An additional eight elements show significant uptake by the sampler (Fe, Cr, Ba, Sr, U, V, Mo, Ti). For the latter elements, we need more research to better understand how the distribution of species with pH affect the uptake in the DGT to get more reliable diffusion coefficients. When extracting the Chelex gels with concentrated nitric acid we get elution efficiencies from 95 to 99% for most metals. This elution procedure is nearly fully quantitative, and the uncertainty of the elution factor achieved is only ∼1-2% for most metals taken up by the sampler. We achieved reasonably low detection limits from 0.001 to 1 ng mL-1 even at only 24-h exposure times and only moderate precautions for contamination control. Further reductions in detection limits may be possible by more careful cleaning of the absorbent as well as general improvements of laboratory procedures to prevent contamination. Our experiments and theoretical predictions of the DBL indicate that at fluid velocities of ∼0.1 m s-1 the measured and estimated thickness of the DBL layer is ∼0.1 mm, i.e., ∼10% of the total diffusion length of the sampler. The uncertainty of the estimate imposes an uncertainty of the total diffusion length of 4-8%. At lower flow velocities, the DBL will be thicker and impose larger uncertainties in the estimate of the diffusion length term. This is the first study where lanthanides have been examined in such detail. The very good performance observed for these metals may help to establish these metals as new performance test parameters for the DGT. The chemical properties of the lanthanides are similar: all are small trivalent cations with almost equal diffusion rates and quantitative absorption by the Chelex receiving absorbent. The lanthanides are very easily determined with ICPMS. They may become very convenient for performance testing of the DGT sampler in the laboratory. They may also be used as indicators of general sampling performance in situ, as we have found good precision for sampling of the lanthanides at levels of 0.1-10 pg mL-1 in surface water.21 ACKNOWLEDGMENT This work was supported by research grants from the Norwegian Research Council (NFR) for 2001-2002 on O.R.’s NFR MU-PROFO project 140375/720, “Passive sampling of metals ions in water using DGTs”. Norwegian Institute for Water Research, Oslo, was host institution and supported the project with 30% of total costs from its own strategic institutional funding. Norwegian University for Science and Technology, (NTNU), Trondheim, hosted Ø.G.’s cand. scient. study. We thank Torill Sjøbakk, NTNU, for doing the HR-ICPMS analysis, professor Razi Naqvi, NTNU, for help with the DBL model, and our partners, Professor William Davison and NERC research fellow Hao Zhang, Environmental Sciences, Lancaster University, U.K., for knowledge transfer on DGTs and kind assistance at many occasions. Received for review December 1, 2002. Accepted April 30, 2003. AC026374N
