Environ. Sci. Technol. 2000, 34, 5177-5183
Sensing Dissolved Sediment Porewater Concentrations of Persistent and Bioaccumulative Pollutants Using Disposable Solid-Phase Microextraction Fibers P H I L I P P M A Y E R , * ,† W O U T E R H . J . V A E S , †,§ F E M K E W I J N K E R , † KARIN C. H. M. LEGIERSE,‡ RIK (H.) KRAAIJ,† JOHANNES TOLLS,† AND JOOP L. M. HERMENS† Environmental Toxicology and Chemistry, RITOX, Utrecht University, P.O. Box 80058, NL-3508 TB Utrecht, The Netherlands, and RIKZ, P.O. Box 8039, 4330 EA Middelburg, The Netherlands
Polymer coated glass fibers were applied as disposable samplers to measure dissolved concentrations of persistent and bioaccumulative pollutants (PBPs) in sediment porewater. The method is called matrix solid-phase microextraction (matrix-SPME), because it utilizes the entire sediment matrix as a reservoir for an equilibrium extraction: a glass fiber with a 15 µm coating of poly(dimethylsiloxane) (PDMS) was placed in a sediment sample until the PBPs reached their equilibrium distribution between the PDMS and the sediment matrix (1-30 days). PBP concentrations in the PDMS were determined by gas chromatography, and they were divided by PDMS water partition coefficients to derive at dissolved porewater concentrations. This approach was applied to measure porewater concentrations of spiked as well as field sediment, and several hydrophobic organic substances (log KOW 5.27.5) were measured with high precision in the pg to ng/L range. Simple equilibrium partitioning is the basis for the substantial concentration factors that are built into matrix-SPME and for the low demands in materials and operation time. Matrix-SPME was in this study directed at the determination of dissolved porewater concentrations in sediment, and it is further expected to be applicable to other environmental media, to field sampling, and to the sensing of fugacity.
Introduction Persistent and bioaccumulative pollutants (PBPs) are of high concern for environmental risk assessors, because these substances remain in the environment even when the emission is terminated and because they can build up * Corresponding author fax: +31-15-257 2649; e-mail: mayer@ voeding.tno.nl. Present address: Department of Environmental Toxicology, TNO Voeding, P.O. Box 6011, 2600 JA Delft, The Netherlands. † RITOX, Utrecht University. ‡ RIKZ. § Present address: TNO Voeding, P.O. Box 360, 3700 AJ Zeist, The Netherlands. 10.1021/es001179g CCC: $19.00 Published on Web 11/01/2000
2000 American Chemical Society
considerable concentrations in biota even at low ambient concentrations. Some of these substances can also be subject to long-range transport (1), meaning that they can reach remote areas and civilizations that are not responsible for their production and usage. Polychlorinated biphenyls (PCBs) and the pesticide DDT are prominent examples of such substances. In general, these chemicals are poorly soluble in water and air and well retained by organic matter, and a major part of their global amounts is consequently present in soil and sediment. Only a minute fraction of PBPs in soil and sediment is typically present as freely dissolved molecules in the porewater. Nevertheless, the concentration of the dissolved form is often considered the effective concentration, as it controls several diffusive mass transfer processes such as evaporation, sorption, and uptake into macro- and microorganisms. The determination of dissolved porewater concentrations is therefore crucial for the understanding and correct modeling of the distribution, the transport, the toxicity, and the biodegradation in sediment and soil. The measurement of freely dissolved porewater concentrations represents an analytical challenge, because it is difficult to discriminate the dissolved form of an analyte from its sorbed forms. A substantial number of studies have thus been devoted to develop approaches to distinguish freely dissolved from sorbed forms. These approaches include dialysis (2), fast elution on a solid-phase column (3), solubility enhancement (4), and measurement of headspace concentration (5) and fluorescence quenching (6). Several techniques are based on the passive partitioning into some kind of lipophilic phase such as solvent filled semipermeable membrane devices (SPMD) (7, 8), C18 extraction disks (9), and solid-phase microextraction fibers (SPME) (10-13). These approaches have been successfully applied for different purposes; however, it remains an analytical challenge to measure freely dissolved concentrations of PBPs in dense environmental matrices such as soil and sediment. Our study introduces a new approach that is based on the principle of solid-phase microextraction (SPME), which is a solvent free extraction technique that has been introduced by Arthur and Pawliszyn (14). Our approach is basically (1) to insert a small piece of polymer coated fiber into a sediment sample, (2) to wait until the PBPs reached their equilibrium distribution between the sediment matrix and the polymer, and then (3) to measure the PBP concentration in the polymer. This approach differs from other SPME methods, because it utilizes the entire sediment matrix as a reservoir for an equilibrium extraction, and it is hence called matrix solid-phase microextraction (matrix-SPME). The theoretical considerations behind matrix-SPME and crucial prerequisites of the approach will be described first. Subsequently, the applicability of matrix-SPME is investigated in a stepwise manner and finally demonstrated in a “real world” sample.
Theoretical Considerations Matrix-SPME is an equilibrium extraction technique that can be considered as nondepletive. “Nondepletive” refers to an extraction that is limited to a minor part of the analyte and which consequently does not deplete the analyte concentration. “Equilibrium” refers to extraction times that are sufficiently long to bring the sampling phase into its thermodynamic equilibrium with the surrounding matrix. Nondepletive Extraction. The method aims at sensing dissolved concentrations without depleting them. The “nondepletive” criterion is normally met by restricting the SPME extraction to a negligible fraction of the dissolved amount VOL. 34, NO. 24, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1: Working Principle of Matrix-SPME Is Illustrated by Comparing It to Conventional Exhaustive Extraction and to Negligible Depletion SPME (nd-SPME) conventional solvent extraction
nd-SPME (10, 13)
matrix-SPME
extraction mode
exhaustive extraction
kinetic/equilibrium extraction
equilibrium extraction
dissolved analytes
completely extracted
reservoir for negligible depletion extraction
eventually in equilibrium with sampling phase
bound analytes
completely extracted
not extracted
reservoir for negligible depletion extraction
calibration
standard in solvent
standard in water
standard in solvent + partition coefficient
measurement endpoint
total concentration
dissolved concentration
dissolved concentration (or fugacity)
from an aqueous sample (13). This is difficult to achieve for equilibrium extractions of hydrophobic organic substances in porewater, because it would require unpractical large sediment sample volumes of several liters. The nondepletive criterion was therefore extended in this study: the sorption capacity of the sampling phase is kept well below the sorption capacity of the entire matrix. Porewater concentrations might then be temporally depleted, whereas final equilibrium porewater concentrations remain unaffected. Equilibrium Extraction. Kinetic SPME measurements are feasible on aqueous samples, because extraction kinetics from water can be sufficiently defined and controlled (13, 15). Extraction kinetics in sediment are more difficult to define and also more difficult to control, because they involve diffusion, turbulent mixing, and desorption from matrix constituents (16, 17). However, the extraction reaches eventually its well-defined equilibrium, at which the fugacity (or chemical potential) in the fiber coating is the same as in the matrix (18). This is the reason for the potential of matrixSPME to sense fugacities as well as dissolved concentrations, and it is the reason for the long extraction times of matrixSPME. These long extraction times call for parallel rather than serial sampling, which in turn requires a large number of SPME fibers. Further, the direct contact with the matrix can cause wear of the polymer coating (11), which calls for disposable usage. Disposable usage of commercial SPME devices would be very costly (>50 U.S. $ per extraction), and we therefore applied technical grade optical fibers, that normally are used in cables for data transmission (about 0.1 U.S. $ per sample). Determining Dissolved Porewater Concentrations. Nonpolar hydrophobic organic substances diffuse into the PDMS coating, where they are retained by absorption (19-22). The uptake of dissolved analytes into the PDMS coating is thus a real bulk to bulk partitioning process, and the equilibrium is consequently described by the partition coefficient KPDMS,water. Dissolved porewater concentrations (cporewater,dissolved) can therefore be derived from the measured concentration in the PDMS coating (cPDMS) and the appropriate partition coefficient KPDMS,water (20):
cporewater,dissolved ) cPDMS/KPDMS,water
(1)
Table 1 summarizes the characteristics of matrix-SPME in comparison to two other extraction approaches.
Materials and Methods SPME Fiber, Chemicals, and Solvents. Fiberguide Industries (Stirling, NJ) supplied PDMS coated glass fibers with a core diameter of 200 µm and a coating thickness of 15 µm (15 µm PDMS fiber). The volume of the polymer coating was calculated to be 10.1 µL PDMS per meter of fiber. We selected PDMS as sampling phase, because it is thermally stable and a suitable extraction phase for a wide range of hydrophobic analytes (15). We selected the 15 µm PDMS fiber, that normally is used for data transmission, due its thin coating 5178
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and its low price (1.2 U.S. $ per meter). The fiber was cut into 100-mm pieces, and these were washed twice in analytical grade methanol (10 min) and twice in Millipore grade water. The following hydrophobic analytes were obtained from different suppliers in analytical grade at a purity of at least 98%: hexachlorobenzene, lindane, 1,2,3,5,-tetrachlorobenzene, pentachlorobenzene, 1,3,5-tribromobenzene, p,p′DDE, phenanthrene, fluoranthene, benzo[a]pyrene, 2,2′,5,5′PCB (PCB-52), 2,3,5,6-PCB (PCB-65), 2,2′,4,5,5′-PCB (PCB101), 2,3,3′,4,4′-PCB (PCB-105), 2,3,3′,5,6-PCB (PCB-112), 2,3′,4,4′,5-PCB(PCB-118),2,2′,3,4,4′,5′-PCB(PCB-138),2,2′,4,4′,5,5′-PCB (PCB-153), 2,2′,4,4′,5,6′-PCB (PCB-154), 2,2′,4,4′,6,6′-PCB (PCB-155), 2,3,3′,4,4′,5-PCB (PCB-156), 2,3,3′,5,5′,6-PCB (PCB165), and 2,2′,3,4,4′,5,5′-PCB (PCB-180) (PCB numbering according to IUPAC). Spiked Sediment. We used sieved (500 µm mesh) freshwater sediment from Oostvaardersplassen (OVP, Netherlands) with an organic carbon content of 2.7%. This sediment has been used in several studies because of its relatively low contamination level (e.g. refs 23-25). The sediment was suspended in water (7:5, Vsediment:Vwater) and then spiked with an acetone solution that contained 21 organic pollutants including 9 PCBs, hexachlorobenzene, and the DDT metabolite p,p′-DDE. The nominal contamination level of individual PCBs and p,p′-DDE was 20-30 µg/kg dry sediment. The sediment was then poisoned with NaN3 and HgCl2 in order to inhibit biodegradation, and it was finally stored at 10 °C for 2 years. Field Sediment. Freshwater sediment was provided by the Dutch Institute for Inland Water Management and Wastewater Treatment. This sediment from lake Ketelmeer (Netherlands) was sampled November 27, 1996 (sampling coordinates X ) 181.194; Y ) 513.087, upper 0-30 cm), sieved (500 µm mesh), intensively mechanically homogenized, and then stored at 10 °C. The organic carbon content was determined by the National Institute for Coastal and Marine Management (Middelburg, The Netherlands) at 3.75% as described in ref 26. The partitioning of PBPs within this sediment has recently been studied (23, 27). SPME Extraction. Fifteen milliliter vials with silicone/ PTFE septum caps were prefilled with a sediment sample. The sediment sample was sized to have a sorption capacity of at least 50 times the sorption capacity of the extraction phase. This was in practice achieved by using sediment sample volumes containing at least 50 mg of organic carbon (VPDMS‚KPDMS,water , mOC‚KOC,water, V ) volume, K ) partition coefficient, m ) mass). The 100-mm fibers were inserted into the vial through a syringe needle that pierced the septum. The needle was immediately withdrawn, and the position of the fiber was adjusted so that 70 mm of the fiber remained in the vial during the entire extraction. The extraction was terminated by retracting the fiber through the septum. Gas Chromatography (GC). Within 20 s after completion of the sampling the fibers were introduced into a splitless 1078 Universal Capillary Injector of a Varian 3600 CX or 3400 CX (Varian, Palo Alto, CA) gas chromatograph for thermal
desorption. The fibers entered the injector through a needle that pierced the injector septum. The upper part of the fiber was sealed and positioned with a piece of septum, so that analytes from the lower 56 mm (( 2 mm) of the fiber () 0.57 µL PDMS) were directed to the column (according to the construction drawing of the injector), whereas analytes from the middle part of the fiber were vented by the septum purge. The injector was programmed to increase from 50 °C to 250 °C at a rate of 150 °C per minute to accomplish the thermal desorption process from the fiber, which was facilitated using a narrow bore insert liner. Fifteen min after sample introduction, the injector was programmed to cool to its initial temperature and to return to the split mode. The desorbed chemicals were focused at 50 °C for 15 min on a 30 m × 0.32 mm fused silica DB 5.625 column (J&W Scientific, Folson, CA) with a 0.25 µm film thickness. Subsequently, the oven temperature increased with a rate of 30 °C/min to 100 °C, 5 °C/min to 175 °C, and 10 °C/min to 300 °C where it was maintained for 5 min. Helium was used as the carrier gas. Detection was based on electron capture detection except for the final field sediment measurements that were based on mass chromatography. The 63Ni electron capture detector (ECD) was operated at 350 °C using N2 as the makeup gas. The mass spectrometer was operated in the full-scan mode (100-400 m/z), using a Varian Saturn 2000 ion trap (multiplier voltage 1600 V, AGC target value of 15 000). Analytes were quantitated using the three most abundant m/z ratios, except for PAHs, where only the molecular ion was used. Solvent standards were used to calibrate the analyte amounts injected into the GC. By comparing the area counts of the syringe and the fiber injection, we determined the amount that was contained in the thermally desorbed part of the fiber. Cleaned fibers served as blanks, and no detectable amount of target analytes was found on the cleaned fibers.
Feasibility Experiments Three feasibility experiments were conducted with increasing degree of difficulty. The first experiment was designed to demonstrate the working principle of matrix-SPME in the absence of matrix interference. An octadecyl Empore disk was applied to control the fugacity of an aqueous solution, in analogy to sediment organic matter that controls the fugacity of sediment porewater. The second experiment was performed with sediment that was spiked with known analytes in order to ease the chromatographic analysis. The third and final experiment was conducted on field sediment, which meant a lower contamination level of unknown analytes. Empore Disk Experiment. Dissolved aqueous concentrations of five halogenated aromatics were established by partitioning from a C18 Empore disk as described by Mayer and co-workers (28). During the experiment, SPME extractions “consumed” the dissolved concentrations that were “refilled” from the Empore disk. This and the sediment experiments were carried out with closed vials that were agitated on a rotary shaker at about 200 rpm with an orbit of 3 mm. A 25-mm C18 Empore disk with about 50 µL of octadecyl (C18) was contaminated with hexachlorobenzene, lindane, 1,2,3,5-tetrachlorobenzene, pentachlorobenzene, and 1,3,5tribromobenzene at about 1 µmol/L C18 as described in ref 28. A 40-mL vial was filled with 30 mL of Millipore grade water, the contaminated disk was added, and the vial was closed with a silicone/PTFE septum cap. This vial was agitated overnight, and stable aqueous concentrations were established according to Cwater ) CC18/KC18,water (28). Agitated SPME extractions were conducted for different exposure times while avoiding any direct contact with the Empore disk. 1,2,3,5tetrachlorobenzene was not detectable in this experiment due to an interference caused by the bleeding of the PDMS
coating at the desorption temperature of 275 °C. This bleeding was substantially reduced at a desorption temperature of 250 °C, which was used in all subsequent experiments. Spiked Sediment Experiment. Fifteen milliliter vials were filled with about 10 mL of sediment, which was quite fluid due to its high water content of about 80% (w/w). Three cleaned fibers were inserted through the septum of each vial. These vials were stored at 25 °C under either static conditions or they were shaken (200 rpm, 3-mm orbit). SPME extractions lasted for up to 14 days in the agitated sediment and for up to 28 days in the static sediment. The fibers were then withdrawn through the silicone/PTFE septum that removed any visible sediment traces from the fiber, and they were immediately analyzed. We limited our analysis to analytes (1) that were spiked to the sediment, (2) that we could measure with the ECD detector, and (3) for which KPDMS,water values were available. Hexachlorobenzene was not quantitated because its peaks exceeded the linear range of the detector. Uptake profiles were generated by fitting a first-order onecompartment uptake model with the rate constant k to the ECD peak areas (A):
A(t) ) Asteady state‚(1 - e-k‚t)
(2)
This model is not necessarily a precise reflection of the uptake process into the fiber. However, it is expected sufficient for the fitting of experimental data in order to estimate the extraction time to reach 90% of the equilibrium concentration in the PDMS (t90% ) ln(10)/k). Field Sediment Experiment. Matrix SPME was applied to field sediment in order to investigate the feasibility of the approach on a real world sample. This sediment had a water content of 50-60% (w/w), and it was thus less fluid than the spiked sediment/water suspension. Triplicate SPME extractions of up to 7 weeks were conducted as described for the spiked sediment. Additionally, 8-week extractions were performed, and fibers were analyzed by means of GC-MS in order to identify and quantitate analyte concentrations in the PDMS fiber coating. Some fibers broke during the manual injection. Therefore only duplicate measurements were reported for the time points of the uptake profile. The coelution of PCB congeners with identical molecular mass cannot be excluded. However, this is a general issue concerning gas chromatography of PCBs, whereas it is considered of less importance for the scope of the present study, and it will thus not be further addressed. We expected PAH contamination in the field sediments and included therefore phenanthrene, fluoranthene, and benzo[a]pyrene in the five-point calibration. Quantitation of Dissolved Concentration. The quantitation of dissolved concentrations requires two steps: (1) the quantitation of analytes in the PDMS as described in the previous paragraph and (2) the subsequent calculation using PDMS water partition coefficients. These partition coefficients were carefully determined for the 15 µm PDMS fiber and 17 aromatic hydrocarbons, which are presented in another paper (20). These experimental partition coefficients formed the basis for the linear regression of log KPDMS,water against log KOW (20):
log KPDMS,water ) 1.00‚log KOW - 0.91 (log KOW: 4.5-7.5, n ) 17, r 2 ) 0.99, s ) 0.10) The aqueous concentration in the Empore disk experiment was established and controlled by partitioning from a C18 Empore disk. Dissolved aqueous concentrations can thus also be determined using C18 water partition coefficients (KC18,water): VOL. 34, NO. 24, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Empore disk experiment: Uptake into the 15 µm PDMS coating from agitated water. Aqueous concentrations were maintained during the extraction by partitioning from a C18 Empore disk. Extractions were done in singlet except for the triplicate measurement after 24 h (( SD). The ECD detector was operated in the constant frequency setting.
FIGURE 2. Empore disk experiment that combines “partitioning driven administering” and “solid-phase microextraction”. Aqueous concentrations were established and controlled by partitioning from a C18 Empore disk, and concentrations in the PDMS coating were established by equilibrium solid-phase microextraction.
Cwater,dissolved ) CC18/KC18,water
(3)
Such partition coefficients have been reported in the literature (28, 29), and they form the basis for the linear regression of log KC18,water against log KOW (29):
log KC18,water ) 1.00‚log KOW + 0.70 (log KOW: 2.5-5.4, n ) 17, r 2 ) 0.93, s ) 0.27) The regression for log KPDMS,water and for log KC18,water have both a slope of 1.00, and they are thus parallel. The constant offset of 1.6 log units suggests equilibrium concentrations in the PDMS to be approximately factor 40 lower than in the C18.
Results Empore Disk Experiment. This experiment can be looked at as a SPME extraction from a constant aqueous concentration, and this extraction reached its equilibrium within a few 5180
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FIGURE 3. Spiked sediment experiment: ECD response of agitated extractions plotted against contact time. Averages of triplicate measurements were fitted to a first-order one compartment model, and estimated k values (day-1) are presented in parentheses. The ECD detector was operated in the constant current setting. Relative standard deviations were on average 15% at 1 day, 4% at 4 days, and 3% at 14 days.
FIGURE 4. Spiked sediment experiment: Agitated and static SPME extractions. Bars represent average areas ( SE for triplicate extractions within one vial. Log KPDMS,water from ref 20 are presented in brackets. hours as shown in Figure 1. Alternatively, the experiment can be looked at as a three-phase system. Figure 2 illustrates and summarizes the partitioning among the three phases: Empore disk, water, and PDMS. An almost equimolar mixture within the reservoir phase (0.73-0.89 µmol/L C18) resulted in a wide range of low concentrations in the conducting medium (estimated: 0.42-20 pmol/L water) and again a narrow concentration range in the sensing phase (0.0180.037 µmol/L PDMS). Equilibrium concentrations in the PDMS were 1.3-1.6 log units below concentrations in the C18, which is close to the prediction that were based on the regressions for KPDMS,water and KC18,water. Spiked Sediment Experiment. Samples of the spiked sediment were extracted with SPME fibers (triplicate) under agitated or static conditions. GC areas from agitated extractions were plotted against the extraction time in Figure 3, and these areas were fitted to the one compartment model. Rate constants ranged between 2.4 day-1 and 0.22 day-1, which lead to steady-state times (t90%) of 1-11 days. Extraction kinetics in the static sediment were slower than in the agitated sediment. ECD areas from the 28-day static extraction and from the 14-day agitated extraction are compared in Figure 4. GC areas from the static extraction were lower for analytes
TABLE 2: Analyte Amounts in the PDMS Coating (nPDMS) as Determined by GC-MS and Dissolved Porewater Concentrations (cwater) as Calculated with KPDMS,water Values from Ref 20a Ketelmeer
FIGURE 5. Spiked sediment experiment: Relative standard deviations between triplicate fibers plotted against the progress of extraction. Relative standard deviation were based on peak areas, and the progress of extraction was calculated as the extracted amount relative to the estimated steady state.
fluoranthene pentachlorobenzene PCB #52 PCB #65 PCB #101 PCB #118 p,p′-DDE PCB #138 PCB #153 PCB #180
nPDMS [pg]
KPDMS,water [L/L]
169 ( 5 6.0 ( 0.6
1.8‚104 1.9‚104
16.4 ng/L 0.57 ng/L
193 ( 7 37.6 ( 1.8 75.3 ( 2.5 44.3 ( 1.6 23.1 ( 2.2 31.0 ( 4.7 123 ( 7 13.0 ( 0.9
2.4‚105 2.2‚105 5.1‚105 7.4‚105 7.6‚105 1.6‚106 1.4‚106 2.5‚106
1.42 ng/L 0.30 ng/L 0.26 ng/L 106 pg/L 54 pg/L 38 pg/L 138 pg/L 9.1 pg/L
a Analyte amounts are presented as average ( SD of triplicate extractions.
steady-state times (t90%) to range from 30 to 6 days. Relative standard deviations of field sediment extractions were on average 8% at the beginning of the extraction (day 1 and 3), and they were on average 4% at (near) equilibrium (day 23 and 51). Measured analyte amounts on the SPME fiber, literature KPDMS,water values (20), and calculated dissolved concentrations are presented in Table 2. Dissolved porewater concentrations ranged between 16.4 ng/L for the least hydrophobic fluoranthene and 9.1 pg/L for the most hydrophobic PCB #180.
Discussion
FIGURE 6. Field sediment experiment: ECD response of agitated extractions plotted against contact time. Duplicate measurements were fitted to a first-order one compartment model, and rate constants (day-1) are presented in parentheses. The ECD detector was operated in the constant frequency setting. with KPDMS,water > 106 (one tailed t-test, p < 0.05), whereas they were similar for analytes with KPDMS,water < 106 (one tailed t-test, p > 0.05). The data presented in Figures 3 and 4 suggest that (1) 14-day agitated extractions were sufficient to reach (near) equilibrium, that (2) 28-day static extractions were insufficient to reach equilibrium for the most hydrophobic analytes, and (3) that the moderate agitation did not significantly alter the sorption properties of the sediment. One-day and 4-day extractions of agitated sediment were terminated and analyzed at the same day. Relative standard deviations of GC peak areas were in the 5% range for steadystate extractions, while they were substantially higher in the kinetic phase of the extraction (see Figure 5). Field Sediment Experiment. Extraction profiles from the Ketelmeer sediment were obtained for seven chlorinated target analytes (see Figure 6). Rate constants ranged from 0.078 day-1 for the most hydrophobic PCB #180 to 0.38 day-1 for the least hydrophobic hexachlorobenzene, suggesting
Matrix SPME was developed to measure dissolved concentrations in dense environmental matrices, and we applied it in the present study to sediment. There is no reference method for such measurements, which could be used for an external validation of the approach. Results of three feasibility experiments have therefore been presented, and their implications for the performance of matrix-SPME will subsequently be discussed. Extraction Kinetics and Sampling Times. Equilibrium extractions required a few hours in the Empore disk experiment, up to 11 days in the spiked sediment experiment, and up to 30 days in the field sediment experiment. The substantially faster extraction kinetics in water compared to sediment were supposedly due to the less hydrophobic analytes and the lower viscosity of water. The faster extraction kinetics in the spiked sediment relative to the field sediment were presumably due to its higher water content making it less viscous. Extraction time curves are essential in order to identify the time required for an equilibrium extraction. Matrix-SPME relies on the “refilling” of the aqueous concentration by desorption from the sediment. Nevertheless, this desorption is not expected to be the rate limiting step for the extraction, because the sediment has a substantially higher sorption capacity and a high exchange area relative to the fiber coating. Instead diffusion through the aqueous boundary layer near the fiber coating is considered the rate-limiting step, as reported for the diffusive uptake of hydrophobic organics from water (20, 30, 31). Equilibration times do then increase with (1) increasing hydrophobicity of the substance, with (2) increasing thickness of the unstirred boundary layer, and with (3) a decreasing surface-to-volume ratio (30). (1) The target analytes of our study are highly hydrophobic. (2) The turbulence level in the sediment was low and the viscosity high, which will result in relatively thick unstirred boundary layers. (3) The 15 µm PDMS coating has a high surface-to-volume ratio of 7.1‚104 m2/m3. This means that the observed extraction kinetics were the result of “slow” VOL. 34, NO. 24, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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analytes, that were extracted from a “slow” matrix, but with a “fast” sampling phase. Extraction times of days to weeks require stable porewater concentrations, and matrix-SPME measurements of highly dynamic systems should therefore be evaluated with caution. However, sediment porewater concentrations can generally be expected rather constant, because they either are (near) equilibrium concentrations, or because they are stabilized steady-state concentrations resulting from different processes such as desorption, biodegradation and evaporation. Measurement Endpoint and Calibration. Stable concentrations in the PDMS indicated equilibrium between the coating and the porewater, and it also indicated the return of porewater concentrations to their initial level. The chemical potential or the fugacity in the coating is then by definition the same as in the sediment porewater. We applied KPDMS,water values that have been determined in distilled water to derive the “concentration in distilled water at the final fugacity of the sediment porewater”. Water serves in that manner as a reference phase, and it also serves as a link to effect and exposure assessments that are traditionally based on aqueous concentrations. The obtained dissolved concentrations are expected to be very valuable within the context of bioavailability, because only dissolved molecules generally cross biological membranes and because the fugacity of the sediment porewater controls the equilibrium partitioning into a sediment organism. Nevertheless, the dissolved concentrations should not be called “bioavailable concentrations” because biological uptake can involve more than diffusive uptake and equilibrium partitioning. An external calibration series of hexane solutions was used to calibrate the GC operation in order to determine analyte concentrations in the PDMS. This was straightforward for the applied GC settings, because the entire analyte amount that reached the injector also was directed to the detector. The partitioning into the fiber coating was calibrated with PDMS water partition coefficients (eq 1). The accuracy of such a calibration depends on how accurate the partition coefficients were determined and how accurately they describe the partitioning during the extraction (15). Partition coefficients were determined with an accurate methodology that minimizes the influence of experimental artifacts; and they were obtained for the same batch of fiber and at the same temperature as for the extraction (20). The obtained precision was highly satisfying especially for manually operated extractions with disposable fibers. Relative standard deviations for equilibrium extractions within one vial were generally well below 10 % between fibers. We expect the main source for these deviations to be due to differences in the thermally desorbed length of the fiber, and the precision might thus be improved when desorbing whole fiber pieces. Sensitivity and Selectivity of Matrix SPME. Concentrations of pg/L to ng/L were measured in relatively small sediment samples with matrix-SPME. Matrix SPME has the potential to measure low concentrations (ng to pg/L) in small samples because of three reasons. (1) A substantial concentration factor is built into matrix-SPME, because dissolved aqueous concentrations are concentrated by the factor KPDMS,water into a PDMS extract. (2) Desorption from the sediment matrix “refills” the porewater concentration. (3) The entire analyte amount on the lower part of the fiber is introduced to the GC and directed to the detector. The following model calculation can give an indication of potential detection limits (Cdet water)
Cdet water )
Cdet PDMS ndetection ) KPDMS,water VPDMS‚KPDMS,water
(4)
where ndetection denotes the required analyte amount for the 5182
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production of a detector signal and VPDMS denotes the volume of the polymer coating. Let us assume a sampling phase of 1 µL of PDMS and a conservative detector requirement of 10 pg. The required concentration in the PDMS (cPDMS) is then generally 10 µg/L, whereas the required dissolved aqueous concentration is hydrophobicity dependent. Such estimated detection limits are 550 pg/L for the least hydrophobic fluoranthene and 4 pg/L for the most hydrophobic PCB-180. Actual detection limits can be affected by interference. Interference by elemental sulfur was observed during the GC-ECD measurement of the spiked and poisoned OVP sediment, and it was not observed during field sediment measurements. A range of approaches has been developed to deal with sulfur contamination of liquid extracts (32), and further research is required to identify a suitable approach for PDMS extracts. Substantial bleeding from the PDMS coating was observed with ECD detection at a desorption temperature of 275 °C, and moderate bleeding was observed with MS detection at a desorption temperature of 250 °C. These bleeding problems are less for commercially available SPME devices with PDMS coatings that can withstand temperatures higher than 300 °C. Suppliers of chromatographic products are encouraged to provide such fibers at a reasonable price, as this would enhance the applicability and performance of matrix-SPME. Matrix-SPME aims at measuring trace concentrations within environmental matrices that always will contain interfering substances. This calls for a selective extraction, a selective chromatographic separation, and a specific detection. Equilibrium SPME extractions are in fact much more selective than exhaustive extractions and also more selective than kinetic extractions, because they take full advantage of differences in the coating to matrix distribution coefficients to separate hydrophobic target analytes from interferences (15). The hydrophobic target analytes were concentrated by up to 2.5 million times into the fiber, whereas most interferences will have much lower partition coefficients. The selectivity of the chromatographic separation and the specificity of the detection were both less than optimal, and this leaves room to improve the overall performance of matrix-SPME. General Features and Future Developments. There are a number of other implications of this new approach. The extraction of pollutants from sediment and into a polymeric coating took place in closed vials. This minimized the transfer of potentially toxic substances out of the sample (occupational exposure), and it minimized the transfer of substances from the laboratory into the sample (sample contamination). Further, the use of organic solvents was substantially less than for conventional extraction methods. Finally, the actual hands on time of the approach was much shorter than for most conventional methods, and matrixSPME should therefore be suitable for large scale studies with many samples as well as for routine measurements. The manual sample introduction into the GC injector was the most time-consuming step in the present study, and a thermal desorption autosampler might thus be employed in order to increase productivity. The method can easily be modified for particular applications. However, optimizing the method for a certain criterion should be done with care, because it can change the performance with regard to other criteria. The sensitivity of the method can for instance be improved with thicker polymer coatings. However, thicker coatings would also slow the extraction and thus require longer sampling times. Parameters that can be adjusted are for instance (1) coating thickness, (2) agitation method, and (3) detector type and setting. This study was directed at the determination of dissolved porewater concentrations in sediment samples, which are
difficult to measure with conventional methods. Two other applications of matrix-SPME are expected to have merit for environmental monitoring and environmental risk assessment. (1) Matrix-SPME is expected suitable for in situ sampling in different environmental media, because it is independent of sample size and factors that might affect extraction kinetics. This is due to matrix-SPME being a nondepletive equilibrium extraction. Such equilibrium sampling with matrix-SPME would be a supplement to the sampling with semipermeable membrane devices, which due to their slow uptake kinetics typically are limited to kinetic sampling (31, 33). (2) Matrix-SPME can potentially also be applied to sense fugacities, because analytes at equilibrium have the same fugacity in the fiber coating as in the surrounding media (34). Such fugacity measurements are valuable, because fugacities (rather than concentrations) determine the direction and the extent of diffusive mass transport within our heterogeneous multimedia environment (35-38).
Acknowledgments We would like to thank Enaut Urrestarazu, Agnes Oomen, Leon van der Wal, Dick Sijm, Willem Seinen, and Linda McPhee for fruitful discussions and helpful comments. Thanks are also given to Bea A. Vrind and Gerard Cornelissen for supplying the sediment and to Alex Heikens and Theo Sinnige for technical assistance. We acknowledge the funding of a pilot study by the RIKZ, and P.M. acknowledges the financial support from the Danish Research Academy.
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Received for review April 14, 2000. Revised manuscript received September 12, 2000. Accepted September 20, 2000. ES001179G
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