Anal. Chem. 2007, 79, 2885-2891
Analysis of Freely Dissolved Alcohol Ethoxylate Homologues in Various Seawater Matrixes Using Solid-Phase Microextraction Steven T. J. Droge,* Theo L. Sinnige, and Joop L. M. Hermens
Institute for Risk Assessment Sciences, Utrecht University, Yalelaan 2, 3584 CL Utrecht, the Netherlands
Solid-phase microextraction fibers (SPME) were tested as tools to determine freely dissolved alcohol ethoxylate (AE) surfactants in seawater matrixes. Partitioning of a wide range of AE homologues into a 35-µm polyacrylate fiber coating was linearly related to aqueous concentrations as low as submicrograms per liter, with high reproducibility. The exposure time needed to reach equilibrium between aqueous phase and the SPME fiber depended on the fiber-water partitioning coefficient (Kfw) of the AE homologue. Specific attention was given to the influence of various matrixes on the analysis via SPME. The presence of sediment increases the uptake kinetics of AE homologues for which diffusion in the aqueous phase is rate limiting. The Kfw in equilibrated systems was not affected by the presence of other homologues, micelles, or varying amounts of sediment phase. SPME is therefore a suitable tool for analysis of AE in sorption studies and sediment toxicity tests. A strong linear relation was observed between Kfw and the hydrophobicity of the AE homologue, using estimated octanol-water partition coefficients. This relation can be used to predict the partitioning coefficient of any AE homologue to the SPME fiber, which facilitates the analysis of complex mixtures. Alcohol ethoxylates (AE) are nonionic surfactants used worldwide in large quantities, mainly for laundry cleaning products. Commercial products are complex mixtures of AE homologues with alkyl chain lengths usually ranging between 10 and 16 carbon atoms and a polar chain of usually 1-20 ethoxylate (EO) units. The alkyl chain of AE is generally linear, but can consist up to 50% of monobranched (2-alkyl-substituted) alkyl chains, and ∼10% of the total AE production has highly branched alkyl chains.1 The long alkyl chains make most AE relatively hydrophobic, and as a consequence, once in the environment they tend to accumulate in the sediment phase. Coastal sediments close to industrialized Spanish areas all contained AE with total concentrations ranging between 37 and 1300 µg/kg.2 A surface sediment from the interior of the Bay of Cadiz (southwest Spain) showed a AE distribution * Corresponding author. Tel: (+31)30 2535314. Fax: (+31)30 2535077. E-mail:
[email protected]. (1) Marcomini, A.; Zanette, M.; Pojana, G.; Suter, M. J.-F. Environ. Toxicol. Chem. 2000, 19, 549-554. (2) Petrovic, M.; Ferna´ndez-Alba, A. R.; Borrull, F.; Marce, R. M.; Gonza´lezMazo, E.; Barcelo´, D. Environ. Toxicol. Chem. 2002, 21, 37-46. 10.1021/ac0620260 CCC: $37.00 Published on Web 03/06/2007
© 2007 American Chemical Society
with enrichment of the longer alkyl chain AE, with total concentration of 35 µg/kg for the C12-ethoxymers, up to 282 µg/kg for C18-ethoxymers, including branched homologues.3 Although detailed studies on effluent concentrations in North America and Europe were recently published,4,5 data on AE concentrations in freshwater systems is very limited.6 The sediment is the main environmental compartment where alcohol ethoxylates accumulate. From a risk assessment point of view, it is important to develop proper sampling techniques for analyzing concentrations of ethoxylates in sediment pore water, because this compartment is generally considered to be the main route of exposure to sediment dwelling organisms. Obtaining reliable aqueous concentrations (Caq) in sediment phases is difficult though. When centrifugation of sediment samples is used to collect pore water, the supernatant may still contain nonseparable particulate matter (NSM) and dissolved organic matter (DOM) with a substantial amount of AE bound to it.7 As hydrophobic chemicals have a high affinity for DOM, and clay minerals can be strong sorbents for polar or ionized organic molecules, this appears to be a relevant problem when determining truly aqueous concentrations of AE from sediment suspensions or other matrixes. The freely dissolved concentration in a sediment-water suspension can also be determined indirectly via passive samplers,8-11 which do not require separation of the water and sediment phase. Solid-phase microextraction (SPME) is a sampling technique based on passive diffusion of freely dissolved chemicals from the water phase into a thin polymer coating around a piece of glass fiber. The affinity of a chemical for the polymer (3) Lara-Martin, P. A.; Go´mez-Parra, A.; Gonza´lez-Mazo, E. Int. J. Environ. Anal. Chem. 2005, 85, 293-303. (4) Wind, T.; Stephenson, R. J.; Eadsforth, C. V.; Sherren, A.; Toy, R. Ecotoxicol. Environ. Saf. 2006, 64, 42-60. (5) Eadsforth, C. V.; Sherren, A. J.; Selby, M. A.; Toy, R.; Eckhoff, W. S.; McAvoy, D. C.; Matthijs, E. Ecotoxicol. Environ. Saf. 2006, 64, 14-29. (6) Dyer, S. C.; Sanderson, H.; Waite, S.; Van Compernolle, R.; Price, B. P.; Nielsen, A.; Evans, A.; DeCarvalho, A. J.; Hooton, D.; Sherren, A. Environ. Monit. Assess. 2006, 120, 45-63. (7) Schrap, M. S.; Opperhuizen, A. Chemosphere 1992, 24, 1259-1282. (8) Mayer, P.; Vaes, W. H. J.; Wijnker, F.; Legierse, K. C. H. M.; Kraaij, R. H.; Tolls, J.; Hermens, J. L. M. Environ. Sci. Technol. 2000, 34, 5177-5183. (9) Leppanen, M. T.; Kukkonen, J. V. K. Aquat. Toxicol. 2000, 49, 227-241. (10) Verbruggen, E. M. J.; Van Loon, W. M. G. M.; Tonkes, M.; Van Duijn, P.; Seinen, W.; Hermens, J. L. M. Environ. Sci. Technol. 1999, 33, 801-806. (11) Jonker, M. T. O.; Koelmans, A. A. Environ. Sci. Technol. 2001, 35, 37423748.
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phase depends mainly on the hydrophobic character of the molecule, resulting in a unique polymer-water partitioning coefficient (Kfw) for each chemical. With a well-established Kfw, the freely dissolved concentration in the suspension can be derived directly from the concentration in the polymer phase. The extraction-phase volume for SPME is small compared to the sample volume. Consequently, in most cases, a negligible fraction from the aqueous phase will be extracted. In pure aqueous solutions, the depletion of the aqueous phase is determined by the fiber-water partition coefficient and the volume ratio fiber coating versus the volume of the aqueous phase. In sediment suspensions, negligible depletion will also be obtained if the volume of sediment is high enough to balance the loss of chemical from the aqueous phase due to uptake into the fiber. In those cases, the depletion will often be completely negligible, because the fiber volume is often much lower than the volume of the sediment. The concentration in the aqueous phase can then simply be calculated from the concentration in the fiber and the fiberwater partition coefficient. This specific tool to measure truly dissolved concentrations is therefore also called negligible depletion SPME (nd-SPME).8,12 Furthermore, equilibrium is reached relatively fast compared to other passive samplers since the thickness of polymer coating is typically in the range of 7-100 µm. Although partitioning kinetics depends strongly on the compound’s molecular properties and agitation of the test solution, several studies indicated that uptake kinetics of a contaminant from the aqueous phase to the SPME fiber can also be influenced by matrix effects.13-16 Particularly when tests are performed under nonequilibrium conditions, such phenomena may influence the outcome of SPME analysis. Since matrix effects often involve increased kinetics, for some compounds they could shorten the necessary equilibration times. Although well validated for many nonpolar organics (e.g., refs 8, and 17-19), SPME has only once been applied to a single alcohol ethoxylate in solution.20 In that study, SPME was part of an on-line detection system. Several studies using SPME have been performed on nonylphenol ethoxylates (NPEO). However, these studies include either only the shortest ethoxylates21,22 or the complex mixtures.23 The aim of this study was to test the ability of the SPME method to analyze the freely dissolved concentration of individual nonbranched AE homologues in seawater and marine sediment(12) Vaes, W. H. J.; Urrestarazu Ramos, E.; Verhaar, H. J. M.; Seinen, W.; Hermens, J. L. M. Anal. Chem. 1996, 68, 4463-4467. (13) Heringa, M. B.; Hogenvonder, C.; Busser, F.; Hermens, J. L. M. J. Chromatogr., B 2006, 834, 35-41. (14) Kopinke, F. D.; Georgi, A.; Mackenzie, K. Acta Hydrochim. Hydrobiol. 2001, 28, 385-399. (15) Oomen, A. G.; Mayer, P.; Tolls, J. Anal. Chem. 2000, 72, 2802-2808. (16) Mayer, P.; Karlson, U.; Christensen, P. S.; Johnsen, A. R.; Trapp, S. Environ. Sci. Technol. 2005, 39, 6123-6129. (17) Ter Laak, T. L.; Durjava, M.; Struijs, J.; Hermens, J. L. M. Environ. Sci. Technol. 2005, 39, 3736-3742. (18) Vaes, W. H. J.; Hamwijk, C.; Urrestarazu Ramos, E.; Verhaar, H. J. M.; Hermens, J. L. M. Anal. Chem. 1996, 68, 4458-4462. (19) Van der Wal, L.; Jager, T.; Fleuren, R. H. L. J.; Barendregt, A.; Sinnige, T. L.; Van Gestel, C. A. M.; Hermens, J. L. M. Environ. Sci. Technol. 2004, 38, 4842-4848. (20) Aranda, R.; Burk, R. C. J. Chromatogr., A 1998, 829, 401-406. (21) Dı´az, A.; Ventura, F.; Galceran, M. T. J. Chromatogr., A 2002, 963, 159167. (22) Dı´az, A.; Ventura, F.; Galceran, M. T. Anal. Chem. 2002, 74, 3869-3876. (23) Boyd-Boland, A. A.; Pawliszyn, J. B. Anal. Chem. 1996, 68, 1521-1529.
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water systems via nd-SPME. The optimization of the method included first the selection of an appropriate polymer phase. Subsequently, equilibration times of several AE were determined in seawater via kinetic studies. Testing a set of individual alcohol ethoxylates, differing in either ethoxylate chain length or alkyl chain lengths, allowed us to examine the relationship between the partition coefficient to the SPME and the hydrophobicity of the compounds. The calibration of the SPME relative to the dissolved concentration was extended for several alcohol ethoxylate homologues to aqueous concentrations above the critical micelle concentration (cmc), to determine how measurements of freely dissolved analytes are affected in the presence of micelles. Further investigations on possible matrix effects on the SPME method included experiments with simple and more complex mixtures of pure homologues and studying the effects of the presence of varying amounts of a sediment phase, and therewith suspended particles and dissolved organic matter. MATERIALS AND METHODS Experimental Design. A small set of pure alcohol ethoxylate homologues were used as model compounds for the development of the SPME method. The first goal of this study was to determine which polymer phase could best be used as a partioning-based extraction phase. Because we would like to apply the technique to environmental samples as well as in laboratory studies, we are in particular interested in a technique that can cover a broad range of concentrations. Moreover, because we want to avoid the possibility of competition effects in case of application to mixtures, the sampling method should be based on absorption. After examination of the time needed to reach equilibrium for one of the compounds, C14EO8, between three different types of custommade SPME fiber materials, the fibers were then exposed to a broad range of concentrations of C14EO8 in seawater for a sufficiently long equilibration period. Based on these findings, one of the SPME fibers was selected to study the uptake kinetics of several other homologues, C10EO8, C12EO8, C14EO4, C14EO11, and C16EO8. With the optimized equilibration periods, measurements obtained for individual solutions of C10EO8, C12EO8, and C14EO8 were then compared to SPME extractions from mixtures of these three AE. Since commercial AE mixtures often contain up to hundreds of different AE homologues, we also examined the seawater-fiber partitioning coefficients of a complex mixture of AE, which consisted of C12 homologues with a range between 2 and 18 EO units. To test the influence of different seawater matrixes on the analysis of freely dissolved individual AE with SPME, we determined (i) isotherms covering concentrations both below as above cmc for C14EO8, C12EO8, and C14EO11; (ii) uptake kinetics for C14EO8 in sediment solutions; and (iii) isotherms for C10EO8, C12EO8, and C14EO8 in sediment slurries. Chemicals, SPME Fibers, and Sediment. The individual poly(ethylene glycol) alkyl ethers C10EO8, C12EO8, C14EO4, C14EO6, C14EO8, C14EO11, and C16EO8 and the AE mixture of C12ethoxymers (C12EO∼9) were purchased at Fluka Chemie GmbH (Buchs, Germany) and were of at least 98% purity (TLC) or higher. The actual molar EO distribution pattern of C12EO∼9 was determined experimentally using several pure homologues as reference standards. Except for KBr (Sigma-Aldrich, Zwijndrecht, The Netherlands), the salts used for the preparation of artificial
seawater (GP224) were from Merck (Amsterdam, The Netherlands). The biocide NaN3 (Merck, Darmstadt, Germany) was added to the seawater at 100 mg/L, formaldehyde (37% solution without methanol) was from Fluka. Methanol was always HPLC quality (Labscan, Dublin, Ireland), and highly pure deionized water (R g 18 MΩ) was prepared by a water purification system (Millipore Waters, Amsterdam, The Netherlands). SPME fibers with an internal diameter of 110 µm and either a 28.5-µm poly(dimethylsiloxane) (PDMS) coating (volume 12.4 µL/m), 35-µm polyacrylate (PA) coating (15.4 µL/m), or a 7-µm PA coating (2.6 µL/m) were purchased from Polymicro Technologies (Phoenix, AZ), cut to the desired length (generally 2.0-6.0 cm, depending on the expected partitioning coefficient) and further used as received. An off-shore North Sea sediment (north of the “Friese Front” area) was collected using a box corer, and an estuarine sediment was collected at a mud flat in the Eastern Scheldt (“Oesterput” location) by wet sieving over 500 µm. Organic carbon contents (NA 1500 NCS elemental analyzer, Fisons) were 0.27 and 1.2% of the dry weight, respectively, and the fraction of particles of 90% of the nominal concentration (data not shown). Minimizing the air-water interface (and probably also contact time of this phase with the glass wall) to only a small air bubble strongly reduced this problem to a minimum. Test vials were therefore always filled up as much as possible. To dissolve AE from the glass wall and obtain steady concentrations, the test vials were kept for 48 h on a shaking device (Rock ‘n’ roller, Snijders Scientific B.V., Tilburg, The Netherlands), before introducing SPME fibers. After taking out exposed SPME fibers, they were blotted dry on a tissue, quickly wiped along a wetted tissue, and put in HPLC vials with PTFE septum caps (Bester BV, Amsterdam, The Netherlands). The AE was extracted from the polymer phase in 0.5-1 mL of methanol and left at least 24 h at -20 °C before analysis. Negligible amounts of AE were found in repeated fiber extracts. SPME fibers were discarded after use. To isolate the AE from seawater samples, SPE tubes (500 mg of C18 Supelclean ENVI from Supelco) were used that were activated by passing 5 mL of methanol followed by 5 mL of pure water. After elution of 10 mL of the aqueous sample, the columns were flushed with 10 mL of pure water to remove the excessive amount of salt and taken to dryness by applying vacuum for a several seconds. AE was eluted with 8 mL of methanol, which was collected in glass vials and kept at -20 °C until analysis. The recovery of the SPE procedure was tested in triplicate for all AE by spiking a weighed amount of methanol stock (
