Anal. Chem. 1997, 69, 597-600
Solid Phase Microextraction for Determining the Distribution of Chemicals in Aqueous Matrices Juergen Poerschmann,*,‡ Zhouyao Zhang,† Frank-Dieter Kopinke,‡ and Janusz Pawliszyn*,†
Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, and Center for Environmental Research, Permoserstrasse 15, D-04318 Leipzig, Germany
Solid phase microextraction (SPME) can be applied to measure the distribution of chemicals in different speciation states. The standard SPME apparatus and procedures can be applied to measure free concentrations of a target compound spiked into pure water and spiked into water containing another pseudophase, such as dissolved humic organic matter (HOM). Based on a comparison of results obtained for the two samples, the partitioning of the target analyte between water and the pseudophase is calculated. The samples investigated in the present study were from heavily contaminated coal wastewater containing dissolved organic polymers with properties similar to those of HOM. External calibration provides information about freely dissolved analytes. After calibration of the SPME signal by addition of an internal standard (e.g., a deuterated surrogate), the results indicate the total concentration of the target analyte in the sample due to the identical partitioning of the internal standard. The concentration determined in this way coincides well with data obtained from liquid-liquid extraction (LLE). Both methods, SPME with internal calibration and LLE, measure total concentration, composed of a freely dissolved portion and a portion that is reversibly bound onto a pseudophase (HOM). SPME can be applied to measure the distribution of an analyte in a matrix of two or more components, as proposed in refs 1 and 2. Measuring the distribution in a structured matrix is necessary for studying chemical pathways in diverse fields such as environmental processes, biological metabolism, food quality, and industrial processes. In many cases, a distribution can favorably be described by means of partition coefficients between the matrix components,3-6 such as the KDOM,m value, which describes the partition of a compound between water and dissolved organic matter (DOM). In this paper, SPME is used to measure partitioning of a compound between water and dissolved organic polymers with properties similar to those of humic organic matter (HOM). The partitioning of pollutants between water and HOM is based on †
University of Waterloo. Center for Environmental Research. (1) Pawliszyn, J. J. High Resolut. Chromatogr. 1993, 16, 565. (2) Kopinke, F.-D.; Po ¨rschmann, J.; Remmler, M. Naturwissenschaften 1995, 82, 28. (3) Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241. (4) Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Environ. Sci. Technol. 1983, 17, 227. (5) Young, T. M.; Weber, W. J. Environ. Sci. Technol. 1995, 29, 92. (6) Kile, D. E.; Chiou, C. T.; Zhou, H.; Li, H.; Xu, O. Environ. Sci. Technol. 1995, 29, 1401. ‡
S0003-2700(96)00978-X CCC: $14.00
© 1997 American Chemical Society
reversible interactions, which may be specific or hydrophobic in nature. Hydrophobic partitioning depends primarily on pollutant hydrophobicity and the properties of hydrophobic domains in the sorbent (see ref 7 and references cited therein). “Hydrophobic interaction” can be considered as a combination of van der Waals binding forces and a substantial entropic term which drives the hydrophobic molecules out of the water phase.8,9 To measure HOM-water partitioning, it has been necessary up to now to use different methods depending on whether dissolved or particulate HOM was under investigation. Obviously, when investigating particulate HOM, a physical separation of one phase from the matrix by centrifugation is a relatively simple procedure. These methods, whether measuring the freely dissolved or reversibly bound concentration of the pollutant, should not disturb the sorption equilibrium and should be adaptable to multicomponent determination. The commonly used methods, including fluorescence quenching,10 dialysis,11 headspace partitioning,12 RP (C18)-HPLC,13 and fast solid phase extraction (SPE),14 more or less fail to meet all these requirements individually. For example, the method based on fluorescence quenching, although distinguished by outstanding sensitivity and zero impact on the sorption equilibrium, is restricted to a single fluorophor and hindered by experimental difficulties such as innerfilter effects or simultaneous occurrence of static and dynamic quenching.15 The chromatographic approaches (such as RP-HPLC and SPE) do not have fast enough response times to monitor kinetics of sorption or desorption. Moreover, the removal of the freely dissolved analyte fraction is a severe disturbance of the original sorption equilibrium. The partitioning kinetics is another important issue. Interaction of nonpolar organic analytes with dissolved polymers is considered to be fast because physisorption as the normal process does not have high activation energies.16,17 These processes have not been extensively investigated with dissolved HOM.18 In (7) Kopinke, F.-D.; Poerschmann, J.; Stottmeister, U. Environ. Sci. Technol. 1995, 29, 941. (8) Schlautman, M. A.; Morgan, J. J. Environ. Sci. Technol. 1993, 27, 961. (9) Weber, W. J.; McGinley, P. M.; Katz, L. E. Water Res. 1991, 25, 499. (10) Gauthier, T. D.; Seitz, W. R.; Grant, C. L. Environ. Sci. Technol. 1987, 21, 243. (11) Mc Carthy, J. F.; Jimenez, B. D. Environ. Sci. Technol. 1985, 19, 1067. (12) Jota, M. A. T.; Hassett, J. P. Environ. Toxicol. Chem. 1991, 10, 483. (13) Maxin, Ch.; Ko ¨gel-Knaber, I. Eur. J. Soil Sci. 1995, 46, 193. (14) Landrum, P. R.; Nilhart, S. R.; Eadie, B. J.; Gardner, W. Environ. Sci. Technol. 1994, 18, 187. (15) Kumke, M. U.; Lo ¨hmannsro ¨ben, H.-G.; Roch, Th. Analyst 1994, 119, 997.3. (16) Pignatello, J. J.; Ferrandino, F. J.; Huang, L. Q. Environ. Sci. Technol. 1993, 27, 1573. (17) Brusseau, M. L.; Larsen, T.; Christensen, T. H. Water Resour. Res. 1991, 27, 1137. (18) Hasset, J. P.; Milicri, E. Environ. Sci. Technol. 1985, 19, 638.
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contrast, investigation of sorption and desorption kinetics on particulate HOM has been a subject of intense research.19,20 In this paper, SPME is also applied to analyze the kinetics of sorption on dissolved HOM. Rapid extraction from a stirred HOM-water sample can monitor concentration changes of target analytes with respect to time for short time intervals. In the present study, samples are taken from a coal wastewater from a pond of about 2 × 106 m3 with a dissolved HOM concentration of about 350 mg/L, about two-thirds fulvic and onethird humic acids.21 The SPME method is developed for two classes of the wastewater pollutants, phenols and polycyclic aromatic hydrocarbons (PAHs). Conventional and headspace SPME are discussed for determining concentrations of organic pollutants in water rich in HOM. SPME is compared with LLE, and from the results pollutant partition coefficients could be calculated. THEORY In a sample with several components (true phases or pseudophases, e.g., colloidally dissolved HOM) i ) 1, 2, 3, ..., the nominal concentration of a target analyte (m) in a component (i) can be written
nm
Ci,m ) Vi +
∑V K j
(1) i,j,m
j*i
where nm is the total amount of analyte in the sample, Ki,j,m is its partition coefficient between the components i and j (Ki,j,m ) Ci,m/ Cj,m), and Vi is the volume of component i. Volume can be replaced by mass or surface area if a sample component is better expressed in those terms, with Ki,j,m in appropriate units. Suppose a sample consists of water and one other pseudophase, such as dissolved organic matter (DOM). With the fiber not present, the free concentration of the analyte (m) in water (w) at equilibrium would be
Cw,m )
nm Vw + mDOMKDOM,m
(2)
assuming the analyte concentration in headspace is neglibible, where Vw is the volume of the water phase, mDOM is the weight of DOM, nm is in grams, and KDOM,m is the analyte’s partition coefficient between DOM and water, usually defined as KDOM,m ) Cm in DOM/Cm in water and expressed in mL/g, if Cm in DOM is in g/g and Cm in water is in g/mL. With the fiber present, the free concentration at equilibrium would be
Cw,m′ )
nm Vw + mDOMKDOM,m + VFKF,m
(3)
where VF is the fiber volume and KF,m (dimensionless, [g/mL]/ [g/mL]) is the fiber-water partition coefficient. Thus, the ratio of the free concentration of the target analyte before SPME to that after SPME is (19) Ball, W. P.; Roberts, P. V. Environ. Sci. Technol. 1991, 25, 1223; 1237. (20) Grathwohl, P.; Reinhard, M. Environ. Sci. Technol. 1993, 27, 2360. (21) Wienhold, K.; Hanschmann, G. Chem. Technol. 1991, 43, 232.
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Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
VFKF,m Cw,m )1+ Cw,m′ Vw + mDOMKDOM,m
(4)
For the free concentration of the target analyte in water to change less than 10%, the criterion is
VFKF,m 1 - 1 ) 0.111 < Vw + mDOMKDOM,m 0.9
(5)
If the value of the sorption term mDOMKDOM,m is unknown, then this criterion is certain to be satisfied if
KF,m
0.0061KF,m for a fiber with a 100 µm thick coating, Vw > 0.00026KF,m for a 7 µm thick coating. The value of KF,m can be determined by performing SPME on samples of deionized pure water spiked with analyte standards. If sample volume is 40 mL, the KF,m of the analyte (m) for a 7 µm fiber must not be greater than 155 500 in order to avoid a more than 10% disturbance of the partitioning equilibrium by the fiber uptake. According to data published recently in ref 22, this condition is met even for highly hydrophobic PAHs. Thus, SPME can extract analyte without significantly changing its concentration in the water of a DOM-water sample with sufficient volume of water and, hence, can determine the concentration of analytes freely dissolved in water in a DOM-water sample. Equation 2 can be rearranged to eq 7 such that the relationship between the concentrations of freely dissolved analyte, reversibly sorbed analyte, and its total concentration is explicitly expressed:
Ctotal,m ) Cw,m[1 + KDOM,m(mDOM/Vw)]
(7)
assuming the volume of the sample equals Vw. On the basis of eq 7, the total concentration of an analyte, Ctotal,m, in a sample, the sum of the freely dissolved and the reversibly bound portions, can be determined by SPME using isotopically labeled internal standards (st):
GCm Ctotal,m ) Ctotal,st GCst
(8)
where GCm and GCst are the GC responses of the nonlabeled analyte and the labeled standard, respectively, produced by SPME. The total concentration of the deuterated standard is a known value. Nondeuterated analytes and deuterated surrogates assume to have identical partition coefficients. Another prerequisite is that the partitioning equilibrium establishes rapidly, in the time between spiking and beginning SPME extraction. Alternatively, total concentrations can be determined by other methods, such as exhaustive LLE. In contrast to the internal calibration of SPME by means of deuterated surrogates, the conventional external calibration using spiked water samples with no DOM gives the freely dissolved (22) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1996, 68, 144.
concentration. The knowledge of an analyte’s total concentration (either from internal calibration or from LLE), the concentration of its freely dissolved fraction, and the concentration of DOM allows calculation of KDOM, the analyte’s partition coefficient between water and DOM. This technique to measure analyte partition coefficients between two components of a sample can be extended to many other systems, such as air and air particulate, or even air and water. The method uses simple and commercially available apparatus and is cost and time efficient, and SPME has been shown to be sensitive for many analytes in air and water. EXPERIMENTAL SECTION Coal wastewater was stored under nitrogen at 5 °C to minimize microbial processes. Humic and fulvic acids were isolated from the coal wastewater by the method explained in ref 21. Humic substance was isolated from sediment on the bottom of the pond by extracting with 1 N NaOH according to the guidelines of the International Humic Substances Society.23 Alkaline extract is over 90% humic acids, and the rest is fulvic, for all sediment samples we have tested. Naphthalene, fluorene, anthracene, and pyrene (>99% purity) were obtained from Promochem (Wesel, Germany). Deuterated naphthalene, acenaphthene, phenanthrene, chrysene, and perylene (>99% purity) were obtained from Supelco (Munich, Germany). Analytical reagent grade solvents were purchased from Merck (Darmstadt, Germany). SPME syringe and fibers with 7 and 100 µm PDMS coatings were purchased from Supelco. Fibers were conditioned at 270 °C under helium overnight prior to use. For LLE and SPME, all glassware was thoroughly washed with acetone and silanized prior to use. The wastewater was diluted 1:1 with deionized water before extraction to minimize formation of emulsions. Deuterated standards were spiked into the solvent (benzene, toluene, methyl isobutyl ketone, or chloroform) instead of the wastewater to avoid losses due to irreversible binding to HOM.22 For the internal calibration of SPME, deuterated internal standards were spiked to the diluted wastewater to give 10 ppm phenols (phenol-d5, o-cresol-d7, 2,4-dimethylphenol-d3) and 10 ppb PAHs (naphthalene-d8 through chrysene-d12) each. LLE was carried out by vigorously shaking 25 mL of diluted wastewater with 25 mL of solvent for 20 min. The ratio of solvent to diluted wastewater was 1:1, to give a high extraction efficiency and to break emulsions. Extracts were concentrated by rotary evaporation at 40-75 °C, depending on solvent, to about 200 µL. For conventional SPME, aqueous samples filled almost completely 5 mL amber vials and were agitated with a custom-made stir bar at 1000 rpm. For headspace SPME, 25 mL aqueous samples were placed in 40 mL amber vials capped with PTFEcoated septa (Pierce, Rockford, IL) and were agitated with a 14 mm diameter star-head stir bar (Fisher Scientific, Ottawa, ON, Canada) at 1000 rpm. SPME quantification was by internal deuterated standards or by external calibration using spiked deionized water. Extraction time for external calibration with deionized water was 1 h. The concentration range tested was 1-100 ppm for phenols and 0.1-25 ppb for PAHs. SPME was also used to trace sorption kinetics. To study partitioning between water and humic materials without interferences from suspended mineral particulates, solutions of DOM (23) Kuwatsuka, S.; Watanabe, A.; Itoh, K.; Arai, S. Sci. Plant Nutr. 1992, 38, 23-30.
Table 1. Liquid-Liquid Extraction of Phenols and PAHs with Toluene from a Wastewater (Wastewater 1:1 Diluted, Water:Toluene ) 1:1 in Each Step, Data as ppm for Phenols and ppb for PAHs)
phenol o-cresol m/p-cresol 3,4-dimethylphenol naphthalene fluorene phenanthrene anthracene fluoranthene pyrene
extraction 1
extraction 2
ratio of yields
77.6 5.6 31.1 1.6 25.2 7.6 12.2 3.4 4.9 5.2
21.0 0.4 3.3 0.1 0.5 0.2 0.1
0.26 0.07 0.10 0.06 ∼0.02 ∼0.02 ∼0.01
were made with deionized water and the isolated HOM. A solution of 100 ppm DOM was spiked to give 5 ppb each of naphthalene, fluorene, anthracene, and pyrene and then allowed to equilibrate overnight. Of this, 5 mL was transferred into a vial and stirred at over 1500 rpm. A 100 µm PDMS fiber was dipped into the solution for 10 s and then exposed to desorb for 10 s in a GC injector connected to a 3 m × 0.1 mm fused silica capillary deactivated with phenyl methyl polysiloxane (Hewlett-Packard, Palo Alto, CA) at 270 °C for rapid GC/MS analysis. After deuterated PAHs were spiked, the fiber was exposed for 10 s and then analyzed by rapid GC/MS, 20 times in 15 min, to monitor ion traces of deuterated and nonlabeled PAHs. This experiment was done with solutions of fulvic acid from the wastewater, humic acid from the wastewater sediment, and diluted wastewater with its native PAHs. A 5971B GC/MS (Hewlett Packard) and a Saturn I GC/ITMS (Varian, Mississaugua, ON, Canada) were used with 30 m × 0.25 mm × 0.25 µm columns, HP-5 (Hewlett-Packard) and SPB-5 (Supelco), respectively. GC injection was at 300 °C for the 7 µm fiber, or 280 °C for the 100 µm fiber and syringe injections; oven 40 °C for 3 min, 10 °C/min to 290 °C, hold 15 min. HP 5971B data acquisition was in SIM mode. RESULTS AND DISCUSSION Table 1 gives the results of consecutive LLE extractions using toluene. The results (not presented here) were similar for benzene, chloroform, and methyl isobutyl ketone. A third extraction step (not included in Table 1) detected only 5 ppm phenol, but no more PAHs. Changing wastewater pH in the range 2-13 did not significantly affect LLE efficiency of PAHs using any of the solvents. Dissolved HOM has a stretched-out structure at high alkalinity and a coiled structure at high acidity, but the portion of pollutants accessible to LLE did not change in these experiments, indicating that all reversibly bound analytes were removed. Table 2 gives concentrations determined by SPME with internal and external calibration and by a calculation based on eq 7. Data are averages of five replicates. Relative standard deviations (RSDs) were not determined for SPME of heavy PAHs because of fiber alterations caused by the dissolved HOM, discussed below. Total concentrations determined by SPME using internal calibration with deuterated surrogates (column 3) agree reasonably well with those determined by exhaustive LLE (column 2). SPME was found, however, to have a higher sensitivity than LLE. Concentrations of the freely dissolved Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
599
Table 2. Concentrations of Pollutants in a Wastewater Determined by LLE and SPME and Calculated According to Eq 7 (Data as ppm for Phenols and ppb for PAHs)
pollutant
SPME with SPME (%RSD) LLE (%RSD) internal with external log KDOM from Table 1 standards calcd calibrationa (refs 2, 7)
phenol 104.0 (8.6) o-cresol 6.0 (7.4) m/p-cresol 34.5 (7.2) naphthalene 25.7 (3.8) fluorene 7.8 (4.0) phenanthrene 12.3 (6.1) anthracene 3.4 (6.3) fluoranthene 4.9 (12.4) pyrene 5.2 (10.3) chrysene 2.4 (16.1)
94.0 6.0 b 27.5 7.3 b 3.5 b 4.9 1.8
97.3 6.2 35.9 26.1 7.7 8.9 3.0 4.1 4.4 1.3
97.0 (5.3) 6.2 (3.7) 35.6 (3.8) 21.6 (5.9) 3.4 (6.6 2.1 0.55 0.40 0.35 0.2
0.88 1.35 1.35 2.79 3.58 3.98 4.11 4.44 4.53
a This column gives concentrations of freely dissolved pollutants, in contrast to columns 2 and 3, which give total concentrations. b Deuterated surrogate was not available.
fraction of analytes were determined by SPME with external calibration (column 5), as outlined in the Theory section. For calculating total concentrations from those of freely dissolved fractions according to eq 7, we estimated KDOM values from results in previous papers on the same type of HOM.24 An analyte’s KDOM, usually not available, can be roughly estimated from known correlations between it and the octanol-water partition coefficient (Kow),7 e.g., KDOM ≈ 0.25Kow. Kow values of many compounds are tabulated in the literature.25-27 The calculated total concentrations (column 4 in Table 2) agree well with the other two estimates of total concentration, considering that log KDOM values are supposed to have an uncertainty of (0.2. Figure 1 shows results of the kinetic experiment for naphthalene and anthracene in a solution of 100 ppm humic acid from sediment. Sorption equilibrium is established very rapidly, within less than 1 min. The data presented in Figure 1 do not rule out, (24) Po¨rschmann, J.; Stottmeister, U. Chromatographia 1993, 36, 207. (25) Herbert, B. J.; Dorsey, J. G. Anal. Chem. 1995, 67, 744. (26) Kamlet, M. J.; Doherty, R. M.; Abraham, M. H.; Marcus, Y.; Taft, R. W. J. Phys. Chem. 1988, 92, 5244. (27) Daylight Chemical Information Systems. Daylight Software: CLOGP-4.34; Irvine, CA 1994. (28) Zhang, Z.; Poerschmann, J.; Pawliszyn, J. Anal. Commun. 1996, 33, 219. (29) Zhang, Zh.; Pawliszyn, J. J. High Resolut. Chromatogr. 1993, 16, 689.
600 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
Figure 1. Ratio of GC signals for deuterated and nondueterated compounds produced by short SPME extractions at several times after spiking deuterated standards into a contaminated HOM-water sample. This plot shows kinetics of the sorption of naphthalene and anthracene onto 100 ppm DOM measured by SPME. (, 136/128; 9, 188/178.
however, that there may be a second sorption process with a much lower rate. Some experimental difficulties were observed, caused by fiber adsorption of hydrophobic materials from a sample. These nonvolatile polymers are partially decomposed during the desorption step in the GC injector and give rise to fouling of the PDMS fiber. The problems could be solved by enclosing the fiber in a hollow fiber membrane28 or by extracting with the fiber from the headspace over the sample.29 Both techniques avoid direct contact between the DOM and the fiber. SPME minimizes disturbance of the natural matrix, requires only a small volume of sample, and has a fast response time. SPME can sample matrices directly in their natural environment. SPME is applicable to many compounds, uses commercially available apparatus, can be coupled to various conventional instruments including mass spectrometers, and can be applied to any matrix including soils. ACKNOWLEDGMENT This work was supported by the Natural Sciences and Engineering Research Council of Canada, the Deutsche Forschungsgemeinschaft of Germany, Supelco, and Varian. Received for review September 24, 1996. November 8, 1996.X
Accepted
AC9609788 X
Abstract published in Advance ACS Abstracts, January 1, 1997.
