Article pubs.acs.org/est
Can SPME Fiber and Tenax Methods Predict the Bioavailability of Biotransformed Insecticides? Amanda D. Harwood,† Peter F. Landrum,† and Michael J. Lydy*,† †
Fisheries and Illinois Aquaculture Center, Department of Zoology, Southern Illinois University, Carbondale, Illinois 62901, United States S Supporting Information *
ABSTRACT: Recent studies recognize the ability of chemical techniques such as solid phase microextraction (SPME) fibers and Tenax extraction to predict bioavailability more effectively than exhaustive chemical extractions for sediment-associated organic contaminants. While the majority of research using these techniques studied legacy compounds such as PCBs and PAHs, there is great potential for these methods to work well for highly toxic, rapidly biotransformed compounds such as pyrethroid insecticides. The current study compared the ability of the two techniques to predict the bioavailability of permethrin and bifenthrin to two benthic invertebrates (Lumbriculus variegatus and Hexagenia sp.). In addition, variations in the application of the two techniques, specifically duration and conditions of exposure of the SPME fibers and duration of extraction with Tenax, were explored. The SPME fiber concentrations correlated strongly to both 6 and 24 h Tenax concentrations. The SPME fiber concentrations and 6 h and 24 h Tenax extractable concentrations correlated with both the parent permethrin and bifenthrin concentrations in the tissues of both species at steady state. Parent compound tissue concentrations for both species could be predicted with a single relationship for individual pyrethroids. This demonstrated the potential value of these methods to predict the bioavailability of compounds subject to biotransformation and application to multiple species.
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INTRODUCTION Predicting sediment toxicity of hydrophobic organics using sediment concentrations obtained by exhaustive chemical extraction is impractical due to variations in bioavailability among sediments resulting from differences in the composition of sediments and the resultant variation in the binding of organic compounds to sediment constituents. The modern approach to predict bioavailability is to use chemical techniques, which take bioavailability into consideration. Chemical techniques that work well at predicting the bioavailability and toxicity of hydrophobic organics such as polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and some insecticides in sediments, include using solid phase microextraction (SPME) fibers and Tenax extraction.1−16 The SPME fibers measure the chemical activity in sediment and provide estimates of the freely dissolved pore water concentrations, whereas Tenax measures the amount of chemical which desorbs from the sediments. So, SPME fiber and Tenax concentrations represent the bioavailable and bioaccessible fractions, respectively.17,18 The ability of these methods to accurately predict bioavailability has been well documented and was reviewed in You et al.11 These techniques are also effective using different variations in the application of the methods. For example, SPME fibers can either be coexposed in test chambers with animals (e.g., refs 5, 7, and 14) or separately exposed on a shaker table,8,16 which © 2012 American Chemical Society
provides agitation theoretically decreasing the time to equilibrium.1,15 In addition, single-point Tenax extractions are representative of the readily bioaccessible contaminant and are typically either 6 or 24 h in duration.19 Both extraction times have proven effective predictors of the bioavailability of hydrophobic organics.7,10,12,18,20,21 This is likely because both extraction periods are proportional to the rapidly desorbing fraction, which is considered the bioavailable fraction.19 While SPMEs and Tenax have been directly compared, 7,9 a simultaneous comparison of the two methods including variations in the methodologies has not yet been conducted. Additionally, the majority of past studies were conducted using compounds or species for which biotransformation is expected to be minimal. When bioavailability-based methods are used to predict contaminant concentrations for biotransformed compounds, the estimates can overpredict tissue residues of parent compounds.12 Therefore, it remains unknown if these techniques would be useful for predicting bioavailability and toxicity for highly toxic, rapidly biotransformed compounds with specific modes of action such as pyrethroid insecticides. Pyrethroid insecticides are used in high Received: Revised: Accepted: Published: 2413
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allow binding of the pesticides to the sediments. Sediments were then distributed into eight replicate 600 mL beakers (∼150 g dw), four 20 mL scintillation vials (∼15 g dw), and eight test tubes. In permethrin experiments, 20 mL tests tubes with ∼0.2 g dw sediment were used, whereas in bifenthrin experiments 50 mL tests tubes with ∼2 g dw sediment were used. Bifenthrin experiments were conducted on a larger scale to ensure quantifiable concentrations on the Tenax. Additional control replicates were prepared to ensure that no contamination existed and ensure animal survival over the testing period. All experimental units were filled with overlying MHW. To each replicate beaker and scintillation vial, SPME fibers were added (20 or 40 cm for permethrin and bifenthrin, respectively). Fibers were contained in stainless steel packets (10 cm per packet). More fiber was added to bifenthrin sediments than to permethrin treatments to ensure measurable concentrations on the fiber. To each replicate beaker either 15 adult L. variegatus or five 15 mm Hexagenia sp. were added. Beakers were placed at 23 °C for 28 d with three automated 100 mL water changes daily. Water quality parameters including dissolved oxygen, temperature, pH, and conductivity were monitored daily using a Yellow Springs Instrument 55 (Yellow Springs, OH, USA) water quality meter and an Oakton Instruments (Vernon Hills, IL, USA) conductivity/pH meter. Scintillation vials were placed on a shaker Table (100 rpm) for 28 days. At the start of the bioassay, the 6 and 24 h Tenax extractions were conducted as described below. At the completion of the bioassay, fibers and animals were processed as described below. Sediment concentrations were quantified at the start and completion of the bioassay. Sediment (∼0.1 g) was placed in a 20 mL scintillation vial with 10 mL of scintillation cocktail and vortexed for 1 min. The extraction efficiency of this method was 95 ± 7% and 97 ± 8% for permethrin and bifenthrin, respectively. Degradation of the compounds in sediments at the completion of the bioassay was measured with methods similar to the biotransformation estimates (see below). All samples, including those described below, were stored in darkness for 24 h prior to LSC analysis to prevent quenching and to allow for additional extraction of the compounds into the cocktail. SPME Fiber Extractions. Fibers were removed from the sediment, rinsed with MHW, and patted dry. The compounds were extracted from SPME fibers by placing the fibers in of 1 mL hexane per 10 fibers and allowing the compound to desorb from the fiber a minimum of 36 h at 4 °C. Then 5 mL of scintillation cocktail was added, and the samples were analyzed using LSC. Tenax Extractions. To ensure measurable concentrations in the bifenthrin studies, the test concentrations were conducted with a proportional increase of sediment and Tenax. Sediment and MHW were added to the test tubes. Tenax (0.05 and 0.5 g for permethrin and bifenthrin, respectively) were added to each replicate. Tubes were then rotated for 6 or 24 h. At the end of the allotted time, the tubes were centrifuged, and Tenax was removed and placed in a 20 mL vial with 5 mL of acetone. The Tenax/solvent mixture was then sonicated for 10 min, after which the extract was removed and replaced with 5 mL of an acetone:hexane (1:1 vol/vol) solution and sonicated an additional 10 min. Extracts were combined, and the Tenax was washed and sonicated a third time with 5 mL of acetone:hexane. The combined extracts were then evaporated to 2 mL, an additional 5 mL of scintillation cocktail was added, and samples were analyzed via LSC.
volumes, and environmental residues in sediments have been detected at concentrations lethal to invertebrates in California, Illinois, and Texas.23−28 Thus, developing an accurate tool to predict their bioavailability in sediments would be a valuable asset for environmental assessments. While the ability of SPME fibers and Tenax to predict bioavailability of pyrethroids has been studied, neither a comparison of techniques nor the issue of biotransformation has been thoroughly addressed. The current study simultaneously compared the ability of the two techniques to predict the bioavailability of two pyrethroids, permethrin and bifenthrin, to Lumbriculus variegatus and Hexagenia sp. The SPME fiber concentrations were determined, while coexposed with each species, as well as using a shaker table system with a concentration series of spiked sediment. The Tenax extractable concentration was determined at 6 and 24 h. The concentration derived using each method was correlated to parent concentrations in the tissue of the animals to provide a direct comparison of these techniques.
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EXPERIMENTAL SECTION Chemicals. Permethrin and bifenthrin were purchased from Moravek Biochemicals (14C-labeled, specific activity 260 mCi/ mmol, Brea, CA, USA) and Institution of Isotopes (14C-labeled, specific activity 41.97 mCi/mmol, Budapest, Hungary), respectively. The purity (≥98%) of the radiolabeled compounds was determined using a Packard TriCarb 2900TR liquid scintillation counter (LSC) (Packard Instrument Company, Meriden, CT, USA) after separating the parent compounds from the degradation products with an Agilent 1100 high-pressure liquid chromatograph (HPLC) (Agilent Technologies, Palo Alto, CA, USA) using methods similar to Harwood et al.29 Solvents including acetone, hexane, and acetonitrile were all pesticide grade (Fisher Scientific, Pittsburgh, PA, USA). The SPME fibers were coated with 10 μm of polydimethylsiloxane (PDMS) with a phase volume of 0.069 μL per cm of fiber (Fiberguide Industries, Sterling, NJ, USA). During testing, fibers were stored in 105 μm stainless steel mesh screen to protect them from damage (You et al. 2006). Tenax TA (60/80 mesh) was purchased from Scientific Instrument Services (Ringoes, NJ, USA). Scintillation cocktail (ScintiSafe 50%) was purchased from Fisher Scientific. Animals. The burrowing mayfly, Hexagenia sp., was obtained from existing cultures at Southern Illinois University (Carbondale, IL, USA). Hexagenia were collected as eggs from adults emerging from western Lake Erie. Eggs were hatched at Southern Illinois University and cultured in reference sediment (Touch of Nature, Carbondale, IL, USA) with overlying U.S. EPA moderately hard water (MHW).30 Upon hatching Hexagenia were raised in 40 L aquaria containing approximately 5−7 cm of reference sediment with overlying MHW. Tanks received biweekly 50% water changes and a mixture of equal parts yeast, alfalfa, and trout chow was added three times weekly (10 mL of 15 g/L). Adult L. variegatus were also obtained from existing cultures at Southern Illinois University (Carbondale, IL, USA). Experimental Design. Reference sediment from Bay Creek (Pope County, IL, 1.42 ± 0.05% total organic carbon) was spiked at 1.5, 3.0, 6.0, and 10 ng/g permethrin or 0.5, 1.0, 2.0, and 4.0 ng/g bifenthrin dry weight (dw). These concentrations were chosen as they represent sublethal concentrations for both species for 28 d exposures which was determined in preliminary experiments. Sediments were homogenized for 4 h and stored (aged) 14 d at 4 °C to 2414
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pyrethroids.29,33 The degree, rate, and products of biotransformation are compound and species dependent; therefore, predicting biotransformation would be difficult. Furthermore, standards developed for the management of pyrethroids and other hydrophobic compounds are based on the parent form and this would be the form measured in environmental samples. For these reasons, it is critical to consider the impact of biotransformation and the amount of parent compound when using chemical techniques to predict bioavailability. So when making assessments of bioavailability using tissue residues, if possible, the parent form should be used, particularly if it is the toxic form. Effect of Method Variation. Exposure method had no significant effect on fiber concentrations (Figure 1). The lack of
Tissue Concentrations. Organisms were removed from the sediment, rinsed with MHW, weighed to the nearest 0.1 mg, and stored at −20 °C prior to tissue analysis. Lumbriculus were depurated in clean water for 6 h to remove gut contents prior to freezing.31 Guts were manually removed from Hexagenia sp. Subsamples of the organisms were collected for whole body tissue analysis, and the remaining organisms were pooled for biotransformation estimates. For whole body concentrations, approximately 0.5 mL of tissue solublizer was added, and tissues were stored in solublizer at 4 °C overnight. The animals were then sonicated for 60 s in 10 mL of scintillation cocktail (Tekmar sonic disruptor, Cincinnati, OH, USA). Lipids were quantified in both species using the quantification method described by Lu.32 To measure the proportion parent, the remaining pooled animals were analyzed to determine the ratio of parent to metabolite using methods described in Harwood et al.29 Briefly, the animals were homogenized using a glass tissue homogenizer, and the extract separated by fraction collection on the HPLC using known retention times of the parent compound. Extraction efficiency for this method was 85−93% in the extract with the remainder of the compound on the pellet. The system mass balance, which included extract plus the pellet, ranged from 86 to 110%. Statistics. All relationships between bioavailability-based measurements (SPME fiber or Tenax extractable concentrations) and tissue concentrations were made using linear regression. Concentrations on the fiber among treatments were compared using one-way ANOVA. All statistics were conducted using SAS Software (SAS Institute 2000, Cary, NC, USA). Significance was set at p ≤ 0.05.
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RESULTS AND DISCUSSION Quality Control. No mortality was observed in any of the bioaccumulation assays. Water quality was maintained within US EPA30 acceptable limits (temperature 23 ± 0.2 °C, conductivity 370 ± 40 μS, dissolved oxygen 6.8 ± 1.2 mg/L, pH 6.85 ± 0.15). Concentrations on control SPMEs, Tenax, and in tissues were below reporting limits. Sediment concentrations did not vary among exposure treatments and did not significantly decrease over the 28 d testing period. The average percent lipids were 1.12 ± 0.235% and 1.53 ± 0.255 for Hexagenia and Lumbriculus, respectively. Use of Parent Compound in Analysis. It is important to note that all comparisons and data analysis in the current study used parent compound. This was done for several reasons. First, since the concentration in the sediment was ≥96% parent at the completion of the experiment, the form of the compound being measured by the chemical techniques was largely parent. Due to the minimal degradation products in the sediment and low concentrations on the SPME fibers and Tenax, the concentrations derived using these methods were assumed to be parent compound. Additionally, in the case of pyrethroids, the parent form is the toxic form and, therefore, the most relevant to use. Therefore, measurement of parent compound is necessary in order for bioavailability-based techniques to be proportional to the toxicologically relevant tissue concentration. In the current study, both compounds were extensively biotransformed by both species. At the conclusion of the 28 d bioassay, the remaining parent permethrin was 7.02 ± 0.56% and 34.4 ± 8.71%, and the remaining bifenthrin was 33.6 ± 9.50% and 66.3 ± 10.9% in L. variegatus and Hexagenia sp., respectively. This was expected as these species as well as other invertebrates have been shown to extensively biotransform
Figure 1. Concentrations on the SPME fibers (ng/mL) exposed under different conditions, coexposed with either Lumbriculus variegatus or Hexagenia sp. and on a shaker table, at each sediment concentration (ng/g) for permethrin and bifenthrin. Error bars equal one standard deviation (n = 4). 2415
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extracted by the Tenax is proportional to the rapidly desorbing fraction, it will be proportional to bioavailability,34 making extraction time inconsequential. Relationship between SPME Fiber and Tenax Extractable Concentrations. There was a significant (p < 0.001) linear relationship between SPME fiber concentrations and the 6 and 24 h Tenax extractable concentration for both permethrin (r2 = 0.99 and r2 = 0.99 for 6 and 24 h, respectively) and bifenthrin (r2 = 0.93 and r2 = 0.97 for 6 and 24 h, respectively) (Figure 3). Since a single-point Tenax
difference among exposure treatments indicates the concentrations derived using a shaker table should provide the same SPME fiber concentration as those coexposed with the animals at equilibrium. It is important to note, however, that this would not be expected for nonequilibrium conditions, and nondepletion conditions must be maintained in all systems. If the nondepletion requirement is not maintained it may alter SPME fiber concentrations.1 In the current study, the maximum depletion was 0.19% and 1.4% for permethrin and bifenthrin, respectively, satisfying the nondepletion requirement. The proportion of depletion was calculated using the total amount of chemical removed from the sediment matrix, including depletion from fibers and animals, divided by the total amount of chemical in the experimental chamber. There was a strong significant (p < 0.001) linear relationship between the 6 and 24 Tenax extractable concentration across compounds (Figure 2). This implies that either extraction
Figure 2. Relationship between 6 and 24 h Tenax extractable concentrations (ng/g OC) for permethrin (closed symbols) and bifenthrin (open symbols). See Table S1. Error bars equal one standard deviation (n = 4).
duration should be effective for predicting bioavailability, the two measurements could be extrapolated from each other, and the same extrapolation could be used for both compounds. For chlorobenzenes, PCBs, and PAHs, the 6 h Tenax extraction represented approximately half of the rapidly desorbing fraction, and on average the 30 h extraction was approximately 1.4 times greater than the rapidly desorbing fraction.19 Therefore, the 30 h extraction would be approximately 2.8 times the 6 h extraction concentration, but this relationship varied by compound. You et al.9 found the rapidly desorbing fraction to be 1.62 times the 6 h extraction time for PCBs. In the current study, the 24 h extraction time was 1.65 ± 0.25 times the 6 h extraction across concentrations and compounds. It is also important to note that the ratio between sediment and Tenax mass differed between the Cornelissen19 and You9 studies, which may have affected the relationship between the single-point Tenax extractable concentration and rapidly desorbing fraction. However, Landrum et al.12 used literature values34 to convert between the two time points. These differences are likely compound class and sediment dependent. So, within compound class and sediment, as long as the amount
Figure 3. Relationship between Log SPME fiber concentration (ng/ mL) and Log 6 and 24 h Tenax extractable concentration (ng/g OC) for permethrin (closed symbols) and bifenthrin (open symbols) individually (solid lines) and in combination (dashed lines). See Table S1.
extractable concentration was correlated to the SPME fiber concentrations, these methods are also proportional to the amount of chemical available to the SPME fiber. However, this relationship was not consistent between compounds. Specifically, the slope was significantly lower for permethrin, and the intercept was significantly lower for bifenthrin. The relationships were, however, similar between the two Tenax extraction times. When the data were combined across compounds, the regression was still significant, but the correlation coefficient decreased from 0.99 and 0.93 to 0.64 and from 0.99 and 0.97 to 0.72 for 6 and 24 h extraction times, respectively (Figure 3). The permethrin relationship was not significantly different from the combined relationship. The bifenthrin regression was 2416
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individual compounds, but the predictive capacity using the combined model would have resulted in more variability and less accuracy than using the individual lines. Previous studies have also compared the predictive capacity of SPME fibers to predict bioaccumulation across compounds. The SPME fibers were particularly effective at predicting bioavailability of multiple compounds using the same relationship for compounds that were not rapidly biotransformed, such as chlorobenzenes and PCBs.7,16,35 In the present study, the compounds were rapidly biotransformed and at different rates, which appeared to inhibit the ability for comparisons between compounds. This was also true for trinitrotoluene, another rapidly biotransformed compound.5 Therefore, while SPMEs may be able to predict across species for a single compound, they are less predictive across compounds when biotransformation provides complications. In contrast, both 6 and 24 h Tenax extractable concentrations yielded single regressions for the respective extraction times that included both species and compounds (Figure 5).
significantly different from the combined regression in both slope and intercept. This indicated that, while there was a strong relationship between the bioavailable and bioaccessible fractions for each compound, this relationship differed among compounds. A relationship was previously compared between the rapidly desorbing fraction and SPME fiber concentrations at equilibrium for several hydrophobic chemicals,9 and the correlation coefficient was similar to the present study (∼0.73). Therefore, extrapolations between the bioavailable and bioaccessible fractions were likely compound-dependent and should be made cautiously. Can Chemical Techniques Predict Bioavailability of Biotransformed Compounds? Landrum et al.12 suggested that chemical techniques would be an overestimate of bioaccumulation when compounds are biotransformed; however, this was based on very limited data. When SPME fiber or Tenax concentrations were correlated to the tissue concentrations of a biotransformed compound, the fiber or Tenax concentration should increase at a faster rate than the tissue concentrations, since the parent tissue concentrations will be altered by biotransformation. So, biotransformation would theoretically decrease the slope of this relationship when the organism is not at steady-state. However, if animals reached a steady-state parent tissue concentration, parent tissue concentrations should be proportional to bioavailable concentrations. In this case, SPME fiber or Tenax concentrations for biotransformed compounds should correlate to parent tissue concentrations in a similar manner as nonbiotransformed compounds. The current study demonstrated regressions supporting the expected relationship. There was a significant linear relationship between the SPME fiber concentration and the lipid normalized parent tissue concentration of the test species for both permethrin and bifenthrin (r2 = 0.73, Figure 4). However, the relationship
Figure 4. Relationship between Log SPME fiber Concentration (ng/ mL) and Log Parent Tissue Concentrations of permethrin (closed symbols) and bifenthrin (open symbols) in Lumbriculus variegatus (triangles) and Hexagenia sp. (squares). See Table S1.
Figure 5. Relationship between Log 6 and 24 h Tenax Concentration (ng/g OC) and Log Parent Tissue Concentrations of permethrin (closed symbols) and bifenthrin (open symbols) in Lumbriculus variegatus (triangles) and Hexagenia sp. (squares). See Table S1.
between SPME fibers and tissue concentrations became stronger when the compounds were separated (permethrin, r2 = 0.98 and bifenthrin, r2 = 0.76). There was also a significant difference in slopes and intercepts between the two compounds. These differences in slope and predictive capacity were similar to those observed in the relationship between SPME fibers and Tenax extractable concentrations (Figure 3). This indicated that SPMEs predicted bioavailability for
There was a slight increase in correlation coefficient using the 24 h extraction time, but either extraction time provided a good prediction of tissue concentrations of both species across compounds. Unlike with SPMEs, the relationship between the Tenax extractable compound and tissue residues was statistically identical for permethrin and bifenthrin. Tenax has 2417
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demonstrated its ability to predict bioavailability of multiple compounds and species using both the rapidly desorbing fraction8,16,19 and single-point extraction.7−9,12,21 Furthermore, Tenax, was able to predict bioavailability across compounds and has also been shown to predict bioavailability across compound classes.7,12,19 Thus, Tenax extraction may have an advantage for predicting bioaccumulation over SPME fibers for pyrethroids based on the limited data from this study. The majority of studies using Tenax to predict residues in L. variegatus were conducted using PCBs8,9,12,16 which were not biotransformed by this species. The relationships between the Tenax concentrations and lipid normalized tissue concentrations have similar slopes with a range of 0.84 to 0.94. In the current study, the slopes of Tenax concentrations correlated to tissue concentrations yielding a comparable slope (0.95). The intercepts for PCBs (0.72 to 0.84) were also comparable to the current study (0.63 and 0.82 for 6 and 24 h, respectively). The intercept for 6 h is lower than the literature range since the majority of studies used the rapidly desorbing fraction which represents a greater amount of compound than 6 h, thus increasing the intercept. The study by You et al.8 which included multiple types of hydrophobic organic compounds had a slightly higher slope of 1.04, but the intercept (0.72) was within the range of other studies. This indicated that bioaccumulation of pyrethroids (biotransformed) was still proportional to bioavailability in a manner similar to PCBs (not biotransformed). Therefore, chemical techniques, such Tenax, can be used to predict bioavailability and ultimately toxicity of biotransformed compounds, such as pyrethroids. Comparison of Methods. There was no significant effect of exposure method on the equilibrium concentration of pyrethroid on the fiber, and the difference between 6 and 24 h Tenax extraction’s ability to predict tissue concentrations was minimal. Thus, within the range of conditions tested, variations in methodologies should not influence the ability of either method to accurately predict bioavailability. Additionally, significant linear relationships across compounds and species were possible using both SPME and Tenax approaches. However, the ability of SPME fibers to predict bioavailability improved when individual compounds were examined. The Tenax extractions were better at predicting pyrethroid bioavailability, particularly across compounds. Since the concentrations on the SPMEs were much lower than those on the Tenax, the variation in these samples decreased the strength of the relationship, particularly in the case of bifenthrin. Thus, analytical limitations are another challenge for using SPME with highly toxic compounds at low concentrations. Overall, the greater sensitivity of Tenax extraction was a definite strength of the method. The increased sensitivity of Tenax was because this method removed a larger fraction of chemical from the matrix, particularly in the 24 h extraction. Tenax was also not limited by a nondepletion requirement. This feature of the Tenax extraction makes it particularly useful for compounds, such as pyrethroids, which are ecologically relevant at very low concentrations. So, while both methods predict bioavailability of individual compounds across species effectively, Tenax was a better method for use with pyrethroids due its ability to predict bioavailability across compounds. Development of an accurate tool to predict the bioavailability of biotransformed insecticides would be a valuable asset for environmental assessments, and these techniques, particularly Tenax, show promise in meeting this need.
Article
ASSOCIATED CONTENT
S Supporting Information *
Equations for the relationships shown in Figures 2−5 are presented in a table. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 618-453-4091. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Keegan Smith, Amanda Rothert, Warren Hanson, and Elizabeth Mackenbach for assistance with experiments. A portion of this research was funded by a Southern Illinois University Dissertation Research Assistant Award.
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REFERENCES
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dx.doi.org/10.1021/es2035174 | Environ. Sci. Technol. 2012, 46, 2413−2419
