Article pubs.acs.org/JAFC
Characterization of Trisiloxane Surfactants from Agrochemical Adjuvants and Pollinator-Related Matrices Using Liquid Chromatography Coupled to Mass Spectrometry Jing Chen and Christopher A. Mullin* Department of Entomology, Center for Pollinator Research, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: Trisiloxane surfactants (TSSs) have been associated with honeybee learning impairment and the ongoing global bee decline. A liquid chromatography−mass spectrometry strategy for the identification of TSSs from agrochemical adjuvants and pollinator-related matrices is introduced here. The strategy incorporates chromatographic retention behavior, isotope ratio, reference to a compiled database of accurate masses, and TSS hydrolysis when necessary. Using this analytical strategy, three TSSs (x = 0, R = H, m = 1, 2, or 3) were identified for the first time from almond flowers of a commercial orchard. The three major purified TSS components in popularly used spray tank adjuvants were identified as TSS (x = 0, m = 0, R = H, CH3, or C(O)CH3) and their structures confirmed by nuclear magnetic resonance spectroscopy. These monitoring tools allow the assessment of the agricultural residues and potential risks of major TSS contaminants to important nontarget species such as honeybee and other essential pollinators. KEYWORDS: trisiloxane surfactant, organosilicone, honeybee decline, LC-MS, adjuvants
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INTRODUCTION Poor honeybee and other pollinator health continues to be a major threat worldwide since 2006,1,2 and factors such as parasites, pesticides, pathogens, or their combinations have been implicated in bee decline.3,4 More recently, the focus has been on the role of pesticide residues in foraging areas or beehives as a primary trigger of bee death.5 However, no direct correlation has been found between any single pesticide and colony collapse.3,6,7 Compared to technical grade pesticides, formulated pesticides are usually more toxic to honeybees and other nontarget species.8 Some formulation additives and solvents are known to be acutely or sublethally toxic to bees, although they are considered and classed as “inerts”.9,10 Organosilicone surfactants used in agriculture are newgeneration adjuvants containing a fully methylated siloxane backbone as the hydrophobic part and mostly one or occasionally more poly(ethylene oxide) (EO)/poly(propylene oxide) (PO) chains as the hydrophilic part. Most often, a propyl group is used to connect the siloxane backbone and EO/PO side chain (Figure 1), but occasionally an alternative alkyl from 1 to 4 carbons or an aryl group can serve as a spacer. According to specific application or the commercial product, the surfactant structure can differ in end group R and the values of x, m, and n,8,11−13 which are generally not revealed by manufacturers. In agro-ecosystem use, the trisiloxane surfactants (TSSs; x = 0 in Figure 1) dominate and are increasingly applied as spray adjuvants due to their superior spreading and penetrating abilities.12,13 Recently, high nanograms per gram levels of TSS residues were detected in North American bee hives,14 and adult bees fed 20 μg of tested TSS adjuvant were strongly impaired in their learning ability to associate a floral odor with an imminent sugar nectar reward.10 Thus, more © XXXX American Chemical Society
Figure 1. General structure of an organosilicone surfactant. Me, EO, and PO represent methyl, ethylene oxide, and propylene oxide, respectively.
attention should be paid to monitoring environmental TSS residues to assess their potential role in the ongoing bee decline. Special Issue: 51st North American Chemical Residue Workshop Received: November 20, 2014 Revised: February 4, 2015 Accepted: February 5, 2015
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Figure 2. Extracted ion chromatograms of trisiloxane surfactant (R = H) oligomers: (A) oligomers containing 1−4 poly(propylene oxide) (PO) units with 8 poly(ethylene oxide) (EO) units; (B) oligomers of 4−8 EO and 1 PO units; (C−E) oligomers of 4−8 EO and 2−4 PO units, respectively.
Major components of the first commercial TSS, Silwet L-77, were previously identified as primarily methoxy- and some hydroxy-capped TSS (Figure 1; x = 0, m = 0, R = CH3 or H; CAS Registry No. 27306-78-1 and 67674-67-3).15 Subsequently, analytical methods were developed to monitor residues of these TSSs in water,16 and in addition the acetyl-capped TSS (x = 0, m = 0, R = C(O)CH3; CAS Registry No. 125997-17-3) in honey, pollen, and beewax.14 New kinds of TSSs are continually being synthesized and reported,17−23 and their possible future use in bee environments can be anticipated. It is important to develop an identification strategy for analyzing TSSs and primary degradates in bee-related samples. Here, a strategy for identifying TSSs and their environmental degradates, based on liquid chromatography coupled to mass spectrometry (LC-MS), is described. Techniques used include low- and high-resolution mass spectrometry (MS), isotope ratio, nuclear magnetic resonance (NMR) spectroscopy, TSS hydrolysis, chromatographic retention behavior, and reference to a compiled database of diagnostic masses. Particular attention was made to distinguishing TSSs from competing polyethoxylate ions in samples. With this strategy, TSSs were identified from almond flower and from commercial highvolume agrochemical adjuvants.
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Mobile phases A and B were water and acetonitrile, respectively, both containing 2 mM ammonium formate and 0.01% formic acid. For almond flower analysis, the gradient was started with 15% B and changed to 70% B at 1 min, then changed to 100% B at 9 min and kept at 100% B until 12 min. The gradient was set back to 15% B at 12.1 min and kept for 5 min. In adjuvant analysis, the gradient was set at 5% B and kept for 3 min. It was changed to 50% B at 25 min, changed to 60% B at 40 min, and kept for 20 min. Then, the gradient was changed to 100% B at 62 min and kept for 3 min. Finally, it was set back to 5% B and kept for 5 min. MS was performed in the positive ion mode. The nebulization gas flow was set to 1.5 L min−1, drying gas flow was set to 10 L min−1, heat block temperature was 300 °C, and DL temperature was set to 250 °C. The interface bias voltage was set to 4500 V. Accurate mass was determined in the Pennsylvania State University Proteomics and Mass Spectrometry Core Facility on a Waters LCT Premier time-of-flight (TOF) MS (Waters Corp. (Micromass Ltd.), Manchester, UK). The nitrogen drying gas temperature was set to 300 °C, and capillary voltage was 2400 V. The MS was set to scan from m/ z 100 to 1000 in positive ion mode, using ESI. Sample Collection and Preparation. Commercial organosilicone adjuvants analyzed included Dyne-Amic, Kinetic HV, and Silwet L-77 from Helena (Collierville, TX, USA), Sylgard 309 from Dow-Corning (Midland, MI, USA), Syl-Tac from Willbur-Ellis (San Francisco, CA, USA), and Silkin from Agriliance LLC (Inver Grove Heights, MN, USA). All adjuvants were dissolved in acetonitrile at 100 ppm. Almond flowers from Stanislaus county, California, USA, were collected in March 2010 on the same day of a pesticide spray treatment and stored at −20 °C. Floral extracts were prepared as before using a QuEChERS method.14 One gram of almond flowers was ground in liquid nitrogen before extraction. TSS Purification. Three TSSs (R = H, CH3, and C(O)CH3) were purified respectively from the corresponding adjuvants using C18 SPE cartridges (500 mg/6 mL, Supelco, Sigma-Aldrich). Fractions were collected and dried under nitrogen. The purified fractions were analyzed by LC-MS and shown to be >95% pure. TSS Hydrolysis. Hydrolysis experiments were performed on 100 ppm of purified TSS fractions and sample extracts in 2 mM ammonium formate buffer (pH 2.75) for at least 30 min at room temperature. Hydrolyzed products were analyzed by LC-MS. NMR Spectroscopy. NMR spectroscopy of the three purified TSS compounds was conducted on a 600 MHz, 14.1 T Bruker AV-III-600 (Bruker-Biospin Corp., Billerica, MA, USA) in the Pennsylvania State
MATERIALS AND METHODS
Chemicals. Acetonitrile (HPLC grade) and formic acid (98%) were purchased from EMD Chemical Inc. (Gibbstown, NJ, USA). Ammonium formate was purchased from Alfa Aesar (Wayne, PA, USA). Magnesium sulfate (MgSO4, anhydrous) and ENVIRO Clean 2 mL dispersed solid phase extraction (SPE) tube (containing 150 mg of MgSO4, 150 mg of CUPSA, and 50 mg of CEC18) were from UCT (Bristol, PA, USA). Sodium acetate (99%, anhydrous) was from Sigma-Aldrich (St. Louis, MO, USA). Chloroform-d was from Isotec Inc. (OH, USA). Water used in all experiments was purified using a MilliDI system (Millipore, MA, USA). LC-MS Conditions. Liquid chromatography coupled to mass spectrometry was performed on an LC-MS 2020 system (Shimadzu, Kyoto, Japan) equipped with an electrospray ionization (ESI) source. A Shim-pack XR-ODS column (100 mm × 2.0 mm) (Shimadzu) was used for LC separation at 50 °C and a flow rate of 0.35 mL/min. B
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Journal of Agricultural and Food Chemistry University NMR Facility. Proton nuclear magnetic resonance (1H NMR) and heteronuclear multiple-quantum coherence (HMQC) spectra were collected. Samples were dissolved in chloroform-d. Reference Database. A database was built of accurate masses for TSS oligomers including [M + H]+, [M + Na]+, [M + K]+, and [M + NH4]+ adducts. TSS oligomers contained 0−20 EO units, 0−10 PO units, end group R = CaH2a+1 (a = 0−18), R = C(O)CaH2a+1 (a = 0− 18), or other functional groups. The database is being updated as new TSS structures are published and is available upon request from the authors.
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RESULTS AND DISCUSSION Chromatography and Mass Spectrometry. TSSs can be separated on the basis of the R end group and respective oligomeric array using gradient elution on a C18 reversed phase column.14 TSS (R = H) oligomers with 8 EO units and 1−4 PO units were eluted in order of increasing number of PO units (Figure 2A). Conversely, oligomers with 4−8 EO and the same PO units increasing from 1 to 4 were eluted in order of decreasing length of EO chain (Figure 2B−E), due to the greater hydrophilic nature of the EO chain. Protonated ([M + H]+) or deprotonated ([M − H]−) molecular ions are usually produced in the ESI source, but these ions cannot be easily observed for TSSs.15,24 Instead, [M + NH4]+, [M + Na]+, or [M + K]+ adducts are usually observed due to a cation trapped in the polyalkylene oxide side chain. The mechanism of forming cation adducts can be explained by the respective chelating effect of the alkali metal.25 Peak areas of respective cation adducts for TSSs are displayed in supplementary Figure S1 in the Supporting Information. In the absence of the ammonium buffer in the mobile phase, all [M + NH4]+, [M + Na]+, and [M + K]+ adduct ions are observed because of trace levels of each cation in glassware and the LC system. Oligomers with longer EO chains show an increased abundance of [M + K]+ adducts (data not shown), which is consistent with results from Bonnington et al.15 By adding 2 mM ammonium formate to the mobile phase, the incidence of [M + Na]+ and [M + K]+ ions is substantially reduced, and [M + NH4]+ ions dominate the mass spectrum (supplementary Figure S1). Therefore, inclusion of 2 mM ammonium formate buffer in the mobile phase simplifies spectral identification and provides better sensitivity. However, a higher buffer concentration can result in ion suppression,26 which reduces LC-MS signal intensity. TSS Identification Using High-Resolution MS. Highresolution MS enables assignment of elemental compositions based on the measured accurate mass. TSS (m = 0, R = CH3) at 250 μg/L was prepared in acetonitrile and injected into the TOF-MS. Three silicon ions were found in each oligomer, and a CH2CH2O difference was found in all neighboring oligomers (data not shown). The [M + NH4]+ structure is hard to elucidate due to lack of MS/MS information because the high stability of the adduct does not allow sufficient fragmentation. Because data from TOF-MS only suggest the elemental composition with three silicon atoms and EO/PO duplicates, other structural analysis tools must be included. Our identification strategy for TSSs is displayed in Figure 3. It can be adapted easily to a low-resolution MS, for example, a single-quadrupole MS, and includes the following steps: Identifying the EO/PO in the Side Chain. TSS with only EO or an EO/PO combination dominated by EO is mainly used in commercial products. The mass differences of neighboring oligomers are 44.03 and 58.04 amu, respectively, for an EO and PO unit. A TSS oligomer with a longer EO chain has a reduced
Figure 3. Trisiloxane surfactant identification strategy.
retention time on a reversed phase column (Figure 2B−E), which is similar to other nonionic surfactants.27 A PO unit further enhances hydrophobicity, and oligomers with increasing PO units have increased retention time (Figure 2A). Combining mass spectral and retention behavior, a prediction for the ratio of EO/PO units in the side chain can be obtained. Distinguishing TSS from Other Nonionic Surfactants. With a low-resolution MS, accuracy of the determined mass does not support calculating the elemental composition. However, the number of silicon atoms can be determined by isotope ratios. A linear relationship is found with increasing EO chain for TSS between the isotope ratio (I[M+1]/I[M]) and m/z (Figure 4A). Surfactants with 0−4 silicon atoms have the same slope values, =0.05, and the intercept value is increased according to the number of silicon atoms. For example, intercept values of nonylphenol ethoxylates (NPEO) and TSSs (m = 0, R = CH3) are around −1.2 and 12 with 0 or 3 silicon atoms in the structure, so by determining the m/z and I[M+1]/I[M] of an unknown surfactant, its intercept value, B, can be calculated using eq 1. B = 100 × I[M + 1]/I[M] − 0.05 × m /z
(1)
By referring to the intercept values in Figure 4 or by comparing those calculated for other similar alkyphenol ethoxylates and alkyl ethoxylates, a putative TSS can be identified and distinguished from other co-occurring nonionic surfactants. Moreover, the relationship of I[M+2]/I[M] ratio and m/z is displayed in Figure 4B. For m/z < 1000, the I[M+2]/I[M] ratios of TSS or higher siloxane surfactant are >13%, whereas they are below 10% in other nonionic surfactants without silicon atoms,14 the latter attribute confirming that the surfactant is a TSS. A similar strategy can be used to distinguish the TSS (m = 0, R = CH3) from octylphenol ethoxylates, in which same mass ions were observed.14,28 Identifying the End Group. New structural types of TSSs with improved spreading ability and stability have been published recently.17−23 We have built a reference database that encompasses the mass ions of various TSSs that are used C
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Figure 5. Proposed major hydrolysis pathways of trisiloxane surfactant (m = 0, R = CH3) in 2 mM ammonium formate buffer, pH 2.75.
honeybee exposures to TSS residues.10 Thus, almond flowers were extracted and analyzed by LC-MS as above, and the total ion chromatogram is displayed in supplementary Figure S2 in the Supporting Information. In the mass range m/z 500−1000, four clusters of oligomers with 44 amu mass differences were found: surfactant 1, m/z 502 + 44n (tR = 3.5−3.7 min); surfactant 2, m/z 532 + 44n (tR = 5.8−6.3 min); surfactant 3, m/z 502 + 44n (tR = 6.3−7.2 min); and surfactant 4, m/z 516 + 44n (tR = 6.7−8.0 min). For each cluster, larger m/z mass ions have lower retention times successively separated by 44 amu, indicating these surfactants have EO units. Equivalent mass ions were found in surfactants 1 and 3. By calculating the I[M+1]/I[M] isotope ratios and using eq 1, the calculated intercept value (∼5) indicated that surfactant 1 had no silicon atom, and it was further confirmed as NPEO using a standard. By parallel analysis, surfactants 2, 3, and 4 all had three silicon atoms. Upon hydrolysis in 0.1% formic acid solution at pH 2.6, all three clusters were lost in the LC-MS analysis, indicating the existence of silicon−oxygen bonds. Concomitant LC-MS analysis of respective peaks from the hydrolysis of surfactants 2, 3, and 4 indicated a TSS with a single side chain of EO and possibly PO units. Mass ion m/z 532 from surfactant 2, based on our reference database, indicated a TSS (n = 5, m = 0, R = CH3) or TSS (n = 4, m = 1, R = H). TSS (n = 5, m = 0, R = CH3) was previously identified as Silwet L-77.14,15 Because the longer retention time for m/z 532 from surfactant 2 is not consistent with our standards for Silwet L-77, we assign this m/ z 532 to the respective EO side chain with a single PO unit, TSS (n = 4, m = 1, R = H), and surfactant 2 was identified as TSS (m = 1, R = H). Similarly, surfactants 3 and 4 were identified as TSS (m = 2, R = H) and TSS (m = 3, R = H), respectively. Identifying TSS and Other Nonionic Surfactants from Agrochemical Adjuvants. TSSs in commercial spray adjuvants are often undisclosed and labeled as a “nonionic surfactant”, “silicone surfactant”, “polyethoxylate”, “methylated silicone polyethoxylate”, etc. Detailed information about TSS structures and their percentages is rarely revealed. On the basis of pounds of usage on California almonds (primary global site for honeybee pollination) reported from the California Pesticide Information Portal,10,31 the top-used organosilicone
Figure 4. Correlations between (A) m/z and I[M+1]/I[M]% and (B) m/ z and I[M+2]/I[M]% for nonylphenol ethoxylates (NPEO) and siloxane surfactants of various EO chain lengths and siloxanes of increasing methylated silicon atoms (0−4 Si).
or potentially used in bee environments. TSSs with 2−20 EO units, 1−10 PO units, and various end groups (hydroxyl, methyl, acetate, etc.) are included in the database. Entry of the m/z value of an unknown TSS reveals candidate oligomers and their identity. Identifying Single or Double Side Chains on the TSS Backbone. Most TSSs used in agrichemical formulations have a single hydrophilic side chain to maintain adequate water solubility, but for some product applications requiring increased polarity, a double alkoxylate side chain may be incorporated. Hydrolysis experiments can be used to identify the number of side chains and to which silicon atoms they are attached. Silicon−oxygen bonds can be hydrolyzed to silanol in acidic or alkaline aqueous solutions.29,30 In 2 mM ammomiun formate (pH 2.75) buffer, TSS mainly hydrolyzes to silanols with various EO chain lengths (Figure 5). For example, TSS (x = 0, m = 0, R = CH3) with a single side chain attached to the middle silicon atom hydrolyzes to oligomers of m/z 168 + 44n (n > 2), whereas those with a side chain attached to the terminal silicon atom hydrolyze to oligomers of m/z 166 + 44n (n > 2). If a TSS has two side chains attached to both the middle and end silicons, two oligomer families representing degradation from the either side chain can be detected during LC-MS analysis. If the two side chains are on both terminal silicons, only one degradate of m/z 168 + 44n (n > 2) is produced, but the ratio of “hydrolyzed product/TSS” is double that of the single-chain TSS. Identifying TSS from Almond Flower Samples. Almond production provides the largest honeybee pollination event in the United States, and agrochemical sprays containing organosilicone adjuvants are applied intensively at bloom, resulting in D
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Figure 6. Identification results from top-used spray adjuvants (see text for product details). NPEO and OPEO are nonylphenol ethoxylates and octylphenol ethoxylates, respectively.
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ACKNOWLEDGMENTS We are grateful to Maryann T. Frazier and Sara A. Ashcraft for providing the almond flower samples used here, and to the USDA-NIFA-AFRI Foundational Award program (nos. 201167013-30137 and 2014-67013-21591) for funding this work.
surfactant adjuvants were selected and analyzed by LC-MS. On the basis of the identification strategy herein, major TSS and other nonionic surfactants were identified from Dyne-Amic, Kinetic HV, Silkin, Silwet L-77, Sylgard 309, and Syl-Tac (Figure 6). TSS (m = 0, R = H), TSS (m = 0, R = CH3), and TSS (m = 0, R = C(O)CH3) dominated these popularly used adjuvants. Chemical structures of these TSS were confirmed by 1 H NMR and HMQC spectroscopy (Supporting Information, supplementary Figure S3) using purified fractions. Trisiloxane surfactants were recently detected in every beeswax sample up to 390 ng/g and in 60% of pollen samples up to 39 ng/g.14 Interest in applications for higher homologues, particularly the tetrasiloxanes (Figure 1, x = 1), is increasing.32 The prevalence of organosilicone surfactants in agrochemicals including tank adjuvants13 and other industrial products, along with uncertainty of their environmental fate, advocates more robust efforts for their biomonitoring. A structural identification strategy for TSSs is described here based on mostly low- and some high-resolution LC-MS. TSSs were identified from a range of commercial spray adjuvants using accurate mass and isotope ratios, a reference database, and hydrolytic experiments. To verify the utility of our protocol, TSS (m = 1, R = H), TSS (m = 2, R = H), and TSS (m = 3, R = H) were identified for the first time from a California almond flower sample. Major TSS components in popularly used spray adjuvants were identified as TSS (m = 0, R = H), TSS (m = 0, R = CH3), and TSS (m = 0, R = C(O)CH3), and structures were confirmed by NMR. Exploring TSS exposures in bee environments helps us better evaluate this risk factor for honeybees.
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ABBREVIATIONS USED amu, atomic mass unit; CAS, Chemical Abstracts Service; ESI, electrospray ionization; EO, poly(ethylene oxide); HMQC, heteronuclear multiple-quantum coherence; LC-MS, liquid chromatography−mass spectrometry; NMR, nuclear magnetic resonance; NPEO, nonylphenol ethoxylate; PO, poly(propylene oxide); QuEChERS, quick, easy, cheap, effective, rugged, and safe; SPE, solid phase extraction; TOF, time-offlight; tR, retention time; TSS, trisiloxane surfactant
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(1) Steinhauer, N. A.; Rennich, K.; Wilson, M. E.; Caron, D. M.; Lengerich, E. J.; Pettis, J. S.; Rose, R.; Skinner, J. A.; Tarpy, D. R.; Wilkes, J. T.; vanEngelsdorp, D. A national survey of managed honey bee 2012−2013 annual colony losses in the USA: results from the Bee Informed Partnership. J. Apic. Res. 2014, 53, 1−18. (2) Spivak, M.; Mader, E.; Vaughan, M.; Euliss, N. H. The plight of the bees. Environ. Sci. Technol. 2011, 45, 34−38. (3) vanEngelsdorp, D.; Evans, J. D.; Saegerman, C.; Mullin, C.; Haubruge, E.; Nguyen, B. K.; Frazier, M.; Frazier, J.; Cox-Foster, D.; Chen, Y. P.; Underwood, R.; Tarpy, D. R.; Pettis, J. S. Colony collapse disorder: a descriptive study. PLoS One 2009, 4, No. e6481. (4) Cox-Foster, D. L.; Conlan, S.; Holmes, E. C.; Palacios, G.; Evans, J. D.; Moran, N. A.; Quan, P. L.; Briese, T.; Hornig, M.; Geiser, D. M.; Martinson, V.; vanEngelsdorp, D.; Kalkstein, A. L.; Drysdale, A.; Hui, J.; Zhai, J. H.; Cui, L. W.; Hutchison, S. K.; Simons, J. F.; Egholm, M.; Pettis, J. S.; Lipkin, W. I. A metagenomic survey of microbes in honey bee colony collapse disorder. Science 2007, 318, 283−287. (5) Johnson, R. M.; Ellis, M. D.; Mullin, C. A.; Frazier, M. Pesticides and honey bee toxicity − USA. Apidologie 2010, 41, 312−331. (6) Mullin, C. A.; Frazier, M.; Frazier, J. L.; Ashcraft, S.; Simonds, R.; vanEngelsdorp, D.; Pettis, J. S. High levels of miticides and agrochemicals in North American apiaries: implications for honey bee health. PLoS One 2010, 5, No. e9754. (7) Frazier, J.; Mullin, C.; Frazier, M.; Ashcraft, S. Managed pollinator coordinated agricultural project: Pesticides and their involvement in colony collapse disorder. Am. Bee J. 2011, 151, 779− 784. (8) Mullin, C. A.; Chen, J.; Fine, J. D.; Frazier, M. T.; Frazier, J. L. The formulation makes the honey bee poison. Pestic. Biochem. Physiol. 2015, in press, DOI: 10.1016/j.pestbp.2014.12.026.
ASSOCIATED CONTENT
S Supporting Information *
Supplementary Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*(C.A.M.) Phone: (814) 865-2435. Fax: (814) 865-3048. Email:
[email protected]. Notes
The authors declare no competing financial interest. E
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