A New Film-Based Passive Sampler for Moderately Hydrophobic

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A New Film-based Passive Sampler for Moderately Hydrophobic Organic Compounds Wenjian Lao, You-wei Hong, David Tsukada, Keith A. Maruya, and Jay J. Gan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04750 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 27, 2016

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Environmental Science & Technology

Table of Contents (TOC) Art

7000

Concentrations in PMMAfilms (ng/g)

B

Fipronil desulfinyl Fipronil sulfide Fipronil Fipronil sulfone

6000 5000 4000 3000 2000 1000

O

0 Control

MeOH

MeOH/EE

IPA

EE

O 4.8

1.2e+7 1.0e+7

Fipronil desulfinyl Fipronil sulfide Fipronil Fipronil sulfone Regression line

n PMMA

8.0e+6 6.0e+6

4.6

Measured log Kpw

4.4

Regression line +/- Standard error

4.2

Log Kpw

Concentrations on PMMA films (ng/L)

1.4e+7

4.0 3.8 3.6

4.0e+6

3.4 2.0e+6

3.2 0.0

3.0 0

10

20

30

Time (h)

40

50

60

2.8 2

3

4

5

6

Log Kow

ACS Paragon Plus Environment

7

8

9


1

A New Film-based Passive Sampler for Moderately Hydrophobic Organic Compounds

2

Wenjian Lao†, Youwei Hong†‡§, David Tsukada†, Keith A. Maruya† and Jay Gan‡*

3 4 †

5 6

‡

7

§

Southern California Coast Water Research Project Authority, Costa Mesa, 92626, CA

Department of Environmental Sciences, University of California, Riverside, 92521, CA

Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, 361021, China

8 9 10 11 12 13 14 15 16 17 18

19

20

21 1


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22

Abstract

23

Passive samplers for moderately hydrophobic organic compounds (MHOCs) (i.e., log Kow

24

ranging from 2 to 5) are under-developed compared to those that target polar or strongly

25

hydrophobic compounds. The goal of this study was to identify a suitable polymer and

26

develop a robust and sensitive film-based passive sampler for MHOCs in aquatic systems.

27

Polymethyl methacrylate (PMMA) exhibited the highest affinity for fipronil and its three

28

metabolites (i.e., fipronils) (log Kow 2.4-4.8) as model MHOCs compared with polyethylene

29

and nylon films. In addition, a 30-60 min treatment of PMMA in ethyl ether was found to

30

increase its sorption capacity by a factor of 10. Fipronils and 108 additional compounds (log

31

Kow 2.4 - 8.5) reached equilibrium on solvent-treated PMMA within 120 h under mixing

32

conditions and their uptake closely followed first-order kinetics. PMMA-water partition

33

coefficients and Kow revealed an inverse parabolic relationship, with vertex at log Kow of 4.21

34

± 0.19, suggesting that PMMA was ideal for MHOCs. The PMMA sampler was tested in an

35

urban surface stream, and in spiked sediment. The results demonstrated that PMMA film,

36

after a simple solvent swelling treatment, may be used as an effective passive sampler for

37

determining Cfree of MHOCs in aquatic environments.

38 39

Keywords: Polymethyl methacrylate; moderately hydrophobic organic compounds;

40

equilibrium passive sampling; swelling treatment; partition coefficient (Kpw); fipronil

2



41 50

INTRODUCTION

51

Risk assessment and toxicity evaluation require a convenient and effective tool to

52

measure the freely dissolved concentration (Cfree) of hydrophobic organic compounds

53

(HOCs) in water and sediment, because Cfree characterizes chemical activity that is relevant

54

to chemical transport throughout different matrices and acute toxicity. Passive samplers

55

have found widespread use in measuring time-weighted average Cfree in aquatic

56

environments over the last two decades.1, 2 As polarity of organic contaminants spans from

57

ionic to strongly hydrophobic (i.e., log Kow > 5), various passive samplers have been

58

proposed and tested for Cfree measurement.3, 4 Passive samplers composed of nonpolar

59

polymeric sorbents, e.g., poly(dimethylsiloxane) (PDMS), low-density polyethylene (PE),

60

polyoxymethylene (POM) in thin film, sheet or fiber configurations, are compatible with

61

strongly hydrophobic compounds. Complementary passive samplers are available for polar

62

and ionizable chemicals, such as the polar organic chemical integrative sampler (POCIS),5

63

solid-phase adsorption toxin tracking (SPATT),6 polar organics version of Chemcatcher

64

and diffusion gradients in thin-films (o-DGT).7, 8 Based on their specific configurations,

65

passive samplers made of PDMS-coated fibers, or PE, POM or silicone film/sheets may be

66

used in both water and sediment and operated in the equilibrium or kinetic sampling mode.9

67

Passive samplers for moderately hydrophobic organic compounds (MHOCs) (i.e., log

68

Kow 2-5) are comparatively less developed, even though many emerging contaminants fall

69

into this classification.10 A few materials, including polyacrylate-coated SPME fiber, 11, 12

70

POM,13 and a customized plastic film poly(ethylenecovinylacetate-co-carbon monoxide) 3


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(PEVAC),14 were previously tested for MHOCs with some success. However, these

72

samplers have drawbacks including slow diffusion rates of HOCs in POM, lack of

73

commercially produced PEVAC and disposable SPME fiber, and fragility of the SPME

74

fiber assembly.

75

The goal of this study was to develop a film-based sampler for Cfree determination of

76

MHOCs in aqueous matrices. Using the insecticide fipronil and its three biologically active

77

metabolites (log Kow 2.4-4.8) as model MHOCs, we undertook an investigation to first

78

evaluate different film types for their uptake capacities, and then optimize the selected

79

polymer film (polymethyl methacrylate, or PMMA) for equilibrium sampling. Additionally,

80

we discovered that pretreatment of PMMA film by swelling with solvents drastically

81

enhanced its capacity to absorb MHOCs.15, 16 A dataset of PMMA-water partition

82

coefficients (Kpw) for 112 organic compounds (Kow 2.4-8.5) was experimentally derived,

83

allowing the evaluation of relationships between Kpw and Kow. The derived method was

84

then successfully tested in situ for surface water under field conditions and ex situ for

85

sediment spiked with the model MHOCs.

86 87 88

EXPERIMENTAL SECTION The sampler development involved a series of step-wise experiments: 1) comparison of

89

PMMA, PE, and nylon-6 for their sorption capacities for fipronils as model MHOCs;17, 18 2)

90

enhancement of PMMA sorption capacity by a simple solvent swelling pre-treatment; 3)

91

determination of PMMA-water partition coefficients (Kpw) at equilibrium for fipronils and

92

a wide range of organic compounds (log Kow 2.4-8.5) for PMMA; and 4) application of the 4



93

PMMA sampler in an urban stream to measure Cfree of fipronils in situ, and in spiked

94

sediments to estimate Cfree in porewater ex situ. These experiments are briefly described

95

below.

96

Evaluation of film types for sorption of fipronils. Samples of PMMA (75 µm

97

thickness, Kaneka, Hyogo, Japan) and nylon-6 (25 µm thickness, Honeywell, Morristown,

98

NJ, USA) were cleaned in hexane, and then in deionized water by sonication (15 min × 3)

99

before use. Polyethylene film (25 µm thickness, Covalence, Minneapolis, MN, USA) was

100

cleaned in dichloromethane, methanol, and then deionized water by sonication (15 min × 3).

101

Fipronil, fipronil sulfone, fipronil sulfide, and fipronil desulfinyl (fipronils) (>98%,

102

AccuStandard, New Haven, CT, USA) were all found to be stable in water in the laboratory

103

in our preliminary test. However, to prevent interconversion, fipronil, and its three

104

degradates were separately spiked into water at 1000 ng/L in different containers in the

105

following experiments.

106

The spiked water was transferred to several 500-mL flat-bottom flasks. The flasks

107

were filled to minimize headspace and kept in the dark for 48 h. Prior to introducing the

108

film, the spiked water was mixed with a glass coated magnetic stir bar at 250 rpm for 2 h.

109

Three pieces of PMMA, PE or nylon film (0.035 g each) were added into each flask. All

110

flasks were then closed with ground-glass stoppers, wrapped in aluminum foil, and kept in

111

the dark at room temperature (22 °C). Preliminary experiments showed that the partition

112

between the film and water reached an apparent steady state after 72 h of equilibration for

113

all three film types. After 72-h exposure, the films were retrieved, rinsed with deionized

114

water, and air-dried. For analysis, recovery surrogates 4,4'-dibromooctoflurobiphenyl 5


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(DBOFB) and PCB208 (a polychlorinated biphenyl congener) were added and each

116

PMMA film was extracted by sonication for 15 min (× 3) in hexane/methanol/acetone

117

(8:1:1, v/v). The nylon and polyethylene films were similarly extracted using

118

dichloromethane. The extract was concentrated and solvent-exchanged into hexane (1.0

119

mL). After the addition of the internal standard (PCB205) at 50 ng/mL, samples were

120

analyzed for fipronils. Preliminary tests showed that fipronils could be quantitatively

121

extracted from the films using the above extraction procedures, with recoveries > 75%.

122

Modification of PMMA by solvent swelling treatment. The above experiment

123

resulted in PMMA being the best film type for sampling fipronils from water.

124

Solvent-induced film swelling was attempted as a novel approach to further enhance the

125

sorption capacity of PMMA. From preliminary experiments, ethyl ether, methanol,

126

2-propanol and ethyl ether/methanol (1:1, v/v) mixture were selected for evaluation.

127

Pre-cleaned PMMA strips (2.2 by 2.2 cm, or 0.043 g each) (n=3) were soaked in one of the

128

solvents for 30, 60, 120, 240, or 360 min at the room temperature. After air-drying

129

overnight, the solvent-treated PMMA samples were found to return to the original weight,

130

suggesting that residual solvent was completely evaporated from the film. The

131

solvent-treated films were similarly tested in fipronil-spiked water to determine their

132

sorption capacities. Pre-cleaned PMMA film without solvent treatment was included as the

133

control.

134

Topographic images of PMMA film before and after the solvent swelling treatment

135

were taken using an atomic force microscope (AFM) (Dimension 5000, Veeco Instruments,

136

Plainview, NY, USA), from which Mean Roughness (Ra) and surface area were measured. 6



137

The Ra is the arithmetic average of the absolute values of the profile height deviations from

138

the mean in a selected region. The surface area is the three dimensional area of a selected

139

region.

140

Determination of equilibrium time and Kpw. Ethyl ether-treated PMMA pieces (2.5

141

by 1.3 cm, or 0.028 g each) were used to derive Kpw for fipronils and a suite of 108

142

chemicals (log Kow 2.4 - 8.5), including musk ketone, galaxolide, pyrethroid insecticides,

143

polycyclic aromatic hydrocarbons (PAHs), PCBs, organochlorine pesticides (OCPs), and

144

polybrominated diphenyl ethers (PBDEs) (see Supporting Information for a complete list of

145

compounds). The exposure setup was similar to that used in above sorption experiment. A

146

flask containing the spiked water and a stir bar served as “no film” control. At 2, 4, 8, 12,

147

24, and 48 h for the fipronils, or 4, 8, 12, 24, 36, 48, 96, and 120 h for the 108 chemicals, a

148

single flask was sacrificed, and PMMA strips (n=3) were collected and extracted as

149

described above. The water phase was liquid-liquid extracted for three times using

150

dichloromethane (40 mL) and the extract analyzed to quantify the mass balance and derive

151

Cfree for the calculation of Kpw. The recovery surrogates and internal standard were used for

152

quality control.

153 154

At equilibrium, Kpw was calculated using Cfree, =

(1)

155 156

The analyte uptake profile could be described by the first-order kinetics equation19

157 158

=

×

× (1 − × ) 7


(2)

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159 160

with

161

=

162

× ! ×

=

×

+ #$

(3)

163 164

where %& is the initial aqueous concentration, CPMMA is the concentration (ng/L) on film

165

at time t, k1 (1/h) is the uptake rate constant, k2 (1/h) is the elimination rate constant, Vp is

166

the polymer volume of film, Vw is the volume of water, and t is the contact time (h)

167

between the film and analyte in the aqueous phase. Regression analysis using the

168

time-series data would yield k1 and k2, from which Kpw may be further calculated as,

169

=

170

(4)

!

171 172 173

Equilibrium time of passive sampling was defined as the time needed to reach 90% of equilibrium (t90%),20

174

'(&% =

175

*+,&

(5)

!

176 177 178 179 180

Regression analyses were performed by SPSS (version 19.0) for Windows (IBM, Armonk, NY, USA). Applications for Water and Sediment Sampling. The PMMA samplers (14 by 5.3 cm or ~0.67 g each) were prepared, including swelling by ethyl ether, as described above. 8



181

The PMMA samplers were deployed in the surface water for 2 d at a location in San Diego

182

Creek in Irvine, California (33º 39’ 19.20” N, 117º 50’ 43.13” W) that drains an urban

183

watershed. The samplers were suspended at 20 cm from the surface, using copper wire

184

mounted on a weight. On day 0, 1, and 2, water was collected into two 1-L amber glass

185

bottles. The film sampler was retrieved at the end of 2 d deployment. The samples were

186

transported to the laboratory on ice, and immediately analyzed for fipronils to measure

187

CPMMA on the PMMA, and total concentration (Ctotal) in the grab samples by liquid-liquid

188

extraction with methylene chloride (without filtration). The Cfree was calculated from

189

CPMMA using Eq. 1. Water flow velocity was measured at the sampling site once a day from

190

deployment to retrieval during the sampling event. Dissolved organic carbon (DOC) was

191

determined to be 33.2 ± 10.3 mg/L after analysis of sample aliquots on a Shimadzu

192

TOC-VCSH analyzer (Kyoto, Japan).

193

A batch experiment was further conducted to evaluate the application potential of

194

PMMA samplers in sediment. A sediment sample was collected from Dana Point,

195

California. The sediment contained 0.63% total organic carbon (TOC) with a textural

196

composition of 26.1% sand, 66.2% silt and 7.8% clay. Four subsamples of the sediment

197

were separately spiked with each fipronil compound. The spiked sediment samples in glass

198

jars were rotated on a roller for 24 h after spiking, and stored at 4 ºC and rolled 2 h once per

199

week for 28 d to achieve uniform distribution and phase equilibrium.22 The bulk

200

concentration was measured to be 11.6 µg/kg for fipronil, 62.4 µg/kg for fipronil desulfinyl,

201

35.5 µg/kg for fipronil sulfide, and 67.2 µg/kg for fipronil sulfone on a dry weight basis.21

202

Twelve pieces of pre-treated PMMA film (1.8 by 1.8 cm, or 0.028 g each) were inserted 9


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into the spiked sediment (300 g, wet weight) in a glass jar and the samples were rotated at

204

60 rpm at the room temperature. Three PMMA films were retrieved from the sediment at 4,

205

6, 10, and 16 d after the sampler placement and analyzed for fipronils on the PMMA film,

206

from which Cfree was calculated using the derived Kpw in Eq. 1.

207

Instrumental analysis and quality assurance/quality control. The final extracts

208

were analyzed on a gas chromatography/mass spectrometry system (GC/MS) (Agilent

209

7890GC/5975C MS, Wilmington, DE). Detailed instrument conditions are given in the

210

Supporting Information. All analytical data were subjected to strict quality

211

assurance/quality control. Procedural blanks, matrix spike (standards spiked into matrix,

212

e.g., PMMA and water), and sample triplicates were used. No detectable target analytes

213

were found in the procedural blanks. The overall recoveries (PMMA and water) in the

214

batch experiments were >75%, indicating acceptable mass balances.23 Overall mean

215

recoveries of surrogate standards were 62-97% (DBOFB) and 82%-105% (PCB208) for the

216

extraction of PMMA samplers, water, and sediment under the used conditions.

217 218 219

RESULTS AND DISCUSSION Selection of suitable polymer. Among the three polymer films evaluated in this study,

220

PMMA consistently absorbed an order of magnitude more fipronil compounds than

221

polyethylene or nylon film (Fig. S1), suggesting that PMMA has a higher affinity for

222

fipronils under the same conditions. Even though nylon contains the polar amide bond, it

223

did not show superiority over PMMA in the sorption of fipronils. This may be attributed to

224

two factors. The relative density of the polar group in nylon that has 4 and 6 methylene 10



225

groups between the amide groups on the repeating unit is lower than that in PMMA that has

226

only 2 methylene groups. The position of the amide bond in nylon also makes it spatially

227

less accessible for the target molecule, as compared to the pendant ester bond in the PMMA

228

structure.24

229

Enhanced sorption after solvent treatment. To explore possibilities for further

230

enhancing the sorption of MHOCs, pre-cleaned PMMA film was subjected to swelling

231

treatment by solvents. As shown in Fig. 1, sorption of fipronils by PMMA increased

232

dramatically after contact in ethyl ether or isopropanol for 30 min. In particular, the

233

treatment with ethyl ether increased the sorption by approximately tenfold. In contrast,

234

swelling in methanol did not appreciably increase sorption of fipronils, while treatment in

235

methanol/ethyl ether (1:1, v/v) only caused a limited increase. The treatment with ethyl

236

ether or isopropanol appeared to have resulted in relatively higher standard deviationsas

237

compared to the methanol or methanol/ethyl ether treatment (Fig 1). I

238

An additional test was carried out by extending the solvent treatment to 6 h in ethyl

239

ether. The sorption increases were similar when the treatment time was 30 or 60 min, but

240

no significant additional enhancement was observed when the solvent exposure time was >

241

2 h. Consequently, contact in ethyl ether for 30 to 60 min was considered to be effective for

242

maximizing the sorption capacity of PMMA samplers.

243

To the best of our knowledge, this was the first attempt to use solvent swelling to

244

enhance the performance of passive samplers.16, 25 Given that sorption of polymer-based

245

membranes or fibers is often low for polar or MHOCs, solvent pretreatment may be a

246

highly viable solution to developing more efficient passive samplers. 11


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Atomic force microscopic images of PMMA film before and after 30-min ethyl ether

248

treatment showed distinct differences in the membrane’s surface topography (Fig. 2). The

249

PMMA film after solvent treatment showed a uniform and consistent texture. The Ra of the

250

PMMA film increased from 2.32 to 9.44 nm; however, the surface area was only expanded

251

slightly, from 1.07 to 1.12 cm2 cm-2. Therefore, sorption enhancement of PMMA film after

252

the solvent treatment was likely attributable to enhanced absorption into the polymer rather

253

than adsorption.15, 26, 27

254

It was argued that sorption material used for equilibrium passive samplers should

255

have a lower glass transition temperature (Tg) than the ambient temperature of sampling.14

256

This is true for the rubbery polymers such as PDMS, PE, POM and PEVAC that have very

257

low Tgs (< 0 ºC). However, PMMA is a glassy polymer with Tg about 110 ºC. During

258

solvent swelling treatment, it is likely that solvent molecules diffused into the film and

259

relaxed the polymer structure, creating and expanding voids for organic molecules to

260

diffuse into the polymer phase.27, 28 Penetration of organic solvents through PMMA was

261

previously characterized using several instruments including AFM, laser interferometry,

262

ellipsometer, fluorescent force-sensitive and quartz crystal microbalance.29, 30 Using

263

fluorescence analysis, Mayer et al.31 and Magner et al.14 demonstrated that sorption of

264

fluoranthene into PDMS and PEVAC was due to absorption, instead of adsorption.

265

Kpw values for MHOCs. The uptake time series for fipronils by the PMMA film

266

showed apparent equilibrium in 24 h under mixing conditions (Fig. 3). The Kpw values were

267

subsequently calculated using Eq. 1 with averages of data at 24 and 48 h (Table 1). The

268

CPMMA versus the exposure time fitted well to Eq. 2 (R2 = 0.83-0.87), allowing the 12



269

estimation of Kpw values from Eq. 4. Both methods gave identical Kpw values, confirming

270

that the sampling reached equilibrium in 24 h. The average t90% was estimated to be 16 ± 2

271

h for fipronil compounds under the experimental conditions. The log Kpw values estimated

272

from the curve fitting were used in the following experiments for calculating Cfree, because

273

of their relatively smaller standard deviations and the fact that the whole time series was

274

used in deriving the values.

275

The average log Kpw of fipronils was 3.37 ± 0.32, as compared to their average log Kow

276

value of 3.44 ± 0.97 (Table 1) and average (excluding fipronil desulfinyl) log Kpw of 4.34 ±

277

0.31 for polyacrylate-coated (85 µm thickness) SPME fiber. 18 The Kpw values were

278

similarly derived for the entire list of organic compounds considered in this study (Table

279

S1). The fit of data was generally good, with average R2 of 0.89 ± 0.07. Therefore, sorption

280

of HOCs onto PMMA closely followed first-order kinetics, which is in agreement with that

281

observed with other equilibrium passive samplers, including PDMS fibers and polyethylene

282

membranes.4 32 The t90% ranged from 11 to 120 h for the tested chemicals under the

283

experimental conditions. Considering the wide range of log Kow (2.4 to 8.5), the relatively

284

short equilibrium time implies that the solvent-treated PMMA may be suitable for

285

equilibrium sampling of a large number of HOCs, including those that are strongly

286

hydrophobic. The short equilibrium time offers several advantages. As sampling may be

287

done under equilibrium conditions, it eliminates the need for using approaches such as

288

performance reference compounds to account for non-equilibrium sampling.33 The short

289

sampling time is highly desirable for less stable compounds, such as those that have

290

relatively short half-lives due to rapid hydrolysis or photolysis.34 13


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291


Plots of log Kpw versus log Kow (Fig. 4a), and (log Kow - log Kpw) versus log Kow (Fig.

292

4b) gave parabolic curves. Good correlation between log Kpw and log Kow were obtained

293

with a very high R2 (0.97) and a small standard error (0.19) for the entire list of chemicals

294

(N = 112):

295

log 0% = −0.10 (log 3% )$ + 1.08 log 3% + 1.28

296

(6)

297 298

In contrast, linear relationships were observed between log Kpw and log Kow on rubbery

299

polymers (e.g., PDMS, PE, POM and PEVAC) in previous studies for given Kow ranges.13,

300

14, 35, 36

301

or log Kpw 4.20 ± 0.19, suggesting that sorption of PMMA for HOCs increased with

302

increasing hydrophobicity until approximately log Kow of 5.40. For highly hydrophobic

303

compounds with log Kow > 5.40, PMMA enrichment factor declined with increasing log

304

Kow. The maximum log Kpw value (i.e., 4.20 ± 0.19) appeared to be the enrichment

305

limitation of the PMMA film.

306

The vertex of the log Kpw versus log Kow curve (Fig. 4) was found at log Kow ~ 5.40

The term (log Kow - log Kpw) increased monotonically with respect to log Kow. At log

307

Kow 4.0, log Kpw equaled to log Kow. When log Kow was < 4.0, log Kpw was greater than log

308

Kow, indicating that less hydrophobic compounds tended to diffuse more favorably into the

309

solvent-treated PMMA membrane than into n-octanol. This observation clearly suggests

310

that PMMA is a good sorbent polymer for sampling MHOCs, and has advantages over the

311

other materials due to its high enrichment factors and short equilibrium times.

312

The nonlinear relationship of log Kpw versus log Kow may be attributed to several 14



Page 16 of 28

313

factors, such as non-equilibrium, poor mass balances, chemical instability, unaccounted

314

chemical losses due to sorption to container surfaces, and intrinsic properties of PMMA

315

and analytes.36, 37 As both aqueous phase and PMMA film were simultaneously analyzed

316

for the target analytes in this study, which eliminated the influence of most of the listed

317

factors, the curvilinear relationship was most likely caused by physicochemical properties

318

of both PMMA and analytes. Unlike the rubbery polymers, the glassy PMMA film has

319

relatively rigid network with small cavities. Even though the PMMA film underwent

320

solvent swelling, formation of cavities and interactions of organic molecules with the

321

cavities in the PMMA film would require energy. Difference of equilibrium constants (log

322

Kow - log Kpw) may be deduced from the Gibbs free energy relationship using the following

323

equation. 37

324 325

log 6 − log = (∆89:; @6AB:=

A New Film-Based Passive Sampler for Moderately Hydrophobic

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