Effective Enrichment and Detection of Trace Polycyclic Aromatic

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New Analytical Methods

Effective Enrichment and Detection of Trace Polycyclic Aromatic Hydrocarbons in Food Samples based on Magnetic Covalent Organic Framework Hybrid Microspheres Ning Li, Di Wu, Na Hu, Guangsen Fan, Xiuting Li, Jing Sun, Xuefeng Chen, yourui suo, Guoliang Li, and Yongning Wu J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b00869 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Journal of Agricultural and Food Chemistry

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Effective Enrichment and Detection of Trace Polycyclic Aromatic Hydrocarbons in Food Samples based on Magnetic Covalent Organic Framework Hybrid Microspheres

4

Ning Li†§, Di Wu∥, Na Hu‡, Guangsen Fan#, Xiuting Li#, Jing Sun‡, Xuefeng Chen†,

5

Yourui Suo‡, Guoliang Li †§┴* and Yongning Wu┴

6 7 8 9 10 11 12 13 14 15 16

†

17

∥

School of Food and Biological Engineering, Shaanxi University of Science and

Technology, Xi’an 710021, China ‡

Qinghai Key Laboratory of Qinghai-Tibet Plateau Biological Resources, Northwest

Institute of Plateau Biology, Chinese Academy of Sciences, Xining 810001, China §

Key Laboratory of Life-Organic Analysis of Shandong Province, Qufu Normal

University, Qufu 273165, China. #

Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing

Technology and Business University, Beijing 100048, China ┴

Key Laboratories of Chemical Safety and Health, China National Center for Food

Safety Risk Assessment, Beijing 100050, China Yangtze Delta Region Institute of Tsinghua University, Zhejiang 314006, China

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Corresponding Author *E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT

34

The present study reported a facile, sensitive and efficient method for enrichment

35

and determination of trace polycyclic aromatic hydrocarbons (PAHs) in food samples

36

by employing new core-shell nanostructure magnetic covalent organic framework

37

hybrid microspheres (Fe3O4@COF(TpBD)) as the sorbent followed by HPLC-DAD.

38

Under mild synthetic conditions, the Fe3O4@COF(TpBD) were prepared with the

39

retention of colloidal nanosize, larger specific surface area, higher porosity, uniform

40

morphology and supermagnetism. The as-prepared materials showed an excellent

41

adsorption ability for PAHs, and the enrichment efficiency of the Fe3O4@COF(TpBD)

42

could reach 99.95%. The obtained materials also had fast adsorption kinetics and

43

realized adsorption equilibrium within 12 min. The eluent was further analyzed by

44

HPLC-DAD, and good linearity was observed in the range of 1-100 ng/mL with the

45

linear correlation being above 0.9990. The limits of detection (S/N=3) and limits of

46

quantitation (S/N=10) for 15 PAHs were in the range of 0.83-11.7 ng/L and 2.76-39.0

47

ng/L, respectively. For the application, the obtained materials were employed for the

48

enrichment of trace PAHs in food samples, and exhibited superior enrichment

49

capacity and excellent applicability.

50

KEY WORDS: Polycyclic aromatic hydrocarbons, Magnetic covalent organic

51

framework hybrid microspheres, Magnetic solid phase extraction, HPLC-DAD, Food

52

samples

53

INTRODUCTION

54

Polycyclic aromatic hydrocarbons (PAHs) as one of the most widespread class of

55

environmental and food contaminants are a great threat to human health due to their

56

carcinogenesis, tetratogenesis and mutagenesis, etc.1-3 Contamination of food by 2


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PAHs arises from environmental sources (e.g. deposition from the air on the surface

58

of plants, intake by marine organisms in polluted zones, etc.) and food preparation

59

(e.g. grilling, roasting or smoking). Epidemiological studies have proved the exposure

60

to these compounds is one of the important factors responsible for the increase in

61

cancer.4 In order to protect public health, almost all countries have regulated the

62

presence of PAHs in foods. Due to the complexity of food samples, extremely low

63

concentration of PAHs in food samples and the relatively low maximum residue limit

64

(MRL) defined by the legislation, the efficient and sensitive determination of PAHs

65

has been a challenging task.

66

The general procedure for the analysis of PAHs in foods comprises three steps:

67

extraction, clean-up, and quantification. Compared to quantification, extraction and

68

clean-up steps have been the breakthrough for accurate and sensitive determination of

69

PAHs in foods, because the main technique for quantification is still liquid

70

chromatography (LC), equipped with a diode array detector (DAD), fluorescence

71

detector (FLD) or a mass spectrometer (MS), or gas chromatography (GC) with

72

different detectors, and the room for improvement is very limited.5 In recent years,

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various methods were utilized for PAHs enrichment, such as supercritical fluid

74

extraction (SFE)6, the alkaline saponification in alcoholic medium7 and solid phase

75

extraction (SPE)8. SFE is an environmentally friendly technique for extraction of

76

PAHs, but there are some limitations in the application on account of its low

77

robustness and initial high-cost.9 It is not a suitable method for a wide range of

78

applications though it has many outstanding features. The alkaline saponification in

79

alcoholic medium is also often used for PAHs extraction, but it is too time-consuming,

80

requiring about 2-4 h.7 SPE is another important technique for extraction of PAHs, but 3



81

it is difficult for rapid separation after treatment from a large volume solution, which

82

is a serious problem to overcome. Magnetic solid phase extraction (MSPE) is based

83

on the universal dispersion of a magnetizable or magnetic material as the sorbent,

84

realizing a rapid isolation by an external magnet.10 Additionally, applying an external

85

magnetic field facilitates the removal of non-adsorbed species in the adsorption and

86

desorption steps, while allowing the enrichment of adsorbed species.11 Compared with

87

traditional SPE, MSPE has many obvious advantages including reduction of the time

88

taken for analysis by reducing the steps in the extraction procedure, low consumption

89

of organic solvents, and easy separation of the analyte using an external magnet. Also

90

the high back pressure caused by tightly packed SPE cartridges can be avoided,3, 12

91

which expands its application in extracting analytes from real samples.

92

Porous materials have received burgeoning attention over the past decade in the

93

field of sample preparation because of their outstanding performance.1, 13-15 Among

94

them, metal organic frameworks (MOFs), which are a class of crystalline materials

95

formed by the self-assembly between metal ions and organic ligands,16, 17 have been

96

extensively employed in the field of enrichment18-21 and separation22, 23. However,

97

there are still some restrictions in the application of MOFs due to poor stability. Many

98

MOFs are lacking in chemical stability when they are exposed to solvents, especially

99

moisture, since coordination bonds are the linkage mode between organic ligands and

100

metal moieties of MOFs. More recently, covalent organic frameworks (COFs) as a

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novel kind of porous materials are emerging into the advanced materials field, which

102

are assembled reversibly through strong covalent bonds (C-C, C-O, B-C and

103

Si-C).24-26 Compared with MOFs, COFs possess the obvious advantages of greater

104

specific surface, better structure stability27 and π - π stacking interaction, etc. Due to 4


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its merits, the applications of COFs in separation science have gained increasing

106

attention recently.

107

In the current work, we present a facile strategy for the synthesis of core-shell

108

architecture magnetic COFs (Fe3O4@COF(TpBD)) for efficient enrichment of trace

109

PAHs in smoked pork, wild fish, grilled fish, smoked bacon, coffee and water.

110

MATERIALS AND METHODS

111

Reagents and Chemicals.

112

Ferric chloride hexahydrate (FeCl3·6H2O), ammonium acetate (NH4OAc) and

113

sodium citrate were purchased from Sinopharm Chemical Reagent Co., Ltd.

114

(Shanghai, China). Benzidine (BD) and 1,3,5-triformylphloroglucinol (Tp) were

115

purchased from Sigma-Aldrich (Shanghai, China). Eight standards (Figure 1)

116

including naphthalene, 1, acenaphthylene, 2, fluorene, 3, phenanthrene, 4, anthracene,

117

5, fluoranthene, 6, pyrene, 7 and perylene, 8 were purchased from Aladdin Industrial

118

Corporation

119

benzo[a]anthracene, 9, chrysene, 10, benzo[b]fluoranthene, 11, benzo[k]fluorathene,

120

12, benzoapyrene, 13, dibenz[a,h]anthracene, 14 and benzo[g,h,i]peryrene, 15 were

121

purchased from Sigma-Aldrich (Shanghai, China) with purity > 99%. The standard

122

analytes were dissolved in acetonitrile to prepare stock solutions. All standard

123

solutions were stored in a refrigerator at 4 ºC in darkness when not in use. HPLC

124

grade acetonitrile, acetone, n-hexane, methanol, dichloromethane and ethanol were

125

purchased from Yucheng Chemical Reagent Co. (Shandong Province, China). Pure

126

distilled water was purchased from Watson (Guangzhou, China). Ethylene glycol and

127

tetrahydrofuran (THF) were purchased from Shanghai Chemical Reagent Co.

128

(Shanghai, China).

(Shanghai,

China).

Seven

standards

5


(Figure

1)

including


129 130

Apparatus and HPLC Separation. Transmission electron microscope (TEM) images were obtained on a FEI Tecnai

131

G20 (FEI Company, Hillsboro). A MiniFlex 600 diffractometer (Rigaku, Tokyo, Japen)

132

was applied to characterize the X-ray diffraction (XRD) patterns of prepared materials

133

with Cu Kα radiation (λ=0.15418 nm), and a scanning rate of 5 deg/min was applied

134

to record the patterns in the 2θ range of 2-80°. Fourier transform infrared spectra

135

(FT-IR) was operated on a NEXUS-470 fourier transform infrared (FT-IR)

136

spectrometer (Thermo Nicolet, Madison, Wisconsin). Magnetic hysteresis curves were

137

conducted on an MPMS-XL-7 (Quantum Design, USA). The surface area of the

138

obtained materials was determined on an ASAP 2020 micropore physisorption

139

analyzer (Micromeritics, Norcross, GA) at 77 K.

140

Separation experiments were performed using a 1260 HPLC system (Agilent

141

Technologies, Palo Alto, CA, USA), which was equipped with a diode array detection

142

(DAD). The column used was a 150 mm x 4.6 mm i.d., 3 µm, Thermo Hypersil Gold

143

RP-18. The mobile phases were (A) 5% acetonitrile in water and (B) acetonitrile, and

144

the gradient elution condition of mobile phase was as follows: 0 min, 50% B; 30 min,

145

65% B; 35 min, 70% B and 40 min, 100% B. The solvent flow rate was constant at 1

146

mL/min. The detection wavelength was 254 nm. The column was equilibrated with

147

the initial mobile phase for 5 min before the next injection and the injection volume

148

was 10 µL. Peaks were identified and quantified by comparison of retention times and

149

areas using a standard solution containing 15 PAHs with certain concentrations.

150

Preparation of Fe3O4.

151

The Fe3O4 nanocrystal clusters were obtained via a solvothermal method

152

according to a previous report.28 Specifically, 1.35 g of FeCl3.6H2O, 3.85 g of 6


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NH4OAc and 0.4 g of sodium citrate were dissolved in 70 mL of ethylene glycol. The

154

mixture were stirred continuously for 1 h at 100 ºC and then transferred into a

155

Teflon-lined stainless-steel autoclave (100 mL capacity). The autoclave was heated at

156

200 ºC and maintained for 16 h, and then it was cooled to room temperature.

157

Subsequently, the product was poured into a 150 mL beaker and collected by applying

158

a magnet to the outer wall of the beaker. The product was washed with ethanol and

159

water. The process of collection and washing was repeated three times. The final

160

product was dried at 45 ºC for further use.

161

Preparation of Fe3O4@COF(TpBD).

162

The Fe3O4@COF(TpBD) materials were synthesized according to the reported

163

method.27 16 mg of the obtained Fe3O4, 11 mL of THF and 16 mg of BD were mixed

164

in a vial. The mixture was sonicated for 10 min and then refluxed with stirring for 30

165

min at 50 ºC. Subsequently, a THF solution of Tp (12 mg, 4 mL) was dropped into the

166

mixture at a rate of 400 µL/min. The mixture was stirred at 50 ºC for 12 h after

167

completion of the dropwise addition, and the solvent was evaporated on a rotary

168

evaporator at 45 ºC to give the product.

169

Sample preparation.

170

Sample preparation was performed according to reported methods

29-31

with

171

some modifications. Smoked pork, wild fish, grilled fish, smoked bacon and coffee

172

were purchased from local market in Qufu, China. 2 g of meat samples triturated with

173

a grinder were weighed into a round bottom flask. Then 20 mL of a 2M solution of

174

potassium hydroxide in water-ethanol mixture (1:9, v/v) was added to hydrolyze the

175

sample.29 The obtained mixture was refluxed in a water bath at 70 ºC for 2 h. After

176

that, PAHs were extracted from the hydrolyzed sample with 20 mL acetonitrile under 7



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ultrasonic assistance for 20 min. The extracts were centrifuged at 4000 rpm for 10 min

178

and the supernatant was collected. The residue was extracted by 20 mL of acetonitrile

179

twice and the extracts were combined in the flask. The extracts were condensed to

180

dryness using a rotary vacuum evaporator at 50 º C. Then the extracts were

181

redissolved in 1 mL of acetonitrile and were diluted with 9 mL water for the MSPE.30

182

3 g coffee samples were dissolved in 10 mL of hot pure water for 5 min, then filtered

183

and cooled to room temperature.31 River water samples were collected from Yi River

184

in Qufu, China and filtered into a glass bottle, and then stored in a refrigerator at 4 º

185

C.

186

Magnetic Solid Phase Extraction Procedure.

187

As shown in Figure 2, 5 mg of Fe3O4@COF(TpBD) were added to 10 mL of

188

standard solution or sample solution. The solution was vortexed for 12 min. The

189

Fe3O4@COF(TpBD) were then collected by applying a magnet to the outer wall of

190

the vial, and eluted with 1 mL of acetonitrile under ultrasound for 15 min. The

191

supernatant was collected and filtered through a 0.22 µm membrane to eliminate

192

particulate matter before HPLC analysis.

193

Adsorption Characteristics of the Fe3O4@COF(TpBD).

194

The adsorption characteristics of the materials to PAHs were investigated, and

195

their affinity to the Fe3O4@COF(TpBD) was evaluated and the enrichment factors

196

were investigated. The adsorption capacity was calculated as follows:

197

qt=(C0 - Ct)V/m

(1)

198

where qt (mg/g) is the adsorption capacity of the adsorbent. C0 (g/L) is the initial

199

concentration of PAHs, and Ct (g/L) is the concentration of PAHs at adsorption time t.

200

V (mL) and m (g) are the volume of the solution and the weight of sorbent, 8


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201

respectively.

202

Validation of the proposed method

203

The proposed method was validated in terms of linearity, limit of detection

204

(LOD), limit of quantification (LOQ), accuracy and precision. A calibration curve for

205

each PAH was constructed by plotting the peak area versus concentration at eight

206

different concentrations, in the range of 1-100 ng/mL under the optimized

207

experimental conditions. The LODs and LOQs were estimated in accordance with the

208

baseline noise method at a signal-to-noise ratio (S/N) of 3 and 10, respectively. The

209

accuracy of the method was evaluated by the recovery. Recovery studies were carried

210

out for three replicates by spiking a known amount of standard to samples. Recoveries

211

were calculated from the differences in total concentrations between spiked and

212

unspiked samples. The precisions were expressed as the intra-day and inter-day

213

relative standard deviations (RSDs), which were assessed by the analysis of 15 PAHs

214

in a standard solution. Intra-day precision was determined by running a standard

215

solution with three replicates in the same day, and inter-day precision was obtained by

216

analyzing the same sample every two days with three replicates.

217

RESULTS AND DISCUSSION

218

Characterization of the Fe3O4@COF(TpBD).

219

Transmission electron microscope (TEM) was utilized to characterize the

220

as-obtained Fe3O4 and Fe3O4@COF(TpBD). Figure 3A is the TEM image of the

221

Fe3O4 nanomaterials, showing the uniform spherical shape. Figure 3B is the TEM

222

image of the Fe3O4@COF(TpBD) nanomaterials. It can be seen that the covalent

223

organic frameworks deposited on the Fe3O4 particles after polymerization, forming an

224

about 100 nm thick organic shell with continuous and smooth appearance. And the 9



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microspheres evidently displayed the well-defined core-shell appearance, and the

226

uniform spherical shape, without observation of any impurities.

227

The

chemical

composition

and

structure

of

the

Fe3O4@COF(TpBD)

228

nanomaterials were confirmed by Fourier transform infrared (FT-IR) spectroscopy

229

(Figure 3C). Sample pellets were prepared using anhydrous KBr and the

230

corresponding materials. The transmittance of the pellets was measured from 4000 to

231

400 cm-1. Compared to the spectra of COF(TpBD), that of Fe3O4@COF(TpBD)

232

showed additional adsorption bands at 606 cm-1, which was attributed to the Fe-O-Fe

233

vibrations of magnetite.32 The characteristic stretching bands of O-H group and

234

aromatic C=C units were at 3432 cm-1 and 1442 cm-1, respectively. The signals at

235

1292 cm-1 and 1605 cm-1 were compatible with the presence of C-N stretching27 and

236

C=N bonds33.

237

To further evaluate the functionalization of COF(TpBD) on the surface of Fe3O4,

238

the XRD patterns of the Fe3O4, COF(TpBD) and Fe3O4@COF(TpBD) nanomaterials

239

were obtained. As shown in Figure 3D, the most intense peak at 7.3º and 27.4º

240

corresponding to the (200) and (001) reflection plane was the same as the reported

241

data,27 with the other diffraction peaks at 30.3º, 35.7º, 43.1º, 54.1º, 57.5º and 62.9º

242

indexed to the cubic structure of Fe3O4 crystals by the Mercury 3.6 software. In

243

contrast, COF(TpBD) had only two very weak shoulders at about 6.3º and 27.4º,

244

confirming the successful preparation of Fe3O4@COF(TpBD).27

245

To evaluate the porous structure, N2 adsorption measurements were carried out at

246

77 K.27 As shown in Figure 3E, the Fe3O4@COF(TpBD) microspheres showed typical

247

type IV characteristics, which were indicative of a mesoporous character.32 The

248

Brunauer-Emmett-Teller (BET) surface area and pore volume were calculated to be as 10


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high as 114.55 m2/g and 0.31 cm3/g, respectively, and the average pore diameter of the

250

Fe3O4@COF(TpBD) was approximately 5.34 nm. The above results demonstrated the

251

enhanced surface area and porosity in Fe3O4@COF(TpBD).

252

The magnetic hysteresis curve (Figure 3F) indicated that the Fe3O4@COF(TpBD)

253

possessed superparamagnetic properties with a saturated magnetization value of 61.1

254

emu/g. Compared with the pure Fe3O4 (87.8 emu/g), the magnetic content in the

255

Fe3O4@COF(TpBD) reached 69.6%. Such high saturation magnetization endowed the

256

Fe3O4@COF(TpBD) with a fast response to an external magnetic field.

257

Optimization of MSPE conditions.

258

In order to realize the rapid and efficient enrichment, the major experimental

259

parameters that may affect the performance of MSPE were investigated, including

260

eluent, amount of sorbent, adsorption time and desorption time. The performance of

261

MSPE could be evaluated by determining the extraction recovery (ER), which is

262

defined as the ratio of the total amount of the PAHs in the desorption solution after

263

MSPE extraction and the total amount of PAHs originally present in standard solution

264

before MSPE extraction.

265

It is well known that the type of eluent is critical to MSPE extraction efficiency.

266

Therefore, different types of eluent were applied including dichloromethane,

267

acetonitrile, acetone and n-hexane. Acetonitrile gave the highest ER, and so it was

268

selected as the eluent in the following MSPE experiments.

269

Extraction efficiency is usually enhanced by increasing the adsorption time until

270

reaching the extraction equilibrium. In this work, the effect of adsorption time on

271

extraction efficiency was conducted by varying the oscillation time in the range of 2

272

to 24 min with a fixed desorption time of 20 min and a fixed sorbent amount of 10 mg. 11



273

The ER increased in the range of 2 to 12 min, but further prolonging the extraction

274

time there was no significant change. Therefore, 12 min was selected as the optimal

275

extraction time for further work.

276

Desorption time was also studied in the range of 5-30 min with a fixed

277

adsorption time of 12 min and a fixed sorbent amount of 10 mg. The ER increased in

278

the range of 5-15 min, and the further extension of desorption time has no significant

279

change. Therefore, 15 min was selected as the optimal desorption time in the

280

following extractions.

281

The effect of the sorbent amount on extraction efficiency was studied in the range

282

of 1-10 mg under the optimized adsorption and desorption time. With the increase of

283

Fe3O4@COF(TpBD), the ER was enhanced for PAHs when the sorbent amount

284

increased from 1 to 5 mg. Further increase in the amount of Fe3O4@COF(TpBD) did

285

not result in an obvious change. So, 5 mg was selected as the optimal condition in the

286

following MSPE.

287

Investigate on adsorption kinetics

288

The investigation of adsorption kinetics can assist the research of the rate of

289

PAHs adsorption for the Fe3O4@COF(TpBD) sorbent. In this work, we calculated the

290

amount of PAHs adsorbed by the Fe3O4@COF(TpBD) under the condition of room

291

temperature. The Fe3O4@COF(TpBD) achieved adsorption saturation at 12 min.

292

For further explication, we utilized different kinetic models to simulate the

293

experimental data, such as pseudo-first-order model (introduced initially by Lagergren)

294

and pseudo-second-order model32. The pseudo-first-order model can be expressed as:

295 296

ln(qe-q) = lnqe- k1t34

(2)

where the constants t (min) and q (mg/g) are the adsorption time and the 12


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adsorption capacity of the sorbent, respectively, qe is the adsorption capacity of the

298

sorbent at equilibrium, and k1 is the rate constant of pseudo-first-order model. This

299

equation is also the intraparticle diffusion equation34-37 and is usually used to describe

300

the physisorption behavior between the sorbent and target analyte.

301

The pseudo-second-order model can be expressed as: t/q= 1/(k2qe2) + t/qe34

302

(3)

303

where the constants t (min) and q (mg/g) are the adsorption time and the

304

adsorption capacity of the sorbent, respectively, qe is the adsorption capacity of the

305

sorbent at equilibrium, and k2 is the rate constant of pseudo-second-order model. The

306

pseudo-second-order model is often used to describe the process controlled by the

307

adsorption reaction at the liquid/solid interface in the adsorbent38.

308

In order to evaluate which model the system fits best, we compared R2 values for

309

the two different models: pseudo-first-order (y(1) = ln(qe-q)); and pseudo-second-order

310

(y(2)=t/q). The determination coefficient R2 should be computed for the same function

311

y=q in both cases, so as to get a reliable estimate for the comparison of these two

312

models.34

313

(R2=0.6625-0.9395) and pseudo-second-order model (R2=0.9902-0.9977) indicated

314

that the pseudo-second-order model was better at describing the behavior of

315

Fe3O4@COF(TpBD) sorbent for PAHs adsorption, suggesting that the adsorption was

316

based on the adsorption capacity of the surface sites on Fe3O4@COF(TpBD) at

317

equilibrium.33

318

Reusability of the Fe3O4@COF(TpBD).

The

comparison

of

the

R2 between

pseudo-first-order

model

319

The reusability of the Fe3O4@COF(TpBD) for adsorption of PAHs was tested in

320

several successive runs, with the Fe3O4@COF(TpBD) being washed three times with 13



321

ethanol and air-dried at room temperature before the next use). The reusability of the

322

Fe3O4@COF(TpBD) was evaluated by normalizing the adsorption capacity of the

323

PAHs at each adsorption run in a ratio to the adsorption capacity in the first cycle. We

324

observed that the adsorption capacities of the Fe3O4@COF(TpBD) were only

325

decreased by 2-13% after three runs of PAHs adsorption, indicating the mechanical

326

stability and excellent reusability of Fe3O4@COF(TpBD).

327

Method validation and comparison

328

Good linearity was achieved for 15 PAHs with the linear correlation coefficients

329

(R2) being above 0.9990. The LODs and LOQs for these PAHs were calculated at S/N

330

ratio of 3 and 10, ranging from 0.83 to 11.70 ng/L and 2.76 to 39.00 ng/L, respectively.

331

The real samples unspiked and spiked with PAHs at two concentration levels (1 and

332

10 µg/L for water sample, 1 and 10 µg/kg for solid sample) were analyzed by the

333

proposed method (n=3). The results of recovery are summarized in Table 1.

334

Recoveries of the PAHs at two concentration levels were in the range of 84.3-107.1%,

335

with RSDs within 4.3%, indicating the good accuracy of the proposed method. The

336

intra-day precision for the tested sample ranged from 1.6-3.7%, while the inter-day

337

precision was 2.5-4.3%. The above results indicated that the proposed method is

338

reliable for the determination of PAHs in food samples.

339

In order to clearly describe the advantages of the proposed method, the

340

comparisons of the developed method to previously reported methods for PAHs

341

detection is summarized. The proposed method exhibited several advantages. First,

342

compared to the reported materials, less sorbent, eluent and time were required to

343

concentrate the trace targeted PAHs from larger volumes of samples, indicating the

344

prepared Fe3O4@COF(TpBD) with higher extraction efficiency and better 14


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applicability for targeted PAHs determination. The recovery of the proposed method

346

was higher than most of the reported methods. Our new method also offered lower

347

LODs than the methods based on HPLC-UV or HPLC-DAD. The results proved that

348

the proposed method was facile and sufficiently sensitive for PAHs detection in

349

complex food samples.

350

Analysis of PAHs in real samples.

351

The proposed method was evaluated for its potential application for PAHs

352

extraction in real samples, including smoked pork, wild fish, grilled fish, smoked

353

bacon, coffee and water. Real samples unspiked and spiked with 15 PAHs at two

354

concentration levels were analyzed by the proposed method (n=3). The results are

355

shown in Table 1 and the representative HPLC chromatograms are shown in Figure 4.

356

In coffee sample, only phenanthrene was found, while six PAHs were detected in the

357

smoked bacon sample with a content range of 0.001-0.006 µg/kg. Additionally,

358

fluorene and benzoapyrene in smoked pork sample were found with concentrations of

359

5.24 and 2.82 µg/kg respectively. As indicated in Table 1, fluorene, phenanthrene and

360

anthracene were determined in wild fish, and higher value of PAHs was observed in

361

grilled fish sample than other tested foods samples. Five PAHs were detected in the

362

water, ranging from 0.02-1.43 µg/L.The above results revealed that the proposed

363

method showed excellent applicability and feasibility for trace PAHs analysis in

364

complex food samples.

365

Fe3O4@COF(TpBD) as a sorbent in MSPE for the extraction of PAHs from food

366

samples. We anticipate the present study may provide new insights and inspirations

367

for the detection of trace yet hypertoxic PAHs.

This is the first report of the

368 15


application of


369

Page 16 of 27

Supporting Information

370

This material is available free of charge via the Internet at http://pubs.acs.org.

371

Supporting information include the recovery of different extraction solvents for

372

smoked bacon and grilled fish, calibration curves, LODs, LOQs, enrichment

373

efficiency and precision data for the method, determination and recoveries of 15 PAHs

374

in river water samples, comparison of our method with the reported methods,

375

optimization of MSPE conditions, and the reusability of the Fe3O4@COF(TpBD).

376

Notes

377

The authors declare no competing financial interest.

378

Acknowledgments

379

This work was supported by The National Natural Science Foundation of China

380

(No. 21677085, 21537001 and 81472986), the Natural Science Foundation of

381

Shandong Province (ZR201702150005), the Project funded by China Postdoctoral

382

Science Foundation (No ZR2017JL012) and the Development Project of Qinghai Key

383

Laboratory (No. 2017-ZJ-Y10).

384 385 386 387 388 389 390 391 392 393 394 395 396 397

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Page 21 of 27


Figure Caption Figure 1. Chemical structures of naphthalene, 1, acenaphthylene, 2, fluorene, 3, phenanthrene, 4,

anthracene, 5,

fluoranthene,

pyrene,

6,

7, perylene, 8

benzo[a]anthracene, 9, chrysene, 10, benzo[b]fluoranthene, 11, benzo[k]fluorathene, 12, benzoapyrene, 13, dibenz[a,h]anthracene, 14 and benzo[g,h,i]peryrene, 15. Figure 2. Schematic fabrication process of Fe3O4@COF(TpBD) and application to magnetic solid phase extraction. Figure 3. (A) TEM images of Fe3O4 nanocrystal clusters; (B) TEM images of Fe3O4@COF(TpBD);

(C)

FT-IR

spectra

of

Fe3O4,

COF(TpBD)

and

Fe3O4@COF(TpBD);

(D)

XRD

spectra

of

Fe3O4,

COF(TpBD)

and

Fe3O4@COF(TpBD); (E) The N2 adsorption-desorption isotherms of as-synthesized Fe3O4@COF(TpBD); (F) Magnetic curves of Fe3O4 and Fe3O4@COF(TpBD). Figure 4. Typical chromatograms obtained after MSPE from spiked real samples. Peak identification: 1, naphthalene; 2, acenaphthylene; 3, fluorene; 4, phenanthrene; 5, anthracene; 6, fluoranthene; 7, pyrene; 8, perylene; 9, benzo[a]anthracene; 10, chrysene; 11, benzo[b]fluoranthene; 12, benzo[k]fluorathene; 13, benzoapyrene; 14, dibenz[a,h]anthracene; 15, benzo[g,h,i]peryrene.

21



Page 22 of 27

Table 1 Determination and Recoveries of 15 PAHs in Real Samples (n=3). Analytes

Smoked pork

Wild fish

Grilled fish

Smoked bacon

Coffee

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Found (µg/kg)

N.D.

N.D.

5.24

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

2.82

N.D.

N.D.

Recoverya (%)

88.7±1.4

86.6±2.5

99.2±1.7

98.4±3.4

97.1±2.6

98.3±3.2

96.8±1.3

100.2±2.1

95.6±2.5

93.4±1.8

89.6±1.1

93.5±3.4

91.7±3.7

95.2±1.5

102.3±2.7

Recoveryb (%)

99.3±1.7

100.2±2.3

102.3±2.5

97.8±3.1

87.6±1.6

102.5±1.8

95.4±1.5

98.6±2.6

89.8±3.2

89.3±2.4

95.1±1.4

95.3±3.5

89.4±2.4

99.2±2.1

95.7±2.4

Found (µg/kg)

N.D.

N.D.

18.72

12.02

12.03

N.D

N.D

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

Recoverya (%)

86.9±3.5

95.8±2.7

91.1±1.7

98.3±2.5

97.3±2.4

96.7±2.7

98.4±1.4

94.9±1.7

99.2±1.9

101.2±2.1

89.1±1.9

102.9±2.3

94.5±2.3

99.6±2.7

101.4±3.8

Recoveryb (%)

96.7±2.1

88.1±3.9

86.9±2.7

94.2±3.3

95.6±2.7

95.5±3.5

102.6±1.3

101.6±2.4

99.5±3.2

105.1±2.3

101.3±2.6

92.6±2.1

99.8±2.6

96.1±3.5

103.7±3.1

Found (µg/kg)

N.D.

N.D.

89.12

76.38

70.45

N.D.

23.26

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

Recoverya (%)

86.7±2.4

92.1±1.5

89.3±4.1

91.5±3.4

94.2±2.6

88.5±1.8

96.9±3.5

95.7±1.9

99.4±2.9

96.7±2.5

101.2±2.4

89.8±1.8

93.4±2.7

94.8±3.4

100.3±3.6

Recoveryb (%)

89.6±1.6

102.4±4.0

91.3±1.4

94.6±2.3

96.1±3.1

87.4±3.7

88.7±2.5

93.6±3.6

95.6±1.7

102.3±3.4

94.8±1.9

103.4±2.8

93.7±1.7

96.9±2.7

98.5±2.4

Found (µg/kg)

0.001

0.001

0.001

0.001

N.D.

0.006

N.D.

N.D.

N.D.

0.001

N.D.

N.D.

N.D.

N.D.

N.D.

Recoverya (%)

85.7±2.1

84.6±2.3

99.2±1.5

88.4±2.4

95.1±0.6

95.3±3.4

86.8±1.3

104±2.1

97.6±2.5

91.4±1.8

86.6±2.1

95.5±2.4

101.7±2.7

98.2±2.5

102.1±3.7

Recoveryb (%)

100.3±0.7

100.2±1.3

104.3±2.5

87.8±2.1

84.6±2.6

103.5±1.5

85.4±3.5

88.93±1.6

89±3.6

86.3±2.4

85.1±1.4

85.3±3.2

88.4±3.4

100.2±2.1

85.7±2.4

Found (µg/kg)

N.D.

N.D.

N.D.

0.05

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

N.D.

Recoverya (%)

88.7±1.5

94.5±1.8

104.3±1.2

86.8±3.2

85.2±0.8

98.1±2.1

89.1±1.8

97.8±1.7

88.5±4.3

95.5±2.9

85.7±2.1

84.4±3.6

99.8±1.5

101.1±3.4

88.5±2.7

Recoveryb (%)

84.3±3.2

84.8±1.6

99.2±2.1

87.6±2.1

84.8±3.2

103.2±4.3

85.4±3.6

87.1±2.4

85.8±1.5

89.6±1.5

85.2±1.7

85.7±2.9

87.3±4.1

96.9±2.8

98.1±3.5

N.D.: not detected; Recoverya and Recoveryb: spiked with 10 µg/kg level and 1 µg/kg of PAHs, respectively; Analytes: 1, naphthalene; 2, acenaphthylene; 3, fluorene; 4, phenanthrene; 5, anthracene; 6, fluoranthene; 7, pyrene; 8, perylene; 9, benzo[a]anthracene; 10, chrysene; 11, benzo[b]fluoranthene; 12, benzo[k]fluorathene; 13, benzoapyrene; 14, dibenz[a,h]anthracene; 15, benzo[g,h,i]peryrene.

22


Page 23 of 27


Figure 1.

23



Figure 2.

24


Page 24 of 27

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Figure 3.

25



Figure 4.

26


Page 26 of 27

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Table of Contents Graphic

27


Effective Enrichment and Detection of Trace Polycyclic Aromatic

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