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Antioxidative properties and interconversion of tertbutylhydroquinone and tert-butylquinone in soybean oils Jun Li, Yanlan Bi, Huifang Yang, and Donghai Wang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b04517 • Publication Date (Web): 12 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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Journal of Agricultural and Food Chemistry

Antioxidative properties and interconversion of tert-butylhydroquinone and tert-butylquinone in soybean oils

Jun Li a,b, Yanlan Bia,*, Huifang Yanga, Donghai Wangb,*

a

Lipid Technology and Engineering, School of Food Science and Engineering,

Henan University of Technology, Lianhua Road, Zhengzhou 450001, Henan, China. b

Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, US

* The correspondence author Yanlan Bi: Tel: +86-0371-67758022 Fax: +86-0371-67758022 E-mail: [email protected] Donghai Wang: Tel: 785-532-2919 Fax: 785-532-5825 E-mail: [email protected]

1

ACS Paragon Plus Environment


1

Abstract

2

During the process of antioxidation of tert-butylhydroquinone (TBHQ) in oil and fat

3

systems, tert-butylquinone (TQ) can be formed, which has higher toxicity than TBHQ.

4

The changes of TBHQ and TQ in edible oils at room temperature (RT) or under

5

thermal treatment were investigated. Under thermal treatment, volatilization was the

6

main pathway of TBHQ loss in edible oils. TQ was the main oxidation product of

7

TBHQ under thermal treatment as well as at RT. The amount of TQ in thermal treated

8

oils was much less than that in oils stored at RT due to the decreased amount of

9

oxygen dissolved in oils and easy volatilization of TQ at high temperature. In addition,

10

TQ can be reduced to TBHQ by reduction components in edible oils, but the

11

conversion amount was very small. Thus, TQ, theoretically having no antioxidative

12

property, presented a very weak antioxidative activity equivalent to that of BHA due

13

to the presence of insignificant amount of TBHQ formed from TQ in edible oils. The

14

narrow potential difference of 0.059 between oxidation and reduction peaks of TBHQ

15

and TQ resulted in easy interconversion of TBHQ and TQ under the action of

16

common oxidation and reduction substances which have a higher oxidation potential

17

or a lower reduction potential than them.

18 19

Keywords: TBHQ; TQ; Oxidation; Reduction

20

2


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Introduction

22

Tert-butylhydroquinone (TBHQ) is one of the most common synthetic

23

antioxidants usually applied in oils and oleaginous foods to delay the oxidative

24

rancidity of oils and extend their shelf life.1,2 To gain an in-depth understanding to the

25

antioxidative properties of TBHQ, its fate in different types of frying oils has

26

rekindled extensive interests of scholars for recent decades. The results shows that a

27

large amount of TBHQ lose from oils in the form of volatilization under thermal

28

treatment, and the amount of TBHQ loss varies considerably, depending on the types

29

of oils, the added amounts of TBHQ, and thermal treatments etc.3-6 Moreover, Kim et

30

al.7 reported that tert-butylquinone (TQ) was the major oxidation product of TBHQ at

31

frying temperature in the model system. Similar results were reported by Li et al.5,6

32

During the storage of TBHQ-containing oils and oleaginous foods at room

33

temperature (RT), besides participating in the lipid radical reactions, TBHQ is easily

34

oxidized to TQ in the presence of oxygen, metal ions (such as Fe3+ and Cu2+), and

35

hydroperoxides, etc.8 These results are coincident with the fact of the easy

36

interconversion of phenols and quinones. Relevant toxicological studies9-11 have

37

demonstrated that TQ has higher toxicity than TBHQ, which causes a higher potential

38

risk to human health in comparison with TBHQ. Therefore, it’s essential to not only

39

study the changes of TBHQ and TQ in edible oils at RT or under thermal treatment,

40

but also develop analytical methods capable of accurately detecting the contents of

41

TBHQ and TQ in edible oils.

42

Reverse-phase high performance liquid chromatography (RP-HPLC) is the most 3



43

common analytical technique for TBHQ detection in edible oils. Thus, the accuracy of

44

this technique for the detection of TBHQ and TQ has been investigated and reported

45

in our previous research.12 RP-HPLC is not suitable for the detection of TQ because

46

TQ can be reduced to TBHQ by the reduced hydrogen ([H]) in the mobile phase

47

(water and acid solution) of the RP-HPLC system during the injection (Figure S1 of

48

the Supporting Information), resulting in the measured TQ being lower and TBHQ

49

higher than the actual concentrations. To solve this, a normal-phase (NP)-HPLC was

50

successfully developed for the simultaneous and accurate detection of TBHQ and TQ

51

without the oxidation reaction of TBHQ and the reduction reaction of TQ due to no

52

[H] existing in the mobile phase of NP system (Figure S2 of the Supporting

53

Information).

54

Many research works about the changes of TBHQ in edible oils under thermal

55

treatment have been reported.1,3,5-7 To date, however, no relevant studies on the

56

changes of TQ in edible oils at RT or under thermal treatment have been found as

57

well as the changes of TBHQ in edible oils at RT. The primary objective of this study

58

was to investigate the changes of TBHQ and TQ in edible oils at RT and under

59

thermal treatment. Also, the relevant reasons for the interconversion of TBHQ and TQ

60

in the above process were investigated.

61 62

Materials and Methods

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Chemicals. Standards of TBHQ (purity > 99.0%) and TQ (purity > 98.0%) were

64

purchased from Sigma-Aldrich (St. Louis, MO). Fresh refined soybean oil (SBO) 4


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(acid value of 0.13 mg of KOH/g, peroxide value of 1.22 mmol/kg) with no synthetic

66

antioxidants was obtained from Henan Sunshine Oils and Fats Co., Ltd. (Zhengzhou,

67

China). Liquid chromatography (LC) solvents (methanol, n-hexane, isopropanol, and

68

ethyl acetate) were of HPLC grade, obtained from VBS Biologic Inc. (New York),

69

and used after filtration through a 0.45 µm organic membrane. Glacial acetic acid

70

(purity > 99.8%, HPLC grade) was purchased from Kerch Chemical Reagent Co., Ltd.

71

(Tianjin, China). High purity helium (5.0 grade) was purchased from Henan Branch

72

Beneficial Gas Co., LTD. (Zhengzhou, China). All other solvents were

73

analytical-grade and used without further purification.

74 75

Apparatus. The RP-HPLC spectra were measured with a 2695 separation

76

module and an ultraviolet (UV)/visible-2489 detector (Waters, Milford, MA). The

77

NP-HPLC spectra were measured with a 2695 separation module and an

78

UV/visible-2487 detector (Waters, Milford, MA). The GC spectra were measured

79

with a 7820A separation module and a flame ionization detector (FID) (Agilent, Santa

80

Clara, CA). The oxidative stability index (OSI) was measured with a 743 oxidative

81

stability instrument (Metrohm, Herisau, Switzerland). The potential was measured

82

with a CHI-660D electrochemical workstation, and an Ag/AgCl (saturated KCl)

83

electrode, a commercial glassy carbon electrode, and a platinum wire (3 mm diameter)

84

as the reference, working, and auxiliary electrodes, respectively, purchased from

85

Shanghai Chenhua instruments Co., Ltd. (Shanghai, China). A vortex mixer (Staufen,

86

German), SCQ-250B ultrasonic apparatus (Shanghai, China), and TDL-80-2B 5



87

low-speed centrifuge (Shanghai, China) were also used in the experiments.

88 89

Preparation of SBO Containing 2000 mg/kg of TBHQ or TQ. A total of 1.00

90

g (accurately weighed to 0.1 mg, similarly hereafter) of TBHQ or TQ was added to

91

499.00 g of refined SBO, and then shaken until homogeneous.

92 93

Preparation of SBO Containing 50, 100, and 200 mg/kg of TBHQ or TQ. A

94

total of 25.00, 50.00, and 100.00 g of SBO containing 2000 mg/kg of TBHQ or TQ

95

were diluted to 1000 g with control SBO and then shaken until homogeneous.

96 97

Heating Experiment. 5 copies of 500 g of SBO containing 200 mg/kg of TBHQ

98

were held at 105, 120, 135, 150, and 180 oC for 24 h, respectively. Samples were

99

taken at intervals of 0, 1, 2, 4, 6, 8, 10, 12, 16, and 24 h.

100 101

Storage Experiment. 5 copies of 60 g of SBO containing 200 mg/kg of TBHQ

102

or TQ were sealed and stored at RT. 1 copy of stored SBO samples was taken to

103

determine TBHQ and TQ contents every two weeks.

104 105

Extraction Procedure. The extraction procedure was similar to previous

106

methods.6,13,14 A total of 0.50 g of each of oil samples was weighed into a 10 mL test

107

tube, and 4 mL of methanol was added. The mixture was agitated for 3 min using a

108

vortex mixer and then centrifuged at 3000 rpm for 2 min at RT. The supernatant was 6


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quantitatively transferred to a 10 mL volumetric flask. The extraction procedure was

110

repeated twice more, adding 3 mL of methanol each time. Finally, sufficient methanol

111

was added to achieve a 10 mL solution. The solution was filtered through a 0.45 µm

112

membrane before HPLC analysis.

113 114

HPLC Analysis. The conditions of RP-HPLC analysis were as follows:6,15,16 the

115

injection volume for TBHQ or TQ samples was 20 µL; the column was Symmetry C18

116

(4.6 × 250 mm, 5 µm, Waters); the solvent system was methanol-H2O containing 0.5%

117

AcOH (65:35, v/v); the flow rate was 0.8 mL/min; the detection wavelength was set at

118

280 nm; the column temperature was 35 oC. Methanol and H2O containing 0.5%

119

AcOH were both degassed for 15 min by an ultrasonic apparatus.

120

The conditions of NP-HPLC analysis were as follows:12 the injection volume for

121

TBHQ or TQ samples was 20 µL; the column was Sunfire Prep Silica (4.6 × 250 mm,

122

5 µm, Waters); the solvent system was n-hexane containing 5% ethyl acetate (solvent

123

A) and n-hexane containing 5% isopropanol (solvent B); the gradient elution program

124

was isocratic mode of 8% solvent B for 10 min, linear gradient from 8 to 100%

125

solvent B for 5 min, and then isocratic mode of 100% solvent B for 12 min; the flow

126

rate was 0.8 mL/min; the dual detection wavelengths were set at 280 nm for TBHQ and

127

310 nm for TQ, respectively; the column temperature was 30 oC. Solvents A and B

128

were both degassed for 40 min by an ultrasonic apparatus.

129

Identification of compounds was achieved by comparing their retention time to

130

those of standards. Data were collected and processed using Empower 2.0 software 7



131

(Waters Corporation).

132 133

GC Analysis. The conditions of GC analysis were as follows: samples (0.8 µL of

134

TBHQ or TQ standard solution, and 5 µL of oil samples) were injected in split mode.

135

The split ratio was 50:1 (v/v) for TBHQ or TQ standard solution, and 25:1 (v/v) for

136

oil samples. The capillary chromatographic column was a HP-5 MS (Agilent

137

Technologies), 30 m × 0.32 mm i.d., film thickness 0.25 µm. The column, injector,

138

and detector were set at 180, 200, and 300 oC, respectively. Hydrogen- and air-flow

139

rates were 40 and 400 mL/min, respectively. Helium gas was used as the carrier gas at

140

a rate of 0.5 mL/min.

141 142

OSI Analysis. The OSI analysis was determined as defined in the AOCS Official

143

Method.17 The instrument was run at 100, 110, 120, 130, and 140 oC, respectively.

144

Sample size used was 5.00 g. The volume of deionized water in measuring vessel was

145

50 mL. Air-flow rate was set at 9 L/h for all analyses.

146 147

The potential analysis. The concentrations of TBHQ and TQ methanol solutions

148

were both 10 µg/mL. The pH value of Britton-Robinson buffer was 2.0. The scanning

149

rate was 100 mV/s.

150 151

Statistical Analyses. All experiments were performed at least in duplicate. All

152

data were presented as the mean ± the standard deviation (SD). The significance of 8


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the differences was assessed using a one-way analysis of variance (ANOVA). Data

154

evaluation was performed using SPSS software for Windows (version rel. 16.0, SPSS

155

Inc., Chicago, IL, USA). Differences were considered significant when the p value

156

was < 0.05.

157 158

Results and Discussion

159

Changes of the contents of TBHQ in SBO containing TBHQ under thermal

160

treatment. As temperatures and heating time increased, the remaining contents of

161

TBHQ in SBO decreased gradually (p < 0.05) (Figure 1). For example, when oil

162

samples were held at 135 oC for 2, 4, 8, 12, 16, and 24 h, the remaining contents of

163

TBHQ in SBO were 195.04, 168.04, 115.61, 71.18, 39.51, and 10.26 mg/kg,

164

respectively. When oil samples were held at 105, 120, 135, 150, and 180 oC for 12 h,

165

the remaining contents of TBHQ in SBO were 170.55, 106.25, 71.18, 53.07, and

166

38.25 mg/kg, respectively. This indicated that a large amount of TBHQ were lost from

167

SBO under heating conditions. The main reason was its easy volatilization (Figure S3

168

of the Supporting Information), which is the major pathway for TBHQ loss under the

169

heating conditions as described in our previous studies.5,6 It was also reported that TQ

170

was the major transformation product of TBHQ.5,12 However, the contents of TQ in

171

heated oils were tiny due to the decreased amount of oxygen dissolved in oils and

172

easy volatilization of TQ at high temperature.

173 174

Changes of the contents of TBHQ and TQ in SBO containing TBHQ at RT. 9



175

As the storage time prolonged, the contents of TBHQ in SBO decreased gradually (p

176

< 0.05) and the contents of TQ in SBO increased gradually (p < 0.05) (Figure 2),

177

respectively. For example, when SBO samples were stored for 4 and 8 weeks at RT,

178

the contents of TBHQ decreased from initial 197.35 to 182.98 and 178.59 mg/kg,

179

respectively, and the contents of TQ increased from initial non-existence to 14.17 and

180

16.15 mg/kg, respectively. This indicated that partial TBHQ was converted to TQ

181

while TBHQ played an antioxidative role in SBO.

182

The contents of TBHQ loss in SBO were almost equivalent to the contents of TQ

183

formed in SBO (Figure 2), further indicating that TQ was the major transformation

184

product of TBHQ. As reported by our previous studies5,12 (Figure S4 of the

185

Supporting Information), there are two pathways for the conversion of TBHQ to TQ

186

in edible oils. One pathway is that TBHQ is directly oxidized to TQ by the oxygen

187

dissolved in the edible oils. Another is that TBHQ reacts with lipid radicals (R·) or

188

lipid peroxide radicals (ROO·) and finally TQ is formed.

189 190

Antioxidative properties of TQ. A slight difference of oxidation induction time

191

(OIT) was observed between SBO with and without TQ (p < 0.05) (Figure 3). For

192

example, the OIT of SBO containing 200 mg/kg of TQ at 100, 110, and 120 oC was

193

12.27, 6.12, and 3.24 h, respectively, and the OIT of control SBO was 11.92, 5.68,

194

and 3.17 h, respectively. This indicated that TQ had an antioxidative effect on SBO,

195

but very weak. It was also observed that the OIT of SBO containing TQ was much

196

less than that of SBO containing TBHQ (p < 0.05). 10


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By comparing the OIT of SBO containing TBHQ, TQ, butylated hydroxytoluene

198

(BHT), and butyl hydroxyl anisd (BHA) at 120 oC (Figure 4), it was found that the

199

antioxidative property of TQ was equivalent to that of BHA (p > 0.05). Similar results

200

were reported by Kureci et al.18,19 However, the antioxidative property of TQ was less

201

than that of BHT and, particularly, TBHQ (p < 0.05).

202

Based on the chemical structure of TQ (Figure S5 of the Supporting Information),

203

it can be seen that theoretically TQ itself has no antioxidative property. Therefore, a

204

hypothesis was proposed that the weak antioxidative effect presented by TQ might be

205

because a fraction of TQ was reduced to TBHQ by the reduction components in SBO

206

and then this small amount of TBHQ presented a weak antioxidative property.

207 208

Changes of TQ in SBO containing TQ under thermal treatment. To

209

investigate the changes of TQ in SBO containing TQ under thermal treatment, 30.00 g

210

of SBO with 2000 mg/kg of TQ was weighed into a 100-mL flat-bottom flask and

211

then heated for 1 h at 120 oC under a reflux condensation mode. After the end of the

212

reaction, it was observed that a large amount of light yellow volatiles from heated

213

SBO had condensed on the inner surface of the end of the condenser (Figure 5). To

214

analyze the composition of these volatiles, the condenser was eluted with methanol

215

and all the methanol solution was collected and concentrated to 3 mL using a

216

nitrogen-blowing method at RT. Finally, the concentrated samples were analyzed by

217

GC with helium as the carrier gas (Figure 6A). The peaks of TQ and TBHQ were

218

clearly observed at 5.05 and 8.29 min with the relative contents of 97.19 and 2.81%, 11



219

respectively. Also, the methanol extracting solutions of the heated SBO were analyzed

220

by GC with the same carrier gas (Figure 6B). The peaks of TQ and TBHQ were also

221

observed clearly at 5.05 and 8.29 min with the relative contents of 93.27 and 6.73%,

222

respectively. These results confirmed the hypothesis mentioned above that partial TQ

223

was reduced to TBHQ by the reduction components from the heated SBO under

224

thermal treatment, compared to the original contents of TBHQ (1.44%) and TQ

225

(98.56%) in TQ standard. This indicated that the antioxidative effect of TQ was due to

226

the role of TBHQ from reduced TQ in heated SBO. In addition, the relative contents

227

of TQ in heated SBO were less than that in volatiles and the relative contents of

228

TBHQ in heated SBO were more than that in volatiles, suggesting that TQ was more

229

volatile than TBHQ under heating conditions.

230 231

Changes of the contents of TBHQ and TQ in SBO containing TQ at RT. As

232

the storage time prolonged, the contents of TQ in SBO decreased gradually (p < 0.05)

233

and the contents of TBHQ in SBO increased gradually (p < 0.05) (Figure 7),

234

respectively. For example, when SBO samples were stored for 4 and 8 weeks at RT,

235

the contents of TQ decreased from initial 196.11 to 181.27 and 147.42 mg/kg,

236

respectively, and the contents of TBHQ increased from initial non-existence to 5.41

237

and 7.81 mg/kg, respectively. This further indicated that partial TQ was reduced to

238

TBHQ by the reduction components in SBO. In addition, the contents of TQ loss in

239

SBO were much more than that of TBHQ formed in SBO (Figure 7), suggesting that

240

only an insignificant amount of TQ was reduced to TBHQ in SBO. This further 12


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241

confirmed that the antioxidative effect presented by TQ was very weak in SBO.

242 243

The potential of TBHQ and TQ. Based on the above analysis, it was observed

244

that the oxidation reaction of TBHQ and the reduction reaction of TQ easily occurred,

245

which was in accordance with the easy interconversion of phenols and quinones. This

246

largely depends on their potential. As shown in Figure 8 and Table 1, the oxidation

247

and reduction potential of TBHQ was 0.490 and 0.431, respectively, and the oxidation

248

and reduction potential of TQ was 0.486 and 0.427, respectively. There only had a

249

narrow potential difference of 0.059 between oxidation and reduction peaks, resulting

250

in the easy interconversion between TBHQ and TQ under the action of common

251

oxidation and reduction substances which have a higher oxidation potential or a lower

252

reduction potential than them.

253 254

Abbreviations used

255

TBHQ, tert-butylhydroquinone; TQ, tert-butylquinone; SBO, soybean oil; HPLC,

256

high performance liquid chromatography; GC, gas chromatography; RP, reverse phase;

257

NP, normal phase; RT, room temperature; SD, standard deviation; RSD, relative

258

standard deviation.

259 260

Acknowledgement

261

The authors declare no competing financial interest. The authors also thank Dr.

262

Richard Akins for help in reviewing this paper. 13



263 264

Funding sources

265

This work was financially supported by National Natural Science Foundation of

266

China (No. 31671818).

267 268

Supporting Information Description

269

Supporting Information Available: Superimposed RP-HPLC chromatograms of 5, 10,

270

and 20 µg/mL of TBHQ (profile A) and TQ (profile B) standard solutions, NP-HPLC

271

chromatograms of TBHQ (profile A) and TQ (profile B) standard solutions,

272

Condensed volatiles from SBO containing TBHQ, Two pathways of TBHQ

273

converting to TQ, Chemical structures of TBHQ and TQ.

274

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products of tertiary butyl hydroquinone (TBHQ) (II). J. Am. Oil Chem. Soc. 1983,

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Figure captions:

335

Figure 1. Changes of the contents of TBHQ in SBO containing 200 mg/kg of TBHQ

336

under heating conditions. (Oil samples were analyzed with RP-HPLC.)

337

Figure 2. Changes of the contents of TBHQ and TQ in SBO containing 200 mg/kg of

338

TBHQ at RT. (Oil samples were analyzed with NP-HPLC.)

339

Figure 3. Comparison of antioxidative effect of TBHQ and TQ on SBO at different

340

temperatures. (The contents of TBHQ and TQ added in SBO were both 200 mg/kg.)

341

Figure

342

butylated-hydroxytoluene (BHT), and butyl-hydroxyl-anisd (BHA) on SBO at 120 oC.

343

(The contents of TBHQ, TQ, BHT, and BHA added in SBO were 200 mg/kg.)

344

Figure 5. Condensed volatiles from SBO containing 2000 mg/kg of TQ at 120 oC.

345

(Conditions: a total of 30.00 g of SBO with 2000 mg/kg of TQ was weighed into a

346

100-mL flat-bottom flask and then heated for 1 h at 120 oC under the reflux

347

condensation mode.)

348

Figure 6. GC chromatograms of condensed volatiles from SBO containing 2000

349

mg/kg of TQ at 120 oC (A), methanol extracting solution of SBO containing 2000

350

mg/kg of TQ at 120 oC (B), TBHQ methanol solution (C) and TQ methanol solution

351

(D) with helium as carrier gas. (The relative contents of TQ and TBHQ in TQ

352

methanol solution were 98.56 and 1.44%, respectively. The relative contents of TQ

353

and TBHQ in TBHQ methanol solution were 0.47 and 99.53%, respectively.)

354

Figure 7. Changes of the contents of TBHQ and TQ in SBO containing 200 mg/kg of

355

TQ at RT. (Oil samples were analyzed with NP-HPLC.)

4.

Comparison

of

antioxidative

effect

18


of

TBHQ,

TQ,

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Figure 8. The potential of TBHQ and TQ.

19



Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Table 1. The potential of TBHQ and TQ. Oxidation peak

Reduction peak Potential difference

Potential/V

Current/µA Potential/V Current/µA

TBHQ

0.490

4.808

0.431

5.679

0.059

TQ

0.486

6.962

0.427

8.592

0.059

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Table of contents (TOC) Graphics

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Antioxidative Properties and Interconversion of tert ... - ACS Publications

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