Optimization of Fiber Coating Structure Enables Direct Immersion

Subscriber access provided by University Libraries, University of Memphis

Technical Note

Optimization of Fibre Coating Structure Enables Direct Immersion Solid Phase Microextraction and High Throughput Determination of Complex Samples Janusz Pawliszyn, and Erica Silva Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac301305u • Publication Date (Web): 25 Jul 2012 Downloaded from http://pubs.acs.org on July 26, 2012

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

1

Optimization of Fibre Coating Structure Enables

2

Direct Immersion Solid Phase Microextraction and

3

High Throughput Determination of Complex

4

Samples

5 6 7 8 9

Érica A. Souza Silva and Janusz Pawliszyn*

10 11

Department of Chemistry, University of Waterloo, Ontario, N2L 3G1, Canada

12 13

*Corresponding author: [email protected]

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

14

ABSTRACT

15

This study presents a new approach for improving the structure, and hence the

16

robustness, of the SPME fibre coating applied for GC analysis. It involves application of

17

an external layer of PDMS over the commercial PDMS/DVB extraction phase. The fibre

18

provided extraction capabilities similar to that exhibited by the original PDMS/DVB fibre

19

towards triazole pesticides from water samples. Furthermore, the fibre could be utilized

20

for over 100 extractions in direct contact with a complex food matrix such as whole grape

21

pulp, with no sample pre-treatment required. The amount of extracted pesticides from

22

whole

23

extraction/desorption/conditioning cycles which is a dramatic improvement when

24

compared to commercial PDMS/DVB fibre coating applied in food analysis facilitating

25

high throughput automation.

grape

pulp

had

RSD

values

below

20

%

throughout

130


Page 3 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


26

INTRODUCTION

27

Recently there has been growing effort towards the development and improvement of

28

sample preparation methods for simple and fast analysis of complex matrices. Such a

29

situation has forced analysts to develop better, less labour-intensive, faster and more

30

accurate analytical procedures in different fields such as biological, food and

31

environmental analysis.1

32

In the early 1990’s Pawliszyn and co-workers developed solid-phase microextraction

33

(SPME), which integrates sampling, extraction, concentration and sample introduction

34

into a single solvent-free step.2 SPME addresses the need to facilitate rapid sampling.

35

However, the appropriate selection of the extraction coating is one of the most critical

36

steps in SPME method development. The suitability of the coating for a specific analyte

37

of interest is determined by the polarity of the coating and its selectivity towards the

38

analytes in contrast to other matrix components.3 Given the great diversity of analyte-

39

matrix combinations, significant developments are still being made in some critical areas.

40

For instance, due to the complex nature of matrices, direct immersion (DI-SPME) can be

41

difficult, and some pretreatment or clean-up of the sample prior to SPME extraction may

42

be necessary to protect the coating and avoid the fouling of the extraction phase. Fouling

43

is caused by irreversible adsorption of macromolecules from the complex matrix at the

44

coating surface which could lead not only to a substantial decrease in the fibre lifetime,

45

making it unusable for more than a few samples, but could also change the coating

46

extraction properties.4

47

Therefore, the search for new coatings to improve the performance of DI-SPME coupled

48

to GC analysis in complex matrices is an active research topic.5-6. In spite of the

49

drawbacks presented by the commercially available coatings, those are still a first choice

50

for routine and inter-laboratory validations. Even though PDMS is preferred for the

51

extraction of nonpolar pesticides, it has been extensively used for the extraction of a wide

52

range of analytes in complex matrices.7-8 The main premise supporting this fact is that

53

PDMS, as a non-porous liquid coating, suffers less from the irreversible fouling effect

54

caused by the matrix components when compared to solid coatings. PDMS coating is the

55

most robust option for directly analyzing complex matrices, making it preferred

56

regardless of the sensitivity of this coating towards the analytes of interest. Jahnke et al.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 21

57

corroborate this hypothesis in a recent publication investigating the effects of non-volatile

58

matrix macromolecules fouling the PDMS, concluding that the sorptive properties of the

59

PDMS were not modified, and that PDMS is suitable for sampling of highly complex

60

matrices.9 Moreover, the authors mentioned that fouling of the PDMS might still occur in

61

highly complex matrices, but a physical cleaning of the polymer is sufficient to

62

circumvent this problem.

63

Additionally, PDMS/DVB fibre coating has also been extensively used for DI-SPME-GC

64

analysis of pesticides from fruits and vegetables after sample dilution. This coating

65

presents increased retention capacity, high distribution coefficient, smaller diffusion

66

coefficient, and high selectivity.8,10 However, it suffers of irreversible fibre fouling

67

damage when placed in direct contact to the matrix. This can be very problematic as it

68

can change the chemistry of the coating, thus affecting the uptake of the analyte and the

69

reproducibility of extraction, resulting in poor accuracy and decreasing extraction

70

efficiency of the fibre upon repeated use.11-12

71

All aforementioned limitations, together with our experimental findings, motivated us to

72

explore the possibility of modifying existing commercial SPME fibre coatings with a thin

73

layer of PDMS to create a new type of SPME fibre, achieving matrix-compatibility while

74

retaining the original coating sensitivity towards the analytes of interest, for GC analysis

75

of complex samples. In the present study, grapes were chosen as a model of complex

76

matrix, and triazole pesticides, which are vastly applied in vineyards, were chosen as

77

model analytes. The modified SPME fibre was tested for extraction efficiency and

78

robustness when directly subjected to grape matrix.

79 80

EXPERIMENTAL SECTION

81

Chemicals and Materials

82

Triazole pesticides standards (triadimefon, penconazole, triadimenol, hexaconazole and

83

diniconazole) were Pestanal grade purchased from Sigma-Aldrich (Oakville, ON,

84

Canada). Individual solutions (c.a. 20 mg/mL) of each pesticide were prepared in

85

methanol. A mixture standard stock solution was prepared containing 2000 mg/L of each

86

pesticide in methanol. Different working standards solutions (0.1 to 200 ng/µL of each

87

pesticide) were prepared by dilution in the same solvent.


Page 5 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


88

Sylgard 184 (PDMS prepolymer and curing agent) was purchased from Dow Corning

89

(Midland, MI, USA). Commercial SPME fibres (PDMS 100µm, PDMS/DVB 65µm)

90

were purchased from Supelco (Bellefonte, PA, USA).

91 92

GC Analysis

93

Analysis of triazole pesticides was performed on a Varian Saturn 3800 GC/2000 ITMS

94

system fitted with a HP-5MS column (30 m, 0.25 mm i.d., 0.25 µm film thickness)

95

(Hewlett-Packard, Avondale, PA). Helium as the carrier gas was set to 1.5 mL/min. The

96

1079 injector was set to at a temperature of 260 °C (unless otherwise specified). The

97

column temperature program was initially set at 70 °C for 2 min, ramped at 40 °C/min to

98

235 °C for 1 min, ramped at 3 ºC/min to 250 ºC and then ramped at 40 ºC/min to 280 ºC

99

and held for 12.12 min giving a total run time of 24 min. For water samples analysis, the

100

ion trap was operated in full scan mode (MS), whereas, for the grape pulp matrix the

101

analyzer was operated in tandem mode (MS/MS). The MS operational conditions were as

102

follows: electron ionization (EI) was always 70 eV; temperatures of 180, 50 and 260 °C

103

for the trap, manifold and transfer line respectively; initially a mass range of 55-325 m/z

104

was scanned to confirm the retention times of analytes. The multiplier voltage (1x105

105

gain) was 1600 V with a multiplier offset of +200V. Automatic gain control (AGC) was

106

turned on with an AGC target value of 20,000 counts for EI-MS and 2000 counts for EI-

107

MS/MS; the emission current was 10 µA for MS and 80 µA for MS/MS. For MS/MS, the

108

AGC pre-scan ionization time was 1500 µs and the isolation window was 3m/z (except

109

for diniconazole where a 5 m/z window was used). All specific MS/MS conditions for the

110

studied triazole pesticides are listed in Supplementary Information Table S1. Automated

111

analysis was performed using a CTC CombiPal autosampler (Zwingen, Switzerland)

112

using the associated Cycle Composer software (Version 1.4.0). The PAL was equipped

113

with a SPME fibre holder, a temperature controlled six-vial agitator tray, and a fibre-

114

conditioning device.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 21

115

Preparation of PDMS-modified fibre

116

Sylgard 184® PDMS pre-polymer and curing agent were mixed at a 10:1 ratio, according

117

to the manufacturer’s manual, into a polypropylene centrifuge tube and subjected to

118

centrifugation for 3 min at 4000 rpm for degassing. The coating procedure consisted of

119

immersing the commercial PDMS/DVB fibre into the PDMS solution and subsequently

120

pulling out at a slow rate of approximately 0.5 mm s-1. Passing it through a micropipette

121

tip of about 350 µm diameter aperture ensured that a thinner layer was formed, with the

122

excess polymer being removed. After the coating process, the coated fibre was placed in

123

a vacuum oven at 50 ºC under N2 flow for 12 hrs. The coating/curing process was

124

repeated twice to assure complete and uniform coverage. Prior to use the fibre was

125

conditioned in a GC injection port (PTV) under helium flow from 100 ºC (hold for 5 min)

126

to 250 ºC (hold for 30 min) at 5 ºC/min. The fibre was then conditioned again at 250 ºC

127

for 10 min. The 10-min conditioning cycle was repeated a few more times until a stable

128

GC baseline was obtained.

129

After curing and conditioning, the modified coatings were inspected using optical stereo

130

microscope to ensure that a thin layer of smooth surface was achieved. In order to verify

131

the topography of the coating surface as well as the thickness of the PDMS outer layer,

132

scanning electron microscopy (SEM) images were acquired using an LEO 1530 field

133

emission (Carl Zeiss NTS GmbH, Germany).

134 135

SPME procedure

136

Triazoles determination in grapes

137

Uncontaminated white grapes, purchased at a local market in Waterloo (ON Canada),

138

were manually stemmed, washed with de-ionized water, dried, and crushed using a

139

blender. A sample aliquot (9 g) was weighed in a 10-mL vial, fortified at 100 ng g-1. A 5

140

min incubation of the sample was performed in the autosampler agitation unit at 500 rpm

141

and at 30°C, followed by a 30 min extraction at 30ºC, while stirring at 500 rpm.

142

Following extraction, the fibre was rinsed in DI water at 30 ºC while stirring at 500 rpm

143

in the autosampler agitator unit for 50 s. Subsequently, the fibre was placed in the GC

144

injection port for desorption for 7 min at 260 ºC.

145


Page 7 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


146

Triazoles determination in water

147

This approach was employed to compare the extraction kinetics between the commercial

148

PDMS/DVB fibre and the PDMS-modified fibre. An aliquot of 18 mL of an aqueous

149

solution containing c.a. 5.5 ng mL-1 of each triazole was placed in a 20-mL vial. A 5 min

150

incubation of the sample was performed in the stir plate while stirring at 1200 rpm and at

151

30 °C. Extraction time ranged from 5 min to 1440 min. Following extraction, the fibre

152

was placed in the GC injection port for desorption for 7 min at 260 ºC. All extraction

153

time points were performed in duplicate.

154 155

RESULTS AND DISCUSSION

156

Initial Assessment: Commercial PDMS/DVB fibre coating

157

In an attempt to overcome the problem with irreversible fibre fouling, a rapid rinsing of

158

the fibre in de-ionized water after extraction and prior to desorption was implemented. It

159

was observed that no significant loss of analyte occurred up to 50 s of rinsing, thus, it was

160

chosen for further experiments. Subsequently, grape samples were subjected to extraction

161

applying a 50 s rinsing prior to desorption. Overall, the obtained results demonstrate an

162

ineffectual improvement in the fibre lifetime. In agreement with De Jager et al.12, after 20

163

extraction/desorption cycles in grape matrix the PDMS/DVB fibre was blackened and a

164

substantial decrease in extraction efficiency was observed, resulting in very

165

irreproducible results. After 10 extractions, the extraction efficiency had decreased by 3

166

to 41% and by the 20th extraction, the efficiency had dropped by 83 to 89%. The same

167

experimental set up was repeated for PDMS fibre to evaluate the performance of PDMS

168

coating, since the ability of PDMS to withstand complex matrix without changes in its

169

sorptive properties has been subject to study.9 The results obtained for both sets of

170

experiments are shown in Figure 1. Responses were normalized taking first extractions as

171

100 %. The results for PDMS coating indicated that, after 20 extractions, the extraction

172

efficiency had dropped by 8 to 14 %. Indeed, PDMS coating offered improved

173

repeatability and robustness to direct immersion in complex matrix, despite its low

174

sensitivity towards the studied analytes. Therefore, at this point the question of sacrificing

175

sensitivity for robustness and vice-versa arises. Nevertheless, in the present work, a new



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 21

176

approach is attempted, in which the benefits of the high sensitivity exhibited by the

177

PDMS/DVB and the robustness of PDMS are combined.

178


Page 9 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


179 180

Figure 1.Repeatability of commercial fibres in grape matrix (20 extraction cycles using a

181

single fibre). (A) PDMS/DVB 65 µm; (B) PDMS 100 µm. Normalization performed

182

taken extraction efficiency of first extraction for each analyte as 100 %. Extractions were

183

peformed for 30 min at 30 ºC, from 9 g grape pulp with triazole pesticides at a

184

concentration of 100 ng g-1.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 21

185

PDMS-modified coating preparation and characterization

186

It was necessary to optimize the coating method (spraying or dipping) and the overall

187

parameters such as addition of solvent, rate of pulling and aperture diameter. The

188

spraying method resulted in highly irregular coatings, thus, the work proceeded using

189

dip-coating. The optimized procedure is presented in the experimental section. The

190

optimized PDMS outer layer was obtained for two layers of PDMS which resulted in

191

optimum surface coverage of the original coating. The study showed that thinner coatings

192

(1 layer) did not ensure total surface coverage, resulting in a coating that still exhibited a

193

porous surface. In addition, thicker coatings (3 layers) resulted in non-uniform surface

194

coverage in terms of thickness throughout the coating length and also rendering weaker

195

physical stability due to excessive thickness that could lead to stripping of the coating

196

when withdrawn inside the fibre needle. Figure 2 shows the SEM images of the coatings

197

after being covered with a 10 nm layer of gold on its surface. The SEM image shows the

198

formation of a thin PDMS film on the surface of the PDMS/DVB fibre. The image

199

presents a uniform, non-porous, and smooth surface throughout the coating. The PDMS

200

outer layer thickness for the optimized coating was estimated to be approximately 25-30

201

µm.

202

In order to investigate the reproducibility of the optimized overcoating procedure, the

203

intra-fibre (n=4), as well as the inter-fibre reproducibility (three fibres, 3 replicates each)

204

in both water and grape pulp matrices were found to be very good as indicated by R.S.D.

205

values ranging from 0.1 to 11.4%. The results obtained are summarized in Supplementary

206

Information Table S2.


Page 11 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


207 208

Figure 2.SEM image of the PDMS-modified coating (2 layers): Surface morphology and

209

estimation of coating thickness using 900x magnification.

210



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 21

211

Triazoles Extraction

212

At first, DI-SPME of triazole compounds from water matrix was used to evaluate the

213

effect of the external PDMS layer on extraction capabilities of the PDMS/DVB coating

214

by comparing its extraction time profiles with those obtained with commercial

215

PDMS/DVB 65 µm fibre. Profiles of extraction obtained with the PDMS-modified fibre

216

shown in Figure 3 resulted very similarly for all the triazole pesticides studied. This is

217

compared to non-modified PDMS/DVB fibre, where only marginal differences could be

218

observed at shorter extraction times as a result of the additional step of diffusion of the

219

analytes through the PDMS outer layer. Such an effect is better illustrated in

220

Supplementary Information Figure S1, where it can be noted that the slopes of the initial

221

stage of the adsorption profiles for triazole pesticides were only slightly decreased for

222

PDMS overcoated fibre as compared to the non-modified fibre coating. However,

223

statistical analysis of both extraction time profiles by means of t-test indicates that the

224

kinetics of extraction was not influenced, by the additional barrier. Therefore, in the

225

studied matrix-analyte-coating system, the diffusion through the thin PDMS outer layer

226

does not statistically reduces the rate of mass transfer even at pre-equilibrium extraction

227

times. From a practical point of view, the obtained results are of utmost importance,

228

since most SPME applications are performed in pre-equilibrium conditions. In the present

229

work, the application of such modified coating in pre-equilibrium conditions does not

230

jeopardize the overall sensitivity towards triazoles that would be achieved by the

231

commercial PDMS/DVB coating. Additionally, as Figure 3 illustrates, the amount of

232

analyte extracted at equilibrium or near equilibrium by the PDMS-modified coating is

233

statistically equal to the amount extracted by the commercial PDMS/DVB. Moreover, the

234

results obtained suggest that there was no blockage of the extraction sites on the surface

235

of the original PDMS/DVB coating by the additional PDMS layer, thus, no impairing of

236

extraction capabilities of the original coating. In the present study, it seems that the

237

PDMS layer does not substantially change either kinetic or thermodynamic parameters

238

associated with the original coating.

239

It is worth emphasize that in this new configuration, the analytes after crossing the

240

boundary layer present in the matrix must first diffuse through the PDMS overcoating

241

prior to the adsorption in the solid DVB coating. Since this in-between phase is a liquid


Page 13 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


242

polymer, if the distribution constant for a given analyte is substantially lower as

243

compared to DVB phase, the mass transfer is considerably slowed down, which can limit

244

in some cases the kinetics of the extraction process (i.e. short extraction times

245

applications). The reduction in the rate of mass transfer is dependent on the thickness of

246

the overcoating, as well as on the differences in the distribution constant between the

247

overcoated polymer and the extraction polymer. Therefore, according to SPME

248

fundamentals, one might expect that different behaviors could be obtained for thicker

249

PDMS outer layer as well as for analytes bearing different physical-chemical properties

250

(i.e. Log P values).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 21

251

252 253

Figure 3.Comparative extraction time profiles for triazoles pesticides using commercial

254

PDMS/DVB fibre and PDMS/DVB/PDMS fibre. (A) Full extraction time profile (up to

255

24h). (B) Shorter range extraction time profile (up to 1h). DI-SPME performed in 18 mL

256

nanopure water (n=2) at 5.5 ng mL-1.


Page 15 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


257

Fibre life-time

258

The stability of the coatings over time is yet another practical parameter of top

259

importance in SPME technique. To determine endurance and reusability, the modified

260

PDMS/DVB/PDMS fibre was subjected to a series of 130 successive DI-SPME cycles in

261

whole grape pulp. Each cycle consisted of 15 min extraction at 30 ºC; 50 s rinsing in de-

262

ionized water prior to desorption; 7 min desorption at 260 ºC; post-desorption washing in

263

de-ionized water for 2.5 min; and 2.5 min fibre conditioning at 250ºC in the autosampler

264

conditioning station device. In the present experiment, the fibre was constantly inspected

265

under electronic microscope (every 10 cycles) and, when needed, manually freed of any

266

possible debris attached to its surface by simply using a KimWipe® tissue. No

267

irreversible damage on the surface was observed. Moreover, quality control (QC)

268

consisting of water samples spiked with triazole pesticides were distributed along the

269

batch to ensure that the fibre performance was not altered.

270

As presented in Figure 4, the fibre endurance measured as the amount of analyte

271

extracted presented RSDs below 20%, which taking into account the complexity of the

272

studied matrix, is an impressive achievement with performance much superior to that

273

exhibited by the original commercial fibre, which exhibited over 80% drop in signal by

274

the 20th extraction. It is also worth noting that the amount of analyte extracted by SPME

275

is proportional to the free (unbound) concentration of analyte in the sample matrix. In

276

addition, in the present study a short pre-equilibrium extraction time was employed,

277

hence the small amount of absolute recoveries observed for all analytes. If sensitivity is

278

an issue, this can be overcome by applying longer extractions time. In terms of

279

reusability, due to the very complex matrix, there was a drop in the amount extracted

280

after the 90th extraction, but the amount extracted remained reproducible from the 90th to

281

the 130th extraction for most of analytes. Only one triazole pesticide (triadimefon)

282

exhibited pronounced variations throughout this study. One possible explanation for this

283

behaviour could be the fact that triadimefon can undergo biotransformation in rich

284

organic matter media. Moreover, it is expected that by employing the right calibration

285

technique, i.e. the use of internal standard to compensate for any possible variation in the

286

method, the newly developed coating could be easily reusable over 100 times in complex

287

food matrices such as whole grape pulp.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 21

288

It should also be noted that an accumulation of high molecular pigments compounds on

289

the inner DVB phase, causing discoloration of this phase, was observed along the slight

290

trend down on the extraction efficiency.

291

Finally, the improvement achieved by the newly modified coating is illustrated in Figure

292

5. Microscope pictures of both fibres before extraction from grape pulp, as well as SEM

293

pictures of surface morphologies for commercial PDMS/DVB coating fibre and PDMS-

294

modified coating fibre are presented. The extent of fouling on the surface of the coating is

295

dramatically reduced by the application of the PDMS outer layer, as presented in Figure

296

5.


Page 17 of 21

2.5

2 Absolute recovery (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

Triadimefon (14.2%) Penconazole (13.4%)

1

Triadimenol (14.0%) Hexaconazole (14.3%) Diniconazole (10.8%)

0.5

0 0

20

40

60 80 Extraction #

100

120

140

297 298

Figure 4.Robustness of the PDMS/DVB/PDMS fibre in DI-SPME mode in grape pulp

299

for studied triazole pesticides. DI-SPME performed in 9 g of whole grape pulp (130

300

extractions using a single fibre) at 100 ng g-1. Numbers in brackets and solid lines

301

represent the pooled R.S.D. (%) values and average values over the 130 extractions,

302

respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A

B

C

D

Page 18 of 21

303

304 305

Figure 5. Microscope picture of the commercial PDMS/DVB coating before extractions

306

(A); Microscope picture of the PDMS/DVB/PDMS coating before extractions (B); SEM

307

images of the PDMS/DVB coating after 20 extractions cycles in grape (C); and

308

PDMS/DVB/PDMS coating after over 130 extractions cycles in grape (D). SEM surface

309

morphology using 580x magnification.


Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


310

CONCLUSIONS

311

A new concept of modified SPME fibre coating suitable for direct immersion in complex

312

matrix is presented. A procedure for preparing the new modified fibre was developed.

313

Subsequently, its extraction capabilities towards triazole pesticides from water samples

314

were proven to be similar to those exhibited by the original commercial PDMS/DVB

315

coating. In the present study, the PDMS layer has not substantially changed the kinetic

316

and thermodynamic parameters associated with the original coating for the target

317

analytes. At the same time, the results show that modified PDMS/DVB/PDMS coating

318

provides enhanced robustness in a highly complex matrix such as whole grape pulp when

319

compared to the original commercially available PDMS/DVB. It is here demonstrated

320

that it is possible to perform automated DI-SPME in a complex matrix such as whole

321

grape pulp for over 100 extractions using a single fibre without the use of any sample

322

pre-treatment. The practical aspects of the PDMS-modified coating demonstrated here

323

create new opportunities for the application of DI-SPME-GC in analysis of complex

324

samples not only laboratory high throughput, but also in-vivo determinations. The

325

creation of perfectly smooth, uniform, non-fouling surfaces is one of the major

326

prerequisites for high-throughput food applications, and to date, no commercially

327

available fibre is suitable for such applications. Further investigation of the PDMS-

328

modified coating capabilities towards analytes bearing different physical-chemical

329

properties from food matrices of varied compositions, as well as research into the

330

manufacturing of coatings with different outer layer chemistries and thicknesses is

331

currently ongoing with the aims to better understand the kinetics and thermodynamic

332

parameters involved in the application of such coatings, as well as improve the robustness

333

in complex food and biological matrices.

334 335 336

ACKNOWLEDGMENTS

337

The authors thank the Natural Sciences and Engineering Research Council of Canada

338

(NSERC), Agilent Technologies Foundation and Sigma-Aldrich Corporation for the

339

financial support. The help of Dr. Viorica Lopez-Avila was also greatly appreciated in

340

this research.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

341

Page 20 of 21

REFERENCES

342

(1) Ramos, L. J. Chromatogr. A 2012, 1221, 84-98.

343

(2) Pawliszyn, J. Solid Phase Microextraction: Theory and Practice, Wiley-VCH, new

344

York, 1997.

345

(3) Risticevic, S.; Lord, H.; Górecki, T.; Arthur, C. L.; Pawliszyn, J. Nat. Protoc. 2010,

346

5, 122-139.

347

(4) Ridgway, K.; Lalljie, S. P. D.; Smith, R. M. J. Chromatogr. A 2007, 1153, 36-53.

348

(5) Augusto, F.; Carasek, E.; Silva, R. G. C.; Rivellino, S. R.; Batista, A. D.; Martendal,

349

E. J. Chromatogr. A 2010, 1217, 2533-2542.

350

(6) Zeng, J.; Chen, J.; Chen, W.; Huang, X.; Chen, L.; Chen, X. Food Sci. Biotechnol.

351

2009, 18, 579-585.

352

(7) Beltran, J.; López, F. J.; Hernández, F. J. Chromatogr. A 2000, 885, 389-404.

353

(8) Kataoka, H.; Lord, H. L.; Pawliszyn, J. J. Chromatogr. A 2000, 880, 35-62.

354

(9) Jahnke, A.; Mayer, P. J. Chromatogr. A 2010, 1217, 4765-4770.

355

(10)

356

152-161.

357

(11)

358

Pawliszyn, J. Anal. Chim. Acta. 2009, 638, 175-185.

359

(12)

360

36-40.

Abdulra’uf, L. B.; Hammed, W. A.; Tan. G. H. Crit. Rev. Anal. Chem. 2012, 42,

Vuckovic, D.; Shirey, R.; Chen, Y.; Sidisky, L.; Aurand, C.; Stenerson, K.;

De Jager, L. S.; Perfetti, G. A.; Diachenko, G. W. J. Chromatogr. A 2008, 1192,

361


Page 21 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

362


For TOC only

363


Optimization of Fiber Coating Structure Enables Direct Immersion

Recommend Documents