Rapid Multiplug Filtration Cleanup with Multiple-Walled Carbon

Feb 10, 2014 - Beijing Station of Agro-Environmental Monitoring, Test and Supervision Center of Agro-Environmental Quality, MOA, Beijing 100029, Peopl...
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Rapid Multiplug Filtration Cleanup with Multiple-Walled Carbon Nanotubes and Gas Chromatography−Triple-Quadruple Mass Spectrometry Detection for 186 Pesticide Residues in Tomato and Tomato Products Pengyue Zhao,† Baoyong Huang,§ Yanjie Li,† Yongtao Han,† Nan Zou,† Kejia Gu,† Xuesheng Li,# and Canping Pan*,† †

Department of Applied Chemistry, College of Science, China Agricultural University, Beijing 100193, People’s Republic of China Beijing Station of Agro-Environmental Monitoring, Test and Supervision Center of Agro-Environmental Quality, MOA, Beijing 100029, People’s Republic of China # Institute of Pesticide and Environmental Toxicology, Guangxi University, Nanning 530005, People’s Republic of China §

ABSTRACT: This study reports the development and validation of a novel rapid cleanup method based on multiple-walled carbon nanotubes in a packed column filtration procedure for analysis of pesticide residues followed by gas chromatography− triple-quadruple tandem mass spectrometry detection. The cleanup method was carried out by applying the streamlined procedure on a multiplug filtration cleanup column with syringes. The sorbent used for removing the interferences in the matrices is multiple-walled carbon nanotubes mixed with anhydrous magnesium sulfate. The proposed cleanup method is convenient and time-saving as it does not require any solvent evaporation, vortex, or centrifugation procedures. It was validated on 186 pesticides and 3 tomato product matrices spiked at two concentration levels of 10 and 100 μg kg−1. Satisfactory recoveries and relative standard deviations are shown for most pesticides using the multiplug filtration cleanup method in tomato product samples. The developed method was successfully applied to the determination of pesticide residues in market samples. KEYWORDS: multiple-walled carbon nanotubes, multiplug filtration cleanup, tomato products, pesticide multiresidue analysis

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INTRODUCTION Tomato (Lycopersicon esculentum) is one of the most widely grown vegetables in the world.1 An increasing number of consumers have been paying more attention to the benefits of tomatoes for its highly appreciated sensory properties and nutritional value. Tomato is one of the basic components of the Mediterranean,2 American, and Asian diets, which is used almost daily, raw, cooked, or processed as a canned product, juice, or ketchup. Many studies have been published relating a regular tomato intake with a decreased risk of prostate cancer or cardiovascular diseases.3 The long-term selection process imposed to improve tomato productivity and quality has made tomato crops less resistant to diseases, pests, and adverse environmental conditions.4 To maintain a high tomato production yield, the use of pesticides is considered to be a necessary, economic, and conventional agricultural practice. Violating good agricultural practice (GAP) use of pesticides in tomato farming may cause potential health risk to consumers or force unnecessary pressure to the environment. Many regulations such as maximum residue limits (MRLs) have been established for tomatoes by several international organizations and countries (as well as for other cultivars), and MRL values may be different in different countries. For example, in Japan the MRL of metalaxyl for tomato is 2 mg kg−1 and in the United States it is 1 mg kg−1, but China and Codex set a lower MRL value of 0.5 mg kg−1, and the EU sets the lowest MRL value as 0.2 mg kg−1. The different MRL values of the same pesticide and matrix will be © 2014 American Chemical Society

used for trade, regulatory enforcement, risk assessment, and so on in different countries in the world. Therefore, pesticide residue determination in tomato and tomato products is a very demanding task in public health safety and trade. Pesticide residue analysis is advancing very rapidly, which is required by the monitoring market of various countries in domestic and international trading, risk assessment of dietary intakes, or environmental research, etc. The key procedure of this technique is cleanup of sample extracts to avoid interferences from complicated matrices. The most common extraction technique was solid-phase extraction (SPE).5−8 However, conventional SPE is not selective enough to develop a comprehensive method for multiresidue analysis. The chemistries of the various pesticides would require multiple extraction methods to screen the large number of possible pesticide residues.9 Many other techniques were also reported such as solid-phase microextraction,10 pressurized liquid extraction,11 frozen microextraction,12 supercritical fluid extraction,13 and ultrasound-assisted extraction.14 The QuEChERS method (quick, easy, cheap, effective, rugged, and safe) has been widely used as a pesticide multiresidue method in vegetables and fruits since it was Special Issue: 50th North American Chemical Residue Workshop Received: Revised: Accepted: Published: 3710

November 20, 2013 February 4, 2014 February 10, 2014 February 10, 2014 dx.doi.org/10.1021/jf405240j | J. Agric. Food Chem. 2014, 62, 3710−3725

Journal of Agricultural and Food Chemistry

Article

Table 1. MRM Transitions and Other GC-MS/MS Parameters for the Compounds in Groups A and B no. 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

pesticide acetochlor acrinathrin alachlor aldrin anilofos atrazine azaconazole benalaxyl bendiocarb benoxacor α-BHC β-BHC δ-BHC bifenox bifenthrin bitertanol bromophos-ethyl bupirimate buprofezin butachlor cadusafos carbaryl carbofuran chlordimeform chlorfenapyr chlorpyrifos-methyl clodinafop-propargyl coumaphos cyanophos cyflufenamid cyfluthrin I cyfluthrin II cypermethrin IV DCPA o,p′-DDD o,p′-DDE p,p′-DDE o,p′-DDT DEF deltamethrin diazinon dichlofenthion dichlorvos diclobutrazol diclofop-methyl dicloran dieldrin difenoconazole I difenoconazole II dimethipin dimethoate dimethylvinphos diphenylamine disulfoton dithiopyr edifenphos EPN fenitrothion fludioxonil fluvalinate-τ I

quantification transitiona

RT (min) (A) Group A 20.59 32.05 21.21 19.89 31.83 17.89 28.39 29.29 15.61 20.26 15.30 17.51 23.15 31.64 29.86 33.37 26.77 28.21 27.15 26.25 13.91 23.76 17.88 13.67 29.13 20.00 29.45 34.81 19.56 27.95 34.23 34.52 34.83 22.92 27.14 24.73 26.06 27.54 26.47 38.79 16.71 19.12 6.21 28.23 29.57 18.06 26.52 38.29 38.36 22.26 19.98 24.17 13.21 17.71 22.02 29.60 31.22 23.80 30.60 37.61 3711

174.0→146.1 (10) 207.8→181.1 (10) 188.1→160.2 (10) 262.9→192.9 (35) 225.9→184.0 (10) 214.9→200.2 (10) 217.0→173.1 (15) 148.0→77.0 (10) 166.0→151.1 (10) 120.1→65.1 (20) 216.9→181.1 (5) 216.9→181.1 (5) 217.0→181.1 (5) 189.1→126.0 (20) 181.2→166.2 (10) 170.1→141.1 (10) 358.7→302.8 (15) 272.9→193.1 (10) 105.0→104.1 (10) 188.1→160.2 (10) 158.8→97.0 (10) 144.0→115.1 (10) 164.2→149.1 (10) 195.9→181.0 (5) 136.9→102.0 (10) 124.9→47.0 (15) 266.0→91.0 (15) 210.0→182.0 (10) 242.9→109.0 (10) 118.1→90.0 (10) 162.9→90.9 (10) 162.9→90.9 (10) 163.1→127.1 (10) 298.9→221.0 (25) 235.0→165.2 (20) 246.0→176.2 (30) 246.1→176.2 (30) 235.0→165.2 (20) 169.0→57.1 (5) 181.0→152.1 (10) 137.1→84.0 (10) 278.9→222.9 (15) 109.0→79.0 (5) 269.8→158.9 (20) 253.0→162.1 (15) 206.1→176.0 (10) 262.9→193.0 (35) 322.8→264.8 (15) 322.8→264.8 (15) 124.0→76.0 (5) 92.9→63.0 (10) 294.9→108.9 (15) 169.0→168.2 (15) 88.0→60.0 (5) 353.9→306.0 (5) 172.9→109.0 (5) 169.0→141.1 (10) 125.1→47.0 (10) 248.0→182.1 (10) 250.0→55.0 (40)

confirmation transitiona 146.0→131.1 (10) 181.0→152.0 (30) 160.0→132.1 (10) 254.9→220.0 (20) 225.9→157.0 (10) 214.9→58.1 (10) 219.0→175.0 (15) 148.0→105.1 (10) 126.0→52.1 (15) 258.9→120.1 (15) 181.0→145.0 (15) 181.0→145.0 (15) 181.0→145.0 (15) 340.9→309.9 (10) 181.2→165.2 (10) 170.1→115.0 (10) 302.8→284.7 (15) 272.9→108.0 (10) 105.0→77.0 (10) 176.1→147.1 (10) 158.8→131.0 (10) 144.0→116.1 (10) 149.1→121.1 (5) 151.9→117.1 (10) 246.9→227.0 (15) 124.9→78.9 (10) 238.0→130.0 (15) 361.9→109.0 (15) 124.9→47.0 (10) 118.1→89.0 (10) 162.9→127.0 (10) 162.9→127.0 (10) 163.1→91.0 (10) 300.9→223.0 (25) 237.0→165.2 (20) 248.0→176.2 (30) 315.8→246.0 (15) 237.0→165.2 (20) 202.0→147.0 (10) 252.9→93.0 (15) 137.1→54.0 (10) 222.9→204.9 (15) 184.9→93.0 (10) 271.8→160.9 (15) 339.9→252.9 (10) 160.1→124.1 (10) 277.0→241.0 (5) 264.9→202.0 (20) 264.9→202.0 (20) 118.0→58.0 (5) 86.9→46.0 (15) 296.9→108.9 (15) 168.0→167.2 (15) 153.0→96.9 (10) 305.9→286.1 (5) 108.9→65.1 (15) 169.0→77.1 (10) 125.1→79.0 (10) 248.0→154.1 (10) 181.0→152.0 (10)

dx.doi.org/10.1021/jf405240j | J. Agric. Food Chem. 2014, 62, 3710−3725

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Table 1. continued no.

pesticide

61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92

fluvalinate-τ II furathiocarb iprobenfos lindane p,p′-DDD p,p′-DDT pentachloronitrobenzene phorate piperonyl butoxide procymidone pronamide prothiofos pyridaben pyrimethanil quinalphos silafluofen sulfotep tebuconazole tecnazene cis-tefluthrin terbacil terbufos tetraconazole tetradifon tolclofos-methyl triadimefon triadimenol triallate triazophos trifloxystrobin trifluralin vinclozolin

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

ametryn benfluralin butafenacil cafenstrole α-chlordane γ-chlordane chlorobenzilate chlorpropham chlorpyrifos cloquintocet-mexyl cyhalofop-butyl cyprodinil diflufenican dimepiperate dimethenamid diphenamid α-endosulfan β-endosulfan esprocarb ethalfluralin ethion ethoprophos etofenprox etrimfos fenarimol fenobucarb fenothiocarb

quantification transitiona

RT (min) (A) Group A 37.77 31.16 18.97 21.98 28.53 28.94 15.94 14.46 29.19 26.84 19.37 26.27 32.83 16.96 25.42 34.08 14.76 30.81 11.88 16.97 23.33 16.37 26.24 31.65 20.70 24.58 27.34 16.68 29.89 29.17 14.30 21.67 (B) Group B 21.45 14.43 35.05 35.97 25.70 25.39 28.12 14.62 22.37 30.54 32.13 23.87 29.97 24.57 20.58 25.50 25.09 28.65 20.82 13.93 28.71 13.13 33.79 17.82 32.59 13.65 13.65 3712

confirmation transitiona

250.0→55.0 (40) 163.1→135.1 (10) 203.9→91.0 (5) 216.9→181.0 (5) 235.0→165.2 (20) 235.0→165.2 (20) 236.9→142.9 (10) 121.0→65.0 (10) 176.1→131.1 (10) 96.0→67.1 (10) 173.0→145.0 (15) 113.0→94.9 (10) 147.2→117.1 (10) 198.0→183.1 (10) 146.0→118.0 (10) 179.2→151.1 (10) 201.8→145.9 (10) 250.0→125.0 (20) 260.9→203.0 (10) 177.1→127.1 (15) 160.0→117.1 (5) 230.9→129.0 (10) 336.0→217.9 (20) 158.9→131.0 (10) 265.0→250.0 (10) 208.0→181.1 (10) 168.0→70.0 (10) 268.0→184.1 (20) 161.2→134.2 (10) 116.0→89.0 (15) 305.9→264.0 (5) 187.0→124.0 (20)

181.0→152.0 (10) 163.1→107.1 (10) 121.9→121.0 (15) 181.0→145.0 (15) 237.0→165.2 (20) 237.0→165.2 (20) 236.9→118.9 (10) 121.0→47.0 (10) 176.1→103.1 (10) 96.0→53.1 (10) 175.0→147.0 (15) 266.9→239.0 (5) 147.2→132.2 (10) 198.0→118.1 (10) 146.0→91.0 (10) 286.0→258.1 (10) 237.8→145.9 (10) 125.0→89.0 (15) 214.9→179.0 (10) 197.0→141.1 (10) 161.1→144.1 (10) 230.9→175.0 (10) 170.9→136.0 (10) 226.9→199.0 (15) 265.0→93.0 (10) 208.0→111.0 (10) 128.0→65.0 (25) 142.9→83.0 (15) 161.2→106.1 (10) 172.0→145.1 (15) 264.0→160.1 (15) 197.9→145.0 (15)

227.0→58.1 (10) 292.0→264.0 (15) 331.0→180.0 (15) 100.0→72.0 (15) 271.9→236.9 (20) 271.7→236.9 (15) 251.1→139.1 (15) 153.0→125.0 (10) 198.9.0→171.0 (15) 192.0→190.0 (10) 256.2→120.0 (10) 225.2→224.3 (20) 266.0→238.0 (15) 119.0→91.0 (10) 230.0→154.1 (10) 167.1→165.0 (10) 206.9→172.0 (15) 206.9→172.0 (15) 222.0→91.0 (15) 275.9→202.1 (10) 152.9→96.9 (15) 157.9→97.0 (10) 163.0 → 135.1 (10) 181.0→153.1 (5) 219.0→107.0 (10) 121.0→ 77.0 (10) 160.1→72.0 (15)

185.0→170.0 (10) 292.0→206.0 (15) 180.0→124.0 (15) 188.0→118.9 (15) 372.9→265.9 (20) 372.8→265.8 (15) 139.0→111.0 (15) 153.0→90.0 (10) 196.9→169.0 (15) 192.0→162.0 (10) 120.1→91.0 (10) 224.2→208.2 (20) 266.0→246.0 (15) 118.0→117.0 (10) 154.1→111.1 (10) 167.1→152.0 (10) 194.9→158.9 (15) 194.9→158.9 (15) 91.0→65.1 (15) 315.9→275.9 (10) 124.9→96.9 (15) 157.9→114.0 (10) 163.0 → 107.1 (10) 168.0→153.1 (5) 251.0→139.0 (10) 121.0→ 103.1 (10) 253.4→169.1 (10)

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Table 1. continued no. 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 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87

pesticide fenoxycarb fensulfothion fenvalerate fipronil flamprop-methyl flucythrinate flumioxazin fluquinconazole flusilazole flutolanil fosthiazate I fosthiazate II halfenprox isofenphos isoprocarb isoprothiolane isoxathion kresoxim-methyl lactofen malathion mepronil metalaxyl methacrifos methamidophos methidathion p,p′-methoxychlor metolachlor metominostrobin mevinphos molinate myclobutanil napropamide nitrothal-isopropyl oxadiazon oxyfluorfen paclobutrazol parathion parathion-methyl penconazole pendimethalin phenthoate permethrin phenthoate phosalone piperophos pirimiphos-methyl prochloraz profenofos promecarb prometryn propachlor propargite propazine propham propiconazole propoxur pyrazophos pyributicarb pyridaphenthion pyrimidifen

RT (min) (B) Group B 26.14 29.77 36.56 29.64 28.35 35.43 38.70 33.73 28.52 28.51 26.17 26.27 33.38 25.47 12.08 27.84 28.17 27.40 32.66 23.69 29.72 22.25 10.16 7.51 26.91 30.47 23.18 27.66 9.49 10.13 29.26 27.18 23.94 27.39 28.37 27.68 24.73 22.56 25.77 24.80 25.70 32.25 20.37 31.22 21.48 27.16 34.49 26.97 15.66 21.27 13.70 29.57 17.74 9.47 29.71 14.40 32.50 30.02 31.30 34.78 3713

quantification transitiona

confirmation transitiona

255.2→186.2 (10) 156.0→141.0 (15) 167.0→125.1 (5) 350.8→254.8 (20) 105.0 → 77.1 (10) 198.9→157.0 (10) 354.0→325.9 (10) 340.0 → 298.0 (10) 233.0→165.0 (15) 173.0→145.1 (20) 195.0 → 103.0 (10) 195.0 → 103.0 (10) 265.0→117.0 (10) 212.9→121.0 (20) 121.0→77.1 (5) 162.1→85.0 (20) 105.0→77.0 (15) 116.0→89.0 (15) 343.8→222.9 (10) 126.9 → 99.0 (10) 119.1→91.0 (15) 234.0→146.1 (20) 207.9→180.1 (5) 141.0→95.0 (10) 144.9→85.0 (15) 227.0 → 169.1 (10) 238.0→162.2 (10) 191.2→160.2 (20) 127.0→109.0 (10) 126.2→55.1 (5) 179.0→125.1 (15) 128.0→72.0 (20) 236.0 → 194.1 (10) 174.9→112.0 (15) 252.0→196.0 (15) 236.0→125.0 (15) 290.9→109.0 (10) 262.9→109.0 (10) 248.0→157.1 (10) 251.8→162.2 (10) 274.0 → 121.0 (10) 183.1→168.1 (15) 274.0→121.0 (20) 182.0→111.0 (15) 320.0→122.0 (10) 290.0→125.0 (20) 180.0→138.0 (15) 338.8→268.7 (20) 135.1→91.0 (15) 226.0→184.0 (10) 120.0→77.1 (5) 135.0→107.0 (15) 214.2→172.2 (10) 136.9→93.0 (10) 172.9→109.0 (10) 110.0→63.0 (5) 221.0→193.0 (10) 165.0→108.0 (10) 340.0→199.0 (10) 184.1→169.1 (10)

186.2→158.2 (10) 140.0→125.0 (15) 208.9→141.1 (5) 366.8→212.8 (20) 105.0 → 51.1 (10) 156.9→107.1 (10) 354.0→107.0 (30) 108.0 → 57.0 (10) 233.0→91.0 (15) 280.9→173.0 (20) 195.0 → 60.0 (10) 195.0 → 60.0 (10) 262.9→169.0 (10) 212.9→185.0 (20) 136.0→121.1 (5) 162.1→134.0 (20) 105.0 → 51.0 (10) 116.0→63.0 (15) 343.8→299.9 (10) 172.9 → 99.0 (10) 119.1→65.1 (15) 220.0→192.1 (20) 124.9→47.1 (10) 95.0→79.0 (10) 144.9→58.1 (15) 227.0 → 141.1 (10) 162.2→133.2 (10) 196.2→77.1 (20) 127.0→95.0 (10) 126.2→83.1 (5) 179.0→90.0 (15) 128.0→100.0 (20) 194.0 → 148.1 (10) 174.9→76.0 (15) 252.0→146.0 (15) 125.1→89.0 (15) 138.9→109.0 (10) 125.0→47.0 (10) 248.0→192.1 (10) 251.8→161.1 (10) 274.0 → 125.0 (10) 183.1→165.1 (15) 274.0→125.0 (20) 182.0→102.0 (15) 140.0→98.0 (10) 232.9→151.0 (20) 195.9→96.9(15) 207.9→63.0 (20) 150.1→135.2 (15) 199.0→184.1 (15) 176.1→57.1 (5) 149.9→135.0 (15) 229.1→58.1 (10) 119.0→91.0 (10) 172.9→145.0 (10) 110.0→64.0 (5) 232.0→204.0 (10) 108.0→93.0 (10) 204.0→203.0 (10) 186.0→171.1 (10)

dx.doi.org/10.1021/jf405240j | J. Agric. Food Chem. 2014, 62, 3710−3725

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Table 1. continued no. 88 89 90 91 92 93 94 a

pesticide quinoclamine quinoxyfen ronnel simeconazole tebufenpyrad terbutryn thiazopyr

RT (min) (B) Group B 25.00 28.78 20.53 23.01 30.28 22.01 23.76

Collision energy (eV) is given in parentheses.

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introduced by Anastassiades and Lehotay et al. in 2003.15,16 Its main advantage is comprehensiveness, being useful for the analysis of various pesticides. A great deal of literature suggests that better results are obtained by the QuEChERS method with efficient removal of matrix compounds and higher recoveries of pesticides with minimal solvent.9,17−20 The first step of the QuEChERS method is to extract the analytes from samples with a small amount of acetonitrile, followed by liquid−liquid partition by salting-out with sodium chloride and magnesium sulfate. Then, a cleanup step is often carried out by mixing the acetonitrile extract with loose sorbents, which was based on reversed-dispersive solid phase extraction (r-DSPE) to absorb the interference substances in the matrices, rather than the analytes. In most cases, primary−secondary amine (PSA), graphitized carbon black (GCB), or C18 was used as r-DSPE sorbent to remove the interference substances in the matrix such as fatty acid compounds, pigments, sterols, and other nonpolar interfering substances.21−23 Carbon nanotubes (CNTs), first reported by Iijiama in 1991,24 are interesting carbonaceous materials as new carbonbased nanomaterials. According to the carbon atom layers in the wall of the nanotubes, CNTs can be divided into singlewalled carbon nanotubes (SWCNTs) and multiple-walled carbon nanotubes (MWCNTs). MWCNTs have been widely used as sorbent materials in inorganic elements, pollutants, and veterinary drugs.25−27 In our previous study, MWCNTs were used as alternative reversed-dispersive solid phase extraction materials in pesticide multiresidue analysis with the QuEChERS method.28,29 They were also mixed with other sorbents such as PSA and GCB for dispersive cleanup of acetonitrile extracts from tea samples.30 A more practical way to perform the r-DSPE method is to use cartridges (like most SPE cartridges) where the sample matrix compounds in the acetonitrile extraction interact with the sorbents.31 In the work presented here, an r-DSPE cleanup method for tomato product extracts was developed using multiplug filtration cleanup (m-PFC) followed by gas chromatography−triple-quadruple tandem mass spectrometry (GC-MS/MS) analysis. The cartridges of m-PFC contained MWCNTs and anhydrous magnesium sulfate to efficiently clean up QuEChERS extracts prior to analysis. The proposed technique based on MWCNTs intends to adsorb the interfering substances in the matrices, rather than the analytes, and anhydrous magnesium sulfate could remove water from the extract. The m-PFC method is shown to be very rapid to perform without any solvent evaporation. It is expected to facilitate routine analyses of pesticides in fresh vegetable/fruit samples such as tomato and its processed products.

quantification transitiona

confirmation transitiona

207.0 → 172.1 (10) 271.9→237.1 (10) 285.0→269.9 (15) 121.0 → 101.1 (10) 332.9→171.0 (10) 241.1→170.2 (15) 327.1→277.0 (10)

209.0 → 172.1 (10) 237.0→208.0 (10) 286.9→272.0 (15) 121.0 → 75.1 (10) 275.9→171.0 (10) 185.0→170.0 (15) 349.0→329.0 (10)

MATERIALS AND METHODS

Standards, Reagents, and Materials. Analytical standards of the pesticides in the study were provided by the Institute of the Control of Agrochemicals, Ministry of Agriculture, Peoples’ Republic of China. The purities of the standard pesticides were from 95 to 99%. Stock solutions of 10 mg L−1 for pesticide mixtures were prepared in acetonitrile and stored at −20 °C. The working solutions were prepared daily. HPLC grade acetonitrile was obtained from Fisher Chemicals (Fair Lawn, NJ, USA). Analytical reagent grade anhydrous sodium chloride (NaCl) and magnesium sulfate (MgSO4) were obtained from Sinopharm Chemical Reagent (Beijing, China). MWCNTs with average external diameters of 10−20 nm were provided by Tianjin Bonna-Agela Technologies Co., Ltd. (China). MPFC tips were packed with 10 mg of MWCNTs and 150 mg of anhydrous magnesium sulfate with assistance from Tianjin BonnaAgela Technologies. GC-MS/MS Analytical Conditions. Determinations were performed using an Agilent 7000B triple-quadrupole mass spectrometer interfaced to an Agilent 7890A GC. An Agilent Technologies capillary column HP1701 MS analytical column (30 m × 250 μm × 0.25 μm film thickness) was used for GC separation, with helium (99.9999%) as carrier gas at a constant flow rate of 1.2 mL min−1. The column temperature was initially at 80 °C (hold for 1 min), increased to 150 °C at a rate of 30 °C min−1 and then to 210 °C at a rate of 3 °C min−1, and finally increased to 290 °C at a rate of 10 °C min−1, holding for 12 min. The temperature of the injector port was 260 °C, and a volume of 1 μL was injected in splitless mode. The total running time was 43 min. The mass spectrometer was operated in electron ionization mode (70 eV). Default instrument settings of collision gas flow of N2 at 1.5 mL min−1 and He at 2.25 mL min−1 and quadrupole temperature of 150 °C were used in all MS/MS experiments. The detector voltage was automatically set by the instrument after automated MS/MS tuning, which was typically 1250 V. A full autotune of the mass spectrometer using the default parameters of the instrument was performed before each sequence. Agilent MassHunter was used for instrument control and data acquisition/processing. For the final multiple reaction monitoring (MRM) acquisition method, two ion transitions at the experimentally optimized collision energy (CE) were monitored for each analyte. Both pairs of the MRM transitions were used for confirmation analysis, which meets the EU Decision,32 and the most sensitive transitions were selected for quantification analysis. To obtain better separation efficiency, the 186 compounds were divided into two groups: groups A and B consisted of 92 and 94 compounds, respectively. Therefore, the 186 pesticides were detected by two GC-MS/MS analyses. Table 1 summarizes the optimized MS/ MS conditions for the individual analytes and their typical retention times (RT) in groups A and B. Sample Preparation. The tomato, tomato juice, and ketchup samples were obtained from a local supermarket, and tomato samples were homogenized with a blender for 1 min at room temperature. For recovery determination, the homogenized samples (10.0 ± 0.1 g) were spiked by the addition of the standard stock solutions at two concentration levels of 10 and 100 μg kg−1. The spiked samples were set aside for 30 min before extraction. 3714

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Journal of Agricultural and Food Chemistry

Article

An amount (10.0 ± 0.1 g) of ground tomato, tomato juice, or ketchup samples was weighed into a 50 mL centrifuge tube, and 10 mL of acetonitrile was added. The resulting solution was shaken by the vortex for 1 min. One gram of sodium chloride and 4 g of anhydrous magnesium sulfate were added for tomato and ketchup extracts, and 3 g of sodium chloride and 4 g of anhydrous magnesium sulfate were added for tomato juice extracts. The tube was cooled to room temperature in an ice−water bath immediately. The centrifuge tube was shaken vigorously for 1 min to prevent salt agglomeration before centrifugation at 3800 rpm for 5 min. The supernatant was used for further m-PFC. M-PFC Procedures. A 1 mL aliquot of the initial extracts was used for extraction, which was introduced into a 2.0 mL microcentrifuge tube. The m-PFC procedure was carried out with a 2.0 mL syringe and an m-PFC tip. The sorbent in the tip was 10 mg of MWCNTs mixed with 150 mg of MgSO4. Schematic diagrams of an m-PFC tip and the procedures are shown in Figure 1. As shown in Figure 2, the syringe needle was kept under

sucked into the m-PFC tip and filtered through the sorbent. Step 2, push the piston. All of the extracts were pressed into the microcentrifuge tube and filtered through the sorbent a second time. Step 3, repeat steps 1 and 2 twice. Step 4, remove the needle and press the extracts into an LC vial to carry out the chromatographic analysis. At the same time, the acetonitrile layer was filtered through a 0.22 μm filter membrane. Method Performances. Three kinds of tomato products were selected for validation purposes: tomato, tomato juice, and ketchup. Validation data were obtained for each type of sample. The following parameters were determined during validation of the analytical method: linearity, limit of quantification (LOQ), limit of detection (LOD), accuracy, and precision. Linearity was studied using matrixmatched calibration by analyzing samples of tomato, tomato juice, and ketchup. The recovery and reproducibility experiments were carried out for each sample in five replicates at two fortification levels (10 and 100 μg kg−1). The LODs were determined as the concentration of analyte giving a signal-to-noise ratio (S/N) of 3 for the target ion (the less intense transition); LOQs were determined as the concentration of analytes giving a S/N of 10 for the target ion (the less intense transition).

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RESULTS AND DISCUSSION Optimization of GC-MS/MS Conditions. Optimization of triple-quadrupole MS/MS was a rather demanding task because specific experimental conditions were required for each target compound to conduct analyses.33 To find the retention times and the best resolution between the analyte peaks, preliminary experiments were carried out systematically in full scan mode using compound standard solutions. A tandem mass detector has high selectivity and sensitivity and provides an effective solution. The relevant consideration included the choice of precursor ions, product ions, and optimization of collision energies for best response. The mass spectrometric parameters option was initially performed by full scan for the compounds. After that, the precursor ion for each analyte was selected, and then the collision energy voltages (potential on second quadrupole) were optimized to generate MS/MS product ions. The characteristic ion transition and collision energy for each compound during MRM acquisition are listed in Table 1. The collision energy was optimized for two selective ion transitions for every pesticide. Both pairs of MRM transitions were used for confirmation analysis, which meets the EU Decision,32 and the most sensitive transitions were selected for quantification analysis. Because of the large number of compounds to be detected, the 186 pesticides were divided into two groups and analyzed twice by GC-MS/MS. Groups A and B consisted of 92 and 94 compounds, respectively. The conditions of gas chromatography for the two groups were the same, but the MS/MS conditions were different due to the various pesticides. The developed method was highly selective with the monitoring of specific MRM of each analyte, which was essential to reduce the risk of false-positive results. Extraction Procedure Optimization. In the original QuEChERS method, the amounts of acetonitrile and sodium chloride were 10 mL and 1 g per 10 g of sample, respectively. In our pre-experiments, the original QuEChERS extraction procedure with 1 g of sodium chloride and 10 mL of acetonitrile was used, and good recoveries (>70%) could be obtained for most pesticides in the matrices of tomato and ketchup. However, when it came to tomato juice, the recoveries for 82 pesticides (of the 186 pesticides) decreased below 60%, probably due to the high content of water in juice. To obtain better recoveries for tomato juice, different amounts of sodium

Figure 1. Schematic diagram of an m-PFC tip: 1, syringe; 2, column; 3, PE frits (upper); 4, PE frits (low); 5, MWCMNs (10 mg) and anhydrous magnesium sulfate (150 mg); 6, syringe needle; 7, 2.0 mL microcentrifuge tube.

Figure 2. Schematic diagram of the rapid m-PFC method. the surface of the extract, and then the syringe piston was pushed and pulled as follows: Step 1, pull the piston. All of the extracts were 3715

dx.doi.org/10.1021/jf405240j | J. Agric. Food Chem. 2014, 62, 3710−3725

Journal of Agricultural and Food Chemistry

Article

Figure 3. Chromatograms for tomato extracts after m-PFC: (a) total ion current (TIC) chromatogram for a typical blank sample of the target analytes in group A; (b) TIC chromatogram for a typical blank sample spiked at 10 μg kg−1 of the target analytes in group A; (c) TIC chromatogram for a typical blank sample of the target analytes in group B; (d) TIC chromatogram for a typical blank sample spiked at 10 μg kg−1 of the target analytes in group B.

chloride (1, 2, and 3 g) were employed and analyzed in triplicate at one calibration point (100 μg kg−1). It was shown that the salting-out effect of 3 g of sodium chloride was notably higher in comparison with the other amounts, and the recoveries for most compounds were satisfactory and acceptable. Therefore, 3 g of sodium chloride was added for saltingout after acetonitrile extraction, which was employed as the optimum amount for the matrix of tomato juice in the study. The original QuEChERS extraction procedure (1 g of sodium chloride) was used for the other two matrices of tomato and ketchup. M-PFC Process. The m-PFC process was performed to remove the sample matrix in the extract prior to chromatographic analysis. The removal of interference substances in the matrices is necessary to ensure reproducible peak intensities for quantitative analysis. The proposed cleanup method was based on the m-PFC procedure with the mixture of anhydrous magnesium sulfate and MWCNTs. Anhydrous magnesium sulfate was used to remove the water in the matrices, and

MWCNTs were used to remove other interference substances such as fatty acids, pigments, and other matrix compounds. In our previous study, we found that MWCNTs could be used as alternative r-DSPE materials in pesticide multiresidue analysis with the QuEChERS method.28 The optimal amount of MWCNTs was 10 mg, which was suitable for cleanup. A smaller amount of MWCNTs (5 mg) could not obtain as good a cleanup performance as 10 mg of MWCNTs and 15 mg or more MWCNTs could lead to low recoveries (15%, which still could not meet the requirements for the analysis of pesticides. As a result, the m-PFC based on the MWCNTs process might not be suitable for the analysis of those compounds containing many rings or benzoheterocycle. It is possible to incorporate an alternative sorbent to extract these compounds using m-PFC technology. Real Samples Analysis. To evaluate the proposed method, it was applied to the analysis of real samples. It was used for the analysis of pesticide residues in 10 tomato, 5 tomato juice, and 5 ketchup samples, which were obtained from local markets and supermarkets in Beijing. All of the samples were extracted and analyzed as described under Materials and Methods. From the analytical results, chlorpyrifos (1.6−8.1 μg kg−1), procymidone (17−51 μg kg−1), flucythrinate (5.6−8.7 μg kg−1), and metalaxyl (2.3−11.2 μg kg−1) were detected in six tomato samples. Procymidone (2.5 μg kg−1) was detected in one tomato juice sample. Chlorpyrifos residues were detected in one ketchup sample, with the concentration of 3.3 μg kg−1. A rapid and sensitive method for the analysis of multiresidues in tomato products has been developed using m-PFC technology combined with the QuEChERS method. The mPFC process provided an effective method for cleanup of pesticide extracts from matrices. The cleanup procedure after extraction was focused on removing the interfering substances in the matrices, rather than extracting and isolating the analytes. In most cases, satisfactory recoveries of 70−130% were obtained with relative standard deviations of