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Determination of triazine herbicides in vegetables by ionic liquid foam floatation solid phase extraction high performance liquid chromatography
Journal of Chromatography B, 953–954 (2014) 132–137
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Journal of Chromatography B journal homepage: www.elsevier.com/locate/chromb
Determination of triazine herbicides in vegetables by ionic liquid foam floatation solid phase extraction high performance liquid chromatography Liyuan Zhang a,b , Runzhong Yu c , Zhibing Wang a , Na Li a , Hanqi Zhang a , Aimin Yu a,∗ a
College of Chemistry, Jilin University, Qianjin Street 2699, Changchun 130012, PR China College of Food, Heilongjiang Bayi Agricultural University, Xinfeng Lu 5, Daqing 163319, PR China c Department of Computer Application Engineering, Daqing Vocational College, Huoju Lu, Daqing 163300, PR China b
a r t i c l e
i n f o
Article history: Received 20 August 2013 Received in revised form 21 January 2014 Accepted 10 February 2014 Available online 19 February 2014 Keywords: Ionic liquids Foam floatation Triazine herbicides Solid phase extraction Vegetable
a b s t r a c t The ionic liquid foam floatation solid phase extraction was established and applied to the extraction of six triazine herbicides, including desmetryn, secbumeton, terbumeton, terbuthylazine, dimethametryn and dipropetryn, in vegetable samples. To obtain the optimized experimental parameters, the effects of pH value of sample solution, the type and concentration of ionic liquid, the flow rate of carrier gas, foam floatation time, the type of solid phase extraction cartridge, the type and volume of elution solvent on the recoveries of the analytes were examined. The high performance liquid chromatography was applied to the determination of the analytes. Under the optimized experimental conditions, the linearities for determining the analytes were satisfactory and the limits of detection for desmetryn, secbumeton, terbumeton, terbuthylazine, dimethametryn and dipropetryn were 2.50, 1.75, 2.76, 1.87, 1.36 and 1.44 g kg−1 , respectively. The recoveries of the analytes ranged from 78.64% to 104.37% and the relative standard deviations ranged from 1.44 to 6.45%. The real samples were analyzed and the results were satisfactory. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Triazine herbicides are extensively used around the world by virtue of their properties of inhibiting photosynthetic reactions to help control the growth of grasses and broadleaf weeds [1]. However, they may be dangerous for human health. A large number of researches have proven the toxicity of triazine herbicides, which have been suspected of giving rise to cancer, congenital defect and so on [2–4]. Triazine herbicides and their metabolites and degradation products can pollute the crop itself and the environment of crop growth, such as soil and natural water. When these harmful substances pollute the foodstuffs, they can seriously endanger human health. Many countries and regions have established the criteria of maximum residue limits (MRLs) of triazine herbicides. The Environmental Protection Agency (EPA) has provided that the MRLs of triazine herbicides in most products are 0.25 mg kg−1 , while the European Union (EU) dictated that the MRL of terbuthylazine in vegetables is 0.05 mg kg−1 . The simple, rapid and sensitive method
∗ Corresponding author. Tel.: +86 431 85168399; fax: +86 431 85112355. E-mail address: [email protected] (A. Yu). http://dx.doi.org/10.1016/j.jchromb.2014.02.011 1570-0232/© 2014 Elsevier B.V. All rights reserved.
for extraction, separation and determination of the triazine herbicides in vegetable samples is required. In case of the complex matrix sample and low analyte content, the extraction and concentration should be more important than the determination. In recent years, more and more extraction methods have been developed, such as liquid–liquid extraction (LLE) [5–7], solid phase extraction (SPE) [8–11], solid phase micro-extraction (SPME) [12], pressurized microwave-assisted extraction (PMAE) [13], dynamic microwave-assisted extractionsolidification of floating organic drop (DMAE–SFOD) [14], cloud point extraction (CPE) [15], matrix solid phase dispersion (MSPD) [16], molecularly imprinted solid phase extraction (MISPE) [17], dispersive solid-phase extraction (DSPE) [18] and pressurized liquid extraction (PLE) [19,20]. Recently, magnetic nanoparticles (MNPs) were used as the adsorbents in many fields and successfully used for the pre-concentration and removal of some toxic and hazardous pollutants from water, soil and biological samples [21–31]. The methods based on ionic liquids (ILs) were developed for the extraction of triazine herbicides from different liquid samples (milk, honey, yogurt), such as microwave-assisted ionic liquid micro-extraction (MAILME) [32], dispersive liquid–liquid microextraction (DLLME) [33] and ionic liquid foaming-based solvent
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Fig. 1. Structures of triazine herbicides.
floatation (ILF-SF) [34]. Determination of triazine herbicides in water and food samples, especially in vegetable samples, was predominantly performed by high performance liquid chromatography (HPLC) [20], gas chromatography (GC) [35], HPLC mass spectrometry (HPLC–MS) [36], GC–MS [37] and micellar electrokinetic capillary chromatography (MECC) [38]. In this paper, the extraction and determination of triazine herbicides in vegetable samples by ionic liquid foam floatation solid phase extraction (ILFF–SPE) high performance liquid chromatography (HPLC) was developed. This method was based on the fact that IL has the characteristics of surfactant [39]. The ILFF–SPE was applied to the extraction of triazine herbicides in vegetables. ILbased FF–SPE was applied to the extraction of steroid hormones in water sample [39]. There is no report about application of ILFF–SPE in triazine herbicides in vegetable samples. In the present study, the ILFF–SPE was applied directly to solid and large volume sample. At the same time, sample pretreatment steps and extraction time were reduced because of the combination of extraction method and enrichment method. 2. Materials and methods 2.1. Chemicals and reagents Desmetryn, secbumeton, terbumeton, terbuthylazine, dimethametryn, and dipropetryn were obtained from National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China). The structures of the triazine herbicides are
shown in Fig. 1. 1-Ethyl-3-methylimidazolium tetrafluoroborate ([C2 MIM][BF4 ]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4 MIM][BF4 ]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6 MIM][BF4 ]), 1-Octyl-3-methylimidazolium tetrafluoroborate ([C8 MIM][BF4 ]), 1-butyl-3-methylimidazolium hexafluo([C4 MIM][PF6 ]), 1-hexyl-3-methylimidazolium rophosphate hexafluorophosphate ([C6 MIM][PF6 ]), and 1-octyl-3methylimidazolium hexafluorophosphate ([C8 MIM][PF6 ]) were obtained from Chengjie Chemical Co. Ltd. (Shanghai, China). Chromatographic grade methanol and acetonitrile were purchased from Fisher Scientific Company (UK). All other reagents of analytical grade were purchased from Beijing Chemical Factory (Beijing, China). Pure water was obtained with a Milli-Q water purification system (Millipore Co., USA). 2.2. Apparatus HPLC analysis was carried out on a LC chromatographic system equipped with a binary pump (LC-20AB) and UV detector (Shimadzu Corporation, Kyoto, Japan). Chromatographic separation of target analytes was performed on Agilent Eclipse XDB-C18 column (150 mm × 4.6 mm i.d., 3.5 m, Agilent, USA). Oasis HLB (3 mL, 100 mg) and MCX (3 mL, 100 mg) extraction cartridges were purchased from Waters (Milford, MA, USA). RE-52AA vacuum rotary evaporator (Yarong, Shanghai, China) were used. SPE cartridges, including SuperClean Al2 O3 (3 mL, 100 mg), SuperClean C18 (3 mL, 100 mg), and SuperClean Si (3 mL, 100 mg), were purchased from Supelco (Bellefonte, PA, USA).
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Table 1 Analytical performances. Triazine herbicide
Working curve
Correlation coefficient
Linear range (g kg−1 )
LOD (g kg−1 )
LOQ (g kg−1 )
Desmetryn Secbumeton Terbumeton Terbuthylazine Dimethametryn Dipropetryn
A = 3441.7c + 4524.8 A = 3622.2c + 5821.1 A = 3294.9c + 1464.5 A = 4630.8c + 5596.1 A = 6653.9c + 11129 A = 8144.7c + 12694
0.9989 0.9988 0.9987 0.9983 0.9990 0.9992
5.00–160.00 5.00–160.00 5.00–160.00 3.00–160.00 3.00–160.00 3.00–160.00
2.50 1.75 2.76 1.87 1.36 1.44
8.33 5.84 9.21 6.23 4.53 4.81
2.3. Working standard solutions
2.6. Determination by HPLC
The standard stock solutions for the triazine herbicides at the concentration level of 500 g mL−1 were prepared in methanol. Working standard solutions were prepared every week by diluting stock standard solutions with methanol. Mixed working standard solutions at different concentrations were prepared in the same method as the working standard solutions. The standard stock and working solutions were stored at 4 ◦ C and protected from light.
The mobile phases A and B were water and acetonitrile, respectively. The gradient conditions are as follows: 0–5 min, 40–60% B; 5–10 min, 60–80% B; 10–15 min, 80–80% B; 15–25 min, 80–40% B. The flow rate of the mobile phase was 0.5 mL min−1 . Sample injection volume was 20 L and the column temperature was kept at 30 ◦ C. The UV detection was carried out at a wavelength of 228 nm. 2.7. Method validation
2.4. Sample The vegetable samples were purchased from local large-scale supermarket in September (Changchun, China) and stored at 4 ◦ C. In this study, four kinds of vegetable samples, including cauliflower, cabbage, red cabbage and broccoli, were used. All experimental results were obtained with cauliflower except for the experimental results mentioned in Section 3.2.2, which were obtained with all four samples. The vegetable samples were crushed with stamp mill. The spiked samples containing the analytes were prepared by spiking the mixed working standard solution into the crushed samples and shaken for 10 min. Then all the samples were stored at 4 ◦ C before pretreatment.
2.5. Ionic liquid foam floatation–solid phase extraction 5 g of crushed vegetable sample was added into a breaker. 100 mL of water and 40 L of IL aqueous solution were added into the beaker. The pH values ranging from 2 to 14 of the resulting samples were adjusted with 1 M HCl and 1 M NaOH. Subsequently, the mixture was transferred to a floatation vessel. The ILFF–SPE system is shown in Suppl. Fig. 1. The SPE cartridge was activated with 2.0 mL of methanol and 2.0 mL of water before the extraction. The carrier gas was passed through the sample. The resulting foam was passed through the glass cotton, where the solid particles were not passed through, and introduced into the SPE cartridge. The analytes were extracted from the sample into the solution, diverted from the solution to the SPE cartridge and then adsorbed onto the cartridge. The flow rate of the carrier gas was 800 mL min−1 . The foam floatation time was 12 min. The SPE cartridge was washed with water at the flow rate of 2.0 mL min−1 for 1.0 min and subsequently the carrier gas N2 was passed through the cartridge until all the water was evaporated. The SPE cartridge was then eluted with 1.0 mL of methanol at the flow rate of 0.08 mL min−1 . The eluate was collected in a conical evaporating flask and evaporated to dryness at 35 ◦ C in a vacuum rotary evaporator. The residue was dissolved in 0.4 mL of methanol and then filtered through a 0.22 m filter membrane. Then, 20 L of the eluate was injected into the HPLC system. All experiments were performed in triplicate.
2.7.1. Working curve A certain amount of the mixed working standard solution was added into the vegetable sample. The resulting spiked samples were pretreated by ILFF-SPE mentioned above. 20 L of the eluate was injected in the HPLC system and analyzed under the chromatographic conditions mentioned above. The working curve was obtained by plotting peak areas versus the concentrations of the analyte in the spiked samples. The working curve was applied for calculating extraction recovery and evaluating the performances of the present method. 2.7.2. Precision The precision of the present method was presented by relative standard deviation (RSD). The intra-day and inter-day precision was acquired by analyzing spiked vegetable samples at two different concentration levels (10.00 and 200.00 g kg−1 ). The intra-day precision was obtained by analyzing the sample in triplicate in one day. The inter-day precision was obtained by analyzing the sample once each day over three consecutive days. 3. Results and discussion The effect of experimental parameters, including pH value of sample solution, the type and amount of IL, flow rate of carrier gas, time of foam floatation, the type and volume of solid phase extraction column, the type and volume of elution solvent, was investigated. 3.1. Optimization of ILFF–SPE conditions 3.1.1. Effect of sample solution pH value As shown in Suppl. Fig. 2, the effect of pH values of sample solution ranging from pH 2 to 14 was investigated. The results clearly show that the pH value of sample solution can affect the efficiency of extraction. In acidic condition, the loading capacity of IL foam was limited because of the foaming rate was too rapid. The recoveries obviously increase when the pH increases from 2 to 10 and remain unchanged or slightly decrease in the pH range of 10–14. Hence, pH 12 was chosen in the following experiments. 3.1.2. Effect of type of ionic liquid In ILFF–SPE, it is very important to select the type of ionic liquid. In this experiment, [C2 MIM][BF4 ], [C4 MIM][BF4 ], [C6 MIM][BF4 ], [C8 MIM][BF4 ], [C4 MIM][PF6 ], [C6 MIM][PF6 ], and [C8 MIM][PF6 ]
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were used as the foaming agents. The experimental results are shown in Suppl. Fig. 3. The results indicate that both hydrophilic and hydrophobic ILs can foam in the aqueous solution. ILs should have suitable surface activity and good foaming ability. It is necessary to take into account the length of alkyl chain of ILs. The recoveries obtained with [C6 MIM][PF6 ] are highest. [C6 MIM][PF6 ] was used as foaming agent. 3.1.3. Effect of amount of the ionic liquid The amount of the ionic liquid plays an important role in the extraction of six triazine herbicides from vegetables. 40 L of IL aqueous solutions containing 3.2, 6.4, 12.8, 19.2, 25.6 g of [C6 MIM][PF6 ], respectively were used in this study. The effect of [C6 MIM][PF6 ] amount in the aqueous solution was evaluated. The detailed results can be found in Suppl. Fig. 4. When amount of [C6 MIM][PF6 ] is lower than 12.8 g, [C6 MIM][PF6 ] could not foam in the solution well. In addition, if the amount is too high, the ability of extraction is limited. Finally, the amount of 12.8 g was selected. 3.1.4. Effect of the flow rate of carrier gas N2 was used as carrier gas. The results are shown in Suppl. Fig. 5. The recoveries increase with the increase of carrier gas flow rate. When the flow rate is lower than 800 mL min−1 , six triazine herbicides are not sufficiently transferred from sample solution to the SPE cartridge. When the flow rate of carrier gas is higher than 800 mL min−1 , the recoveries are invariable. Based on the experiment results 800 mL min−1 was selected in the work. 3.1.5. Effect of foam floatation time As can be seen from Suppl. Fig. 6, the recoveries of analytes increase rapidly with increase of foam floatation time, reach maximum when foam floatation time is 12 min and then are almost invariable. Consequently, the optimum time of foam floatation should be 12 min. 3.1.6. Effect of type of SPE cartridge Type of SPE cartridge can significantly affect the extraction and clean up efficiency. In this experiment, Si, HLB, MCX, Al2 O3 and C18 SPE cartridges were examined. The recoveries of six triazine herbicides are shown in Suppl. Fig. 7. When C18 is used, the recoveries of six triazine herbicides are higher than those obtained with the other cartridges. So, C18 (3 mL, 100 mg) was used as SPE cartridge. 3.1.7. Effect of type and volume of elution solvent In order to evaluate the effect of elution solvent, the effect of the type of elution solvent was investigated. The experimental results are shown in Suppl. Fig. 8. Six organic solvents, including acetone, ethyl acetate, ether, methanol, ethanol and acetonitrile, were used as elution solvents in this experiment. On the one hand, under the chosen HPLC conditions, the elution solvents can have good chromatographic behavior. On the other hand, the elution solvents should elute all the target analytes. Based on the above results, methanol was used as elution solvent. The volume ranging from 400 to 1200 L was tested when methanol was used as elution solvent. When the volume of methanol is too small the triazine herbicides cannot be eluted completely. The triazine herbicides can be eluted completely when the volume of methanol was 1000 L. Therefore, 1000 L of methanol was used in this work. 3.2. Evaluation of the method 3.2.1. Analytical performance The working curve was drawn by plotting the peak area (A) measured versus the concentration (C) of the target analyte. The linear regression equations and correlation coefficients are listed in
Fig. 2. Chromatograms of standard solution (a), cabbage (b), cauliflower (c), and spiked cauliflower sample (d). 1, desmetryn; 2, secbumeton; 3, terbumeton; 4, terbuthylazine; 5, dimethametryn; 6, dipropetryn.
Table 1.The limits of detection (LODs) and quantification (LOQs) are indicated in Table 1. The LODs and LOQs are obtained by following equations: LOD = 3 /k ;
LOQ = 10 /k
where is the standard deviation of blank signal, which is obtained by analyzing the non-spiked sample 11 times and k is the slope of the working curve. The LOQs of the six triazine herbicides are lower than the MRLs and the present method is appropriate for practical application. 3.2.2. Analysis of samples Four kinds of samples were analyzed to evaluate the accuracy and precision of the present method. The typical chromatograms of standard solution, cabbage, cauliflower and spiked cauliflower sample are shown in Fig. 2. Analytical results of four vegetable samples, including the recoveries, intra-day and inter-day precision, are listed in Table 2. The recoveries of all triazine herbicides at two concentration levels are from 78.64% to 104.37%. The results indicate that the present method offers good precision (RSD ≤ 6.45). The five triazine herbicides, including desmetryn, secbumeton, terbumeton, terbuthylazine and dipropetryn in four vegetable samples, were undetectable. However, the dimethametryn, which was
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Table 2 Analytical results of vegetable samples. Sample
Triazine herbicide
Added (g kg−1 )
Recovery (%)
Intra-day RSD (%)
Inter-day RSD (%)
Cauliflower
Desmetryn
10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00
85.93 89.11 93.73 90.75 94.56 95.89 93.41 88.76 81.23 83.23 104.37 102.23 81.22 84.23 90.10 92.46 94.35 92.67 91.45 94.29 92.68 94.55 95.43 98.56 79.34 80.56 91.28 90.98 90.38 94.65 95.90 93.47 93.28 92.98 98.67 101.64 78.64 80.29 83.26 84.55 88.76 87.67 90.44 89.35 91.86 93.63 94.38 92.33
3.23 2.83 3.76 3.14 4.23 3.78 4.44 3.33 2.95 2.44 3.87 2.38 4.76 4.87 4.30 3.77 3.23 3.05 4.49 3.96 2.56 2.04 1.98 1.44 5.32 4.74 5.21 4.80 3.99 3.67 3.89 3.56 3.08 2.67 3.23 3.09 4.34 4.06 3.83 3.17 4.58 4.04 2.19 1.68 2.74 2.60 3.15 2.00
4.39 3.88 4.96 4.02 5.38 4.95 5.22 4.21 4.17 3.90 4.86 3.76 6.23 6.45 5.87 4.38 3.88 3.58 5.29 4.44 3.94 2.27 3.26 2.13 6.20 5.78 5.98 5.81 5.34 4.46 5.18 4.87 4.42 3.11 4.64 3.99 5.06 4.77 5.16 4.45 5.82 4.85 3.99 2.53 3.56 3.43 4.37 3.25
Secbumeton Terbumeton Terbuthylazine Dimethametryn Dipropetryn Desmetryn
Cabbage
Secbumeton Terbumeton Terbuthylazine Dimethametryn Dipropetryn Red cabbage
Desmetryn Secbumeton Terbumeton Terbuthylazine Dimethametryn Dipropetryn Desmetryn
Broccoli
Secbumeton Terbumeton Terbuthylazine Dimethametryn Dipropetryn
undetectable in the cabbage, red cabbage and broccoli samples, was detectable in the cauliflower. The result can be seen in Fig. 2. The concentration of the dimethametryn in the cauliflower was 73 g kg−1 .
3.2.3. Comparison of methods As shown in Table 3, the present method is compared with other methods. Compared with the SPE, PMAE, MISPE and PLE, the present method has advantages in the consumption of organic
Table 3 Comparison of the present method with other methods. Matrices
Extraction method
Detection method*
Analytes
Bovine milk cereal foods Cereals
SPE
HPLC–DAD
PMAE DMAE–SFOD
HPLC–DAD HPLC–MS HPLC–UV
MISPE
HPLC–UV
Sulfonylurea herbicides Triazine herbicides Triazine herbicides Triazine herbicides
PLE
HPLC NACE HPLC–UV
Soil and vegetables Cereal and vegetable Vegetables
ILFF–SPE
Triazine herbicides Triazine herbicides
Sample amount (g)
Organic solvent volume (mL)
Recovery (%)
LOD (g kg−1 )
LOQ (g kg−1 )
Reference
5
25
78–96
2–4
7–10
[11]
2
21
66–89
No report
39.1–48.7 0.9–2.0
[13]
80–102
1.1–1.5
3.5–4.8
[14]
1
1.1
10
41
72–116
0.4–2.4
No report
[17]
7
65
87–114
10–15
No report
[19]
79–104
1.3–2.7
4.5–9.2
This work
5
1.4
* DAD, diode array detection; UV, ultraviolet; NACE, non-aqueous capillary electrophoresis.
L. Zhang et al. / J. Chromatogr. B 953–954 (2014) 132–137
solvent and recoveries. Compared with DMAE–SFOD, when the present method was applied, the extraction device was cheaper and the operation was simpler. 4. Conclusions In the present research, the ILFF–SPE was established and successfully applied for the extraction and enrichment of triazine herbicides from vegetable samples. The factors which can affect the ILFF–SPE were investigated. The ILFF–SPE can be directly applied to the treatment of solid samples and is suitable for the treatment of large volume samples. The extraction and enrichment of the analytes were simultaneously performed and therefore, the sample treatment step and time can be reduced. The ILFF–SPE could be applied for pretreatment of other samples. Acknowledgment This work was supported by the Special-funded Programme on National Key Scientific Instruments and Equipment Development (Grant number 2011YQ1401500) and Development of Instrument for Rapid Analysis of Food (Grant number 2009BAK58B01-05). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jchromb.2014.02.011. References [1] C.R. Worthing, S.B. Walker (Eds.), The Pesticide Manual, 7th ed., The Lavenham Press, Lavenham, UK, 1983. [2] F. Hernandez, C. Hidalgo, J.V. Sancho, F.J. Lopez, Anal. Chem. 70 (1998) 3322. [3] C. Federico, S. Motta, C. Palmieri, M. Pappalardo, V. Librando, S. Saccone, Mutat. Res. Genet. Toxicol. Environ. 721 (2011) 89. [4] J.Y. Kim, A. Mulchandani, W. Chen, Anal. Biochem. 322 (2003) 251. [5] M. Battista, A. di Corcia, M. Marchetti, J. Chromatogr. 454 (1988) 233. [6] H.G.J. Mol, P. Plaza-Bola˜nos, P. Zomer, T.C. de Rijk, A.A.M. Stolker, P.P.J. Mulder, Anal. Chem. 80 (2008) 9450. [7] R.A. McLaughlin, V. Michael Barringer, J.F. Brady, R.A. Yokley, M.L. Homer, E.M. Janis, C.B. Orvin, The Triazine Herbicides, Elsevier, San Diego, 2008, pp. 243.
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Determination of triazine herbicides in vegetables by ionic liquid foam floatation solid phase extraction high performance liquid chromatography Liyuan Zhang a,b , Runzhong Yu c , Zhibing Wang a , Na Li a , Hanqi Zhang a , Aimin Yu a,∗ a
College of Chemistry, Jilin University, Qianjin Street 2699, Changchun 130012, PR China College of Food, Heilongjiang Bayi Agricultural University, Xinfeng Lu 5, Daqing 163319, PR China c Department of Computer Application Engineering, Daqing Vocational College, Huoju Lu, Daqing 163300, PR China b
a r t i c l e
i n f o
Article history: Received 20 August 2013 Received in revised form 21 January 2014 Accepted 10 February 2014 Available online 19 February 2014 Keywords: Ionic liquids Foam floatation Triazine herbicides Solid phase extraction Vegetable
a b s t r a c t The ionic liquid foam floatation solid phase extraction was established and applied to the extraction of six triazine herbicides, including desmetryn, secbumeton, terbumeton, terbuthylazine, dimethametryn and dipropetryn, in vegetable samples. To obtain the optimized experimental parameters, the effects of pH value of sample solution, the type and concentration of ionic liquid, the flow rate of carrier gas, foam floatation time, the type of solid phase extraction cartridge, the type and volume of elution solvent on the recoveries of the analytes were examined. The high performance liquid chromatography was applied to the determination of the analytes. Under the optimized experimental conditions, the linearities for determining the analytes were satisfactory and the limits of detection for desmetryn, secbumeton, terbumeton, terbuthylazine, dimethametryn and dipropetryn were 2.50, 1.75, 2.76, 1.87, 1.36 and 1.44 g kg−1 , respectively. The recoveries of the analytes ranged from 78.64% to 104.37% and the relative standard deviations ranged from 1.44 to 6.45%. The real samples were analyzed and the results were satisfactory. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Triazine herbicides are extensively used around the world by virtue of their properties of inhibiting photosynthetic reactions to help control the growth of grasses and broadleaf weeds [1]. However, they may be dangerous for human health. A large number of researches have proven the toxicity of triazine herbicides, which have been suspected of giving rise to cancer, congenital defect and so on [2–4]. Triazine herbicides and their metabolites and degradation products can pollute the crop itself and the environment of crop growth, such as soil and natural water. When these harmful substances pollute the foodstuffs, they can seriously endanger human health. Many countries and regions have established the criteria of maximum residue limits (MRLs) of triazine herbicides. The Environmental Protection Agency (EPA) has provided that the MRLs of triazine herbicides in most products are 0.25 mg kg−1 , while the European Union (EU) dictated that the MRL of terbuthylazine in vegetables is 0.05 mg kg−1 . The simple, rapid and sensitive method
∗ Corresponding author. Tel.: +86 431 85168399; fax: +86 431 85112355. E-mail address: [email protected] (A. Yu). http://dx.doi.org/10.1016/j.jchromb.2014.02.011 1570-0232/© 2014 Elsevier B.V. All rights reserved.
for extraction, separation and determination of the triazine herbicides in vegetable samples is required. In case of the complex matrix sample and low analyte content, the extraction and concentration should be more important than the determination. In recent years, more and more extraction methods have been developed, such as liquid–liquid extraction (LLE) [5–7], solid phase extraction (SPE) [8–11], solid phase micro-extraction (SPME) [12], pressurized microwave-assisted extraction (PMAE) [13], dynamic microwave-assisted extractionsolidification of floating organic drop (DMAE–SFOD) [14], cloud point extraction (CPE) [15], matrix solid phase dispersion (MSPD) [16], molecularly imprinted solid phase extraction (MISPE) [17], dispersive solid-phase extraction (DSPE) [18] and pressurized liquid extraction (PLE) [19,20]. Recently, magnetic nanoparticles (MNPs) were used as the adsorbents in many fields and successfully used for the pre-concentration and removal of some toxic and hazardous pollutants from water, soil and biological samples [21–31]. The methods based on ionic liquids (ILs) were developed for the extraction of triazine herbicides from different liquid samples (milk, honey, yogurt), such as microwave-assisted ionic liquid micro-extraction (MAILME) [32], dispersive liquid–liquid microextraction (DLLME) [33] and ionic liquid foaming-based solvent
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Fig. 1. Structures of triazine herbicides.
floatation (ILF-SF) [34]. Determination of triazine herbicides in water and food samples, especially in vegetable samples, was predominantly performed by high performance liquid chromatography (HPLC) [20], gas chromatography (GC) [35], HPLC mass spectrometry (HPLC–MS) [36], GC–MS [37] and micellar electrokinetic capillary chromatography (MECC) [38]. In this paper, the extraction and determination of triazine herbicides in vegetable samples by ionic liquid foam floatation solid phase extraction (ILFF–SPE) high performance liquid chromatography (HPLC) was developed. This method was based on the fact that IL has the characteristics of surfactant [39]. The ILFF–SPE was applied to the extraction of triazine herbicides in vegetables. ILbased FF–SPE was applied to the extraction of steroid hormones in water sample [39]. There is no report about application of ILFF–SPE in triazine herbicides in vegetable samples. In the present study, the ILFF–SPE was applied directly to solid and large volume sample. At the same time, sample pretreatment steps and extraction time were reduced because of the combination of extraction method and enrichment method. 2. Materials and methods 2.1. Chemicals and reagents Desmetryn, secbumeton, terbumeton, terbuthylazine, dimethametryn, and dipropetryn were obtained from National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China). The structures of the triazine herbicides are
shown in Fig. 1. 1-Ethyl-3-methylimidazolium tetrafluoroborate ([C2 MIM][BF4 ]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4 MIM][BF4 ]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6 MIM][BF4 ]), 1-Octyl-3-methylimidazolium tetrafluoroborate ([C8 MIM][BF4 ]), 1-butyl-3-methylimidazolium hexafluo([C4 MIM][PF6 ]), 1-hexyl-3-methylimidazolium rophosphate hexafluorophosphate ([C6 MIM][PF6 ]), and 1-octyl-3methylimidazolium hexafluorophosphate ([C8 MIM][PF6 ]) were obtained from Chengjie Chemical Co. Ltd. (Shanghai, China). Chromatographic grade methanol and acetonitrile were purchased from Fisher Scientific Company (UK). All other reagents of analytical grade were purchased from Beijing Chemical Factory (Beijing, China). Pure water was obtained with a Milli-Q water purification system (Millipore Co., USA). 2.2. Apparatus HPLC analysis was carried out on a LC chromatographic system equipped with a binary pump (LC-20AB) and UV detector (Shimadzu Corporation, Kyoto, Japan). Chromatographic separation of target analytes was performed on Agilent Eclipse XDB-C18 column (150 mm × 4.6 mm i.d., 3.5 m, Agilent, USA). Oasis HLB (3 mL, 100 mg) and MCX (3 mL, 100 mg) extraction cartridges were purchased from Waters (Milford, MA, USA). RE-52AA vacuum rotary evaporator (Yarong, Shanghai, China) were used. SPE cartridges, including SuperClean Al2 O3 (3 mL, 100 mg), SuperClean C18 (3 mL, 100 mg), and SuperClean Si (3 mL, 100 mg), were purchased from Supelco (Bellefonte, PA, USA).
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Table 1 Analytical performances. Triazine herbicide
Working curve
Correlation coefficient
Linear range (g kg−1 )
LOD (g kg−1 )
LOQ (g kg−1 )
Desmetryn Secbumeton Terbumeton Terbuthylazine Dimethametryn Dipropetryn
A = 3441.7c + 4524.8 A = 3622.2c + 5821.1 A = 3294.9c + 1464.5 A = 4630.8c + 5596.1 A = 6653.9c + 11129 A = 8144.7c + 12694
0.9989 0.9988 0.9987 0.9983 0.9990 0.9992
5.00–160.00 5.00–160.00 5.00–160.00 3.00–160.00 3.00–160.00 3.00–160.00
2.50 1.75 2.76 1.87 1.36 1.44
8.33 5.84 9.21 6.23 4.53 4.81
2.3. Working standard solutions
2.6. Determination by HPLC
The standard stock solutions for the triazine herbicides at the concentration level of 500 g mL−1 were prepared in methanol. Working standard solutions were prepared every week by diluting stock standard solutions with methanol. Mixed working standard solutions at different concentrations were prepared in the same method as the working standard solutions. The standard stock and working solutions were stored at 4 ◦ C and protected from light.
The mobile phases A and B were water and acetonitrile, respectively. The gradient conditions are as follows: 0–5 min, 40–60% B; 5–10 min, 60–80% B; 10–15 min, 80–80% B; 15–25 min, 80–40% B. The flow rate of the mobile phase was 0.5 mL min−1 . Sample injection volume was 20 L and the column temperature was kept at 30 ◦ C. The UV detection was carried out at a wavelength of 228 nm. 2.7. Method validation
2.4. Sample The vegetable samples were purchased from local large-scale supermarket in September (Changchun, China) and stored at 4 ◦ C. In this study, four kinds of vegetable samples, including cauliflower, cabbage, red cabbage and broccoli, were used. All experimental results were obtained with cauliflower except for the experimental results mentioned in Section 3.2.2, which were obtained with all four samples. The vegetable samples were crushed with stamp mill. The spiked samples containing the analytes were prepared by spiking the mixed working standard solution into the crushed samples and shaken for 10 min. Then all the samples were stored at 4 ◦ C before pretreatment.
2.5. Ionic liquid foam floatation–solid phase extraction 5 g of crushed vegetable sample was added into a breaker. 100 mL of water and 40 L of IL aqueous solution were added into the beaker. The pH values ranging from 2 to 14 of the resulting samples were adjusted with 1 M HCl and 1 M NaOH. Subsequently, the mixture was transferred to a floatation vessel. The ILFF–SPE system is shown in Suppl. Fig. 1. The SPE cartridge was activated with 2.0 mL of methanol and 2.0 mL of water before the extraction. The carrier gas was passed through the sample. The resulting foam was passed through the glass cotton, where the solid particles were not passed through, and introduced into the SPE cartridge. The analytes were extracted from the sample into the solution, diverted from the solution to the SPE cartridge and then adsorbed onto the cartridge. The flow rate of the carrier gas was 800 mL min−1 . The foam floatation time was 12 min. The SPE cartridge was washed with water at the flow rate of 2.0 mL min−1 for 1.0 min and subsequently the carrier gas N2 was passed through the cartridge until all the water was evaporated. The SPE cartridge was then eluted with 1.0 mL of methanol at the flow rate of 0.08 mL min−1 . The eluate was collected in a conical evaporating flask and evaporated to dryness at 35 ◦ C in a vacuum rotary evaporator. The residue was dissolved in 0.4 mL of methanol and then filtered through a 0.22 m filter membrane. Then, 20 L of the eluate was injected into the HPLC system. All experiments were performed in triplicate.
2.7.1. Working curve A certain amount of the mixed working standard solution was added into the vegetable sample. The resulting spiked samples were pretreated by ILFF-SPE mentioned above. 20 L of the eluate was injected in the HPLC system and analyzed under the chromatographic conditions mentioned above. The working curve was obtained by plotting peak areas versus the concentrations of the analyte in the spiked samples. The working curve was applied for calculating extraction recovery and evaluating the performances of the present method. 2.7.2. Precision The precision of the present method was presented by relative standard deviation (RSD). The intra-day and inter-day precision was acquired by analyzing spiked vegetable samples at two different concentration levels (10.00 and 200.00 g kg−1 ). The intra-day precision was obtained by analyzing the sample in triplicate in one day. The inter-day precision was obtained by analyzing the sample once each day over three consecutive days. 3. Results and discussion The effect of experimental parameters, including pH value of sample solution, the type and amount of IL, flow rate of carrier gas, time of foam floatation, the type and volume of solid phase extraction column, the type and volume of elution solvent, was investigated. 3.1. Optimization of ILFF–SPE conditions 3.1.1. Effect of sample solution pH value As shown in Suppl. Fig. 2, the effect of pH values of sample solution ranging from pH 2 to 14 was investigated. The results clearly show that the pH value of sample solution can affect the efficiency of extraction. In acidic condition, the loading capacity of IL foam was limited because of the foaming rate was too rapid. The recoveries obviously increase when the pH increases from 2 to 10 and remain unchanged or slightly decrease in the pH range of 10–14. Hence, pH 12 was chosen in the following experiments. 3.1.2. Effect of type of ionic liquid In ILFF–SPE, it is very important to select the type of ionic liquid. In this experiment, [C2 MIM][BF4 ], [C4 MIM][BF4 ], [C6 MIM][BF4 ], [C8 MIM][BF4 ], [C4 MIM][PF6 ], [C6 MIM][PF6 ], and [C8 MIM][PF6 ]
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were used as the foaming agents. The experimental results are shown in Suppl. Fig. 3. The results indicate that both hydrophilic and hydrophobic ILs can foam in the aqueous solution. ILs should have suitable surface activity and good foaming ability. It is necessary to take into account the length of alkyl chain of ILs. The recoveries obtained with [C6 MIM][PF6 ] are highest. [C6 MIM][PF6 ] was used as foaming agent. 3.1.3. Effect of amount of the ionic liquid The amount of the ionic liquid plays an important role in the extraction of six triazine herbicides from vegetables. 40 L of IL aqueous solutions containing 3.2, 6.4, 12.8, 19.2, 25.6 g of [C6 MIM][PF6 ], respectively were used in this study. The effect of [C6 MIM][PF6 ] amount in the aqueous solution was evaluated. The detailed results can be found in Suppl. Fig. 4. When amount of [C6 MIM][PF6 ] is lower than 12.8 g, [C6 MIM][PF6 ] could not foam in the solution well. In addition, if the amount is too high, the ability of extraction is limited. Finally, the amount of 12.8 g was selected. 3.1.4. Effect of the flow rate of carrier gas N2 was used as carrier gas. The results are shown in Suppl. Fig. 5. The recoveries increase with the increase of carrier gas flow rate. When the flow rate is lower than 800 mL min−1 , six triazine herbicides are not sufficiently transferred from sample solution to the SPE cartridge. When the flow rate of carrier gas is higher than 800 mL min−1 , the recoveries are invariable. Based on the experiment results 800 mL min−1 was selected in the work. 3.1.5. Effect of foam floatation time As can be seen from Suppl. Fig. 6, the recoveries of analytes increase rapidly with increase of foam floatation time, reach maximum when foam floatation time is 12 min and then are almost invariable. Consequently, the optimum time of foam floatation should be 12 min. 3.1.6. Effect of type of SPE cartridge Type of SPE cartridge can significantly affect the extraction and clean up efficiency. In this experiment, Si, HLB, MCX, Al2 O3 and C18 SPE cartridges were examined. The recoveries of six triazine herbicides are shown in Suppl. Fig. 7. When C18 is used, the recoveries of six triazine herbicides are higher than those obtained with the other cartridges. So, C18 (3 mL, 100 mg) was used as SPE cartridge. 3.1.7. Effect of type and volume of elution solvent In order to evaluate the effect of elution solvent, the effect of the type of elution solvent was investigated. The experimental results are shown in Suppl. Fig. 8. Six organic solvents, including acetone, ethyl acetate, ether, methanol, ethanol and acetonitrile, were used as elution solvents in this experiment. On the one hand, under the chosen HPLC conditions, the elution solvents can have good chromatographic behavior. On the other hand, the elution solvents should elute all the target analytes. Based on the above results, methanol was used as elution solvent. The volume ranging from 400 to 1200 L was tested when methanol was used as elution solvent. When the volume of methanol is too small the triazine herbicides cannot be eluted completely. The triazine herbicides can be eluted completely when the volume of methanol was 1000 L. Therefore, 1000 L of methanol was used in this work. 3.2. Evaluation of the method 3.2.1. Analytical performance The working curve was drawn by plotting the peak area (A) measured versus the concentration (C) of the target analyte. The linear regression equations and correlation coefficients are listed in
Fig. 2. Chromatograms of standard solution (a), cabbage (b), cauliflower (c), and spiked cauliflower sample (d). 1, desmetryn; 2, secbumeton; 3, terbumeton; 4, terbuthylazine; 5, dimethametryn; 6, dipropetryn.
Table 1.The limits of detection (LODs) and quantification (LOQs) are indicated in Table 1. The LODs and LOQs are obtained by following equations: LOD = 3 /k ;
LOQ = 10 /k
where is the standard deviation of blank signal, which is obtained by analyzing the non-spiked sample 11 times and k is the slope of the working curve. The LOQs of the six triazine herbicides are lower than the MRLs and the present method is appropriate for practical application. 3.2.2. Analysis of samples Four kinds of samples were analyzed to evaluate the accuracy and precision of the present method. The typical chromatograms of standard solution, cabbage, cauliflower and spiked cauliflower sample are shown in Fig. 2. Analytical results of four vegetable samples, including the recoveries, intra-day and inter-day precision, are listed in Table 2. The recoveries of all triazine herbicides at two concentration levels are from 78.64% to 104.37%. The results indicate that the present method offers good precision (RSD ≤ 6.45). The five triazine herbicides, including desmetryn, secbumeton, terbumeton, terbuthylazine and dipropetryn in four vegetable samples, were undetectable. However, the dimethametryn, which was
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Table 2 Analytical results of vegetable samples. Sample
Triazine herbicide
Added (g kg−1 )
Recovery (%)
Intra-day RSD (%)
Inter-day RSD (%)
Cauliflower
Desmetryn
10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00 10.00 200.00
85.93 89.11 93.73 90.75 94.56 95.89 93.41 88.76 81.23 83.23 104.37 102.23 81.22 84.23 90.10 92.46 94.35 92.67 91.45 94.29 92.68 94.55 95.43 98.56 79.34 80.56 91.28 90.98 90.38 94.65 95.90 93.47 93.28 92.98 98.67 101.64 78.64 80.29 83.26 84.55 88.76 87.67 90.44 89.35 91.86 93.63 94.38 92.33
3.23 2.83 3.76 3.14 4.23 3.78 4.44 3.33 2.95 2.44 3.87 2.38 4.76 4.87 4.30 3.77 3.23 3.05 4.49 3.96 2.56 2.04 1.98 1.44 5.32 4.74 5.21 4.80 3.99 3.67 3.89 3.56 3.08 2.67 3.23 3.09 4.34 4.06 3.83 3.17 4.58 4.04 2.19 1.68 2.74 2.60 3.15 2.00
4.39 3.88 4.96 4.02 5.38 4.95 5.22 4.21 4.17 3.90 4.86 3.76 6.23 6.45 5.87 4.38 3.88 3.58 5.29 4.44 3.94 2.27 3.26 2.13 6.20 5.78 5.98 5.81 5.34 4.46 5.18 4.87 4.42 3.11 4.64 3.99 5.06 4.77 5.16 4.45 5.82 4.85 3.99 2.53 3.56 3.43 4.37 3.25
Secbumeton Terbumeton Terbuthylazine Dimethametryn Dipropetryn Desmetryn
Cabbage
Secbumeton Terbumeton Terbuthylazine Dimethametryn Dipropetryn Red cabbage
Desmetryn Secbumeton Terbumeton Terbuthylazine Dimethametryn Dipropetryn Desmetryn
Broccoli
Secbumeton Terbumeton Terbuthylazine Dimethametryn Dipropetryn
undetectable in the cabbage, red cabbage and broccoli samples, was detectable in the cauliflower. The result can be seen in Fig. 2. The concentration of the dimethametryn in the cauliflower was 73 g kg−1 .
3.2.3. Comparison of methods As shown in Table 3, the present method is compared with other methods. Compared with the SPE, PMAE, MISPE and PLE, the present method has advantages in the consumption of organic
Table 3 Comparison of the present method with other methods. Matrices
Extraction method
Detection method*
Analytes
Bovine milk cereal foods Cereals
SPE
HPLC–DAD
PMAE DMAE–SFOD
HPLC–DAD HPLC–MS HPLC–UV
MISPE
HPLC–UV
Sulfonylurea herbicides Triazine herbicides Triazine herbicides Triazine herbicides
PLE
HPLC NACE HPLC–UV
Soil and vegetables Cereal and vegetable Vegetables
ILFF–SPE
Triazine herbicides Triazine herbicides
Sample amount (g)
Organic solvent volume (mL)
Recovery (%)
LOD (g kg−1 )
LOQ (g kg−1 )
Reference
5
25
78–96
2–4
7–10
[11]
2
21
66–89
No report
39.1–48.7 0.9–2.0
[13]
80–102
1.1–1.5
3.5–4.8
[14]
1
1.1
10
41
72–116
0.4–2.4
No report
[17]
7
65
87–114
10–15
No report
[19]
79–104
1.3–2.7
4.5–9.2
This work
5
1.4
* DAD, diode array detection; UV, ultraviolet; NACE, non-aqueous capillary electrophoresis.
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solvent and recoveries. Compared with DMAE–SFOD, when the present method was applied, the extraction device was cheaper and the operation was simpler. 4. Conclusions In the present research, the ILFF–SPE was established and successfully applied for the extraction and enrichment of triazine herbicides from vegetable samples. The factors which can affect the ILFF–SPE were investigated. The ILFF–SPE can be directly applied to the treatment of solid samples and is suitable for the treatment of large volume samples. The extraction and enrichment of the analytes were simultaneously performed and therefore, the sample treatment step and time can be reduced. The ILFF–SPE could be applied for pretreatment of other samples. Acknowledgment This work was supported by the Special-funded Programme on National Key Scientific Instruments and Equipment Development (Grant number 2011YQ1401500) and Development of Instrument for Rapid Analysis of Food (Grant number 2009BAK58B01-05). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jchromb.2014.02.011. References [1] C.R. Worthing, S.B. Walker (Eds.), The Pesticide Manual, 7th ed., The Lavenham Press, Lavenham, UK, 1983. [2] F. Hernandez, C. Hidalgo, J.V. Sancho, F.J. Lopez, Anal. Chem. 70 (1998) 3322. [3] C. Federico, S. Motta, C. Palmieri, M. Pappalardo, V. Librando, S. Saccone, Mutat. Res. Genet. Toxicol. Environ. 721 (2011) 89. [4] J.Y. Kim, A. Mulchandani, W. Chen, Anal. Biochem. 322 (2003) 251. [5] M. Battista, A. di Corcia, M. Marchetti, J. Chromatogr. 454 (1988) 233. [6] H.G.J. Mol, P. Plaza-Bola˜nos, P. Zomer, T.C. de Rijk, A.A.M. Stolker, P.P.J. Mulder, Anal. Chem. 80 (2008) 9450. [7] R.A. McLaughlin, V. Michael Barringer, J.F. Brady, R.A. Yokley, M.L. Homer, E.M. Janis, C.B. Orvin, The Triazine Herbicides, Elsevier, San Diego, 2008, pp. 243.
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