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Dummy-Template Molecularly Imprinted Solid Phase Extraction For Food Chemistry 139 (2013) 24–30 Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem Analytical Methods Dummy-template molecularly imprinted solid phase extraction for selective analysis of ractopamine in pork ⇑ Wei Du a, Qiang Fu a, , Gang Zhao a, Ping Huang b, Yuanyuan Jiao a, Hao Wu a, Zhimin Luo a, Chun Chang a a School of Medicine, Xi’an Jiaotong University, Xi’an 710061, PR China b Xi’an Institute for Food and Drug Control, Xi’an 710061, PR China article info abstract Article history: Molecularly imprinted polymers (MIPs) for selective adsorption of ractopamine hydrochloride (RAC) Received 2 July 2012 were synthesised by an in situ method, in which salbutamol (SAL) was used as the dummy-template Received in revised form 20 December 2012 to avoid the template leakage. Scanning electron microscopy (SEM), mercury porosimerty and Fourier Accepted 28 January 2013 transform infrared spectroscopy (FTIR) were used to investigate the physical and morphological Available online 10 February 2013 characteristics of the dummy-template MIPs. The test of adsorption selectivity indicated that the dummy-template MIPs exhibited high selectivity to RAC. The saturated adsorption capacity for RAC on Keywords: dummy-template MIPs was 90.9 lggÀ1. Based on the dummy-template polymers, a liquid chromatogra- Ractopamine phy–mass spectrometry (LC–MS) method was developed for the selective analysis of RAC in real pork Dummy-template Molecularly imprinting polymers samples. The averages of intra- and inter-day accuracy ranged from 78.9% to 92.2% and from 90.7% to In situ polymerisation 93.1%, respectively. The RSD% of repeatability ranged from 1.9% to 6.3%, and the RSD% of intermediate 1 Solid phase extraction precision ranged from 3.5% to 9.2%, while the limit of detection (LOD) was 0.02 lgkgÀ . Liquid chromatography- mass spectrometry Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. 1. Introduction plasmon resonance-based biosensor inhibition immunoassays (Lu et al., 2012). However, these methods usually require sample pre- Ractopamine (RAC) is a synthetic phenethanolamine b2-adreno- treatment processes, such as solid-phase extraction (SPE) (Dong ceptor agonist, which can be used as clinical medicine for the treat- et al., 2011; Qu et al., 2011; Shao et al., 2009; Wang et al., 2010), ment of asthma. It, however, can be illegally used as a growth and the routine SPE suffered from the disadvantage of low selectiv- promoter for meat-producing animals with high dosage. Therefore, ity and poor recovery. RAC is strictly banned as a feed additive in many countries Recently, molecularly imprinted polymers (MIPs) have at- (Commission of the European Communities, 1996; The Ministry tracted much attention in different areas, attributed to their high of Agriculture, 2002) due to the potential risk to human beings affinity and pre-determined selectivity for target analytes and who consume products made from RAC-treated animals (Brambilla other structural analogues (Tamayo, Turiel, & Martin-esteban, et al., 2000; Smith, Ehrenfried, Dalidowicz, & Turberg, 2002; Xiao, 2007). Molecularly imprinted solid-phase extraction (MISPE), as a Xu, & Chen, 1999). Nevertheless, the use of RAC remains attractive relatively new concept in the clean-up of biological samples, has to swine producers because it can improve feed efficiency. This proved to be an efficient and selective approach for purification makes it essential to establish sensitive and selective analytical and pre-concentration of RAC from complex matrices (Hu, Li, Liu, methods to monitor the residual RAC in food samples. Tan, & Li, 2011; Tang, Fang, Wang, & Li, 2011; Wang, Liu, Fang, Several analytical methods have been developed for the deter- Zhang, & He, 2009; Widstrand et al., 2004). However, the drawback mination of RAC in animal tissues, urine and feed, such as high- of the MIPs is the unavoidable template leaking, which may influ- performance liquid chromatography (HPLC) (Shelver & Smith, ence the accuracy of identification and quantitation of the analytes 2003), gas chromatography–mass spectrometry (He, Su, Zeng, (Tamayo et al., 2007). A strategy to avoid template leaking is the Liu, & Huang, 2007; Wang, Li, & Zhang, 2006), liquid chromatogra- utilisation of a dummy template, structural analogue of target ana- phy–mass spectrometry (LC–MS) (Antignac, Marchand, Le, & lyte itself, during the preparation of MIPs (Yin et al., 2012). To our Andre, 2002; Blanca et al., 2005; Dong et al., 2011; Kootstra best knowledge, there is still no developed method concerning the et al., 2005), ultra-performance liquid chromatography–tandem template leaking of MISPE during the analysis of RAC residue. mass spectrometry (Shao et al., 2009; Zheng et al., 2010), capillary In this study, the MISPE for selective analysis of RAC in pork was electrophoresis (Wang, Zhang, Wang, Shi, & Ye, 2010), and surface prepared by an in situ method, using the analogue, salbutamol (SAL), as the dummy-template. The chemical structures of RAC ⇑ Corresponding author. Tel.: +86 29 82655382. and SAL are shown in Fig. 1. The adsorption characteristics of the E-mail address: [email protected] (Q. Fu). obtained dummy-template MIPs were investigated. The adsorption 0308-8146/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.01.109 W. Du et al. / Food Chemistry 139 (2013) 24–30 25 225 nm, 246 nm, 223 nm and 256 nm, respectively. The injection volume was 5 ll, and the column temperature was maintained at 25 °C by an Automatic AT-330 column heater (Tianjin, China). LC–MS equipment consisted of a Shimadzu mass spectrometry system (Kyoto, Japan), which included three LC-20AD pumps, a DGU-20A3 degasser, a SIL-20A autosampler, a CTO-20A column oven, a SPD-20A UV/VIS detector, a SPD-M20A diode array detec- tor, a LCMS2010EV mass spectrometer, and a LCMS solution work- station. The analysis was performed in the positive electrospray ionisation mode (ESI) at m/z of 302. The column was a VP-ODS col- Fig. 1. Chemical structures: (a) ractopamine and (b) salbutamol. umn (150 Â 2.0 mm I.D., 5 lm), and the mobile phase was acetoni- trile–0.2% formic acid solution (12:88, v/v) at a flow rate of 0.2 ml minÀ1 with a column temperature of 37 °C. MS conditions isotherms were modelled using Langmuir and Freundlich models. were as follows: nebulizer gas (N2, purity > 99.999%), flow rate of À1 The adsorption rate was determined based on Lagergren’s pseudo 1.5 l min , drying gas (N2, purity > 99.999%), pressure of 0.1 first and second order kinetic equations. The application of the MPa, interface temperature of 300 °C, heat block temperature of dummy-template MISPE, coupled with LC–MS method, was devel- 220 °C, and detector voltage of 1.25 kV. The injected volume was oped for selective analysis of RAC in pork. 10 ll. 2.3. Preparation of dummy-template MISPE 2. Materials and methods The dummy-template MISPE was prepared by an in situ poly- 2.1. Reagents and solutions merisation in the SPE cartridge according to the method reported previously (Fu et al., 2011). Briefly, the dummy-template (SAL), tol- RAC was purchased from Sigma–Aldrich (New Jersey, USA). SAL uene, MAA, EDMA, dodecanol, and AIBN were sequentially added was purchased from Cunyi Chemical Co. (Jiangsu, China). Clenbu- to a 10 ml test tube. The mixture was thoroughly mixed before terol hydrochloride was obtained from Jinhe Pharmaceutical Co. use and then degassed for 15 min. After purging with a nitrogen (Wuhan, China). Terbutaline sulphate was purchased from Gang- stream for 15 min, the pre-polymerising solution was transferred zheng Pharmaceutical Co. (Wuhan, China). Adrenaline hydrochlo- into a 10 ml empty SPE cartridge. The cartridge was then sealed ride was obtained from Hefeng Pharmaceutical Co. (Shanghai, and set up vertically. The polymerisation was allowed to proceed China). Methacrylic acid (MAA) was purchased from Tianjin at 50 °C for 20 h. The obtained MISPE cartridge was washed by a Chemical Reagent Plant (Tianjin, China). 4-Vinylpyridine (4-VPY), mixture of methanol–acetic acid (90:10, v/v) with a flow rate of 2-vinylpyridine (2-VPY) and trifluoromethacrylic acid (TFMAA) 0.5 ml minÀ1 to remove the template molecule and residual poro- were obtained from Sigma–Aldrich (New Jersey, USA). MAA, 4- genic solvents. Finally, the MISPE cartridge was washed with meth- VPY and 2-VPY were distilled under vacuum to remove inhibitors anol at a flow rate of 0.5 ml minÀ1 to remove the residual acetic prior to use. Ethylene glycol dimethacrylate (EDMA) was obtained acid. A similar procedure, without dummy-template, was used to 0 from Sigma–Aldrich (New Jersey, USA). 2,2 -Azobisisobutyronitrile prepare the non-imprinted solid-phase extraction (NISPE) (AIBN) was purchased from Shanghai No. 4 Reagent Factory cartridge. (Shanghai, China) and recrystallised in methanol before use. Meth- anol and acetonitrile were of HPLC grade, purchased from Kemite 2.4. Physical and morphological characterisation Co. (Tianjin, China). Water was purified with Molement 1805b (Shanghai, China). All other chemicals were of analytical grade The morphologies of the dummy-template MIPs and NIPs were and obtained from local suppliers. Empty SPE cartridges (10 ml) observed by a JSM-6390A Scanning Microscope (Jeol, Japan). The were purchased from Shenzhen Doudian Co. (Shenzhen, China). porosity, total pore volume, and average pore diameter were mea- Blank pork sample was supplied by a local farmer. Real pork sured by mercury porosimerty with an Auto Pore IV 9510 porosi- samples were obtained from local markets and stored at À20 °C meter (Micromeritics, USA). Fourier transform infrared spectra prior to use. (FTIR) were recorded on an FTIR-8400S spectrometer (Shimadzu, Standard stock solutions of RAC, clenbuterol, terbutaline and Japan) with a scanning range from 400 to 4000 cmÀ1.
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