Pretreatment-free fast ultraviolet detection of melamine in milk products with a disposable microfluidic device
Introduction
Recently, melamine (MEL), a triazine-based industrial chemical, has been found in pet food and many milk products. Due to the harm to health caused by its illegal addition in milk products, its determination has attracted considerable attention [1], [2], [3], [4]. Many methods have been developed for the screening or detection of MEL in various matrices. The traditional method for screening MEL in milk products or animal tissues was high performance liquid chromatography (HPLC) with ultraviolet (UV) detection [4], [5], [6]. Methods such as gas chromatography/mass spectrometry (GC/MS) and liquid chromatography/mass spectrometry (LC/MS) have been developed for more authoritative determination of MEL [7], [8], [9], [10], [11], [12], [13]. The ability of MS techniques for component verification and high sensitive detection enabled them to be extensively used for MEL analysis. For example, a tandem mass spectrometric method combined with ambient ionization using a low-temperature plasma probe has been applied for high-throughput trace melamine analysis in complex mixtures [14], a MS method by surface desorption atmospheric pressure chemical ionization has been used to detect MEL in Milk [15], and a matrix-assisted laser desorption/ionization MS technique has been developed for analyzing melamine cyanurate in urine [16].
Recently two capillary zone electrophoresis (CZE) methods coupled with UV detection [17] or diode array detection [18], two Raman spectroscopic methods [19], [20], and a visual detection method by hydrogen-bonding recognition-induced color change of gold nanoparticles [21] have been developed for melamine detection in raw milk and infant formula. Several commercial enzyme-linked immunosorbent assay (ELISA) test kits for the detection of triazines have also been available [22]. All these methods mentioned above need complex sample pretreatments such as extraction, preconcentration and derivatization. The sample pretreatment procedure is time-consuming, and always needs toxic solvents, e.g. dichloromethane, nitrile, methanol or trichloroacetic acid. Moreover, the extraction process often leads to low recovery of melamine [22]. Although several pretreatment-free methods such as extractive electrospray ionization MS [23] and near-/mid-infrared spectroscopic method [24] have recently been developed for in situ analysis of MEL, they are relatively expensive for general investigation of MEL in milk products. Moreover, the infrared spectroscopy is relatively weak in quantitative and sensitive analysis. Thus it is still urgent to develop cheaper, faster and green techniques for extensive screening, particularly in situ detection, of MEL in milk products. This work presented a novel pretreatment-free method for sensitively screening MEL by using a disposable microfluidic device developed in our group [25].
Microfluidic electrophoresis device (MED) is a powerful tool for analytical application due to its low consumption of reagents, short separation time, high separation efficiency, and low cost [26], [27], [28]. Its fabrication is also cheap and convenient by using polymer materials such as polydimethylsiloxane [29], polymethylmethacrylate (PMMA) [30], or polycarbonate [31] as matrix. In view of the shortage of these matrices in UV detection [32], our previous work used a fused-silica capillary as separation channel to develop a hybrid quartz capillary/PMMA MED (HQM) by using PMMA as support substrate [33]. This technique avoided the requirements of clean-room facilities, corrosive etchants and time-consuming bonding for the preparation of sampling and separation channels [34]. Based on the integration of all advantages of the HQM [25], [33], [34] with the properties of fracture sampling technique [34], such as negligible sample leakage, efficient sample self-stacking and high separation efficiency, this work developed a fast and environmentally friendly strategy for low-cost and sensitive UV screening of MEL by directly sampling milk sample into separation channel. The ultra narrow sampling fracture and high separation efficiency led to excellent analytical performance of the proposed method, thus it could be widely applied in fast and low-cost in situ screening of MEL in different milk products.
Section snippets
Reagents and materials
MEL with 99% purity was purchased from Acros. All aqueous solutions were prepared using ≥18 MΩ ultrapure water (Milli-Q, Millipore). The phosphate running buffer was passed through a membrane filter (0.22 μm pore size) and dealt with ultrasonic for removing air bubbles prior to use. Fused-silica capillaries (360 μm o.d., 50 μm i.d.) were obtained from Yongnian Optical Fiber Factory (Hebei, China). All other chemicals were of analytical grade. Milk samples were commercially available in Nanjing. The
Properties of HQM
The ultrasonic method for formation of sampling fracture on a capillary excluded the subjective handling influence on the fracture quality [25]. Thus the preparation reproducibility of sampling fracture was good, which could be verified by the reproducible results for MEL detection. The enlarged images of sampling fracture were shown as Fig. 1A and B, at which the width of the sampling fracture was measured to be about 800 nm. Both the two fracture edges and the inner surface were quite smooth
Conclusions
A fast and low-cost approach was developed for the screening of MEL using a disposable microfluidic device with UV detection. The proposed strategy could exclude the interference of microparticles, macromolecules and sugars in the milk suspension, thus avoid large amount of toxic solvents used for the troublesome and time-consuming pretreatments. The ultrasonically formed sampling fracture showed a uniform and smooth surface for producing a very narrow sample plug to achieve highly efficient
Acknowledgements
We gratefully acknowledge the financial support of the National S&T Pillar Program (2007BAK26B06), Key Specific Program (2009ZX10603) and National Basic Research Program (2010CB732400) from Ministry of S&T, the Program for Creative Research Groups (20821063) and the projects (20835006, 20875044) from NNSFC and Natural Science Foundation of Jiangsu Province (BK2008014).
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