Abstract:

New coccine is an azo pigment that is widely used in food. To mitigate potential health issues arising from excessive consumption, China has issued provisions on the allowed addition limit of new coccine in food. Currently, there are certain difficulties with establishing detection methods for such trace pigments in foods; for example, preprocessing is complex and time-intensive. In addition, the low content of the target substance in the sample could be disturbed by food matrix, resulting in poor detection sensitivity. Metal organic frameworks (MOFs), as a novel class of highly efficient adsorbents, have attracted increasing attention because of their stability and large specific surface area. MOFs are porous coordination crystal structures that connect metal clusters with organic ligands via coordination. Owing to their molecule-sized pores, MOFs can be used in various fields such as adsorption, catalysis, and drug dispersion. However, at the same time, their ultra-high specific surface area also leads to ultra-low weight of the material itself; this makes it difficult to collect the material even under high-speed centrifugation. In this study, a MOF material (PCN-222) with a high specific surface area was prepared by the coordination of the carboxyl group in the porphyrin ring and metal zirconium ions. To simplify pretreatment, the nanomaterials were filled into an injection solid phase extraction device for the rapid extraction of new coccine pigments from beverages. The morphology, structure, and properties of the PCN-222 nanomaterials were studied by transmission electron microscopy, particle size analysis, X-ray single-crystal diffraction, infrared spectroscopy, and ultraviolet spectroscopy. The specific surface area of the synthesized material was 979 m²/g. A high specific surface area was conducive to the adsorption of trace target compounds. The surface charge of the material could be controlled by adjusting the pH value of the solution, which was beneficial to the selective adsorption and desorption of ionic pigments. The π-π interaction between the benzene ring of the porphyrin ring and the benzene ring of the azo pigment also promoted extraction. Thus, the extractant exhibited strong enrichment performance for the new coccine anionic pigment. The solid phase extraction conditions were optimized, and it was found that saturated adsorption capacity was achieved by filling 3 mg of extractant. The effect of pH on adsorption was also explored; the adsorption effect was the best at pH 3. In the desorption experiment, N,N-dimethylformamide at pH 11 was conducive to better elution of the target. Further elution volume studies showed that maximum recovery could be achieved by adding 3 mL of eluent. Subsequently, the sample pretreatment time was reduced to 5 min. The enriched sample was separated using a Zorbax eclipse XDB-C18 column (250 mm×4.6 mm, 5 μm), eluted with an ammonium acetate-methanol solvent system, and detected at 254 nm. Under the optimum conditions, the recoveries of the samples at high, medium, and low levels reached 99.5%-109.4%, and the relative standard deviation was less than 3%. The limit of detection (LOD, S/N=3) of this method was 0.1 μg/L and the limit of quantification (LOQ, S/N=10) was 0.3 μg/L. In the actual sample detection experiment, the detection signal of new coccine in the sample was amplified by solid phase extraction to achieve enrichment. In addition, the extraction capacity of PCN-222 remained higher than 90% after four uses, and the synthesized material could be recycled. The high precision and low detection limit indicate that the method is suitable for the enrichment and detection of trace carmine in beverages. The findings of this study will aid in the development of a new solid phase extraction technology for food safety evaluation.

Key words: high performance liquid chromatography (HPLC), one step rapid solid phase extraction, metal-organic frameworks (MOFs), new coccine