Selenium speciation in watermelon by g-C3N4 enrichment combined with capillary electrophoresis-inductively coupled plasma-mass spectrometry

Abstract

Selenium is one of the essential trace elements in the human body, and it plays a critical role in human health. In this work, 2.0 g melamine was placed in an alumina crucible, which was heated in a box-type resistance furnace for 2 h at 600 ℃, at the heating rate of 3 ℃/min, and then cooled to room temperature. After cooling, yellow graphite phase carbon nitride (g-C3N4) nanosheets were obtained. Subsequently, 500 mg of the nanosheets was dispersed in 50 mL water with ultrasonication for 10 h in order to remove the residual un-exfoliated g-C3N4 nanoparticles and large-sized nanosheets. The obtained suspension was centrifuged at about 10000 r/min, followed by drying at 60 ℃ to produce g-C3N4. The prepared g-C3N4 was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), and field emission-environmental scanning electron microscopy (SEM) analyses. Given that the selenium content in actual samples is very low, high sensitivity, and accuracy are imperative for selenium detection. The combination of capillary electrophoresis (CE) with inductively coupled plasma-mass spectrometry (ICP-MS) can greatly improve the sensitivity, accuracy, and speed of the analysis. A novel method based on CE-ICP-MS was established for the determination of selenourea (SeUr), L-selenocystine (SeCys2), DL-selenomethionine (SeMet), selenite (Se(Ⅳ)), selenate (Se(Ⅵ)), and selenoethionine (SeEt) in watermelon. The selenium species in watermelon were extracted by ultrasonication with pepsin as an extractant and g-C3N4 enrichment. The enrichment factor of g-C3N4 ranged from 12 to 29. Six selenium species were completely separated within 11 min in a 100-cm-long capillary with 100 μm internal diameter, at an applied voltage of 22 kV, using a buffer solution of 8 mmol/L NaH2PO4-12 mmol/L H3BO3-0.2 mmol/L cetyl trimethyl ammonium bromide (CTAB; pH 9.2). The interference in the selenium detection was eliminated using a dynamic reaction cell with CH4. The linear correlation coefficients of all the selenium species were greater than 0.9995. Under the optimal conditions, the limits of detection (3 σ, σ for standard deviation, as Se) for SeUr, SeCys2, SeMet, Se(Ⅳ), Se(Ⅵ), and SeEt were 6.2, 30, 11, 8.2, 48, and 5.5 ng/L, respectively. The linear range (as Se) for SeUr, SeCys2, SeMet, Se(Ⅳ), Se(Ⅵ), and SeEt were 0.017-20 μg/L, 0.091-50 μg/L, 0.032-40 μg/L, 0.023-60 μg/L, 0.015-75 μg/L, and 0.015-30 μg/L, respectively. The recoveries ranged from 96.0% to 106%, and the relative standard deviations (RSDs; n=5) were less than 3%. The developed method is simple, rapid, and sensitive, and it is also suitable for the detection of selenium species in other food and environmental samples.