Selenium speciation in watermelon by g-C3 N4 enrichment combined with capillary electrophoresis-inductively coupled plasma-mass spectrometry

doi:10.3724/SP.J.1123.2020.06015

Abstract

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-C₃ N₄ ) 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-C₃ N₄ nanoparticles and large-sized nanosheets. The obtained suspension was centrifuged at about 10000 r/min, followed by drying at 60 ℃ to produce g-C₃ N₄ . The prepared g-C₃ N₄ 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-C₃ N₄ enrichment. The enrichment factor of g-C₃ N₄ 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 NaH₂ PO₄ -12 mmol/L H₃ BO₃ -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.

Key words: capillary electrophoresis (CE), inductively coupled plasma-mass spectrometry (ICP-MS), selenium, speciation analysis, watermelon

FENG Jinsu, CAO Yupin, MO Guichun, TANG Lifu, DENG Biyang. Selenium speciation in watermelon by g-C₃ N₄ enrichment combined with capillary electrophoresis-inductively coupled plasma-mass spectrometry[J]. Chinese Journal of Chromatography, 2020, 38(10): 1224-1231.

Figures/Tables 10

Table 1 Operating conditions for CE and ICP-MS

CE conditions		ICP-MS conditions
CTAB: cetyl trimethyl ammonium bromide.
Electrophoresis buffer	8 mmol/L NaH₂ PO₄ -12 mmol/L	RF power	1100 W
	H₃ BO₃ -0.2 mmol/L CTAB (pH=9.2)	plasma gas flow	15 L/min
Capillary	100 cm×100 μm i. d.	auxiliary gas flow	1.2 L/min
Separation voltage	22 kV	nebulizer gas flow	0.92 L/min
Injection way	0.04 MPa×12 s	pulse voltage	800 V
		CH₄ flow rate	1.1 mL/min
		monitored species	⁸⁰ Se

Fig. 1 Effect of buffer solution concentration on retention time of selenium species a. 4 mmol/L NaH2 PO4 -6 mmol/L H3 BO3 -0.2 mmol/L CTAB; b. 6 mmol/L NaH2 PO4 -9 mmol/L H3 BO3 -0.2 mmol/L CTAB; c. 8 mmol/L NaH2 PO4 -12 mmol/L H3 BO3 -0.2 mmol/L CTAB; d. 10 mmol/L NaH2 PO4 -15 mmol/L H3 BO3 -0.2 mmol/L CTAB; e. 12 mmol/L NaH2 PO4 -18 mmol/L H3 BO3 -0.2 mmol/L CTAB. CSeUr, SeCys2 , SeMet, Se(Ⅳ), Se(Ⅵ), SeEt =30 μg/L. For other conditions, see Table 1 .

Fig. 2 Effect of the pH of buffer solution on retention time of selenium species CSeUr, SeCys2 , SeMet, Se(Ⅳ), Se(Ⅵ), SeEt =30 μg/L. For other conditions, see Table 1 .

Fig. 3 Electropherogram of six selenium species using CE-ICP-MS CSeUr, SeCys2 , SeMet, Se(Ⅳ), Se(Ⅵ), SeEt =30 μg/L. For other conditions, see Table 1 .

Fig. 4 Adsorption percentages of selenium species using g-C3 N4 For detection conditions, see Table 1 .

Fig. 5 Effect of NaOH concentration on the desorption of selenium species Mass concentration: SeUr 20 μg/L, SeCys2 50 μg/L, SeMet 40 μg/L, Se(Ⅳ) 60 μg/L, Se(Ⅵ) 75 μg/L, SeEt 30 μg/L. For detection conditions, see Table 1 .

Fig. 6 Effect of ultrasonic time on adsorption and elution of selenium species CSeUr, SeCys2 , SeMet, Se(Ⅳ), Se(Ⅵ), SeEt =20 μg/L. For detection conditions, see Table 1 .

Table 2 Analytical performance of selenium species using CE-ICP-MS (n =5)

Selenium species	Linear range/(μg/L)	EF	LOD¹⁾ /(ng/L)	RSD/%	LOD²⁾ /(μg/L)
EF: enrichment factor; LOD: 1) this method; 2) reported method.
SeUr	0.017-20	17	6.2	2.2
SeCys₂	0.091-50	21	30	3.3	0.9^[18] , 0.3^[23] , 0.42^[24]
SeMet	0.032-40	18	11	2.4	5.6^[18] , 0.2^[23] , 0.56^[24]
Se(Ⅳ)	0.023-60	18	8.2	1.9	0.4^[18] , 0.15^[23] , 0.36^[24]
Se(Ⅵ)	0.15-75	29	48	3.5	0.4^[18] , 0.15^[23]
SeEt	0.015-30	12	5.5	2.7	1.2^[18] , 0.35^[23]

Fig. 7 Electropherograms of selenium species in (A) watermelon juice and (B) watermelon slag using CE-ICP-MS a. ordinary watermelon; b. selenium-rich watermelon; c. selenium-rich watermelon+10 μg/L SeCys2 . For detection conditions, see Table 1 .

Table 3 Analytical results and recoveries for selenium species in selenium-enriched watermelon samples (n =5)

Selenium species	Original/ (ng/g)	Added/ (ng/g)	Found/ (ng/g)	Recovery/ %	RSD/ %
WJ: watermelon juice. WS: watermelon slag.
SeCys₂ ^WJ	9.10	50.0	60.5	103	2.10
SeMet^WJ	30.0	100	129	99.0	1.60
Se(Ⅳ)^WJ	22.0	100	120	98.0	2.90
Se(Ⅵ)^WJ	7.50	50.0	60.5	106	1.80
SeEt^WJ	6.90	50.0	55.2	96.6	2.40
SeCys₂ ^WS	19.0	50.0	67.0	96.0	2.20
SeMet^WS	5.30	50.0	54.6	98.6	1.90
SeEt^WS	2.71	50.0	53.2	101	1.40

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Selenium speciation in watermelon by g-C₃ N₄ enrichment combined with capillary electrophoresis-inductively coupled plasma-mass spectrometry

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