Field-amplified sample injection and graphene quantum dot dual preconcentration in the analysis of melamine and dicyandiamide by capillary electrophoresis

doi:10.3724/SP.J.1123.2021.08017

Abstract

Abstract:

Sulfur-doped graphene quantum dots (S-GQDs) were prepared by the pyrolysis of citric acid and mercaptopropionic acid. Compared with graphene quantum dots (GQDs), the S-GQDs have improved surface state and chemical reactivity, and thus, exhibited stronger interaction with cations. Based on its excellent affinity for cations, a dual preconcentration technique combining field-amplified sample injection (FASI) and S-GQDs as multianalyte carriers was developed for the determination of melamine and dicyandiamide by capillary electrophoresis (CE). During the FASI process, a large quantity of analytes was introduced into the capillary and accumulated at the capillary inlet. Concurrently, the S-GQDs migrated to the anode and captured the analytes on its surface at the boundary of the sample and buffer solution. The use of S-GQDs allows the capture of abundant analytes, which can amplify the detection signal. This new protocol was evaluated by the quantitative determination of melamine and dicyandiamide in metformin hydrochloride preparations. The effect of volume fraction of the S-GQDs in the buffer solution, the composition and pH of the buffer, and the sample injection time on the preconcentration and separation were investigated. By controlling the pH at 4.6, the sample injection time was prolonged to 450 s. A very large amount of melamine and dicyandiamide, bearing positive electric charges, were injected into the capillary and were captured by S-GQDs. The assay using FASI preconcentration and S-GQDs as enhancer resulted in a 1.6×10⁵-fold improved sensitivity compared with that obtained with traditional 10-kV electrokinetic injection for 10 s. The calibration curves of melamine and dicyandiamide were obtained in the concentration range from 1.0×10^-14 to 1.0×10^-8mol/L, with correlation coefficients (r²) >0.999. The detection limits (S/N=3) were 2.6×10^-15mol/L for melamine and 5.7×10^-15mol/L for dicyandiamide. The recoveries of the two analytes were 95.9%-102.4% and 92.0%-106.0%, respectively, with relative standard deviations (RSDs) of no more than 5%. The RSD values of peak height, peak area, and migration time were all less than 5.6%. This method is reliable, easy, and exhibits a good separation effect. This proves that the S-GQD-enhanced CE method could be developed into a new and sensitive technique for the determination of melamine and dicyandiamide in different preparations of metformin hydrochloride.

Key words: sulfur-doped graphene quantum dots (S-GQDs), capillary electrophoresis (CE), melamine, dicyandiamide

CLC Number:

O658

LI Chao, WANG Qi, ZHANG Zhaoxiang. Field-amplified sample injection and graphene quantum dot dual preconcentration in the analysis of melamine and dicyandiamide by capillary electrophoresis[J]. Chinese Journal of Chromatography, 2022, 40(3): 289-295.

Figures/Tables 10

Fig. 1 (a) Preparation of sulfur-doped graphene quantum dots (S-GQDs) and (b) adsorption of melamine and dicyandiamide on the S-GQDs

Fig. 2 Schematic diagrams of CE separation based on field-amplified sample injection and graphene quantum dot dual preconcentration a. field-amplified sample injection preconcentration; b. preconcentration of S-GQDs as multianalyte carriers; c. CE separation. Veo: velocity of electroosmotic flow; Vep: velocity of electrophoresis.

Fig. 3 (a) Transmission electron microscopy (TEM) image, (b)particle size distribution, and (c) X-ray diffraction (XRD) pattern of the as-prepared S-GQDs

Fig. 4 (a) Full-range X-ray photoelectron spectrum (XPS) of S-GQDs and(b) high resolution XPS of S 2p

Fig. 5 Effect of the volume fraction of S-GQDs in the buffer on the analysis of melamine and dicyandiamide Volume fraction of S-GQDs: a. 0; b. 10%; c. 20%; d. 25%. Peak 1: melamine; peak 2: dicyandiamide.

Fig. 6 Effect of injection time on peak height and separation efficiency of melamine and dicyandiamide

Fig. 7 Comparison of electropherograms without (a) and with (b) FASI and S-GQDs dual preconcentration a. 50 mmol/L phosphate buffer solution; pH 4.6. The concentrations of both melamine and dicyandiamide were 1.0×10-6mol/L. Sample injection volume: 10 kV×10 s. b. Buffer solution (50 mmol/L phosphate+25% (v/v) S-GQDs). The concentrations of both melamine and dicyandiamide were 1.0×10-10mol/L. FASI volume: 10 kV×450 s. Peaks: 1. melamine; 2. dicyandiamide.

Table 1 Regression equations, correlation coefficients (r2), linear ranges, LODs, and precisions (RSDs) of melamine and dicyandiamide (n=5)

Compound	Regression equation	r²	linear range/ (mol/L)	LOD/ (mol/L)	RSDs/%
Compound	Regression equation	r²	linear range/ (mol/L)	LOD/ (mol/L)	Peak height	Peak area	Migration time
Melamine	y=-238.8x+3349.1	0.9996	10^-14-10^-8	2.6×10^-15	2.8	2.6	4.3
Dicyandiamide	y=-219.6x+3074.3	0.9992	10^-14-10^-8	5.7×10^-15	3.2	3.7	5.6
y: peak height; x: -log c; c: concentration, mol/L.

Table 2 Comparison with other methods for the determination of melamine and dicyandiamide

Method	Detection limits				Reference
Method	Melamine		Dicyandiamide		Reference
LC-MS	54	μg/L	5.4	μg/L	[1]
HPLC-MS	0.1	mg/kg	0.4	mg/kg	[2]
Tandem dual solid phase extraction cartridges-HPLC-electrospray	1.48	μg/kg	13.61	μg/kg	[3]
ionization multi-stage MS
Magnetic surface molecularly imprinted polymers-HPLC	15	μg/L	-		[4]
FASI+S-GQDs CE	2.6×10^-15	mol/L	5.7×10^-15	mol/L	this work

Table 3 Spiked recoveries of melamine and dicyandiamide and their RSDs in metformin hydrochloride samples (n=5)

Sample	Melamine					Dicyandiamide
Sample	Original/ (μg/L)	Added/ (μg/L)	Found/ (μg/L)	Recovery/ %	RSD/ %	Original/ (μg/L)	Added/ (μg/L)	Found/ (μg/L)	Recovery/ %	RSD/ %
1^#	82.5	80.0	159.2	95.9	3.3	5.6	5.0	10.2	92.0	2.2
2^#	67.9	80.0	149.8	102.4	2.9	1.2	5.0	6.5	106.0	4.4
3^#	96.2	80.0	175.1	98.6	4.8	2.9	5.0	7.6	94.0	2.8

References

[1]	MacMahon S, Begley T H, Diachenko G W, et al. J Chromatogr A, 2012, 1220:101
[2]	Draher J, Pound V, Reddy T M. J Chromatogr A, 2014, 1373:106
[3]	Zhou Y F, Wang F H, Wang Z L, et al. Chinese Journal of Chromatography, 2018, 36(2):159
[3]	周艳芬, 王芳焕, 王泽岚, 等. 色谱, 2018, 36(2):159
[4]	Sun Z A, Qi Y X, Wang X, et al. Chinese Journal of Chromatography, 2018, 36(8):716
[4]	孙治安, 祁玉霞, 王霞, 等. 色谱, 2018, 36(8):716
[5]	Poma A, Brahmbhatt H, Watts J K, et al. Macromolecules, 2014, 47(18):6322
[6]	Harstad R K, Johnson A C, Weisenberger M M, et al. Anal Chem, 2016, 88:299
[7]	Hu L, Krylova S M, Liu S K, et al. Anal Chem, 2020, 92:14251
[8]	Zhou Q Y, Wang L J, Liu Y, et al. Anal Chem, 2020, 92:10620
[9]	Xiong F F, Jiang D D, Jia Q. Chinese Journal of Chromatography, 2020, 38(1):60
[9]	熊芳芳, 江丹丹, 贾琼. 色谱, 2020, 38(1):60
[10]	Li X, Yan Z F, Li L Z, et al. Chinese Journal of Chromatography, 2021, 39(6):599
[10]	李雪, 晏志凤, 李龙珠, 等. 色谱, 2021, 39(6):599
[11]	Zhang Z X, Li X L, Ge A Q, et al. Biosens Bioelectron, 2013, 41:452
[12]	Zhang Z X, Zhang C Y, Luan W X, et al. Anal Chim Acta, 2015, 888:27
[13]	Zhang Z X, Luan W X, Zhang C Y, et al. Acta Chimica Sinica, 2017, 75(4):403
[13]	张召香, 栾文秀, 张超英, 等. 化学学报, 2017, 75(4):403
[14]	Gupta V, Chaudhary N, Srivastava R, et al. J Am Chem Soc, 2011, 133(26):9960
[15]	Wang F, Gu Z, Lei W, et al. Sensor Actuat B-Chem, 2014, 190:516
[16]	Kumawat M K, Thakur M, Gurung R B, et al. ACS Sustain Chem Eng, 2017, 5(2):1382
[17]	Majumder T, Dhar S, Chakraborty P, et al. Nano, 2019, 14(1):5530
[18]	Li S, Li Y, Cao J, et al. Anal Chem, 2014, 86:10201
[19]	Chen H, Li W, Wang Q, et al. Electrochim Acta, 2016, 214:94
[20]	Chen L, Wu C, Du P, et al. Talanta, 2017, 164:100
[21]	Anh N T N, Doong R. ACS Appl Nano Mater, 2018, 1:2153
[22]	He X Y, Zhou L, Nesterenko E P, et al. Anal Chem, 2012, 84(5):2351