Polydopamine-assisted partial hydrolyzed poly(2-methyl-2-oxazolinze) as coating for determination of melamine in milk by capillary electrophoresis

• PDA/PMOX-EI coating was formed via one-step on inner surface of the capillary.
• Melamine in milk powder was determined by PDA/PMOXA-EI coated capillary using CE.
• Solvent extraction for the pretreatment of sample is not needed in this method.
• CE running procedure is simple and convenient without additional NaOH rinsing step.
• Our coating could be still used to detect melamine after being used for 45 days.

Nitrogen-rich melamine (66% by mass) had been found to be illegally adulterated to milk products and animal feed in order to increase the apparent protein content, and ingestion of melamine may lead to the formation of kidney stones. Hence, the development of reliable analytical methods for the monitoring of melamine in milk products is necessary. The common methods for melamine analysis in milk products are performed via GC–MS or HPLC, however, these procedures require a large of organic solvent and several pretreatment steps. In this study, a new and facile approach was developed to modify a fused-silica capillary inner surface based on poly(dopamine) (PDA) and partial hydrolyzed poly(2-methyl-2-oxazoline) (PMOXA-EI) mixed coating, which was used to determine the melamine in milk powder quantitatively by capillary electrophoresis with ultraviolet (CE-UV) detection with simple analytes pretreatment. Firstly, the influences of the molecular mass of poly(2-methyl-2-oxazoline) (PMOXA), the hydrolysis degree of PMOXA, and the concentration ratio of DA to PMOXA-EI on the separation properties of a mixture of four basic proteins (cytochrome c, lysozyme, ribonuclease A, and α-chymotrypsinogen A) were studied in detail. As a consequence, fast and efficient separation of four proteins was obtained using PDA/PMOXA1k-EI0.4 coating (once coating). Then, a modified facile coating workflow was carried out to acquire a novel twice coating (i.e. PDA/PMOXA1k-EI0.4+PMOXA1k-EI0.4 coated capillary) for highly sensitive and stable analysis of melamine. Under the optimal conditions, the linear response of melamine concentration ranged from 6.25 to 100 µg/mL with high correlation coefficient of 0.9910, and the limit of detection (LOD) for melamine was 0.097 µg/mL. The minimum amount of melamine determined in milk powder with our proposed method was 4 mg/kg. The recoveries and relative standard deviations (RSDs) were 92.13–102.47%, 92.22–100.30%, 2.07–4.98%, and 2.43–3.77%, respectively for the intraday and interday analysis. After being used continually for 45 days, the PDA/PMOXA1k-EI0.4+PMOXA1k-EI0.4 coated capillary could be still used to detect the melamine from milk powder, with the recovery from 91.85% to 106.14%. The technique developed in this study provides a simple and relatively sensitive method for determination of melamine in milk powder.

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