RESUMEN
The atomic-scale nucleation mechanism of vapor-grown carbon nanofibers (CNFs) is investigated using classical molecular dynamics simulations with a developed parameterization. Carbon precipitation and graphene plane formation are simulated, taking into account the carbon concentration (CC) in Ni carbide. The simulated results show that the carbon atoms formed sp(2) networks or sp chains in the Ni nanocrystals and then precipitated onto the Ni surface with distinct precipitation dynamics and time intervals that are dependent on the CC. The lowest-energy configurations of the precipitated carbon atoms exhibit an irregular corrugated network, a defective graphene plane, and separate defective graphene planes under high, medium, and low CC, respectively. These observations are in good agreement with the microstructural characteristics of different types of CNFs from experiments. Pair correlation function calculations show that the precipitated carbon structures exhibit different graphite orderings. The study reveals the atomistic CNF nucleation mechanism and emphasizes the critical role of metal carbide CC in the microstructure formation of CNFs during synthesis.
RESUMEN
At present, new prohibited substances are becoming more common illegal additions in cosmetics. Clobetasol acetate is a new glucocorticoid, which is not covered in the current national standards and is a homologue of clobetasol propionate. A method was established for the determination of clobetasol acetate as a new glucocorticoid (GC) in cosmetics by ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Five common cosmetic matrices were suitable for this new method: creams, gels, clay masks, masks and lotions. Four pretreatment methods were compared: direct extraction by acetonitrile, PRiME pass-through column purification, solid-phase extraction (SPE) purification, and QuEChERS purification. Further, the effects of different extraction efficiencies of the target compound, such as extraction solvents and extraction time, were investigated. The MS parameters, such as ion mode, cone voltage and collision energy of ion pairs of the target compound, were optimized. The chromatographic separation conditions and response intensities of the target compound in different mobile phases were compared. Based on the experimental results, the optimal extraction method was determined to be direct extraction, wherein the samples were vortex dispersed with acetonitrile, ultrasonic extraction over 30 min and filtered by a 0.22 µm organic millipore filter, and then the samples were detected by UPLC-MS/MS. The concentrated extracts were separated on a Waters CORTECS C18 column (150 mm×2.1 mm, 2.7 µm), where the water and acetonitrile were used as the mobile phases for gradient elution. The target compound was detected with the multiple reaction monitoring (MRM) mode under electrospray ionization and positive ion scanning (ESI+). Quantitative analysis was performed by matrix matching standard curve. Under the optimum conditions, the target compound had good linear fitting in the range of 0.9-37 µg/L. The linear correlation coefficient (R2) was greater than 0.99, the limit of quantification (LOQ) of the method was 0.09 µg/g and the limit of detection (LOD) was 0.03 µg/g for these five different cosmetic matrices. The recovery test was conducted under three spiked levels: 1, 2 and 10 times of LOQ. The recoveries of the tested substance were between 83.2% and 103.2% in these five cosmetic matrices, and the relative standard deviations (RSDs, n=6) were between 1.4% and 5.6%. This method was used to screen cosmetic samples of different matrix types, and a total of five positive samples were found, in which the content range of clobetasol acetate was from 1.1 to 48.1 µg/g. In conclusion, the method is simple, sensitive and reliable, and is suitable for high-throughput qualitative and quantitative screening, and the analysis of cosmetics with different matrix types. Moreover, the method provides crucial technical support and a theoretical basis for the establishment of feasible detection standards for clobetasol acetate in China, as well as for the control of the compound in cosmetics. This method has important practical significance to implement management measures of illegal additions in cosmetics.