Ziwei Zhang

Dynamic microwave assisted extraction coupled with dispersive micro-solid-phase extraction of herbicides in soybeans.

Non-polar solvent dynamic microwave assisted extraction was firstly applied to the treatment of high-fat soybean samples. In the dispersive micro-solid-phase extraction (D-µ-SPE), the herbicides in the high-fat extract were directly adsorbed on metal-organic frameworks MIL-101(Cr). The effects of several experimental parameters, including extraction solvent, microwave absorption medium, microwave power, volume and flow rate of extraction solvent, amount of MIL-101(Cr), and D-µ-SPE time, were investigated. At the optimal conditions, the limits of detection for the herbicides ranged from 1.56 to 2.00 μg kg(-1). The relative recoveries of the herbicides were in the range of 91.1-106.7%, and relative standard deviations were equal to or lower than 6.7%. The present method was simple, rapid and effective. A large amount of fat was also removed. This method was demonstrated to be suitable for treatment of high-fat samples.

Measurement of Antioxidant Capacity by Electron Spin Resonance Spectroscopy Based on Copper(II) Reduction.

A new method is proposed for measuring the antioxidant capacity by electron spin resonance spectroscopy based on the loss of electron spin resonance signal after Cu(2+) is reduced to Cu(+) with antioxidant. Cu(+) was removed by precipitation in the presence of SCN(-). The remaining Cu(2+) was coordinated with diethyldithiocarbamate, extracted into n-butanol and determined by electron spin resonance spectrometry. Eight standards widely used in antioxidant capacity determination, including Trolox, ascorbic acid, ferulic acid, rutin, caffeic acid, quercetin, chlorogenic acid, and gallic acid were investigated. The standard curves for determining the eight standards were plotted, and results showed that the linear regression correlation coefficients were all high enough (r > 0.99). Trolox equivalent antioxidant capacity values for the antioxidant standards were calculated, and a good correlation (r > 0.94) between the values obtained by the present method and cupric reducing antioxidant capacity method was observed. The present method was applied to the analysis of real fruit samples and the evaluation of the antioxidant capacity of these fruits.

Determination of antioxidant capacity of diverse fruits by electron spin resonance (ESR) and UV–vis spectrometries

Twenty-one kinds of fruits including strawberry, mulberry, lemon, banana, etc. were measured for antioxidant capacity based on their ability to scavenge 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical. Vitamin C equivalent antioxidant capacity (VCEAC) was used to quantify antioxidant capacity of the studied fruits. The results were expressed as mg of ascorbic acid equivalent per 100g fruit. Each fruit was divided into two parts: harvest part (fresh fruit analyzed immediately), and liquid nitrogen frozen part (fruit frozen and pulverized in liquid nitrogen). Antioxidant capacities of both fresh and frozen fruits were determined, and VCEAC values were proved to have no significant difference. For the frozen fruits, the antioxidant capacities were measured by electron spin resonance spectroscopy (ESR) and UV-vis spectrometry. VCEAC values obtained with UV-vis and ESR range from 11.48 to 345.75mg/100g and 7.01 to 366.26mg/100g. Experimental results indicated that VCEAC values obtained by two methods were highly correlated.

On-site determination of the migration amount of fluorescent whitening agents from paper to finger by fluorescence spectrophotometry

A fluorescence collecting unit was self-assembled in our lab. This unit in combination with a fluorescence spectrophotometer was applied to quantify fluorescent whitening agents (FWAs) in paper samples. The unit is small and portable, containing three light emitting diodes and a lens. The light emitting diodes were used as excitation light sources, and the emitted fluorescence entered the fluorescence spectrophotometer for signal detection after passing through the lens and a fiber. With high sensitivity, not only the total amount of FWAs in the paper samples could be determined but also the trace amount of FWAs migrated from paper to finger could also be obtained. The limit of detection was 0.12 mg m−2. Thirty practical paper notebook samples were analyzed for their FWA content before and after finger touch. For the purpose of comparison, the European standard method was also applied to determine the migration of FWA in these paper notebook samples. The correlation of the analytical results obtained by the present method with those obtained by the well-established European standard method was satisfactory.

Dispersant-assisted dynamic microwave extraction of triazine herbicides from rice

A rapid and effective sample pretreatment method, dispersant-assisted dynamic microwave extraction, was first applied to the extraction of six triazine herbicides from rice. The dynamic microwave extraction was coupled with matrix solid-phase dispersion (MSPD). The dispersant in MSPD was used to disperse the sample and adsorb impurities. Microwave irradiation was used to promote the transfer of the analyte and accelerate the extraction. The extraction and clean-up were performed simultaneously. The effects of experimental parameters, including the solvent type, volume of the extraction solvent, microwave power, type of dispersant, mass ratio of dispersant to rice, and flow rate of the extraction solvent, on the recoveries of the analytes were investigated. The limits of detection for the analytes ranged from 0.62 to 1.79 μg kg−1. The recoveries of the triazine herbicides in spiked rice samples were in the range of 84.3–101%, and the relative standard deviations were lower than 6.2%.

Determination of steroid hormones in milk using aqueous two-phase extraction coupled to liquid chromatography

An acetonitrile (ACN)-salt based aqueous two-phase system was developed and applied to the extraction of steroid hormones in milk. The extraction of hormones was assisted by ultrasonication after adding ACN and sodium chloride and then centrifugation was used to effect the phase separation. The extraction and purification were performed in one step and no further clean-up step is needed. The hormones were determined using high-performance liquid chromatography. The experimental parameters, including type and amount of salt, pH value of the sample solution, extraction temperature and time, and volume of ACN, were optimized. The limits of detection for steroid hormones ranged from 0.90 to 1.52 μg L−1. When the present method was applied to the analysis of milk samples, the recoveries of analytes ranged from 87.3% to 105.8% and relative standard deviations were lower than 7.73%. In one sample, 6.26 μg L−1 of progesterone was detectable.

Determination of antioxidant capacity of thiol-containing compounds by electron spin resonance spectroscopy based on Cu 2+ ion reduction

Electron spin resonance spectroscopy was applied to determining the antioxidant capacity of eight thiol-containing compounds, including reduced glutathione, N-acetyl-L-cycsteine, methimazole, captopril, and tiopronin with one thiol group, 1,4-dithioerythritol and 2,3-dimercapto-1-propanol with two thiol groups, as well as L-cystine with no free thiol group. Cu2+ ion gives an electron spin resonance signal and is reduced to Cu+ ion with no electron spin resonance signal by the free thiol group in the compounds. Trolox equivalent antioxidant capacity (TEAC) was used to evaluate the reducing ability of the thiol-containing compounds and the TEAC values were found to be relevant to the number of thiol groups contained in the compounds. For the purpose of comparison, the UV-vis spectrophotometry, cupric reducing antioxidant capacity (CUPRAC) method, and Ellman assay were applied to the determination of the antioxidant capacity of the thiol-containing compounds. The TEAC values obtained by the present method were very close to those obtained by UV-vis method. However, compared with CUPRAC method, for methimazole the present method gave a more reasonable TEAC value. The present method was also applied to the quantification of N-acetyl-L-cycsteine, methimazole, captopril, and tiopronin in their pharmaceutical formulations.