Voltammetric Determination of Thymol in Oregano Using CeO<Subscript>2</Subscript>-Modified Electrode in Brij® 35 Micellar Medium

Glassy carbon electrode (GCE) modified with CeO₂ nanoparticles dispersed in 0.01 M Brij® 35 (CeO₂-Brij® 35/GCE) has been developed for the determination of thymol in micellar medium. Scanning electron microscopy (SEM) data confirm immobilization of the nanomaterial on the electrode surface. The electrooxidation of thymol on CeO₂-Brij® 35/GCE is an irreversible diffusion-controlled process with participation of two electrons and two protons. Differential pulse voltammetry has been used for the quantification of thymol. The linear dynamic range of the thymol determination is 0.700–10.1 and 10.1–606 μM with the limits of detection and quantification 0.20 and 0.65 μM, respectively. The approach developed has been applied for the quantification of thymol in oregano spices using preliminary micellar extraction with Brij® 35. The results of voltammetric determination are in good agreement with the data of standard spectrophotometric method.     

Enhanced Chemiluminescence Determination of Hydrogen Peroxide in Milk Sample Using Metal–Organic Framework Fe–MIL–88NH<Subscript>2</Subscript> as Peroxidase Mimetic

The presence of hydrogen peroxide (H₂O₂) in milk is a major concern because it constitutes a public health hazard. Here, a sensitive flow injection chemiluminescence method was established for the detection of H₂O₂. As a peroxidase mimetic, metal–organic framework Fe–MIL–88NH₂ was found to significantly enhance the chemiluminescence of luminol–H₂O₂ reaction. The enhancement mechanism could be attributed to peroxidase-like activity of Fe–MIL–88NH₂, which effectively catalyzed the decomposition of H₂O₂ into hydroxyl radical. The experimental conditions for the chemiluminescence reaction were thoroughly investigated. The chemiluminescence intensity was proportional to the concentration of H₂O₂ in the range of 0.1–10.0 μmol/L. The detection limit was 0.025 μmol/L H₂O₂, and the relative standard deviation was 2.6 % for 11 replicated measurements of 1.0 μmol/L H₂O₂ solution. The practicability of this method was demonstrated by determining H₂O₂ in milk samples.     

Optimization and Validation of a UV–HPLC Method for Vitamin C Determination in Strawberries (<Emphasis Type="Italic">Fragaria ananassa</Emphasis> Duch.), Using Experimental Designs

Vitamin C or total ascorbic acid (TAA) in fruits can be assumed as ascorbic acid (AA) plus dehydroascorbic acid (DHA) content. The aim of this work was to optimize and validate, using experimental designs, a new high-performance liquid chromatographic method for vitamin C determination in strawberries. The mobile phase (MP) consisted of a 0.03 M sodium acetate/acetic acid buffer, 5% methanol. For optimization, a Box–Behnken design was used (three factors at three levels: (a) pH of MP, 3.8–5.8; (b) wavelength, 240–270 nm; and (c) flow rate, 0.5–1.2 ml min⁻¹). Responses were: AA and TAA areas, peak widths, and retention times. A global optimization was performed using the Derringer desirability function, and a value of 0.84 was reached for the combination of design factors: A = 5.8, B = 251 nm, and C = 1.15 ml min⁻¹. Method validation, using AA standard solutions, included: linearity study, limits of detection and quantification, and calibration and analytical sensitivity quantifications. Precision and accuracy were studied in strawberry extracts. The coefficients of variation (percent) were: AA, 1.5%; TAA, 1.8%, and DHA, 4.9%. Accuracy was evaluated with AA standard spiked in 30–150% range of the expected amount of analyte in real samples. The joint confidence elliptical region test and t test were employed for the study of the difference between recoveries (percent) and the ideal 100%. The robustness was analyzed using a fractional factorial design (3⁴⁻²), and an AA recovery study after slight changes in operative variables was performed. The results indicate that the optimized method was linear, sensible, precise, accurate, and robust.