Flow Injection Amperometric Evaluation of Trolox Equivalent Antioxidant Capacity of Chocolates with Different Cocoa Content at a Boron-Doped Diamond Electrode

Research background

The objective of this paper is to introduce an instrumentally simple analytical tool for determination of cocoa solid content in chocolates. This electroanalytical method is based on amperometric oxidation of all present antioxidants in chocolates at boron-doped diamond electrode (BDDE) that is integrated in a flow injection analysis (FIA) wall-jet electrode system.

Experimental approach

As part of optimisation, thirteen commonly occurring antioxidants were investigated using cyclic voltammetry at the BDDE in 0.1 mol/L phosphate buffer with different methanol (MeOH) content. Working parameters, such as MeOH volume fraction, flow rate and detection potential, were optimised. Principally, the height of the oxidation peak (current response) representing the oxidation of the sum of antioxidants (total antioxidant content; TAC) was expressed as Trolox equivalents.

Results and conclusions

For analytical purpose, a linear range from 5 to 100 mg/L described by regression equation and characterised by high correlation coefficient R²=0.9994 was achieved. Obtained high positive correlation between the determined values of Trolox equivalent antioxidant capacity (TEAC) and cocoa mass fractions characterised by correlation coefficient of 0.9187 for eight randomly selected samples (one white, two milk, and five dark chocolates) confirmed that cocoa solids represent the main source of antioxidants (reducing agents).

Novelty and scientific contribution

The research demonstrates that TEAC values could be considered as an additional marker of cocoa content in the chocolate analysis to the commonly used theobromine (authenticity of food products). The developed FIA could therefore serve as simple analytical tool in the food quality control.

Flow injection tyrosinase biosensor for direct determination of acetaminophen in human urine

An amperometric biosensor compatible with a flow injection analysis (FIA) for highly selective determination of acetaminophen (APAP) in a sample of human urine was developed. This biosensor is also suitable for use in the routine pharmaceutical practice. To prove this statement, two different commercially available pharmaceutical formulations were analyzed. This nano-(bio)electroanalytical device was made from a commercially available screen-printed carbon electrode covered by a thin layer of non-functionalized graphene (NFG) as amperometric transducer. A biorecognition layer was prepared from mushroom (Agaricus bisporus) tyrosinase (EC 1.14.18.1) cross-linked using glutaraldehyde, where resulting aggregates were covered by Nafion^®, a known ion exchange membrane. Owing to the use of tyrosinase and presence of NFG, the developed analytical instrument is able to measure even at potentials of 0 V. Linear ranges differ according to choice of detection potential, namely up to 130 μmol L^-1 at 0 V, up to 90 μmol L^-1 at -0.1 V, and up to 70 μmol L^-1 at -0.15 V. The first mentioned linear range is described by the equation I_p [μA] = 0.236 - 0.1984c [μmol L^-1] and correlation coefficient r = 0.9987; this equation was used to quantify the content of APAP in each sample. The limit of detection of APAP was estimated to be 1.1 μmol L^-1. A recovery of 96.8% (c = 25 μmol L^-1, n = 5 measurements) was calculated. The obtained results show that FIA is a very selective method for APAP determination, being comparable to the chosen reference method of reversed-phase high-performance liquid chromatography.