Thiago F. Tormin

Fast and direct determination of butylated hydroxyanisole in biodiesel by batch injection analysis with amperometric detection.

We report here, for the first time, application of batch injection analysis (BIA) with amperometric detection for determination of the phenolic antioxidant butylated hydroxyanisole (BHA) in biodiesel. A sample plug was directly injected onto a boron-doped diamond electrode immersed in 50% v/v hydroethanolic solution with 0.1 mol L(-1) HClO(4) using an electronic micropipette. Importantly, the only preparation step required for biodiesel analysis is dilution in the same hydroethanolic electrolyte solution. Our proposed method has several advantages for routine biodiesel analysis, including: a low relative standard deviation between injections (0.29%, n=20), high analytical frequency (120 h(-1)), adequate recovery values (93-101%) for spiked samples, satisfactory accuracy (based on comparative determinations by high-performance liquid-chromatography), and a low detection limit (100 ng of BHA per g of biodiesel). Finally, our method can be adapted for the determination of other antioxidants in biodiesel samples.

Fast simultaneous determination of BHA and TBHQ antioxidants in biodiesel by batch injection analysis using pulsed-amperometric detection

We report the first amperometric method for the simultaneous determination of butylated hydroxyanisole (BHA) and tert-butylhydroquinone (TBHQ) in biodiesel using batch-injection analysis (BIA) coupled to pulsed-amperometry. A sequence of potential pulses was selected in order to detect TBHQ and BHA separately in a single injection step at a glassy-carbon electrode. Samples were diluted in 50% v/v hydroethanolic solution with 0.1 mol L(-1) HClO(4) (supporting electrolyte) before injection using an electronic pipette. The method is highly precise (RSD<1%, n=10), fast (170 injections h(-1)), accurate (recovery between 100 and 110%), presents low detection limits (73 and 75 nmol L(-1) BHA and TBHQ, respectively), and can be easily adapted for on-site determinations.

Direct amperometric determination of tert-butylhydroquinone in biodiesel

The direct amperometric determination of tert-butylhydroquinone (TBHQ) in biodiesel at an unmodified glassy carbon electrode is reported. A biodiesel aliquot was added into an electrochemical cell containing a 75% (v/v) ethanol-water solution under stirring (with final concentration of 50 mmol L(-1) HClO(4)). The amperometric method involved the continuous application of three sequential potential pulses to the working electrode (700 mV for 300 ms, 0 mV for 100 ms and -50 mV for 1s). TBHQ was continuously monitored at the first (direct oxidation) and optionally at the second (reduction) potential pulse while the third potential pulse was applied for cleaning of the electrode surface. For comparison, the samples were also analyzed by high-performance liquid-chromatography and a good agreement between the results was verified. Recovery values for spiked samples were between 90% and 95% and the reproducibility of the proposed method was around 5% (n=5). The proposed method can be easily adapted for on-site analysis.

Batch injection analysis with amperometric detection: application for simultaneous analysis using a single working electrode

In this work, we describe for the first time the association of batch injection analysis (BIA) with dual pulse amperometric (DPA) detection for simultaneous determination of multi-analytes using a single working electrode. The sequence of pulses is selected in such a way that the analytes are detected individually and independently at the same working unmodified electrode (using a correction factor). BIA provides fast response (high-throughput analysis), elevated precision, reduction of sample volumes and minimization of waste. To illustrate the potential of this method, BIA-DPA was applied for the simultaneous determination of biodiesel preservatives (tert-butylhydroquinone and butylated hydroxyanisole) and pharmaceutical compounds (paracetamol and caffeine or dipyrone and caffeine).

Batch-injection analysis with amperometric detection of the DPPH radical for evaluation of antioxidant capacity.

This work proposes the application of batch-injection analysis with amperometric detection to determine the antioxidant capacity of real samples based on the measurement of DPPH radical consumption. The efficient concentration or EC50 value corresponds to the concentration of sample or standard required to scavenge 50% DPPH radicals. For the accurate determination of EC50, samples were incubated with DPPH radical for 1h because many polyphenolic compounds typically found in plants and responsible for the antioxidant activity exhibit slow kinetics. The BIA system with amperometric detection using a glassy-carbon electrode presented high precision (RSD = 0.7%, n = 12), low detection limit (1 μmol L(-1)) and selective detection of DPPH (free of interferences from antioxidants). These contributed to low detection limits for the antioxidant (0.015 and 0.19 μmol L(-1) for gallic acid and butylated hydroxytoluene, respectively). Moreover, BIA methods show great promise for portable analysis because battery-powered instrumentation (electronic micropipette and potentiostats) is commercially available.

Ultrasound-assisted digestion of biodiesel samples for determination of metals by stripping voltammetry

The ultrasound-assisted digestion of biodiesel samples is demonstrated for the first time. The efficiency of the proposed sample preparation method was evaluated for trace metal determination by stripping voltammetric techniques, which are more susceptible to residual carbon content (RCC) in digestates. Biodiesel samples (50–75 mg) were treated in glass tubes, with a mixture of 3.0 mL of concentrated HCl and 4.5 mL of H2O2, placed inside an ultrasonic bath (40 kHz frequency and 120 W) exactly over the region of higher ultrasound intensity and sonicated for 40 min. These parameters were optimized using factorial design experiments for the analysis of spiked samples. Recovery values between 94 and 108% for zinc, cadmium, lead, copper and mercury were verified under optimized conditions. RCC values after US-assisted digestions were in the range of 3.05–3.75% using covered glass tubes, which are acceptable values for the accurate voltammetric determination of metals. These results indicate that the proposed sample preparation method presents comparable performance to traditional or microwave-assisted acid digestions and can be associated with on-site electroanalysis using screen-printed electrodes.

Electrochemical Oxidation of the Fungicide Dimoxystrobin and Its Amperometric Determination by Batch-Injection Analysis

The electrochemical oxidation of dimoxystrobin at a boron-doped diamond electrode is reported. Two well-defined oxidation peaks were observed at around 1350 and 1750 mV in a 0.04 mol L−1 Britton-Robinson buffer solution (pH 10). Based on this electrochemical process, a batch-injection analysis method with amperometric detection was developed for the determination of dimoxystrobin in natural waters. The method presented a high analytical frequency (180 h−1), good precision (RSD < 3%, n = 10), and a detection limit of 0.38 µmol L−1 (124 ng mL−1). Recovery values obtained using analyte fortified water samples were between 80 and 105%. Additionally, a simple strategy to detect the presence of interfering molecules in water samples based on dual-pulse amperometry is presented.

Determinação rápida de hidroquinona usando análise por injeção em batelada (BIA) com detecção amperométrica

A fast analytical method for determination of hydroquinone in pharmaceutical formulations employing batch injection analysis (BIA) with amperometric detection using a boron-doped diamond electrode is described. The supporting electrolyte was a 0.1 mol L-1 H2SO4 solution (the single reagent used for analysis). The method showed good repeatability (RSD of 0.45%, n=20), wide linear range (from 10 to 2000 µmol L-1, R=0.9999), low detection limit (0.016 µmol L-1) and satisfactory recovery values (91-96%). Accuracy of the method was evaluated by comparative analyses using high-performance liquid-chromatography. The ability to replace the electronic pipette by disposable syringes (injection procedure) in BIA systems was also shown.

Controlled release of drugs from cellulose acetate matrices produced from sugarcane bagasse: monitoring by square-wave voltammetry

Abstract In this paper, cellulose triacetate (CTA) was produced from sugarcane bagasse and used as matrices for controlled release of paracetamol. Symmetric and asymmetric membranes were obtained by formulations of CTA/dichloromethane/drug and CTA/dichloromethane/water/drug, respectively, and they were characterized by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Different morphologies of membranes were observed by SEM, and the incorporation of paracetamol was confirmed by lowering of the glass transition temperature (Tg) in the DSC curves. This indicates the existence of interactions between the matrix and the drug. The evaluation of drug release was based on the electrochemical monitoring of paracetamol through its oxidation at a glassy carbon electrode surface using square-wave voltammetry (SWV), which provides fast, precise and accurate in situ measurements. The studies showed a content release of 27% and 45% by the symmetric and asymmetric membranes, respectively, during 8 h.

Highly-sensitive voltammetric detection of trinitrotoluene on reduced graphene oxide/carbon nanotube nanocomposite sensor

This work presents the highly-sensitive detection of 2,4,6-trinitrotoluene (TNT) on reduced graphene oxide/multi-walled carbon nanotube (rGO/MWCNT) nanocomposite sensor. The formation of a thin film of this nanocomposite occurred at the cyclohexane/water immiscible interface of a mixture of MWCNT and rGO in the biphasic solution. The film was transferred to a boron-doped diamond (BDD) electrode for the square-wave voltammetric detection of TNT, which presented improved analytical characteristics in comparison with bare BDD and after modification with precursors. Electrochemical impedance spectroscopy also revealed the faster electron transfer for a redox probe on the nanocomposite modified surface. The synergistic properties of both carbon nanomaterials in the thin film modified surface resulted in a TNT sensor with a detection limit of 0.019 μmol L-1 within a wide linear range (0.5-1100 μmol L-1), with superior performance in comparison with other electrochemical sensors produced with carbon nanomaterials. This new material provides great promises for the highly-sensitive detection of other nitroaromatic explosives as well as other analytes. Moreover, the interfacial method enables the production of homogeneous and stable films on large coated areas as well as the large-scale production of electrochemical sensors.