RESUMO

This work presents the association of cloud point extraction (CPE) and electroanalysis for the selective and sensitive determination of methyl parathion (MP) in honey. The CPE step provided the pre-concentration of MP from a complex sample, in which the optimized extraction parameters (Triton X-100 concentration of 0.75% w/v, NaCl concentration of 1.0% w/v and heating time of 30 min) were investigated using a factorial design (23). The detection of MP was performed using a cathodically pre-treated boron-doped diamond (BDD) working electrode and square wave voltammetry (SWV), after a suitable dilution of the CPE extract in Britton-Robinson buffer pH 6.0 as the supporting electrolyte. MP presented three electrochemical processes over the BDD surface, but only the reduction peak at around -0.7 V was monitored for the MP determination (higher detectability). Improved reproducibility was reached by applying an in situ cleaning step (+2.0 V for 15 s) followed by a re-activation process (-2.0 V for 15 s) between measurements. Using the optimized variables, a linear range between 0.1 and 2.0 µmol L-1 was obtained for MP with a limit of detection of 0.006 µmol L-1, a 6-fold lower value when compared with the value attained without the CPE step. The experimental enrichment factor of MP was 6.1. Also, the optimized CPE allowed the determination of MP in honey samples with good accuracy (recovery between 94 and 106%), which was not possible using direct detection (without CPE) due to the matrix interference. This is the first paper that demonstrates the combination of CPE and electroanalysis for the determination of an organic compound.

RESUMO

We show that fused deposition modelling (FDM) 3D-printed electrodes can be used for quality control of fuel bioethanol. 3D-printing using carbon black/polylactic acid (CB-PLA) filaments resulted in conductive and biodegradable electrodes for biofuel analysis. As a proof-of-concept, copper determination in fuel bioethanol was performed, as such ions catalyse oxidation processes during storage and transport. Square-wave anodic-stripping voltammetry (SWASV) of copper was achieved after sample dilution in 0.1 mol L-1 HCl as supporting electrolyte (resulting in 30:70% v/v ethanol:water). The linear responses were in the range between 10 and 300 µg L-1 (R = 0.999), inter-day precision was lower than 8% (n = 10, for 20 µg L-1) and limits of detection (LOD) and quantification (LOQ) using 180 s as deposition time were 0.097 µg L-1 and 0.323 µg L-1, respectively. Recovery values between 95 and 103% for the analysis of bioethanol spiked with known amounts of copper were obtained. These results show great promise of the application of 3D-printed sensors for the quality control of biofuels. Graphical abstract.

Assuntos

RESUMO

Trace amounts of copper decrease significantly the oxidative stability of biodiesel while antioxidants, mostly tert-butylhydroquinone (TBHQ), are commonly introduced to the biofuel to guarantee induction period values within regulatory limits. Hence, a simple method to monitor the most used antioxidant and the most deleterious contaminant in biodiesel can give a fast response on its quality. In this context, this work presents a simple, efficient, low cost and sensitive method for the simultaneous and direct determination of copper and the antioxidant TBHQ in biodiesel. The biodiesel samples were diluted in 90% (v/v) ethanol and 10% (v/v) water containing 0.1 mol L-1 HCl (final concentration) as the supporting electrolyte generating a homogeneous mixture in which Cu(II) and TBHQ were directly detected using the square wave voltammetry (SWV) on a gold working electrode. The presence of biodiesel in the selected electrolyte facilitated the separation of analyte peaks facilitating the treatment of data and quantification. The linear responses for the simultaneous determination were in the range between 5 and 100 µg L-1 and 4.2-81.5 mg L-1 for copper and TBHQ (R > 0.997), respectively. High precision (RSD = 4.0% and 3.9%, n = 20) and limit of detection (LOD) values of 0.31 µg L-1 and 1.0 mg L-1 (0.030 µg g-1 and 0.102 mg g-1 in biodiesel) for Cu(II) and TBHQ, respectively, were obtained. Recovery values between 97% and 104% for both copper and TBHQ were obtained.

RESUMO

This work presents a portable electrochemical system for the continuous monitoring of corrosion inhibitors in a wide range of matrices including ethanol, seawater and mineral oil following simple dilution of the samples. Proof-of-concept is demonstrated for the sensing of 2,5-dimercapto-1,3,5-thiadiazole (DMCT), an important corrosion inhibitor. Disposable screen-printed graphitic electrodes (SPGEs) associated with a portable batch-injection cell are proposed for the amperometric determination of DMCT following sample dilution with electrolyte (95% v/v ethanol + 5% v/v 0.1molL-1 H2SO4 solution). This electrolyte was compatible with all samples and the organic-resistant SPGE could be used continuously for more than 200 injections (100µL injected at 193µLs-1) free from effects of adsorption of DMCT, which have a great affinity for metallic surfaces, and dissolution of the other reported SPGE inks which has hampered prior research efforts. Fast (180h-1) and precise responses (RSD < 3% n = 10) with a detection limit of 0.3µmolL-1 was obtained. The accuracy of the proposed method was attested through recovery tests (93-106%) and the reasonable agreement of results of DMCT concentrations in samples analyzed by both proposed and spectrophotometric (comparative) methods.