Photochemical oxidation of alcohols: Simple derivatization strategy for their analysis by capillary electrophoresis.

The determination of ethanol is one of the most important parameters in the fermentation industry, influencing not only the production yield and the quality of the product, but also its commercial value. In addition to the traditional approach based on distillation/density, procedure that is considered laborious and time-consuming, methods based on chromatography are widely used. Alternatives using electrochemical, spectroscopic and colorimetric techniques have been also proposed for alcohol analysis. In general, these methods not only offer limited throughput, but also require harsh reaction conditions and/or complex instrumentation. Aiming to address these shortcomings, we propose a fast, simple and clean analytical approach for the determination of primary alcohols based on the photochemical oxidation under UV-LED irradiation in the presence of H2O2. The proposed method was successfully applied to the analysis of 12 different types of alcoholic beverages with an alcohol content ranging from 5% v/v (beer) to 53% v/v (whiskey).

Photochemical and photocatalytic degradation of 1-propanol using UV/H2 O2 : Identification of malonate as byproduct.

1-propanol is a primary alcohol extensively used in the pharmaceutical, chemical, and food industries. It has been also found as a contaminant in the atmosphere and is considered a model compound to mimic the behavior and fate of aliphatic alcohols exposed to environmental conditions. In order to understand that role of relevant variables, this paper presents results obtained with a simple experimental set-up to investigate the reactivity of 1-propanol under mild oxidizing conditions. Coupling this system with CE-C4 D allowed the quantification of the carboxylic acids formed. For the described experiments, aqueous solutions of 1-propanol were placed inside a photoreactor and oxidized upon the addition of TiO2 and/or H2 O2 . According to the described results, the addition of H2 O2 (0.1% w/w) was the most significant variable, roughly tripled the amount of carboxylic acids generated and led to the conversion of up to 70% of the initially available 1-propanol (1 mmol/L). More importantly, the reaction yielded the formation (within 10 min) of propionate (50 µmol/L), acetate (400 µmol/L), formate (50 µmol/L), and malonate (200 µmol/L). The latter is critically important because it represents the first example of the photochemical oxidation of both terminal carbons of the C3 -chain of 1-propanol under mild conditions, and opens new avenues for the production of this important chemical building block.

Gold leaf: From gilding to the fabrication of disposable, wearable and low-cost electrodes.

Gold is among the most used materials in electrocatalysis. Despite this, this noble metal is still too expensive to be used in the fabrication of low cost and disposable devices. In the present work, gold-leaf sheets, usually employed in decorative crafts and wedding candies, is introduced as an inexpensive source of gold. Planar-disc and nanoband gold electrodes were simply and easily manufactured by combining gold leaf and polyimide tape. The planar disc electrode exhibited electrochemical behavior similar to that of a commercial gold electrode in 0.2molL-1 H2SO4; cyclic voltammetry of a 1mmolL-1 solution of potassium ferricyanide (K3[Fe(CN)6]) in 0.2molL-1 KNO3, using this novel electrode, displayed an 80mV difference between the oxidation and reduction peak potentials. The electrode also delivers promising prospects for the development of wearable devices. When submitted to severe mechanical deformation, this electrode exhibited neither loss of electrical contact nor significant variation in electrode response, even after fifteen bending and/or folding cycles. The thickness of the gold-leaf sheet facilitates the production of nanoband electrodes with behavior similar to that of ultramicroelectrodes. The electrode surface is easily renewed by cutting a thin slice off its end with a razor blade; this process led to limiting currents that were reproducible, presenting a relative standard deviation (RSD) of 3.8% (n = 5).

Determination of neutral diols and carboxylic acids formed during glycerol electrooxidation by capillary electrophoresis with dual C4D.

Methods for determination of glycerol and its electrooxidation products (neutral diols and carboxylates) by capillary electrophoresis (CE) with dual capacitively coupled contactless conductivity detectors (C4D) are presented. Glycerol, dihydroxyacetone and glyceraldehyde were detected as anionic borate complexes in less than 3min under counter Electroosmotic Flow (EOF) mode (resolution of the critical pair of 1.8). Limits of detection (LODs) of 15, 15 and 10µmolL-1 were obtained for glycerol, dihydroxyacetone and glyceraldehyde, respectively. Two methods of separation were used for the separation of carboxylates. The first one used the same Back Ground Electrolyte (BGE) containing borate, and the second used a BGE (pH 6.1) composed by 2-(N-morpholino)ethanesulfonic acid (MES), L-Histidine and a flow modifier. Better separation and LODs for carboxylates were obtained using Mes/Histidine as BGE. However, along with the non-applicability of this BGE to the determination of neutral diols, observation of the C4D signals at two different points of the capillary (10 and 50cm apart from the injection tip) revealed interaction of the flow modifier with some species (mesoxalate and glyoxylate). The electrooxidation of a glycerol sample in alkaline media on an 8cm2 gold working electrode was evaluated by the developed methods. After 16h of electrolysis, 87% of the glycerol had been oxidized and formate, glycolate, hydroxypyruvate and glycerate were detected as the main products.

Online monitoring of electrocatalytic reactions of alcohols at platinum and gold electrodes in acidic, neutral and alkaline media by capillary electrophoresis with contactless conductivity detection (EC-CE-C4 D).

An EC-CE-C4 D flow system was applied to the investigation of electrocatalytic processes by monitoring carboxylic acids formed during the electro-oxidation at various potentials of primary alcohols (mixture of 1 mmol/L of ethanol, n-propanol, n-butanol and n-pentanol) in acidic, neutral and alkaline media. The electro-oxidation was carried out on gold and platinum disk electrodes (3 mm of diameter) in a thin-layer electrochemical flow cell. Products were sampled 50 µm apart from the electrode directly into the capillary. All the generated carboxylates were determined in near real time (less than 2 min) by CE-C4 D in counter-flow mode, with Tris/HCl buffer solution (pH 8.6) as BGE. Long sequences of 5-min experiments were run automatically, exploring the applied potential, electrolysis time and solution composition. Electro-oxidation at 1.5 V (versus Ag/AgCl quasi-reference) during 50 s in acidic medium was found appropriate for both Pt and Au electrodes when the determination of alcohols after derivatization is intended. A noteworthy selectivity effect was observed on the Au electrode. The signal corresponding to pentanoate is similar on both electrodes while the signal of ethanoate (acetate) is four times larger on gold than on platinum. The carboxylate signals were lower in alkaline medium (below the determination limit on Pt) than in acidic and neutral media. On gold, the formation of carboxylates was anticipated (0.85 V in alkaline medium versus 1.40 V in neutral medium). The automatic online monitoring of electrochemical processes by EC-CE-C4 D holds great potential to investigate ionic/ionizable intermediates/products of new electrocatalysts and/or alternative fuels.

Analysis of Methanol in the Presence of Ethanol, Using a Hybrid Capillary Electrophoresis Device with Electrochemical Derivatization and Conductivity Detection.

Concurrently with ethanol, many other compounds can be formed during the fermentation of grains and fruits. Among those, methanol is particularly important (because of its toxicity) and is typically formed at concentrations much lower than ethanol, presenting a particular challenge that demands the implementation of separation techniques. Aiming to provide an alternative to traditional chromatographic approaches, a hybrid electrophoresis device with electrochemical preprocessing and contactless conductivity detection (hybrid EC-CE-C4D) is herein described. The device was applied to perform the electro-oxidation of primary alcohols, followed by the separation and detection of the respective carboxylates. According to the presented results, the optimum conditions were obtained when the sample was diluted with 2 mmol L-1 HNO3 and then electro-oxidized by applying a potential of 1.4 V for 60 s. The oxidation products were then electrokinetically injected by applying a potential of 3 kV for 4 s and separated using a potential of 3 kV and a background running electrolyte (BGE) consisting of 10 mmol L-1 N-cyclohexyl-2-aminoethanesulfonic acid (CHES) and 5 mmol L-1 sodium hydroxide (NaOH). n-Propanol was used as an internal standard and the three carboxylate peaks were resolved with baseline separation within <3 min, defining linear calibration curves in the range of 0.10-5.0 mmol L-1. Limits of detection (LODs) of 20, 40, and 50 µmol L-1 were obtained for ethanol, n-propanol, and methanol, respectively. To demonstrate the applicability of the proposed strategy, a laboratory-made sample (moonshine) was used. Aliquots collected along the beginning of the fractional distillation presented a decreasing methanol ratio (from 4% to <0.5%) and a growing ethanol ratio (from 80% to 100%) in the collected volume.

Electrochemical derivatization-capillary electrophoresis-contactless conductivity detection: a versatile strategy for simultaneous determination of cationic, anionic, and neutral analytes.

The simultaneous determination of cationic, anionic, and neutral analytes in a real sample was demonstrated by coupling electrochemical (EC) derivatization with counter-EOF CE-C(4) D. An EC flow cell was used to oxidize alcohols from an antiseptic mouthwash sample into carboxylic acids at a platinum electrode in acid medium. The carboxylates formed in the derivatization process and other sample ingredients, such as benzoate, saccharinate, and sodium ions, were separated in counter-flow mode and detected in one run in Tris-HCl buffer, pH 8.6. Fewer than 5 min were needed to complete each analysis with the automated flow system comprising solenoid pumps for the management of solutions. Insights into the electrochemistry of benzoic acid, present in the sample matrix, were also gained by EC-CE-C(4) D; more specifically, by applying potentials higher than 1.47 V to the platinum electrode, some formiate and minute amounts of salicylate were detected.

From sample processing to quantification: a full electrochemical approach for neutral analyte derivatization, capillary electrophoresis separation, and contactless conductivity detection.

A thin-layer electrochemical flow cell coupled to capillary electrophoresis with contactless conductivity detection (EC-CE-C(4)D) was applied for the first time to the derivatization and quantification of neutral species using aliphatic alcohols as model compounds. The simultaneous electrooxidation of four alcohols (ethanol, 1-propanol, 1-butanol, and 1-pentanol) to the corresponding carboxylates was carried out on a platinum working electrode in acid medium. The derivatization step required 1 min at 1.6 V vs. Ag/AgCl under stopped flow conditions, which was preceded by a 10 s activation at 0 V. The solution close to the electrode surface was then hydrodynamically injected into the capillary, and a 2.5 min electrophoretic separation was carried out. The fully automated flow system operated at a frequency of 12 analyses per hour. Simultaneous determination of the four alcohols presented detection limits of about 5 × 10(-5) mol L(-1). As a practical application with a complex matrix, ethanol concentrations were determined in diluted pale lager beer and in nonalcoholic beer. No statistically significant difference was observed between the EC-CE-C(4)D and gas chromatography with flame ionization detection (GC-FID) results for these samples. The derivatization efficiency remained constant over several hours of continuous operation with lager beer samples (n = 40).

Automated two-dimensional separation flow system with electrochemical preconcentration, stripping, capillary electrophoresis and contactless conductivity detection for trace metal ion analysis.

This paper describes the automation of a fully electrochemical system for preconcentration, cleanup, separation and detection, comprising the hyphenation of a thin layer electrochemical flow cell with CE coupled with contactless conductivity detection (CE-C4D). Traces of heavy metal ions were extracted from the pulsed-flowing sample and accumulated on a glassy carbon working electrode by electroreduction for some minutes. Anodic stripping of the accumulated metals was synchronized with hydrodynamic injection into the capillary. The effect of the angle of the slant polished tip of the CE capillary and its orientation against the working electrode in the electrochemical preconcentration (EPC) flow cell and of the accumulation time were studied, aiming at maximum CE-C4D signal enhancement. After 6 min of EPC, enhancement factors close to 50 times were obtained for thallium, lead, cadmium and copper ions, and about 16 for zinc ions. Limits of detection below 25 nmol/L were estimated for all target analytes but zinc. A second separation dimension was added to the CE separation capabilities by staircase scanning of the potentiostatic deposition and/or stripping potentials of metal ions, as implemented with the EPC-CE-C4D flow system. A matrix exchange between the deposition and stripping steps, highly valuable for sample cleanup, can be straightforwardly programmed with the multi-pumping flow management system. The automated simultaneous determination of the traces of five accumulable heavy metals together with four non-accumulated alkaline and alkaline earth metals in a single run was demonstrated, to highlight the potentiality of the system.

Extending the lifetime of the running electrolyte in capillary electrophoresis by using additional compartments for external electrolysis.

The use of two additional reservoirs to accommodate the electrodes of the power source is proposed to improve the stability of the running electrolyte in capillary electrophoresis. The basic idea is to use salt bridges to connect those reservoirs to the ones containing the capillary ends. Although simple, there are several issues that can be considered in the design and implementation of such system in order to prevent undesired transference of material between the electrolysis and the main reservoirs. The use of a sealed electrolysis reservoir without a gas phase, the use of materials that ensure volume stability, and the use of bridges as long as possible are three basic directions. A compromise is involved in the dimensions of the sectional area of the bridge, because a small area diminishes the amount of a species transferred by diffusion but leads to an undesirable increase of the electrical field during the electrophoretic running. Thus, a bridge composed of a main wide-bore tube connected to a small-bore capillary seems to give the best performance for practical use. A simple electrolysis-separated system was adapted to a preexisting capillary electrophoresis system, and its performance was evaluated with a mixture of tartaric, malic, and succinic acids that was separated in sodium benzoate solution (pH 5.5) using the original equipment and the modified one. Due to the water electrolysis and the small buffering capacity of the electrolyte, there was a significant pH change and consequently changes in the effective mobilities of the analytes and loss of resolution after a few runs using the original equipment. Using the electrolysis-separated system, no significant change in the migration time and resolution was observed even after 15 runs. Besides the freedom to prepare running electrolytes with electroactive species or unbuffered solution, high throughput and the use of small reservoirs, such as the ones used in microfluidic devices, are the main advantages of the system.