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Carbon is due to its electrochemical properties a favourite material for construction of electrodes suitable for detection in flow system. The two most often used techniques (coulometric and amperometric) will be discussed in this Thesis. Carbon paste electrodes (CPE) represent the amperometric detectors. CPE are unstable in the medium with high content of organic solvents, which might complicate their application in HPLC, where the mobile phase contains organic component. It is interesting, that glassy carbon paste electrodes are more stable under such conditions. The change of surface and response of the electrode caused by the effect of methanol are described in this Thesis along with the possible reason of stability of GCPE. Roughing of the surface exposed to methanol is demonstrated by confocal microscopy. This effect leads to increase of height of the peak, background current and noise and to the shift of peak potential to less positive values. These changes are dependent on the time of exposure and the concentration of methanol. Coulometric detectors work with high conversion effectiveness and sensitivity, but in case of their passivation, reactivation is a relatively complicated problem, often leading to the replacement of electrode material. Answer to this problem can be given by using...

Submitted by GRAZIELA CRISTINA SEDENHO null (grasedenho@yahoo.com.br) on 2016-02-12T19:43:04Z No. of bitstreams: 1 Dissertação_Graziela Cristina Sedenho.pdf: 4075574 bytes, checksum: 8d8fa0c583e32e2f42a6d7092c393844 (MD5) / Approved for entry into archive by Juliano Benedito Ferreira (julianoferreira@reitoria.unesp.br) on 2016-02-16T15:44:23Z (GMT) No. of bitstreams: 1 sedenho_gc_me_araiq_par.pdf: 955629 bytes, checksum: 812d933588fa4febf4bd052f9afbeddf (MD5) / Made available in DSpace on 2016-02-16T15:44:23Z (GMT). No. of bitstreams: 1 sedenho_gc_me_araiq_par.pdf: 955629 bytes, checksum: 812d933588fa4febf4bd052f9afbeddf (MD5) Previous issue date: 2016-01-07 / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O etanol é um importante componente da matriz energética brasileira, sendo produzido a partir da cana-de-açúcar. Nesse processo de produção, é produzida vinhaça como subproduto, a qual para cada 1 L de etanol gera-se de 12 L a 18 L. A vinhaça é constituída principalmente de sais de potássio, cálcio e magnésio e compostos orgânicos. Dentre estes, os ácidos carboxílicos tornam-se importantes por estarem presentes em concentrações significativas na vinhaça de cana-de-açúcar e por apresentarem expressiva importância industrial. Sendo assim, nesse trabalho foi desenvolvido um detector eletroquímico baseado em eletrodos de diamante dopado com boro quimicamente modificados com nanopartículas de níquel (NiNPs-BDD) e de cobre (CuNPs-BDD) para determinação dos ácidos lático, málico e tartárico em vinhaça de cana-de-açúcar utilizando a técnica de cromatografia líquida de alta eficiência (CLAE) de troca iônica. Imagens de microscopia eletrônica de varredura e espectros de raios X por dispersão em energia, em conjunto com os experimentos eletroquímicos, mostraram que os eletrodos de BDD foram efetivamente modificados com nanopartículas de óxido-hidróxido de níquel e óxido de cobre com diâmetros médios de 89 nm e 91 nm, respectivamente. Os estudos voltamétricos mostraram que as oxidações dos ácidos lático, málico e tartárico, em meio básico, se dão em potenciais em torno de 0,50 V (vs. Ag/AgCl) e 0,73 V (vs. Ag/AgCl), nos eletrodos NiNPs-BDD e CuNPs-BDD, respectivamente, e são mediadas pelas espécies Ni3+ e Cu3+. O eletrodo de NiNPs-BDD foi utilizado em CLAE com detecção amperométrica pulsada. A coluna de troca aniônica CarboPac PA1 foi utilizada para separação dos ácidos carboxílicos em vinhaça. Empregou-se como fase móvel água deionizada e 0,10 mol L-1 de NaOH em 0,25 mol L-1 de CH3COONa, sob fluxo de 1,0 mL min-1. O potencial de detecção empregado foi 0,48 V vs. Pd. O eletrodo de NiNPs-BDD apresentou boa estabilidade nas condições cromatográficas, tornando viável a sua utilização como detector amperométrico em CLAE. O tempo de análise foi de 27 min e não foi necessário um procedimento complexo e dispendioso de preparo de amostra. As sensibilidades analíticas foram 7,5 × 103 nA L mol-1, 3,5 × 104 nA L mol-1 e 5,4 × 105 nA L mol-1 para os ácidos lático, málico e tartárico, respectivamente. Os limites de detecção foram 1,2 × 10-4 mol L-1, 6,1 × 10-5 mol L-1 e 2,8 × 10-5 mol L-1, respectivamente. As concentrações dos ácidos carboxílicos em vinhaça de cana-de-acúcar foram determinadas pelo método de adição de padrão como sendo (1,2 ± 0,3) × 10-1 mol L-1, (2,7 ± 0,6) × 10-3 mol L-1 e (9,9 ± 1,0) × 10-4 mol L-1 para os ácidos lático, málico e tartárico, respectivamente. Ensaios de recuperação mostraram que o método analítico desenvolvido possui boa exatidão e precisão, podendo ser considerado adequado para a detecção e quantificação dos ácidos lático, málico e tartárico em vinhaça de cana-de-açúcar. O eletrodo de CuNPs-BDD foi empregado na célula amperométrica do cromatógrafo, no entanto não apresentou boa estabilidade nas condições cromatográficas, o que impossibilitou sua utilização na determinação dos compostos de interresse na vinhaça de cana-de-açúcar. / Ethanol is an important component of the Brazilian energy matrix, being produced from sugarcane. In this producing procedure, vinasse is produced as by-product, which for every 1 L of ethanol is generated from 12 L to 18 L. The vinasse is basically composed by salts of potassium, calcium and magnesium and organic compounds. Among these, carboxylic acids are important because they are present in significant concentrations in the sugar cane vinasse and show significant industrial importance. Thus, in this work it was developed an electrochemical detector based on boron-doped diamond electrodes chemically modified with nickel nanoparticles (NiNPs-BDD) and copper nanoparticles (CuNPs-BDD) for the determination of lactic acid, malic acid and tartaric acid in vinasse from sugarcane using high-performance liquid chromatography (HPLC) by ion exchange. Images of scanning electron microscopy and spectra of energy-dispersive X-ray in conjuction with electrochemical experiments showed the electrodes of BDD were effectively modified with nickel oxide-hydroxide nanoparticles and copper oxide nanoparticles with diameters of 89 nm and 91 nm, respectively. The voltammetric studies showed the oxidations of lactic acid, malic acid and tartaric acid, in basic medium, tooke place at potentials around 0.50 V (vs. Ag/AgCl) and 0.73 V (vs. Ag/AgCl) at NiNPs-BDD and CuNPs-BDD electrodes, respectively, and they are mediated by Ni3+ and Cu3+ species. NiNPs-BDD electrode was used in HPLC with pulsed amperometric detection. The anion exchange column CarboPac PA1 was used for separation of carboxylic acids in vinasse. It was employed as mobile phase deionized water and 0.10 mol L-1 NaOH in 0.25 mol L-1 CH3COONa under flow of 1.0 mL min-1. The detection potential was 0.48 V vs. Pd. The NiNPs-BDD electrode showed good stability under chromatographic conditions, becoming feasible its use as amperometric detector for HPLC. The analysis time was 27 min and no complex and expensive sample preparation procedure was required. The analytical sensitivities were 7.5 × 103 nA mol-1 L, 3.5 × 104 L mol-1 nA and 5.4 × 105 L mol-1 nA for lactic, malic and tartaric acids. The detection limits were 1.2 × 10-4 mol L-1, 6.1 × 10-5 mol L-1 and 2.8 × 10-5 mol L-1, respectively. The concentrations of the carboxylic acids in sugar cane vinasse were determined by the standard addition method to be (1.2 ± 0.3) × 10-1 mol L-1, (2.7 ± 0.6) × 10-3 mol L-1 and (9.9 ± 1.0) × 10-4 mol L-1 for lactic acid, malic acid and tartaric acid, respectively. Recovery tests showed the analytical method has good accuracy, being considered suitable for the detection and quantification of lactic acid, malic acid and tartaric acid in sugarcane vinasse. CuNPs-BDD electrode was used in the amperometric cell of the chromatograph, however it did not show good stability in the chromatographic conditions, and it could not be employed in the determination of compounds of interest in sugarcane vinasse. / FAPESP: 2013/09833-5 / FAPESP: 2014/10391-0

Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-05-17T11:46:34Z No. of bitstreams: 1 rafaelasilvalamarca.pdf: 2422143 bytes, checksum: 5faa827f8d908eb13b61ec3fb1c1fd93 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-06-28T13:39:59Z (GMT) No. of bitstreams: 1 rafaelasilvalamarca.pdf: 2422143 bytes, checksum: 5faa827f8d908eb13b61ec3fb1c1fd93 (MD5) / Made available in DSpace on 2016-06-28T13:40:00Z (GMT). No. of bitstreams: 1 rafaelasilvalamarca.pdf: 2422143 bytes, checksum: 5faa827f8d908eb13b61ec3fb1c1fd93 (MD5) Previous issue date: 2015-09-25 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / As substâncias fenólicas têm se tornado objeto de investigação, devido aos inúmeros benefícios à saúde humana e a influência na qualidade alimentícia. Neste trabalho determinou-se os ácidos fenólicos presentes no azeite de dendê, de fabricação artesanal e industrializado. Para estas análises utilizou-se um cromatógrafo líquido de alta eficiência (HPLC Agilent 1100 series), injetor manual com válvula Rheodyne de 20μL, coluna de fase Reversa C18 Inertsil ODS-3, coluna de guarda Inertsil ODS, detector UV-VIS de múltiplos comprimentos de ondas (UV-VIS MWD). Ao canal de resíduos do HPLC foi adaptada uma célula eletroquímica, com capacidade de 1,00 mL, sendo esta composta por três eletrodos: eletrodo de trabalho, carbono vítreo; eletrodo de referência, Ag/AgCl (KCl,sat) e eletrodo auxiliar de aço inox. As medidas eletroquímicas foram realizadas em um potenciostato μAutolab type III, conectado a um microcomputador, utilizando o software GPES 4.9. As análises foram realizadas de forma simultânea em ambos detectores, em uma mesma injeção. A fase móvel otimizada foi capaz de separar os analitos totalmente, sendo esta composta pela mistura uma solução aquosa de H3PO4 pH 2,40 + NaClO4 a 7 mmol.L-1/isopropanol (87:13 v/v), por eluição isocrática. O comprimento de onda utilizado na detecção UV foi 225 nm. As medidas amperométricas foram realizadas no potencial de +1,1 V para todos os analitos. A recuperação das amostras foi realizada através da padronização interna, utilizando o ácido o-cumárico como padrão interno (PI) e o ácido m-cumárico como padrão surrogate (PS). A recuperação do PS foi superior a 75% em ambos detectores para todas as amostras. Os limites instrumentais para detecção UV variaram de 0,019 (ácido siríngico) a 0,029 mg.L-1 (ácido ferúlico) e de quantificação 0,063 (ácido siríngico) a 0,095 mg.L-1 (ácido sinápico); já para a detecção eletroquímica (DE) os limites de detecção encontraram-se na faixa de 0,015 (ácido p-cumárico) a 0,079 mg.L-1(ácido ferúlico) e os de quantificação 0,050 (ácido p-cumárico) a 0,27 mg.L-1(ácido ferúlico). As concentrações dos ácidos fenólicos nas amostras de azeite de dendê de fabricação artesanal, variaram de 0,12 a 10,3 mg.L-1 (detecção UV) e de 1,0 a 11,0 mg.L-1 (DE). Já as concentrações obtidas nas amostras de azeite de dendê industrializadas variaram de 0,21 a 14,7 mg.L-1 (detecção UV) e de 3,0 a 11,9 mg.L-1 (DE). O sistema HPLC-UV-DE mostrou-se promissor nas análises dos ácidos fenólicos nestas e em outras matrizes complexas. / Phenolic substances have become the subject of investigation due to the numerous benefits to human health and the influence in the food quality. In this work, the phenolic acids in homemade and industrialized palm oil were determined. For these analyzes, it was used a high performance liquid chromatograph (HPLC Agilent 1100 series) with a manual 20μL Rheodyne valve, a reverse phase column C18 Inertsil ODS-3, a guard column Inertsil ODS and a UV-VIS detector for multiple wavelengths (UV-VIS MWD). At the end of HPLC waste channel was adapted an electrochemical cell with a capacity of 1.00 mL, with a standard three-electrode configuration, consisting of glassy carbon working electrode, a Ag/AgCl (KCl, sat) reference electrode and stainless steel needle as counter electrode. The electrochemical measurements were made using a μAutolab type III potentiostat, with its data acquisition software (GPES 4.9 version). The detections were performed sequentially, in the same injection. The optimal mobile phase was able to completely separate the analytes by isocratic elution, being composed of an aqueous solution of H3PO4 pH 2.40, containing 7mmol.L-1 of NaClO4 /isopropanol (87:13 v/v). The wavelength used for the UV detection was 225 nm. The amperometric measurements were performed at +1.1 V for all analytes. The recovery of the samples was performed using internal standardization, using o-coumaric acid as internal standard (PI) and m-coumaric acid as standard surrogate (PS). The recovery of PS was over 75% on both detectors for all samples. The instrumental limits for UV detection ranged from 0.019 (syringic acid) to 0.029 mg L-1 (ferulic acid) and quantification limits from 0.063 (syringic acid) to 0.095 mg L-1 (sinapic acid). In the same way, for electrochemical detection (ED) the instrumental limits was found in the range of 0.015 (p-coumaric acid) at 0.079 mg L-1 (ferulic acid), and the quantification limits were between 0.050 (p-coumaric acid) and 0.27 mg L-1 (ferulic acid). The concentration of phenolic acids in homemade palm oil samples ranged from 0.12 to 10.3 mg L-1 (UV detection) and from 1.0 to 11.0 mg L-1 (ED). The concentrations obtained in the industrialized palm oil samples varied from 0.21 to 14.7 mg L-1 (UV detection) and from 3.0 to 11.9 mg L-1 (ED). The HPLC-UV-ED system has shown promise in the analysis of these phenolic acids in this and other complex matrices.

The thesis deals with the testing and optimization of hydrodynamic conditions of the electrochemical detector, which consists of a flow cell containing the electrochemical sensor with an array of electrodes. The flow cell includes a rotating disc-shaped component, which allows radial flow of analytes along the sensor electrodes. This principle imitates the rotating disk electrode, with the difference that here the electrodes are immobile. Hydrodynamic phenomena in this case are similar to RDE. Therefore, the theoretical part of my work deals with hydrodynamics and mass transfer for the well-described case of rotating disk electrode. It was found that in the current set-up under certain conditions, an analyte flowed back from the output cell channel to the sensor, even at the laminar flow. This causes a nonlinear response of the electrodes and their low reproducibility. The solution is to reduce the distance between the rotation component and the electrodes surface and reduce flow speed of analyzed liquid towards the sensor. A novel type of sensor with annular electrodes was designed and tested. It showed four times higher conversion rate of the analyte compared with the originally used AC9 electrochemical sensor having electrodes in the form of full circles lying on a circumference of a common circle. Based on calculations, conversion of the analyte on the plane electrode can be increased up to 100% when using this annular sensor, adjusting flow cell and reducing of the input flow rate. The detector was connected to the liquid chromatograph and its function was tested on two substances - ascorbic acid and dopamine. For the current detector these parameters were set: dynamic range, linearity, noise, detection limit, time constant and the temperature range at which the detector can be operated.