Thermal behavior of food preservative sorbic acid and its derivates.

Sorbic acid and its potassium and calcium salts used as food preservatives and sorbic chloride were submitted to thermal analysis in order to characterize their thermal behavior on heating and cooling processes, using TG/DTG/DTA, TG-MS, DSC, hot stage microscopy and DRX analysis. Sorbic acid melted and decomposed under dynamic heating. Under isothermal it sublimated without decomposition before melting (T < 134 °C). The potassium salt presented a solid-solid phase transition before decomposition. Both potassium and calcium salts decomposed in temperatures higher than the acid without melting, producing the respective carbonates and oxides as final residues. Sorbic chloride evaporate without condensation, on dynamic heating.

Thermal behavior of inosine 5'-monophosphate in acidic form and as alkali and alkaline earth salts.

Inosine 5'-monophosphate in acidic form and its lithium, potassium, magnesium, calcium, strontium and barium were prepared from the sodium salt, characterized by elemental analysis and Fourier transform infrared spectroscopy and submitted to thermogravimetry (TG), differential thermal analysis (DTA), differential scanning calorimetry (DSC) and thermogravimetry coupled to infrared spectroscopy (TG-FTIR) of the volatile products evolved during heating. All the salts were hydrated containing from 4 to 7.5 H2O. After dehydration these salts decomposed releasing the nitrogenous base followed by the ribose group, and producing pyrophosphates as final residue. Evolved Gas Analysis (EGA) reveled the release of water, isocyanic acid and hydrocyanic acid during decomposition of the organic moiety. It was observed only water loss up to 200 °C. At temperatures above 200 °C, the nucleotides were unstable and decomposed, implying that foods containing those additives should be processed below this temperature. Finally, a general mechanism for the decomposition of the inosinates was proposed.

Determination of dopamine in synthetic cerebrospinal fluid by SWV with a graphite-polyurethane composite electrode.

This work describes an electroanalytical investigation of dopamine using cyclic voltammetry (CV) and the graphite-polyurethane composite electrode (GPU). In CV studies, well-defined redox peaks characterize the oxidation process at the GPU electrode, which is indicative of electrocatalytic effects associated with active sites on the GPU electrode surface. A new analytical methodology was developed using the GPU electrode and square wave voltammetry (SWV) in BR buffer solution (0.1 mol L(-1); pH 7.4). Analytical curves were constructed under optimized conditions (f=60s(-1), DeltaE(a)=50 mV, DeltaE(I)=2 mV) and detection and quantification limits of 6.4x10(-8) mol L(-1) (12.1 microg L(-1)) and 5.2x10(-6) mol L(-1) (0.9 mg L(-1)), respectively, were achieved. The precision of the method was checked by performing ten successive measurements for a 9.9x10(-6) mol L(-1) dopamine solution. For intra-assay and inter-assay precisions, the relative standard deviations were 1.9 and 2.3%, respectively. In order to evaluate the developed methodology, the determination of dopamine was performed with good sensitivity and selectivity, without the interference of ascorbic acid in synthetic cerebrospinal fluid, which indicates that the new methodology enables reliable analysis of dopamine.

Evaluation of a new rigid carbon-castor oil polyurethane composite as an electrode material.

Polyurethane resin derived from castor oil was used to prepare a graphite composite intented to be applied in voltammetry. The best graphite/resin content in the composite was evaluated using cyclic voltammetry. The useful potential window was investigated in different electrolytes with pH ranging from 1 to 13, with good results in relation to the glassy carbon electrode. Response of this new electrode material to ferricyanide, hydroquinone, catechol and cadmium(II) ions was investigated in cyclic voltammetry and/or differential pulse voltammetry. The composite showed to be also useful in flow procedures.

PbO2-based graphite-epoxy electrode for potentiometric determination of acids and bases in aqueous and aqueous-ethanolic media.

The construction and analytical evaluation of a PbO2-based graphite-epoxy electrode sensitive to H3O+, based on incorporation of lead(IV) oxide in a graphite-epoxy matrix, are described. The data obtained from a variety of acid-base titrations in aqueous and aqueous-ethanolic media were compared with those obtained by use of a glass electrode under the same conditions. The proposed electrode provides a linear response in the pH range from 1 to 11 with a slope of -58.7+/-0.3 mV pH(-1) and -60.8+/-0.2 mV pH(-1) in aqueous and ethanolic media, respectively. The response time was less than 15 s and the lifetime of the electrode was at least eight months (ca. 5000 determinations) and its performance is good in pH determination and end-point detection in potentiometric acid-base titrations in both aqueous and aqueous-ethanolic media.

Amperometric determination of xanthine and hypoxanthine at carbon electrodes. Effect of surface activity and the instrumental parameters on the sensitivity and the limit of detection.

The performance of active graphite and carbon fiber surfaces produced by different mechanical/electrochemical methods of surface activation has been investigated in the amperometric determinations of xanthine and hypoxanthine under physiologically relevant conditions. The electrodes showed better limits of detection (LOD) when used with differential techniques with a capability of discriminating the analytical signal from the background. Square wave voltammetry and cyclic voltammetry showed the most sensitive response. Electrochemically activated carbon fiber ultramicroelectrodes showed the highest sensitivity (58 A M(-1) cm(-2)) and the LOD in the 200 nM range was observed at the rough pyrolytic graphite electrodes by square wave voltammetry. The results demonstrate the feasibility of the development of new electroanalytical methods for the determination of oxypurines in biological samples.