Evaluation of the activity of ß-glucosidase immobilized on polydimethylsiloxane (PDMS) with a microfluidic flow injection analyzer with embedded optical fibers.

ß-glucosidase from almonds was immobilized on a polydimethylsiloxane (PDMS) microdevice by covalent chain using 3-aminopropyltrietoxysilane and glutaraldehyde. Enzymatic activity was evaluated using p-nitro-phenyl-ß-D-glucopyranoside dissolved in a 0.01 M pH 5.0 phosphate solution at 45 °C measuring the reaction product (p-nitrophenol) at 410 nm. The microdevice consisted of two parts: the one part where the enzymatic reaction was carried out and a second part where pH was adjusted at 10, with NaOH. The reaction product was measured at the microchip exit using two optical fibers which were aligned facing each other with a gap of 7 mm, between both tips using guides located perpendicular to the flow outlet. A water bath was used to carry out the enzymatic reaction on the microdevice at 45 °C. The enzymatic surface of the PDMS microdevice was 1.15 cm2 and the immobilized ß-glucosidase amount on the microdevice was of 1.17 µg/cm2. The calculated kinetics parameters were: Km 2.5 mM; Vmax 2.2 mM/min; Kcat 908.3/min and Kcat/Km 363.3/mM min. The immobilized enzyme is very stable decreasing only 5% the first 15 days; on the 30th day, the activity was 69%, regarding the initial activity.

On-line solid-phase extraction using a C18 minicolumn coupled to a flow injection system for determination of caffeine in green and roasted coffee beans.

A novel and fast method based on the solid phase extraction (SPE) coupled to a flow injection system for the determination of caffeine in coffee beans was developed. The caffeine extraction of coffee beans was carried out with hot water. A C18 reverse-phase mini-column was coupled to a continuous flow manifold to carry out the on-line SPE and the quantification of caffeine from aqueous extracts. Column length, retention time, elution volume, extracting solution and injection volume were evaluated. The retention time was of 90s and the elution was carried out with 400 µL of a methanol:water mixture (25:75). The proposed on-line SPE was compared against a chloroform extraction from aqueous extracts. With the proposed method the sample preparation was minimised and the sample throughput was increased (10 determination/h) because no dilution was required. Green coffee beans and beans with different roasting degree were analyzed.

Determination of bioavailable soluble arsenic and phosphates in mine tailings by spectrophotometric Sequential Injection Analysis.

By using a simple Sequential Injection Analysis (SIA) manifold and in base to the kinetic reaction of the molybdenum with As(V) and P(V) was possible to determine As(III), As(V) and P(V) in simple, binary and ternary samples. The activation energies for the reaction between molybdenum and As(V) and P(V) were of 70.90 kJ mol(-1) and of 19.02 kJ mol(-1), respectively, therefore it was possible to determine both analytes in mixtures by using different reaction temperature. When the analyses were carried out at room temperature, only the P(V) supplied analytical signal; with increased temperature, the kinetics of reaction for As(V) also increased, and a signal was obtained, being 55 degrees C the optimum temperature. In order to determine As(III), it was oxidized into As(V) with KIO(3,) and the reaction was carried out in the same way as for As(V). To resolve mixtures, an equations system from six calibration curves with different sequences of SIA at different temperature was performed. The lineal ranges were between 0.5 microg mL(-1) and 10 microg mL(-1) with a repeatability and reproducibility between 0.7% and 5.2% and detection limits between 0.36 microg mL(-1) and 0.58 microg mL(-1). In binary mixtures of P(V)/As(V) the recoveries were close to 100% for both analytes at ratios lesser than 10:1. For As(V)/As(III) ratios between 1:1 and 5:1 the recoveries were ranged between 85% and 95%. The method was applied in mine tailings and in arsenopyrite. The results showed that the soluble arsenic was found oxidized as As(V). These results were compared with those obtained by atomic absorption spectrometry and both proved to be very close.