Jarbas José Rodrigues Rohwedder

Synchrotron radiation X-Ray fluorescence at the LNLS: beamline instrumentation and experiments

The x-ray fluorescence heamline of the Laboratorio Nacional de Luz Sincrotron (LNLS) is described. The main optical component of the beamline is a silicon (111) channel-cut monochromator, which can tune energies between 3 and 14 keV. A general description of two experimental stations is given. Beam characterization was done by measuring experimental parameters such as vertical profile and monochromatic flux. These results show that the photon flux at 8 keV in an area of 20 mm 2 is 4.2 × 10 9 photons s -1 . The possibility of achieving fine tuning of energies (high resolution) was confirmed. This paper presents some original results derived from the commissioning of the beamline, such as a comparison of detection limits in different experimental conditions, and a novel mechanical system to align capillaries together with a semi-automatic adjustment procedure. So far, there have been several proposals to perform a variety of experiments at this beamline, covering different fields, such as physics, chemistry, geology and biology.

Determination of total sulfur in diesel fuel employing NIR spectroscopy and multivariate calibration

A method for sulfur determination in diesel fuel employing near infrared spectroscopy, variable selection and multivariate calibration is described. The performances of principal component regression (PCR) and partial least square (PLS) chemometric methods were compared with those shown by multiple linear regression (MLR), performed after variable selection based on the genetic algorithm (GA) or the successive projection algorithm (SPA). Ninety seven diesel samples were divided into three sets (41 for calibration, 30 for internal validation and 26 for external validation), each of them covering the full range of sulfur concentrations (from 0.07 to 0.33% w/w). Transflectance measurements were performed from 850 to 1800 nm. Although principal component analysis identified the presence of three groups, PLS, PCR and MLR provided models whose predicting capabilities were independent of the diesel type. Calibration with PLS and PCR employing all the 454 wavelengths provided root mean square errors of prediction (RMSEP) of 0.036% and 0.043% for the validation set, respectively. The use of GA and SPA for variable selection provided calibration models based on 19 and 9 wavelengths, with a RMSEP of 0.031% (PLS-GA), 0.022% (MLR-SPA) and 0.034% (MLR-GA). As the ASTM 4294 method allows a reproducibility of 0.05%, it can be concluded that a method based on NIR spectroscopy and multivariate calibration can be employed for the determination of sulfur in diesel fuels. Furthermore, the selection of variables can provide more robust calibration models and SPA provided more parsimonious models than GA.

Construction and evaluation of an optical pH sensor based on polyaniline-porous Vycor glass nanocomposite

This work describes the preparation of a polyaniline (Pani)‐porous Vycor glass (PVG) nanocomposite and its use as sensing phase in an optical fibre pH sensor. Nanocomposites of Pani‐PVG were prepared by in situ polymerisation of aniline absorbed inside the pores of a PVG (Corning 7930) with an average porous size of 8 nm. The optical sensor was constructed by fixing a PVG slide onto a distal end of bifurcated optical fibre bundle, with a cyanoacrylic resin. The sensor response was found to be reversible in the pH range from 5 to 12 and linear from pH 7.4 to 9.5. Response times of 4, 8 and >16 min were obtained for slide thickness’ of 0.5, 1 and 1.5 mm, respectively. Changes of temperature, ranging from 20 to 408C, showed minor effect on the dynamic range. Similarly, the ionic strength (0.15, 0.30 and 0.50 mol l ˇ1 ) and the nature of the ions (NaCl, KCl and NaClO4) showed minor influence on the sensor response. Leaching of Pani was not observed and the sensor lifetime was determined as being at least 5 months. These results indicate that a Pani‐PVG nanocomposite is suitable for the construction of optical pH sensors, with good analytical performance, since the glass slides can be prepared with good reproducibility and durability. # 2001 Elsevier Science B.V. All rights reserved.

Assessment of infrared spectroscopy and multivariate techniques for monitoring the service condition of diesel-engine lubricating oils

This paper presents two methodologies for monitoring the service condition of diesel-engine lubricating oils on the basis of infrared spectra. In the first approach, oils samples are discriminated into three groups, each one associated to a given wear stage. An algorithm is proposed to select spectral variables with good discriminant power and small collinearity for the purpose of discriminant analysis classification. As a result, a classification accuracy of 93% was obtained both in the middle (MIR) and near-infrared (NIR) ranges. The second approach employs multivariate calibration methods to predict the viscosity of the lubricant. In this case, the use of absorbance measurements in the NIR spectral range was not successful, because of experimental difficulties associated to the presence of particulate matter. Such a problem was circumvented by the use of attenuated total reflectance (ATR) measurements in the MIR spectral range, in which an RMSEP of 3.8cSt and a relative average error of 3.2% were attained.

Sistema de injeção em fluxo espectrofotométrico para monitorar peróxido de hidrogênio em processo de fotodegradação por reação foto-Fenton

Fenton processes (Fe 2+ /H2O2/UV). Sample is injected manually in a carrier stream and then receives by confluence a 0.1 mol L -1 NH4VO3 solution in 0.5 mol L -1 H 2SO4 medium. The product formed shows absorption at 446 nm which is recorded as a peak with height proportional to H 2O2 concentration. The performance of the proposed system was evaluated by monitoring the consumption of H2O2 during the photodegradation of dichloroacetic acid solution by foto-Fenton reaction.

Simultaneous determination of methanol and ethanol in gasoline using NIR spectroscopy: effect of gasoline composition.

Near infrared (NIR) spectroscopy was employed for simultaneous determination of methanol and ethanol contents in gasoline. Spectra were collected in the range from 714 to 2500 nm and were used to construct quantitative models based on partial least squares (PLS) regression. Samples were prepared in the laboratory and the PLS regression models were developed using the spectral range from 1105 to 1682 nm, showing a root mean square error of prediction (RMSEP) of 0.28% (v/v) for ethanol for both PLS-1 and PLS-2 models and of 0.31 and 0.32% (v/v) for methanol for the PLS-1 and PLS-2 models, respectively. A RMSEP of 0.83% (v/v) was obtained for commercial samples. The effect of the gasoline composition was investigated, it being verified that some solvents, such as toluene and o-xylene, interfere in ethanol content prediction, while isooctane, o-xylene, m-xylene and p-xylene interfere in the methanol content prediction. Other spectral ranges were investigated and the range 1449-1611 nm showed the best results.

Chemical oxygen demand (COD) using microwave digestion

Abstract A new approach to determine the Chemical Oxygen Demand (COD) using a microwave digestion to replace the conventional time consuming reflux method is proposed. The procedure uses a microwave oven for the digestion of small volumes of sample (from 500 up to 2000 μ1), in a completely closed 60 ml Teflon vessel. For samples showing COD values up to 1000 mg O 2 1 −1 , the digestion step is completed in 7 min, compared to the 2 h needed for the conventional reflux method. An average precision of 1.0% was obtained using membrane filtered natural samples.

Determination of pentavalent antimony in antileishmaniotic drugs using an automated system for liquid-liquid extraction with on-line detection

An automated system for liquid-liquid extraction flow analysis (LLE-FA) for the determination of Sb(V) in antileishmanial drugs is presented. The method is based on extraction in a 5mL glass extraction chamber of an ion pair formed between hexachloroantimoniate anion and rhodamine B cation into toluene. The detection system consists of a green light emitting diode (LED) and a photodiode. The system is controlled by a microcomputer using a program written in Visual Basic 3.0. The extraction process was optimized and the following experimental parameters were established: sample loop of 150muL; reagent loop of 900muL; stirring time of 100s; phase separation time of 80s; volumetric ratio of 1:1 (aqueous/organic). The method was in-house validated for the determination of Sb(V) in meglumine antimoniate. The following performance criteria were obtained: linearity of 0.9989, linear range of 7.0x10(-5) to 7.2x10(-4)mol Sb(V)L(-1), sensitivity of 1.61x10(6)+/-2 arbitrary units Lmol(-1) (P<0.05), intra-assay precision of 3.5% (n=5; 4.1x10(-4)molL(-1) Sb(V). Whereas the method is selective in the presence of Sb(III), As(III) and Pb(II) at concentrations up to one tenth of the concentration of Sb(V), As(V) interferes. The accuracy of the method was evaluated through comparison of results obtained from analyses of pharmaceutical formulations by the proposed LLE-FA method with those obtained by inductively coupled plasma optic emission spectrometry (ICP OES) and differential pulse polarography for total antimony and Sb(III), respectively. The proposed method presented an analytical frequency of eight analysis per hour and is suitable for Sb(V) determination in the quality control of drugs employed for the treatment of leishmaniasis.

Application of amperometric sol-gel biosensor to flow injection determination of glucose

A glucose biosensor with enzyme immobilised by sol-gel technology was constructed and evaluated. The glucose biosensor reported is based on encapsulated GOX within a sol-gel glass, prepared with 3-aminopropyltriethoxy silane, 2-(3,4-epoxycyclohexyl)-ethyltrimetoxy silane and HCl. A flow system incorporating the amperometric biosensor constructed was developed for the determination of glucose in the 1x10(-4)-5x10(-3) moll(-1) range with a precision of 1.5%. The results obtained for the analysis of electrolytic solution for iv administration and human serum samples showed good agreement between the proposed method and the reference procedure, with relative error <5%.

Differential conductimetry in flow injection. Determination of ammonia in Kjeldahl digests

A differential conductivity meter has been constructed for use in flow injection (Fl) systems. The instrument employs a Wien bridge to generate a sinusoidal alternating electrical potential (2 V, 1.4 kHz) applied to a measurement bridge containing two twin conductance flow cells. The difference in the conductance between the cells is monitored. The instrument can be used to follow the small changes in conductance that occur in samples in which a high background ionic concentration is found. This facility was used for the determination of ammonia in Kjeldahl digests. The de-ionized water acceptor stream, previously used in the Fl system, was replaced by a 5 × 10–4 mol dm–3 HCl solution and the decrease in the conductance caused by the reaction of hydrogen ions with ammonia was monitored. Linear calibration graphs were obtained in the range 0.5–25 mmol dm–3 of ammonium ion and samples can be processed at a rate of 90 h–1. The relative standard deviation for the peak heights of the Fl signals is 1% or lower. The differential system was briefly studied for the determination of carbon dioxide by using NaOH solution as the acceptor stream.