Near Infrared Spectroscopy: fundamentals, practical aspects and analytical applications

This paper intends to review the basic theory of Near Infrared (NIR) Spectroscopy and its applications in the field of Analytical Science. It is addressed to the reader who does not have a profound knowledge of vibrational spectroscopy but wants to be introduced to the analytical potentialities of this fascinating technique and, at same time, be conscious of its limitations. Essential theory background, an outline of modern instrument design, practical aspects, and applications in a number of different fields are presented. This work does not intend to supply an intensive bibliography but refers to the most recent, significant and representative material found in the technical literature. Because this paper has been produced as consequence of the First Workshop on Near Infrared Spectroscopy, whose venue was Campinas - Brazil, as a pre-conference activity of the XI National Meeting on Analytical Chemistry (ENQA), it also depicts the state of the art of NIR spectroscopy in Brazil, pointing out the current achievements and the need to take the technology to a level consistent with this country's economical activities.Este trabalho visa prover uma revisão da teoria da Espectroscopia no Infravermelho Próximo (NIR) e das suas aplicações no campo das Ciências Analíticas. Ele é dirigido ao leitor que não possui um conhecimento profundo sobre espectroscopia vibracional mas que gostaria de ser introduzido às potencialidades analíticas desta técnica fascinante e, ao mesmo tempo, se tornar consciente das suas limitações. A teoria fundamental, uma revisão dos instrumentos modernos existentes e aplicações em inúmeros campos, são apresentados. Este trabalho não pretende suprir uma revisão bibliográfica intensiva mas se refere aos materiais recentes mais significativos e representativos encontrados na literatura técnica. Uma vez que este texto foi produzido em conseqüência do primeiro workshop em espectroscopia NIR, que ocorreu em Campinas - Brasil, como uma atividade preliminar ao 11º Encontro Nacional de Química Analítica (ENQA), ele também apresenta o estado da arte na Espectroscopia NIR no Brasil, mostrando seus resultados mais recentes e apontando a necessidade de se elevar esta tecnologia a um nível consistente com aquele de suas atividades econômicas.19821

Why it is called monosegmented flow analysis

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Operator-free flow injection analyser

A flow injection analyser has been constructed to allow an operator-free determination of up to 40 samples. Besides the usual FIA apparatus, the analyser includes a home-made sample introduction device made with three electromechanical three-way valves and an auto-sampler from Technicon which has been adapted to be commanded by an external digital signal. The analyser is controlled by a single board SDK-8085 microcomputer. The necessary interface to couple the analyser components to the microcomputer is also described. The analyser was evaluated for a Cr(VI)-FIA determination showing a very good performance with a relative standard deviation for 15 signals from the injection of 100 μl of a 1.0 mg.ml-1 standard Cr(VI) solution being equal to 0.5%

Signal-to-noise optimization and evaluation of a home-made visible diode-array spectrophotometer

This paper describes a simple low-cost multichannel visible spectrophotometer built with an RL512G EGG-Reticon photodiode array. A symmetric Czerny-Turner optical design was employed; instrument control was via a single-board microcomputer based on the 8085 Intel microprocessor. Spectral intensity data are stored in the single-board's RAM and then transferred to an IBM-AT 3865X compatible microcomputer through a RS-232C interface. This external microcomputer processes the data to recover transmittance, absorbance or relative intensity of the spectra. The signal-to-noise ratio and dynamic range were improved by using variable integration times, which increase during the same scan; and by the use of either weighted or unweighted sliding average of consecutive diodes. The instrument is suitable for automatic methods requiring quasi-simultaneous multiwavelength detections, such as multivariative calibration and flow-injection gradient scan techniques

Spectrophotometric determination of creatinine by monosegmented continuous-flow analysis

A monosegmented continuous-flow system (MCFS) has been evaluated for determination of creatinine in urine using the Jaffé reaction. The analyser is compact and allows 130 determinations to be performed per hour, with a relative standard deviation of the peak height better than 1.5% (N =10). The results for real samples agree with those obtained by. the standard manual Jaffé procedure and with the kinetic automatic method

A PLS regression model using NIR spectroscopy for on-line monitoring of the biodiesel production reaction

AbstractIn this work, a Partial Least Squares (PLS) regression model using Near-Infrared (NIR) spectroscopy was developed to monitor the progress of the catalyzed transesterification reactions of soybean oil that produce biodiesel. The NIR spectra were collected during the transesterification reaction with a lab made spectrophotometric flow cell. Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy was employed for determining the conversion percentage of glycerides to methyl esters during the transesterification reaction and used as reference to build the PLS calibration model employing NIR spectroscopy data. The model, constructed with selected spectral range has not been tried before and allows the monitoring of the transesterification reaction in terms of conversion ratio for different temperatures. The model was validated and the values of Root Mean Square Error of Prediction (RMSEP) found for two different temperatures were 0.74% and 1.27% (of conversion) for reactions carried out at 20±0.2°C and 55±0.2°C, respectively

An automated system for liquid-liquid extraction in monosegmented flow analysis

An automated system to perform liquid-liquid extraction in monosegmented flow analysis is described. The system is controlled by a microcomputer that can track the localization of the aqueous monosegmented sample in the manifold. Optical switches are employed to sense the gas-liquid interface of the air bubbles that define the monosegment. The logical level changes, generated by the switches, are flagged by the computer through a home-made interface that also contains the analogue-to-digital converter for signal acquisition. The sequence of operations, necessary for a single extraction or for concentration of the analyte in the organic phase, is triggered by these logical transitions. The system was evaluated for extraction of Cd(II), Cu(II) and Zn(II) and concentration of Cd(II) from aqueous solutions at pH 9.9 (NH3/NH4Cl buffer) into chloroform containing PAN (1-(2-pyridylazo)-2-naphthol) . The results show a mean repeatability of 3% (rsd) for a 2.0 mg l-1 Cd(II) solution and a linear increase of the concentration factor for a 0.5mg l-1 Cd(II) solution observed for up to nine extraction cycles

Adaptation of a cold vapour mercury analyser to flow injection analysis

Minor modifications to a Coleman MAS-50A Mercury Analyser System allowed the determination of mercury by flow injection analysis. Using sample volumes of 600 μl it was possible to analyse up to 120 samples per hour, with a detection limit of 0.2 μg. l-1 (120 pg) of mercury. The authors also report on a simple digestion procedure which replaces the time- and reagent-consuming EPA procedure, when the sample content permits

A new detection system for laser induced breakdown spectroscopy based on an acousto-optical tunable filter coupled to a photomultiplier: Application for manganese determination in steel

The development of a new detection system for laser induced breakdown spectroscopy (LIBS), based on a collinear quartz acousto-optical tunable filter (AOTF) for the ultraviolet spectral region coupled to a photomultiplier, is described. It was used in conjunction with a 1064 nm, 5 ns pulse duration neodymiumdoped yttrium aluminium garnet (Nd:YAG) laser source and also employed a radio-frequency signal generator to control the AOTF and a digital delay generator to delay the start of the detection in relation to the instant of the application of the laser pulse. The detection system was optimized for highest detectivity for the manganese peak at 293.9 nm while analyzing a steel sample by LIBS. The resulting signal to background ratio at the optimal conditions of 2 µs delay time, 40 µs integration time gate and 110 mJ pulse energy was similar to that of a commercial echelle-intensified charge-coupled device (echelle-ICCD) detection system. The new detection system was then employed for manganese determination in steel samples, taking the emission signals at just 15 wavelengths, 5 related to the above mentioned manganese peak, another 5 to background emission around 296.0 nm and the others to the iron peak at 297.3 nm (internal standard). The resulting analytical curve for manganese, obtained using 5 samples in the concentration range of 0.214 to 0.939% w/w, presented a correlation coefficient of 0.979 for an exponential regression function. The relative errors of predicting the manganese concentrations, using the calibration curve, for 2 samples, containing 0.277 and 0.608% w/w, were 20.7 and − 1.9%, respectively