Teresa Cristina Bezerra Saldanha

The successive projections algorithm for variable selection in spectroscopic multicomponent analysis

Abstract The “Successive Projections Algorithm”, a forward selection method which uses simple operations in a vector space to minimize variable collinearity, is proposed as a novel variable selection strategy for multivariate calibration. The algorithm was applied to UV–VIS spectrophotometric data for simultaneous analysis of complexes of Co2+, Cu2+, Mn2+, Ni2+ e Zn2+ with 4-(2-piridilazo)resorcinol in samples containing the analytes in the 0.02–0.5 mg l−1 concentration range. A convenient spectral window was first chosen by a procedure also proposed here and applying Successive Projections Algorithm to this range allowed an improvement of the predictive capabilities of Principal Component Regression, Partial Least Squares and Multiple Linear Regression models using only 20% of the number of wavelengths. Successive Projections Algorithm selection resulted in a root mean square error of prediction at the test set of 0.02 mg l−1, while the best and worst realizations of a genetic algorithm used for comparison yielded 0.01 and 0.03 mg l−1. However, genetic algorithm took 200 times longer than Successive Projections Algorithm, and this ratio tends to increase dramatically with the number of wavelengths employed. Finally, unlike genetic algorithm, Successive Projections Algorithm is a deterministic search technique whose results are reproducible and it is more robust with respect to the choice of the validation set.

A method for calibration and validation subset partitioning

This paper proposes a new method to divide a pool of samples into calibration and validation subsets for multivariate modelling. The proposed method is of value for analytical applications involving complex matrices, in which the composition variability of real samples cannot be easily reproduced by optimized experimental designs. A stepwise procedure is employed to select samples according to their differences in both x (instrumental responses) and y (predicted parameter) spaces. The proposed technique is illustrated in a case study involving the prediction of three quality parameters (specific mass and distillation temperatures at which 10 and 90% of the sample has evaporated) of diesel by NIR spectrometry and PLS modelling. For comparison, PLS models are also constructed by full cross-validation, as well as by using the Kennard-Stone and random sampling methods for calibration and validation subset partitioning. The obtained models are compared in terms of prediction performance by employing an independent set of samples not used for calibration or validation. The results of F-tests at 95% confidence level reveal that the proposed technique may be an advantageous alternative to the other three strategies.

Modified microelectrodes and multivariate calibration for flow injection amperometric simultaneous determination of ascorbic acid, dopamine, epinephrine and dipyrone.

Flow injection amperometric quantification of ascorbic acid (AA), dopamine (DA), epinephrine (EP) and dipyrone (DI) in mixtures (in the microgram g-1 range) was successfully performed by using an array of microelectrodes with units modified by the electrodeposition of different noble metals, together with multivariate calibration analysis. The four groups of microelectrodes utilized included a pure gold electrode and electrodes modified by electrodeposition of platinum, palladium or a mixture of platinum + palladium. The array of microelectrodes was inserted in a flow cell and the amperometric data acquisition was performed with a four-channel potentiostat. The analysis of the resulting signals was carried out by a multivariate calibration method, using a group of 16 standard mixtures selected by a two-level factorial design. The analysis of synthetic samples and pharmaceutical compounds containing AA and DI led to very similar values to those obtained by the classical iodimetric analysis. The average absolute errors (in microgram g-1) calculated for each analyte were 0.3, 0.2, 0.4 and 0.4 for AA, DA, EP and DI, respectively.

Simultaneous Spectrometric Determination of Cu 2+ , Mn 2+ and Zn 2+ in Polivitaminic/ Polimineral Drug Using SPA and GA Algorithms for Variable Selection

Este artigo apresenta uma aplicacao do metodo para determinacao espectrofotometrica simultânea dos ions divalentes de cobre, manganes e zinco a analise de medicamento polivitaminico/polimineral. O metodo usa 4-(2-piridilazo) resorcinol (PAR), calibracao multivariada e tecnicas de selecao de variaveis e foi otimizado o empregando-se o algoritmo das projecoes sucessivas (APS) e o algoritmo genetico (AG), para escolha dos comprimentos de onda mais informativos para a analise. Com essas tecnicas, foi possivel construir modelos de calibracao por regressao linear multipla (RLM-APS e RLM-AG). Os resultados obtidos foram comparados com modelos de regressao em componentes principais (PCR) e nos minimos quadrados parciais (PLS). Demonstra-se a partir do erro medio quadratico de previsao (RMSEP) que os modelos apresentam desempenhos semelhantes ao prever as concentracoes dos tres analitos no medicamento. Todavia os modelos RLM sao mais simples pois requerem um numero muito menor de comprimentos de onda e sao mais faceis de interpretar que os baseados em variaveis latentes.

Estudo comparativo sobre filtragem de sinais instrumentais usando transformadas de Fourier e Wavelet

A comparative study of the Fourier (FT) and the wavelet transforms (WT) for instrumental signal denoising is presented. The basic principles of wavelet theory are described in a succinct and simplified manner. For illustration, FT and WT are used to filter UV-VIS and plasma emission spectra using MATLAB software for computation. Results show that FT and WT filters are comparable when the signal does not display sharp peaks (UV-VIS spectra), but the WT yields a better filtering when the filling factor of the signal is small (plasma spectra), since it causes low peak distortion.

Simultaneous Analysis of Co2+, Cu2+ Mn2+, Ni2+ and Zn2+ in The Ultraviolet Region Using 4-(Pyridil-2-AZO) Resorcinol and Multivariate Calibration

ABSTRACT A multicomponent spectrophotometric methodology for the simultaneous determination of Co2+, Cu2+, Mn2+, Ni2+ and Zn2+ in aqueous solution is reported, using (4-(pyridil-2-azo) resorcinol), a diode array spectrophotometer and multivariate calibration by partial least-squares and principal component regerssions. Spectra are recorded in the UV region. The 225 – 320 nm range is selected as optimal, through a criterion based on tederivatives of the differences between individual spectra, which compares favorably with a genetic algorithm. The methodology is applied to the simultaneous determination of the five than 1.5 mgL−1. The best result are obtanied at pH 9.0, with average absolute errors of prediction lower than 0.09 mgL−1

Flow-batch analyser for preparation of calibration standard mixtures in simultaneous multicomponent spectrometric analysis

The application of multivariate calibration techniques to multicomponent analysis by UV-VIS molecular absorption spectrometry is a powerful tool for simul- taneous determination of several chemical species. However, when this methodol- ogy is accomplished manually, it is slow and laborious, consumes high amounts of reagents and samples, is susceptible to contaminations and presents a high opera- tional cost. To overcome these drawbacks, a flow-batch analyser is proposed in this work. This analyser was developed for automatic preparation of standard calibration and test (or validation) mixtures. It was applied to the simultaneous determination of Cu 2+ , Mn 2+ and Zn 2+ in polyvitaminic and polymineral pharmaceutical formula- tions, using 4-(2-piridilazo) resorcinol as reagent and a UV-VIS spectrophotometer with a photodiode array detector. The results obtained with the proposed system are in good agreement with those obtained by flame atomic absorption spectrometry, which was employed as reference method. With the proposed analyser, the prepara- tion of calibration and test mixtures can be accomplished about four hours, while the manual procedure requires at least two days. Moreover, it consumes smaller amounts of reagents and samples than the manual procedure. After the preparation of calibration and test mixtures, 60 samples h -1 can be carried out with the proposed flow-batch analyser.

Análise multicomponente simultânea por espectrofotometria de absorção molecular UV-VIS

This review presents the evolution of simultaneous multicomponent analysis by absorption spectrophotometry in the ultraviolet and visual regions in terms of some qualitative and quantitative analysis techniques, otimization methods, as well as applications and modern trends.

Effects of experimental design on calibration curve precision in routine analysis.

A computational program which compares the effciencies of different experimental designs with those of maximum precision (D-optimized designs) is described. The program produces confidence interval plots for a calibration curve and provides information about the number of standard solutions, concentration levels and suitable concentration ranges to achieve an optimum calibration. Some examples of the application of this novel computational program are given, using both simulated and real data.

Use of near infrared reflectance spectrometry for monitoring water resources: analysis of river seston downstream of an effluent discharge point

Page 8 NIR news Vol. 17 No. 8 (2006) Nowadays, water resources are valuable assets that need to be preserved to ensure an adequate water supply for the population, as well as the natural balance of the aquatic ecosystem. Therefore, regulatory agencies are faced with an increasing need to take mitigating actions in a timely manner upon the onset of environmental problems. For this reason, much effort has been placed on the development of optical techniques aimed at detecting relevant environmental events in real time. In this context, near infrared reflectance (NIRR) spectrometry is an attractive analytical tool owing to its properties of high sample throughput, non-destructiveness and non-chemical consumption. One alternative that has been investigated by our research group involves the NIRR analysis of seston filtered from water samples acquired in a river subject to industrial effluent discharges. The analysis of seston properties in aquatic systems is very important to study the fate and bioavailability of many contaminants that adsorb to the surface of the particulate matter. As compared to mid-infrared spectroscopy, the use of NIRR allows for a simpler sampling strategy and sample preparation procedures. The present work concerned the river Mumbaba, located in the industrial district of the city of João Pessoa (Paraíba, Brazil), which receives effluents from a textile plant. The investigation was aimed at finding possible associations between effluent discharges and alterations in the river seston. For this purpose, four sampling sites were considered: sites 1 and 2 (1100 m and 300 m upstream of the discharge point, respectively), site 3 (discharge point) and site 4 (500 m downstream of the discharge point). For each site, triplicate samples were collected monthly from September 2002 to June 2003. Seston was collected from 500 mL of each sample on Whatman GF/C filters (diameter 47 mm, pore size 1.0 μm) by vacuum filtration. The filters were placed in Petri dishes and heated at 50°C for 6 h, then dried in desiccators at room temperature (24°C). A 10 mm diameter circle was randomly cut from each filter and then subjected to a NIRR measurement. The spectra were recorded using a FT-NIR/MIR spectrometer PerkinElmer GX with a spectral resolution of 4 cm and eight scans. The NIRR region in the range 4014–7398 cm was used. The blank was taken as the spectrum of the Whatman GF/C filter after filtration of distilled water and application of the procedures described above. The measurements were carried out at 24°C and 46% relative humidity by using a PerkinElmer diffuse reflectance accessory. Figure 1(a) shows the NIRR spectra of the triplicate average for the four monitored points in December. In order to minimise noise and baseline drifts, the first derivative of the spectra was used after smoothing by a Savitzky–Golay filter with a 2-order polynomial and a 29point window as shown in Figure 1(b). In order to analyse the effect of the effluent discharge, a SIMCA model was built for site 1 (upstream of the discharge point), which corresponds to the natural state of the river. This model was applied to the classification of sites 2 and 4. It was expected that samples from site 2, which is also upstream of the discharge point, should be classified as belonging to site 1. On the other hand, if the analysis was to be sensitive to changes caused by the effluent, samples from site 4, which is downstream of the discharge point, should be classified as not belonging to site 1. By using eight principal components in the model, all samples from site 2 were classified as belonging to site 1, as expected. However, all site 4 samples (with the exception of the March sample) were also classified as belonging to site 1. Therefore, the SIMCA model was not sensitive to changes in the sestonic properties. Such a result is not adequate because, if this technique were intended for a preliminary screening analysis, high sensitivity would be important to unveil events that require further investigation. At this point, we believed that the sensitivity could be improved by restricting the analysis to spectral features correlated with the effect of the effluent discharges. For this purpose, we developed a selection technique based on the wavelet transform (WT) of the derivative spectra. Let x1, x3 and x4 be the spectra of samples (triplicate averages) from sites 1 (upstream), 3 (effluent discharge point) and 4 (downstream), respectively, at a given month. The WT strategy starts by using the sample from site 1 as a blank signal for the selection procedure, that Use of near infrared reflectance spectrometry for monitoring water resources: analysis of river seston downstream of an effluent discharge point