Marcelo Sena

Avaliação do Uso de Métodos Quimiométricos em Análise de Solos

One of the major interests in soil analysis is the evaluation of its chemical, physical and biological parameters, which are indicators of soil quality (the most important is the organic matter). Besides there is a great interest in the study of humic substances and on the assessment of pollutants, such as pesticides and heavy metals, in soils. Chemometrics is a powerful tool to deal with these problems and can help soil researchers to extract much more information from their data. In spite of this, the presence of these kinds of strategies in the literature has obtained projection only recently. The utilization of chemometric methods in soil analysis is evaluated in this article. The applications will be divided in four parts (with emphasis in the first two): (i) descriptive and exploratory methods based on Principal Component Analysis (PCA); (ii) multivariate calibration methods (MLR, PCR and PLS); (iii) methods such as Evolving Factor Analysis and SIMPLISMA; and (iv) artificial intelligence methods, such as Artificial Neural Networks.

Development and analytical validation of a multivariate calibration method for determination of amoxicillin in suspension formulations by near infrared spectroscopy

This paper proposes a new method for determination of amoxicillin in pharmaceutical suspension formulations, based on transflectance near infrared (NIR) measurements and partial least squares (PLS) multivariate calibration. A complete methodology was implemented for developing the proposed method, including an experimental design, data preprocessing by using multiple scatter correction (MSC) and outlier detection based on high values of leverage, and X and Y residuals. The best PLS model was obtained with seven latent variables in the range from 40.0 to 65.0 mg mL(-1) of amoxicillin, providing a root mean square error of prediction (RMSEP) of 1.6 mg mL(-1). The method was validated in accordance with Brazilian and international guidelines, through the estimate of figures of merit, such as linearity, precision, accuracy, robustness, selectivity, analytical sensitivity, limits of detection and quantitation, and bias. The results for determinations in four commercial pharmaceutical formulations were in agreement with the official high performance liquid chromatographic (HPLC) method at the 99% confidence level. A pseudo-univariate calibration curve was also obtained based on the net analyte signal (NAS). The proposed chemometric method presented the advantages of rapidity, simplicity, low cost, and no use of solvents, compared to the principal alternative methods based on HPLC.

Stationary points for the OH− + CH3F → CH3OH + F− potential energy surface

Abstract Ab initio calculations at the HF, MP2, and CCSD(T) levels of theory, utilizing a range of basis sets including the large bases 6-311++G(2df,2pd) and aug-cc-pVTZ, are used to study the OH − +CH 3 F→CH 3 OH+F − potential energy surface (PES). Structures, vibrational frequencies, and energies are determined for the reactant and product asymptotic limits, the OH⋯CH 3 F ion–dipole potential minimum, the [OH⋯CH 3 ⋯F] − central barrier, and the CH 3 OH⋯F − hydrogen-bonded minimum. This PES does not have a post-reaction F − ⋯CH 3 OH minimum complementary to the pre-reaction OH − ⋯CH 3 F minimum. Except for the CH 3 OH⋯F − minimum, the large basis sets and MP2 theory give a consistent set of structures and frequencies for the stationary points. Neither the structure nor the vibrational frequencies of the CH 3 OH⋯F − minimum are converged by the MP2 and large basis set calculations. RHF theory does not describe the energy of the [OH⋯CH 3 ⋯F] − central barrier nor the reaction exothermicity, however, it does give OH − +CH 3 F→OH − ⋯CH 3 F and F − +CH 3 OH→CH 3 OH⋯F − well depths in good agreement with the CCSD(T) values. Overall good agreement is found between the MP2/6-31+G ∗ and much higher level CCSD(T) energies for the stationary points. The MP2 and CCSD(T) calculations give a reaction exothermicity and F − +CH 3 OH→CH 3 OH⋯F − well depth in good agreement with the experimental values.

Direct determination of trans-resveratrol in human plasma by spectrofluorimetry and second-order standard addition.

Trans-resveratrol (RVT) is an antioxidant found in red grapes and their derivatives, which has been related to the reduction of cardiovascular diseases and cancer incidence. This work developed a new spectrofluorimetric-chemometric method for the direct determination of RVT in human plasma. For each measurement, excitation-emission matrices were obtained from 280 to 360 nm (excitation) and from 380 to 550 nm (emission). The strategy adopted in this work combined data treatment with parallel factor analysis (PARAFAC), for extracting the pure analyte signal, using the standard addition method, which permits determinations in the presence of a strong matrix effect caused by plasma analyte-protein binding. Plasma samples were diluted 10 times and, for each, four standard additions of RVT were performed, in triplicate. A specific PARAFAC model was built for the three replicates of each sample, from three-way arrays formed by five measurements (initial sample plus four additions), 17 excitation wavelengths and 86 emission wavelengths. The best models were selected with four factors and accounted for more than 99.90% of the data variance. The loadings obtained were related to RVT and three interferences. The scores related to the analyte were used for linear regressions and all standard addition curves presented correlation coefficients equal or greater than 0.99. Good results were obtained in the concentration range from 0.10 to 5.00 microg mL(-1), with recoveries between 94.0 and 110.0%. The proposed method was also validated through the estimates of several figures of merit: sensitivity, analytical sensitivity, selectivity, precision, and limits of detection and quantitation.

Análise integrada de sistemas de produção de tomateiro com base em indicadores edafobiológicos

A analise integrada de indicadores edafobiologicos ligados ao manejo do solo constitui uma ferramenta importante para estimar niveis de sustentabilidade do agroecossistema, detectando-se pontos criticos para a devida correcao de manejo. Essa ferramenta foi empregada na avaliacao de sistemas de producao orgânica e convencional de tomate, em cultivo protegido e a campo aberto, no estado de Sao Paulo. Tomaram-se como referencia solos de mata nativa e/ou pastagem natural, dependendo do local de estudo. Em Serra Negra, o solo sob sistema orgânico apresentou maior capacidade de campo e teor de argila dispersa mais baixo, indicativos da estabilidade dos agregados. No sistema convencional observou-se uma elevada condutividade eletrica, evidenciando a alta disponibilidade de sais soluveis. A analise de componentes principais (ACP) permitiu concluir que ha maior grau de similaridade entre o solo sob sistema orgânico e aqueles das bases referenciais, com respeito aos indicadores quimicos e biologicos. Constatou-se que C org, N total, polissacarideos, FDA (hidrolise de diacetato de fluoresceina) e atividade enzimatica de desidrogenase estao positivamente relacionados com o sistema orgânico, a mata nativa e a pastagem. Em contrapartida, a saturacao por bases (V%), pH, teores de Mn, Mg e Ca, bem como a razao de dispersao estao inversamente relacionadas ao manejo orgânico. Ja em Araraquara, os resultados da ACP distinguiram as areas organicamente cultivadas das matas nativas, principalmente, com base nos indicadores biologicos.

PARAFAC: uma ferramenta quimiométrica para tratamento de dados multidimensionais. Aplicações na determinação direta de fármacos em plasma humano por espectrofluorimetria

Since the last decade, the combined use of chemometrics and molecular spectroscopic techniques has become a new alternative for direct drug determination, without the need of physical separation. Among the new methodologies developed, the application of PARAFAC in the decomposition of spectrofluorimetric data should be highlighted. The first objective of this article is to describe the theoretical basis of PARAFAC. For this purpose, a discussion about the order of chemometric methods used in multivariate calibration and the development of multi-dimensional methods is presented first. The other objective of this article is to divulge for the Brazilian chemical community the potential of the combination PARAFAC/spectrofluorimetry for the determination of drugs in complex biological matrices. For this purpose, two applications aiming at determining, respectively, doxorrubicine and salicylate in human plasma are presented.

Ring‐Hydrogen Participation in the Keto‐Enol Isomerization of the Acetophenone Radical Cation

Molecular ions obtained from acetophenone have been observed to undergo proton transfer reactions in competition with unimolecular blackbody dissociation in a Fourier transform ion cyclotron resonance spectrometer provided with an in situ high temperature blackbody source. The ionizing energy dependence of these two processes and generation of the enol molecular ion by fragmentation of butyrophenone reveal that the keto ion undergoes blackbody dissociation exclusively while the enol ion promotes fast proton transfer reactions and undergoes very slow blackbody induced dissociation. Experiments with labeled acetophenone either on the methyl group or on the ring reveal that the enol ions can transfer both H+ and D+ suggesting that the mechanism responsible for the tautomerization process of these radical cations may involve scrambling of the methyl and ring hydrogens, or more than one mechanism. Theoretical calculations at the B3LYP level predict that the most favorable pathway for unimolecular isomerization of the keto ion involves initial migration of an ortho hydrogen to the carbonyl. The subsequent rearrangement to the enol form is calculated to require enough internal energy that would allow hydrogen walk around the benzene ring in agreement with the experimental results. The possibility that isomerization may also occur by a direct 1,3-hydrogen migration is also explored in terms of possible excited electronic states of the ion.

Recent advances in the energetics and mechanisms of gas-phase ionic reactions

Three important aspects are still at the forefront of gas-phase ion chemistry: structure and energetics, mechanisms of gas-phase reactions relevant to condensed phases, and bridging the gap between gas-phase and solution. In this paper, we review some recent results of our laboratories: 1) The use of incoherent infrared radiation to promote multiphoton dissociation of ions is shown to be an efficient and convenient method to determine the energetics of ions and provide structural information of ions. 2) New experimental and theoretical data provide some interesting comparison between nucleophilic reactions at carbon, silicon and germanium centers. In the latter cases, the mechanism involves primarily an attachment-detachment process. 3) The ability to make gas-phase anions attached to neutral molecules provides an interesting approach towards the study of the stability, reactivity, structure and spectroscopy of gas-phase solvated ions.

Determinação espectrofotométrica simultânea de paracetamol e ibuprofeno em formulações farmacêuticas usando calibração multivariada

A simple method was proposed for determination of paracetamol and ibuprofen in tablets, based on UV measurements and partial least squares. The procedure was performed at pH 10.5, in the concentration ranges 3.00-15.00 µg ml-1 (paracetamol) and 2.40-12.00 µg ml-1 (ibuprofen). The model was able to predict paracetamol and ibuprofen in synthetic mixtures with root mean squares errors of prediction of 0.12 and 0.17 µg ml-1, respectively. Figures of merit (sensitivity, limit of detection and precision) were also estimated. The results achieved for the determination of these drugs in pharmaceutical formulations were in agreement with label claims and verified by HPLC.

Determinação simultânea de resíduos de sulfametoxazol e trimetoprima em superfícies de equipamentos de produção

A cleaning validation method was developed and validated, based on swabbing sampling and simultaneous chromatographic determination of sulfamethoxazole (SMX) and trimethoprim (TMP) residues. The method presented limits of detection of 0.06 mg mL-1 for SMX and 0.09 mg mL-1 for TMP. It was considered selective, precise, accurate and robust according to the guidelines from ANVISA, the Brazilian regulatory agency, and International Conference on Harmonization. Mean swab recovery factors of 98.5% for SMX and 97.7% for TMP were obtained for spiked stainless steel plates. The method was successfully applied to the assay of actual swab samples collected from eleven points on an equipment surface.