Quantitation of isomeric ethyl pyridine mixtures by multivariate calibration applied to ion-molecule reaction/collision-induced dissociation triple-stage mass spectra

In reactions of the distonic ion ⁺CH₂OCH₂ with the three isomeric ethyl pyridines, ionized methylene transfer occurs readily yielding distonic N-methylene-ethylpyridinium ions. On-line mass selection and 10 eV collision-induced dissociation (CID) of the CH₂⁺ transfer products yields characteristic fragment ions, which are formed via processes greatly influenced by the ortho, meta or para location of the ethyl substituent in the pyridine ring. Quantitation of mixtures of isomeric 2-, 3-, and 4-ethyl pyridines of varying compositions was then performed by multivariate calibration in the form of the partial least square (PLS) model applied to both single-stage (MS) 70 eV electron ionization (EI) and pentaquadrupole triple-stage sequential ion-molecule reaction/CID product ion mass spectra. The results exemplify the superior ability of combined chemometric analysis and sequential mass spectrometric techniques, which benefits from both characteristic ion chemical reactivity and dissociation behavior, for rapid and accurate quantitation of complex isomeric mixtures.     

Maintenance of a calibration model for near infrared spectrometry by a combined principal component analysis-partial least squares approach

A novel strategy for building and maintaining calibration models has been developed for use when the future boundaries of the sample set are unknown or likely to change. Such a strategy could have an impact on the economics and time required to obtain and maintain a calibration model for routine analysis. The strategy is based on both principal component analysis (PCA) and partial least squares (PLS) multivariate techniques. The principal action of the strategy is to define how “similar” a new sample is to the samples currently defining the calibration dataset. This step is performed by residuals analysis, following PCA. If the new sample is considered to have a spectrum “similar” to previously available spectra, then the model is assumed able to predict the analyte concentration. Conversely, if the new sample is considered “dissimilar”, then there is new information in this sample, which is unknown to the calibration model and the new sample is added automatically to the calibration set in order to improve the model. The strategy has been applied to a real industrial dataset provided by BP Amoco Chemicals. The data consists of spectra of 102 sequential samples of a raw material. The strategy produced an accurate calibration model for both target components starting with only the first four samples, and required a further 17 reference measurements to maintain the model for the whole sampling sequence, which was over a 1-year period.     

Kinetic approach for the enzymatic determination of levodopa and carbidopa assisted by multivariate curve resolution-alternating least squares

A combination of kinetic spectroscopic monitoring and multivariate curve resolution-alternating least squares (MCR-ALS) was proposed for the enzymatic determination of levodopa (LVD) and carbidopa (CBD) in pharmaceuticals. The enzymatic reaction process was carried out in a reverse stopped-flow injection system and monitored by UV-vis spectroscopy. The spectra (292-600 nm) were recorded throughout the reaction and were analyzed by multivariate curve resolution-alternating least squares. A small calibration matrix containing nine mixtures was used in the model construction. Additionally, to evaluate the prediction ability of the model, a set with six validation mixtures was used. The lack of fit obtained was 4.3%, the explained variance 99.8% and the overall prediction error 5.5%. Tablets of commercial samples were analyzed and the results were validated by pharmacopeia method (high performance liquid chromatography). No significant differences were found (α = 0.05) between the reference values and the ones obtained with the proposed method. It is important to note that a unique chemometric model made it possible to determine both analytes simultaneously.     

Simultaneous spectrophotometric determination of ethinylestradiol and levonorgestrel by partial least squares and principal component regression multivariate calibration

Two spectrophotometric methods for the determination of Ethinylestradiol (ETE) and Levonorgestrel (LEV) by using the multivariate calibration technique of partial least square (PLS) and principal component regression (PCR) are presented. In this study the PLS and PCR are successfully applied to quantify both hormones using the information contained in the absorption spectra of appropriate solutions. In order to do this, a calibration set of standard samples composed of different mixtures of both compounds has been designed. The results found by application of the PLS and PCR methods to the simultaneous determination of mixtures, containing 4–11 μg ml⁻¹ of ETE and 2–23 μg ml⁻¹ of LEV, are reported. Five different oral contraceptives were analyzed and the results were very similar to that obtained by a reference liquid Chromatographic method.     

Multivariate calibration applied to the time-resolved chemiluminescence for the simultaneous determination of morphine and its antagonist naloxone

A novel alternative for the simultaneous determination of compounds with similar structure is described, using the whole chemiluminescence-time profiles, acquired by the stopped-flow technique, in combination with mathematical treatments of multivariate calibration. The proposed method is based on the chemiluminescent oxidation of morphine and naloxone by their reaction with potassium permanganate in an acidic medium, using formaldehyde as co-factor. The whole chemiluminescence-time profiles, acquired using the stopped-flow technique in a continuous-flow system, allowed the use of the time-resolved chemiluminescence (CL) data in combination with multivariate calibration techniques, as partial least squares (PLS), for the quantitative determination of both opiate narcotics in binary mixtures.In order to achieve overcoat the additivity of the CL profiles and beside to obtain CL profiles for each drug the most separated as possible in the time, the optimum chemical conditions for the CL emission were investigated. The effect of common emission enhancers on the CL emission obtained in the oxidation reaction of these compounds in different acidic media was studied. The parameters selected were sulphuric acid 1.0 mol L⁻¹, permanganate 0.2 mmol L⁻¹ and formaldehyde 0.8 mol L⁻¹. A calibration set of standard samples was designed by combination of a factorial design, with three levels for each factor and a central composite design. Finally, with the aim of validating the chemometric proposed method, a prediction set of binary samples was prepared. Using the multivariate calibration method proposed, the analytes were determined in synthetic samples, obtaining recoveries of 97-109%.     

Effect of variation in argon content of calibration gases on determination of atmospheric carbon dioxide

Carbon dioxide (CO₂) is a greenhouse gas that makes by far the largest contribution to the global warming of the Earth's atmosphere. For the measurements of atmospheric CO₂ a non-dispersive infrared analyzer (NDIR) and gas chromatography are conventionally being used. We explored whether and to what degree argon content can influence the determination of atmospheric CO₂ using the comparison of CO₂ concentrations between the sample gas mixtures with varying Ar amounts at 0 and 18.6 mmol mol⁻¹ and the calibration gas mixtures with Ar at 8.4, 9.1, and 9.3 mmol mol⁻¹. We newly discovered that variation of Ar content in calibration gas mixtures could undermine accuracy for precise and accurate determination of atmospheric CO₂ in background air. The differences in CO₂ concentration due to the variation of Ar content in the calibration gas mixtures were negligible (<±0.03 μmol mol⁻¹) for NDIR systems whereas they noticeably increased (<±1.09 μmol mol⁻¹) especially for the modified GC systems to enhance instrumental sensitivity. We found that the thermal mass flow controller is the main source of the differences although such differences appeared only in the presence of a flow restrictor in GC systems. For reliable monitoring of real atmospheric CO₂ samples, one should use calibration gas mixtures that contain Ar content close to the level (9.332 mmol mol⁻¹) in the ambient air as possible. Practical guidelines were highlighted relating to selection of appropriate analytical approaches for the accurate and precise measurements of atmospheric CO₂. In addition, theoretical implications from the findings were addressed.     

Vibrational relaxation of CO2 laser excited SiF4 gas in binary mixtures

Vibrational relaxation rates for SiF4 in binary mixtures with CO₂, CH₄, O₂, N₂, He and Ar are determined using CO₂ laser-induced visible fluorescence. A model for the process is discussed.     

Spectrophotometric simultaneous determination of ceratine, creatinine, and uric acid in real samples by orthogonal signal correction–partial least squares regression

Abstract  This work describes a quantitative spectroscopic method for the analysis of ternary mixtures of ceratine (CER), creatinine (CRE), and uric acid (UA) using multivariate data models based upon ultraviolet spectroscopy. By multivariate calibration methods, such as partial least squares regression, it is possible to obtain a model adjusted to the concentration values of the mixtures used in the calibration range. In this study, the calibration model is based on absorption spectra in the 200–260 nm range for 36 different mixtures of CER, CRE, and UA. The unrelated information was removed by the orthogonal signal correction (OSC) method and the results were proved. Evaluation of the prediction errors for the prediction set reveals the OSC-treated data give substantially lower root mean square error of prediction (RMSEP) values than original data. The RMSEP for CER, CRE, and UA with OSC were 1.1686, 0.2195, and 0.3726, and without OSC were 1.9057, 0.3482, and 0.6164, respectively. This procedure allows the simultaneous determination of CER, CRE, and UA in synthetic and real samples. Graphical abstract        

Synthesis of [?CH2C(CO2Et)2CH2Ar?]n polymers and their unique optical properties by through‐space interactions between Ar and CO groups

Polymers of type [? CH₂C(CO₂Et)₂CH₂Ar? ]_n (Ar = 1,4‐phenylene, 2,6‐naphthylene, 9,10‐anthrylene, or 1,4‐phenylene‐ethynylene‐1,4‐phenylene) were synthesized by alkylation of diethyl malonate with XCH₂ArCH₂X (X = Cl or Br). These polymers exhibited unexpectedly enhanced UV absorption and strong, broad, bathochromically shifted fluorescence spectra compared with the parent Ar compounds. The origin of these photophysical characteristics was postulated to be a configuration interaction between the π→π* excitation of the aromatic moiety and the n→π* excitation of the carbonyl moiety on the trimethylene tether via intramolecular charge transfer. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Simultaneous Spectrophotometric Determination of Benzyl Alcohol and Diclofenac in Pharmaceutical Formulations by Chemometrics Method

Diclofenac sodium (DS) is a drug with analgesic, antipyretic, and anti‐inflammatory properties. It is present in numerous pharmaceutical preparations. In injectable forms, it is usually accompanied by benzyl alcohol (BA) as an excipient, which is used as a blocking anesthetic (4%) and an antiseptic (4–10%). In this work a spectrophotometric methodology was applied in order to determine benzyl alcohol and diclofenac in injectable formulations by applying a multivariate calibration method. By a multivariate calibration method such as partial least squares (PLS), it is possible to obtain a model adjusted to the concentration values of the mixtures used in the calibration range. In this study, the concentration model is based on absorption spectra in the 230–320 nm range for 25 different mixtures of benzyl alcohol and diclofenac. Calibration matrix contains 10–95 and 1–50 μg mL^?1 for benzyl alcohol and diclofenac, respectively. The root mean square errors of prediction (RMSEP) for benzyl alcohol and diclofenac were 3.0776 and 1.7557, respectively. The proposed method was validated by using a set of synthetic sample mixtures and subsequently applied to simultaneous determination of benzyl alcohol and diclofenac in two different pharmaceutical formulations.     

Gas transport in polybutadiene treated with aqueous bromine

Diffusion, solubility, and permebility coefficients were measured for He, Co₂, Ar, and CH₄ in polybutadiene (PB) and in polybutadiene reacted in the solid state to various extents with aqueous bromine. Analysis of the sorption curves and X-ray emission spectra showed that the bromination created a heterogeneous membrane with an outer brominated skin and an unreacted core. At relatively low extent of bromination, the diffusion and permeability coefficients for CO₂, Ar, and CH₄ decreased by two orders of magnitude, while the transport coefficients for He were virtually unchanged. The permeability coefficients for CO₂, Ar, and CH₄ became immeasurably small after about 3% bromination. The ideal separation factor for gas pairs with different molecular size increased with bromination, suggesting applications in gas separation processes. © 1993 John Wiley & Sons, Inc.     

Hydrogen Reduction of MoF6 and Molybdenum Carbide Formation in RF Inductively Coupled Low-Pressure Discharge: Experiment and Equilibrium Thermodynamics Consideration

The physical plasma parameters, temperature and electron number density, are studied in the RF-IC (RF inductively coupled) discharge at a reduced pressure of 3 Torr in mixtures of MoF₆ with Ar, H₂ and CH₄. The emission spectra of mixtures are investigated. It is shown that in the presence of argon, the concentration of free electrons in plasma and dissociation rate of MoF₆ increase. A main role of molecular hydrogen is the generation of atomic hydrogen that binds atomic fluorine and leads to the formation of gaseous and solid products. Exhaust gas mixtures exiting the reactor are analyzed by mass spectrometry. It is shown that for all cases, the conversion of MoF₆ into reaction products is close to 100%. A thermodynamic analysis of the equilibrium composition of MoF₆ systems with Ar, H₂ and CH₄ was carried out and the obtained results are in good agreement with experimentally observed composition of the solid and gas phases. Analysis of solid deposits from mixture MoF₆/H₂/Ar revealed the presence of molybdenum powder and large amount of amorphous MoF_x. The deposit obtained from mixtures with methane, MoF₆/H₂/Ar/CH₄, contained crystalline molybdenum carbide, Mo₃C₂.

     

An ensemble of Monte Carlo uninformative variable elimination for wavelength selection

An improved method based on an ensemble of Monte Carlo uninformative variable elimination (EMCUVE) is presented for wavelength selection in multivariate calibration of spectral data. The proposed algorithm introduces Monte Carlo (MC) strategy to uninformative variable elimination-PLS (UVE-PLS) instead of leave-one-out strategy for estimating the contributions of each wavelength variable in the PLS model. In EMCUVE wavelength variables are evaluated by different Monte Carlo uninformative variable elimination (MCUVE) models. Moreover, a fusion of MCUVE and the vote rule can obtain an improvement over the original uninformative variable elimination method. Results obtained from simulated data and real data sets demonstrate that EMCUVE can properly carry out wavelength selection in the course of data analysis and improve predictive ability for multivariate calibration model.     

Analysis of amino acids in complex samples by using voltammetry and multivariate calibration methods

A voltammetric method is proposed for the simultaneous determination of tryptophan, cysteine, and tyrosine using multivariate calibration techniques. Various electrodes and voltammetric techniques were explored to ascertain the optimum measurement strategy. Among them, differential pulse voltammetry (DPV) with a Pt electrode was selected as analytical technique since it provided a suitable compromise between sensitivity and reproducibility while allowing the oxidation peaks of the three compounds to be reasonably discriminated. The sensitivity of DPV with Pt electrode for Trp standards was 8.4×10⁻² A l mol⁻¹, the repeatability 3.7% and the detection limit below 10⁻⁷ M. The lack of full selectivity of the voltammetric data was overcome using multivariate calibration methods on the basis of the differences in the voltammetric waves of each compound. The accuracy of predictions was evaluated preliminarily from the analysis of three-component synthetic mixtures. Subsequently, this method was applied to the analysis of oxidizable amino acids in feed samples. Results obtained were in good concordance with those given by the standard method using an amino acid analyzer.     

Energy transfer pathways in gaseous systems containing CO2 elucidated by thermal lens studies

Pulsed source thermal lens measurement on CO₂ and CO₂/Ar, CO₂/N₂, CO₂/CO gaseous mixtures are reported. A theory relating the thermal lens signal to the V-V and V-T energy transfer processes in the mixtures has been developed and applied to interpret the measurements. The occurrence of convergent and divergent thermal lens signals related to endothermic and exothermic effects in the gas have been quantitatively explained.     

Partial least squares-based multivariate spectral calibration method for simultaneous determination of beta-carboline derivatives in Peganum harmala seed extracts

The partial least squares regression method has been applied for simultaneous spectrophotometric determination of harmine, harmane, harmalol and harmaline in Peganum harmala L. (Zygophyllaceae) seeds. The effect of pH was optimized employing multivariate definition of selectivity and sensitivity and best results were obtained in basic media (pH > 9). The calibration models were optimized for number of latent variables by the cross-validation procedure. Determinations were made over the concentration range of 0.15-10 μg mL⁻¹. The proposed method was validated by applying it to the analysis of the β-carbolines in synthetic quaternary mixtures of media at pH 9 and 11. The relative standard errors of prediction were less than 4% in most cases. Analysis of P. harmala seeds by the proposed models for contents of the β-carboline derivatives resulted in 1.84%, 0.16%, 0.25% and 3.90% for harmine, harmane, harmaline and harmalol, respectively. The results were validated against an existing HPLC method and it no significant differences were observed between the results of two methods.     

Vapour—liquid equilibrium at elevated pressures from the perturbation theory. II. Binary systems

Vapour—liquid equilibria in binary systems of non-polar non-spherical molecule compounds were studied theoretically by combining the perturbation theory of convex molecule fluids with a new variant of the conformal solution theory. The recently proposed equation of state of hard convex body mixtures and the corresponding expressions for the contact values of distribution functions were employed to determine the reference thermodynamic functions and the perturbation terms. Ten binary systems, i.e. ArCH₄, N₂CH₄, N₂C₂H₄, N₂C₂H₆, CH₄C₂H₄, CH₄C₂H₆, C₂H₄C₂H₆, CO₂C₂H₄, CO₂C₂H₆, and ArCO₂ were studied at constant temperatures. Comparison of theoretical predictions with experimental data is given.     

Comparison of partial least squares regression and multi-layer neural networks for quantification of nonlinear systems and application to gas phase Fourier transform infrared spectra

The performance of back-propagation artificial neural networks (NN) and partial least squares (PLS) regression for the calibration of linear and nonlinear systems has been investigated by using six types of synthetic data. Three PLS methods, conventional linear-PLS and two nonlinear-PLS methods, have been used in the study. In all but one of the synthetic data types, the band intensities varied nonlinearly with concentration. These five data types were designed to represent the effect of band shifts with increasing concentration, a nonlinear relationship between peak height and concentration, or a combination of both types of nonlinearities. The results showed that NNs perform better than PLS for all the nonlinear datasets. When a band shift is the major reason for the nonlinearity, the relative performance of NNs and PLS depends on the overlap of the absorption bands. If there is no band overlap, neither NN nor PLS can calibrate the data accurately but the results could be improved by convolving the spectral features with a Gaussian broadening function. The results indicate that a combination of peak position shift and peak height change is the most difficult nonlinearity to calibrate. NN and PLS were also used to determine the concentration of CHCl₃ in pure component and mixtures of CHCl₃ and CH₂Cl₂ using their Fourier transform infrared (FT-IR) spectra, a dataset that has been proved nonlinear in high concentrations due to the nonlinear response of the detector. The best results for the experimental data were obtained by applying one hidden layer NN to the mean-centered absorbance spectra.     

The gas phase “matrix isolation” spectroscopy of CH3F

We report infrared photodissociation spectra for Ne, Ar, Kr, N₂ and CH₄ clusters which contain CH₃F chromophores. The CH₃Fv ₃ mode is excited with a line tunable CO₂ laser. Mass spectrometer detection of changes in the cluster beam intensity serve to partially distinguish the spectra of different size neutral clusters. Many spectra consist of rather broad, inhomogeneous profiles. For intermediate size Ar_nCH₃F clusters a sharp, narrow peak is observed in the spectrum. We assign this peak as due to a cluster in which a central CH₃F molecule is surrounded by at least a full shell of Ar atoms packed in a contracted icosohedral geometry. Because the Ar atoms in a gas phase cluster are unconstrained by an extended crystalline structure, the CH₃F dipole is more fully stabilized (and thus red-shifted) than in a solid matrix. The dependence of the observed spectrum on cluster size is discussed. For comparison, no comparable narrow spectral features are observed in Ar_nC₂H₄ cluster spectra. Clear evidence is also presented that the fragmentation of the neutral clusters upon electron impact ionization is fairly specific. Finally, we note that ionization of Ar_nCH₃F clusters sometimes produces Ar_nF⁺ ions. This is a fragmentation process which does not occur in free CH₃F.