Graciela M. Escandar

Second- and higher-order data generation and calibration: a tutorial.

An introduction to multi-way calibration based on second- and higher-order data generation and processing is provided, with emphasis on practical experimental aspects. After a discussion concerning a proper nomenclature scheme, a suitable classification of the obtainable data, and the general features of the available algorithms and their underlying models, a series of examples is discussed in detail, with the purpose of illustrating the great potentiality of the field for the analytical community. Emphasis is directed toward the most popular multi-way data, i.e., second-order or matrix data, which can be conveniently measured in a variety of instruments. Third-order data are being increasingly studied and are also discussed, along with the less explored field of fourth-order data. The estimation of figures of merit, which analysts need to report during method development, is now sufficiently mature to be provided for the general audience.

Spectrofluorimetric determination of piroxicam in the presence and absence of β-cyclodextrin

The complexation between beta-cyclodextrin (beta-CD) and piroxicam (PX) was investigated by both fluorescence and absorption spectrometry. A 1:2 guest:host stoichiometry for the complex was established, and its association constant was calculated by applying a non-linear regression method to the changes brought about by the presence of beta-CD in both the fluorescence and absorbance spectra of PX. During the study of the influence of the pH on the fluorescence emission of the complex, an efficient enhancement of the signals at acidic pH was observed. This suggests that the protonated form of PX is included more effectively than the ionized form in the beta-CD cavity. Based on the results obtained, spectrofluorimetric methods for the determination of PX were developed. The best limits of detection and quantification were obtained using beta-CD at an acidic pH. The dynamic range in this latter case was 0.02-1 microgram ml-1. The method was applied satisfactorily to the determination of piroxicam in a pharmaceutical preparation.

Spectrofluorimetric determination of diclofenac in the presence of α-cyclodextrin

Abstract This study focuses on the complex formed between α-cyclodextrin (CD) and the anti-inflammatory drug diclofenac in aqueous solution and also on its potential analytical applications. It was corroborated that the fluorescence emission band of diclofenac is significantly intensified in the presence of α-CD. From the changes in the fluorescence spectra, it was concluded that α-CD forms a 1:1 inclusional complex with diclofenac and its equilibrium constant was calculated to be 1.20(3)×103 M−1. With the purpose of characterizing the inclusion complex, the acid–base behaviour of diclofenac in both the presence and absence of α-CD was spectrophotometrically investigated. From the results obtained, it was inferred that both the carboxyl and the secondary amino groups of the guest molecule remain outside the cyclodextrin cavity. Further details on the complex structure was obtained by 1H NMR measurements and semiempirical calculations. In addition to the analysis of the α-CD–diclofenac interaction, a new approach for the quantification of diclofenac in the presence of α-CD is described in the range 0–5 μg ml−1. An application of the method to the determination of the studied drug in pharmaceutical preparations is shown.

Complexation study of diclofenac with β-cyclodextrin and spectrofluorimetric determination

The inclusional complexation between the anti-inflammatory pharmaceutical diclofenac and beta-cyclodextrin (beta-CD) was studied by potentiometry, spectrophotometry and spectrofluorimetry, in both acid and neutral pH. Guest-host 1:1 stoichiometries for the complexes in both media were determined, and their equilibrium constants were calculated. The values obtained from the different methods used are in very good agreement and are in the order of 10(3). From the analysis of the pKa value for diclofenac in both the absence and presence of beta-CD (4.84 and 4.90 respectively), it was inferred that in the inclusion complex the carboxylic group is located outside the cavity. Further structural characterization of the inclusate was carried out by means of 1H NMR spectra and AM1 semiempirical calculations. Based on the obtained results, a spectrofluorimetric method for the determination of diclofenac in the presence of beta-CD was developed in the range of 0-5 micrograms ml-1. Better limits of detection (0.03 microgram ml-1) and quantification (0.1 microgram ml-1) were obtained in this latter case with respect to those obtained in the absence of beta-CD. The method was satisfactorily applied to the quantification of diclofenac in pharmaceutical preparations.

Chemometrics-Assisted Excitation−Emission Fluorescence Spectroscopy on Nylon Membranes. Simultaneous Determination of Benzo[a]pyrene and Dibenz[a,h]anthracene at Parts-Per-Trillion Levels in the Presence of the Remaining EPA PAH Priority Pollutants As Interferences

This work presents a novel approach for the simultaneous ultratrace determination of benzo[ a]pyrene and dibenzo[ a,h]anthracene, the two most carcinogenic polycyclic aromatic hydrocarbons (PAHs), in a very interfering environment, combining the recently discovered ability of the nylon membrane to strongly retain and concentrate PAHs on its surface, the sensitivity of molecular fluorescence, and the selectivity of second-order chemometric algorithms. The fluorescence excitation-emission matrices, directly measured on a nylon-membrane surface, are processed by applying parallel factor analysis (PARAFAC) and unfolded partial least-squares coupled to residual bilinearization (U-PLS/RBL). The superiority of U-PLS/RBL to quantify BaP and DBA at concentrations below 10 ng L (-1) in the presence of the remaining 14 US EPA (United States Environmental Protection Agency) PAHs at total concentrations ranging from 1400 and 14,000 ng L (-1) is demonstrated. The present method successfully faces this complex challenge without using organic solvents, which are to known produce environmental contamination. Finally, the high sensitivity of the present method avoids preconcentration and elution steps, considerably decreasing the analysis time and the experimental errors. Because the instrumental involved in the determination is nonsophisticated, the experiments could be carried out in routine laboratories.

A review on second- and third-order multivariate calibration applied to chromatographic data

Quantitative analytical works developed by processing second- and third-order chromatographic data are reviewed. The various modes in which data of complex structure can be measured are discussed, with chromatographic separation providing either one or two of the data dimensions. This produces second-order data (matrices from uni-dimensional chromatography with multivariate detection or from two-dimensional chromatography) or third-order data (three-dimensional data arrays from two-dimensional chromatography with multivariate detection). The available algorithms for processing these data are classified and discussed, regarding their ability to cope with the ubiquitous phenomenon of retention time shifts from run to run. A summary of relevant works applying this combination of techniques is presented, with focus on quantitative analytical results. Special attention is paid to works achieving the full potentiality of the multidimensional data, i.e., the second-order advantage.

Non-Trilinear Chromatographic Time Retention−Fluorescence Emission Data Coupled to Chemometric Algorithms for the Simultaneous Determination of 10 Polycyclic Aromatic Hydrocarbons in the Presence of Interferences

Multivariate calibration coupled to high-performance liquid chromatography-fast scanning fluorescence spectroscopy (HPLC-FSFS) was employed for the analysis of 10 selected polycyclic aromatic hydrocarbons (PAHs), six of which correspond to heavy PAHs. The goal of the present study was the successful resolution of a system even in the presence of real interferences. Second-order HPLC-FSFS data matrices were obtained in a short time with a chromatographic system operating in isocratic mode. The difficulties in aligning chromatographic bands in complex systems, such as the ones presented here, are discussed. Two second-order calibration algorithms which do not require chromatographic alignment were selected for data processing, namely, multivariate curve resolution-alternating least-squares (MCR-ALS) and parallel factor analysis 2 (PARAFAC2). These algorithms did also achieve the second-order advantage, and therefore they were able to overcome the problem of the presence of unexpected interferences. The study was employed for the discussion of the scopes of the applied second-order chemometric tools, demonstrating the superiority of MCR-ALS to successfully resolve this complex system. The quality of the proposed techniques was assessed on the basis of the analytical recoveries from different types of water and olive oil samples after solid-phase extraction. The studied concentration ranges in water samples were 5.6 x 10(-3)-0.20 ng mL(-1) for heavy PAHs and 0.036-0.80 ng mL(-1) for light PAHs, while in oil samples the PAHs concentrations were 0.13-9.6 and 2.3-49.5 ng mL(-1) for heavy and light PAHS, respectively. All real samples were analyzed in the presence of the studied interferences.

Room-temperature phosphorescence of acenaphthene in aerated solutions in the presence of bromoalcohols and γ-cyclodextrin

Abstract The inclusional complexation between γ-cyclodextrin and acenaphthene in aqueous solution was investigated by spectroscopic techniques. A 1:1 complex was detected and its formation constant was determined. After adding bromoalcohols such as 2,3-dibromopropane-1-ol or 2-bromoethanol both a decrease of the fluorescence and an enhancement of the room-temperature phosphorescence (RTP) of acenaphthene were observed. The RTP signals from acenaphthene included in γ-cyclodextrin were optimized. The apparent formation constant of the ternary γ-cyclodextrin-acenaphthene-2,3-dibromopropane-1-ol complex was determined from RTP data. On the other hand, the inclusion complex formed between γ-cyclodextrin, acenaphthene and 2-bromoethanol was found to be very labile and its equilibrium constant could not be determined with accuracy. Large RTP signals were obtained in both ternary systems, even in the absence of sodium sulfite as deoxygenant agent. This fact would indicate that in these complexes the acenaphthene is protected from the action of quenchers such oxygen.

Second-order multivariate calibration procedures applied to high-performance liquid chromatography coupled to fast-scanning fluorescence detection for the determination of fluoroquinolones

Different second-order multivariate calibration algorithms, namely parallel factor analysis (PARAFAC), N-dimensional partial least-squares (N-PLS) and multivariate curve resolution-alternating least-squares (MCR-ALS) have been compared for the analysis of four fluoroquinolones in aqueous solutions, including some human urine samples (additional four fluoroquinolones were simultaneously determined by univariate calibration). Data were measured in a short time with a chromatographic system operating in the isocratic mode. The detection system consisted of a fast-scanning spectrofluorimeter, which allows one to obtain second-order data matrices containing the fluorescence intensity as a function of retention time and emission wavelength. The developed approach enabled us to determine eight analytes, some of them with overlapped profiles, without the necessity of applying an elution gradient, and thus significantly reducing both the experimental time and complexity. The study was employed for the discussion of the scopes of the applied second-order chemometric tools. The quality of the proposed technique coupled to each of the evaluated algorithms was assessed on the basis of the figures of merit for the determination of fluoroquinolones in the analyzed water and urine samples. Univariate calibration of four analytes led to limits of detection in the range 20-40 ng mL(-1) and root mean square errors for the validation samples in the range 30-60 ng mL(-1) (corresponding to relative prediction errors of 3-8%). The ranges for second-order multivariate calibration (using PARAFAC and N-PLS) of the remaining four analytes were: limit of detection, 2-8 ng mL(-1), root mean square errors, 3-50 ng mL(-1) and relative prediction errors, 1-5%.

Complexation of cobalt(II), nickel(II), and zinc(II) ions with mono and binucleating azo compounds: a potentiometric and spectroscopic study in aqueous solution

Abstract The interaction of Co(II), Ni(II) and Zn(II) ions with a series of aromatic α-hydroxy azo compounds, 1,4-bis-p-sulfonylazo-2,3-dihydroxy naphthalene, 4-(2-hydroxy-1-phenylazo)-benzenesulfonate, 4-(2-hydroxy-1-naphthylazo)-benzenesulfonate, and 4-(9-hydroxy-10-phenanthrylazo)-benzenesulfonate, has been studied in aqueous solution. While both mono and binuclear chelates were found in the complex systems formed by the first azo compound, the existence of mononuclear chelates with different degrees of protonation was verified in the systems formed by the other ones. The equilibrium constants for all metal complexes were evaluated using a combination of potentiometric and spectrophotometric titration data. Within the mononucleating ligands, the magnitudes of the complexes equilibrium constants were to be found similar, even when their aromaticities are different. Both the metal coordination sites and the tautomeric structures of the ligand in each complex have been inferred through spectroscopic analysis.