Gabriela A. Ibañez

Sensitivity equation for quantitative analysis with multivariate curve resolution-alternating least-squares: theoretical and experimental approach.

A new equation is derived for estimating the sensitivity when the multivariate curve resolution-alternating least-squares (MCR-ALS) method is applied to second-order multivariate calibration data. The validity of the expression is substantiated by extensive Monte Carlo noise addition simulations. The multivariate selectivity can be derived from the new sensitivity expression. Other important figures of merit, such as limit of detection, limit of quantitation, and concentration uncertainty of MCR-ALS quantitative estimations can be easily estimated from the proposed sensitivity expression and the instrumental noise. An experimental example involving the determination of an analyte in the presence of uncalibrated interfering agents is described in detail, involving second-order time-decaying sensitized lanthanide luminescence excitation spectra. The estimated figures of merit are reasonably correlated with the analytical features of the analyzed experimental system.

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.

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%.

Standard addition analysis of fluoroquinolones in human serum in the presence of the interferent salicylate using lanthanide-sensitized excitation-time decay luminescence data and multivariate curve resolution

Three fluoroquinolone antibiotics (ciprofloxacin, norfloxacin and danofloxacin) have been determined in human serum in the presence of the potential interferent salicylate, by processing lanthanide-sensitized excitation-time decay matrix data for their terbium (III) complexes. The algorithm employed, multivariate curve resolution-alternating least-squares, is one of the few methodologies which permit the achievement of the second-order advantage in the presence of a high degree of overlapping between the time decay profiles for the analyte and the interferent complexes. Furthermore, the presence of analyte-background interactions makes it necessary to employ the standard addition method for successful quantitation. Both simulations and experiments showed that the modified standard addition method was suitable for this purpose, in which the test data matrix was subtracted from the standard addition matrices, and quantitation proceeded using classical external calibration procedure. The analyte concentration ranges were all within the therapeutic range, i.e., 0-6 mgL(-1) in serum, with final concentrations in the measuring cell in the order of 0.2 mgL(-1).

Three-way partial least-squares/residual bilinearization study of second-order lanthanide-sensitized luminescence excitation-time decay data: Analysis of benzoic acid in beverage samples

Lanthanide-sensitized luminescence excitation-time decay matrices were employed for achieving the second-order advantage using as chemometric algorithms parallel factor analysis (PARAFAC) and multidimensional partial least-squares with residual bilinearization (N-PLS/RBL). The second-order data were measured for a calibration set of samples containing the analyte benzoic acid in the concentration range from 0.00 to 5.00 mg L(-1), for a validation set containing the analyte and the potential interferent saccharin (in the range 0.00-6.00 mg L(-1)), and for real samples of beverages containing benzoic acid as preservant, saccharin, and other potentially interfering compounds. All samples were treated with terbium(III), trioctylphosphine oxide as a synergistic ligand, and contained a suitable imidazol buffer, in order to ensure maximum intensity of the luminescence signals. The results indicate a slightly better predictive ability of the newly introduced N-PLS/RBL procedure over standard PARAFAC, both in what concerns the comparison with nominal analyte concentrations in the validation sample set and with results provided by the reference high-performance liquid chromatographic technique for the real sample set.

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.

Second-order advantage from kinetic-spectroscopic data matrices in the presence of extreme spectral overlapping: A multivariate curve resolution—Alternating least-squares approach

Multivariate curve resolution coupled to alternating least-squares (MCR-ALS) has been employed to model kinetic-spectroscopic second-order data, with focus on the achievement of the important second-order advantage, under conditions of extreme spectral overlapping among sample components. A series of simulated examples shows that MCR-ALS can conveniently handle the studied analytical problem unlike other second-order multivariate calibration algorithms, provided matrix augmentation is implemented in the spectral mode instead of in the usual kinetic mode. The approach has also been applied to three experimental examples, which involve the determination of: (1) the antiparkinsonian carbidopa (analyte) in the presence of levodopa as a potential interferent, both reacting with cerium (IV) to produce the fluorescent species cerium (III) with different kinetics; (2) Fe(II) (analyte) in the presence of the interferent Zn(II), both catalyzing the oxidation of methyl orange with potassium bromate; and (3) tartrazine (analyte) in the presence of the interferent brilliant blue, both oxidized with potassium bromate, with the interferent leading to a product with an absorption spectrum very similar to tartrazine. The results indicate good analytical performance towards the analytes, despite the intense spectral overlapping and the presence of unexpected constituents in the test samples.

A novel second-order standard addition analytical method based on data processing with multidimensional partial least-squares and residual bilinearization.

In the presence of analyte-background interactions and a significant background signal, both second-order multivariate calibration and standard addition are required for successful analyte quantitation achieving the second-order advantage. This report discusses a modified second-order standard addition method, in which the test data matrix is subtracted from the standard addition matrices, and quantitation proceeds via the classical external calibration procedure. It is shown that this novel data processing method allows one to apply not only parallel factor analysis (PARAFAC) and multivariate curve resolution-alternating least-squares (MCR-ALS), but also the recently introduced and more flexible partial least-squares (PLS) models coupled to residual bilinearization (RBL). In particular, the multidimensional variant N-PLS/RBL is shown to produce the best analytical results. The comparison is carried out with the aid of a set of simulated data, as well as two experimental data sets: one aimed at the determination of salicylate in human serum in the presence of naproxen as an additional interferent, and the second one devoted to the analysis of danofloxacin in human serum in the presence of salicylate.

Spectroscopic and potentiometric study of aromaticα-hydroxy azo compounds and their copper(II)complexes

Three aromatic α-hydroxy azo dyes, 4-(2-hydroxy-1-phenylazo)benzenesulfonate, 4-(2-hydroxy-1-naphthylazo)benzenesulfonate and 4-(9-hydroxy-10-phenanthrylazo)benzenesulfonate and their copper(II) complexes have been studied in aqueous solution. The existence of both ground- and excited-state proton transfer leading to equilibria between azo-enolic and keto-hydrazonic tautomers in the free ligands has been investigated using 13 C NMR, electronic absorption and fluoresence emission spectroscopies, together with semiempirical MO/CI calculations. The relative stabilities of the tautomers is explained on the basis of the possibility of aromaticity transfer. The formation constants of the Cu II complexes have been evaluated using a combination of potentiometric and spectrophotometric titration data. The relative stabilities of the complexes formed with the studied ligands is also discussed.

Luminescence Sensors Applied to Water Analysis of Organic Pollutants—An Update

The development of chemical sensors for environmental analysis based on fluorescence, phosphorescence and chemiluminescence signals continues to be a dynamic topic within the sensor field. This review covers the fundamentals of this type of sensors, and an update on recent works devoted to quantifying organic pollutants in environmental waters, focusing on advances since about 2005. Among the wide variety of these contaminants, special attention has been paid polycyclic aromatic hydrocarbons, pesticides, explosives and emerging organic pollutants. The potential of coupling optical sensors with multivariate calibration methods in order to improve the selectivity is also discussed.