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[EN] In this work, a method for the preparation and anchoring of polymeric monoliths in a polytetrafluoroethylene (PTFE) tubing as a column housing for microbore HPLC is described. In order to assure a covalent attachment of the monolith to the inner wall of the PTFE tube, a two-step procedure was developed. Two surface etching reagents, a commercial sodium naphthalene solution (Fluoroetch®), or mixtures of H2O2 and H2SO4, were tried and compared. Then, the obtained hydroxyl groups on the PTFE surface were modified by methacryloylation. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM) confirmed the successful modification of the tubing wall and the stable anchorage of monolith to the wall, respectively. Special emphasis was also put on the reduction of the unwanted effects of shrinking of monolith during polymerization, by using an external proper mold and by selecting the adequate monomers in order to increase the flexibility of the polymer. Poly(glycidyl methacrylate-co-divinylbenzene) monoliths were in situ synthesized by thermal polymerization within the confines of surface-vinylized PTFE tubes. The modified PTFE tubing tightly held the monolith, and the monolithic column exhibited good pressure resistance up to 20 MPa. The column performance was also evaluated via the isocratic separation of a series of alkylbenzenes in the reversed-phase mode. The optimized monolithic columns gave plate heights ranged between 70 and 80 um. The resulting monoliths were also satisfactorily applied to the separation of proteins., This work was supported by projects CTQ2014-52765-R (MINECO of Spain and Fondo Europeo de Desarrollo Regional, FEDER) and PROMETEO/2016/145 (Conselleria d'Educacio, Investigacio, Cultura i Esport, Generalitat Valenciana, Spain). The authors also thank Dr. P. Amoros del Toro from Institute of Materials Science (University of Valencia) and Dr. S. Armenta from Department of Analytical Chemistry for their help in surface area and FT-IR measurements, respectively.

A method for the simultaneous determination of the most frequently used surfactant families –linear alkyl benzenesulphonates (LAS), alkyl ether sulphates (AES), fatty alcohol ethoxylates (FAE) and oleins (soaps, fatty acid salts) – in cleaning products, has been developed. The common reversed phase octyl (C8), pentafluorophenyl and biphenyl columns were not capable of separating the anionic LAS and AES classes; however, since only LAS absorbs in the UV, these two classes were independently quantified using a C8 column and serially connected UV and ELSD detection. The best compromise to resolve the four surfactant classes and the oligomers within the classes was achieved with a C8 column and an ACN/water gradient. To enhance retention of the anionic surfactants, ammonium acetate, as an ion-pairing agent compatible with ELSD detection, was used. Also, to shift the olein peaks with respect to that of the FAE oligomers, acetic acid was used. In the optimized method, modulation of the mobile phase, using ammonium acetate during elution of LAS and AES, and acetic acid after elution of LAS and AES, was provided. Quantitation of the overlapped LAS and AES classes was achieved by using the UV detector to quantitate LAS and the ELSD to determine AES by difference. Accuracy in the determination of AES was achieved by using a quadratic model, and by correcting the predicted AES concentration according to the LAS concentration previously established using the UV chromatogram. Another approach also leading to accurate predictions of the AES concentration was to increase the AES concentrations in the samples by adding a standard solution. In the samples reinforced with AES, correction of the predicted AES concentration was not required. FAE and olein were quantified using also quadratic calibration.

An algorithm that calculates the sensitivity to the systematic error of the fitted parameters of a least-squares regression model, with respect to the known parameters, is developed. The algorithm can be applied to mechanistic and empirical models, obtained by linear and non-linear regression, including principal component and partial least-squares. It can be useful in identifying those parameters or calibration regions that can influence other parameters and the response mostly, and thus, whose accuracy should be particularly procured. Other applications are the weighing of experimental points and the comparison of different models and regression methods in terms of its ability of amplifying as little as possible the systematic errors associated to both the known parameters and the selected regions along the independent variables. Both a simulated (a first order kinetics) and a real (two overlapped chromatographic peaks) experiments showed an excellent agreement between the systematic errors of the fitted parameters when calculated by least-squares with respect to those predicted by the proposed algorithm.

A quantitative assay for guar gum in carob gum, based on the extraction of proteins in acetonitrile–water (7:3), separation by capillary electrophoresis and multiple linear regression (MLR) using the areas of nine selected peaks as predictors, was improved by performing the extraction in the presence of gamanase. In the absence of the enzyme, peak migration times and areas depended on the guar content, which complicated peak identification and evaluation. Manual correction of the migration times by comparison with standard electropherograms obtained with pure carob and carob–guar mixtures was required; however, when the proteins were extracted under sonication at 60 °C for 30 min in the presence of gamanase, the migration times and peak areas did not vary with the composition of the carob–guar mixtures, and its reproducibility improved largely. These effects were attributed to the reduction of the viscosity of the extracts and the removal of the galactomannose interactions with the proteins and the capillary walls. Peak identification and evaluation were easily and directly performed on these electropherograms without further processing. An MLR model constructed with 36 carob–guar mixtures containing up to 20% guar, and by measuring the areas of 12 selected peaks and using eight of them as predictors, yielded a detection limit of 2.8% guar ( α = β =0.05 criterion). A model of similar quality was obtained by partial least-squares (PLS) regression.

A simple and rapid HPLC method for the determination of phenolic antioxidants (propyl and octyl gallates, tert -butylhydroquinone and 3- tert -butyl-4-hydroxyanisole) in sunflower, corn and olive oils, margarine, lard and butter oil is described. The samples are diluted with n -propanol, filtered and injected; solutions containing 30% (w/w) sample can be injected. The analytes are separated with a C18 column and a micellar mobile phase containing 0.1 M SDS, 2.5% n -propanol and 10 mM phosphate of pH 3, and detected at 290 nm. Calibration curves are linear ( r > 0.9999) and the limits of detection range from 0.2 to 1.3 ng, which correspond to antioxidant concentrations well below the levels allowed in food. Mean recovery and overall method reproducibility, as relative standard deviation, for all the samples spiked with 200 μg g −1 is (100 ± 2)% ( n = 24).

A dual-beam thermal lens spectrometer (TLS) with a pulse pump–probe coaxial configuration was coupled to commercially available supercritical fluid extraction equipment using a high-pressure flow cell interface. Because of its feasible critical parameters, moderate cost and good thermooptical properties, carbon dioxide was used as supercritical solvent. Using trans -β-carotene as analyte, several factors related with the extraction process under continuos flow conditions were studied in order to establish their influence in the thermal lens signal magnitude. Under the optimum experimental conditions, the relative TLS signal area showed a linear relationship with the concentration of trans -β-carotene from 1.5×10 −6 to 8×10 −8 M ( n =5, r =0.998) in the supercritical phase. Thus, the viability of on-line detection for supercritical fluid extraction with a pulse thermal lens spectrometer was demonstrated.

A simple and quick procedure for analysis of hydrophobic samples by direct injection in a liquid chromatograph, without previous extraction, has been developed. The sample is solved in a water/sodium dodecyl sulphate/n-pentanol microemulsion without destroying the microemulsion structure, and injected. A micellar mobile phase containing 0.1 M SDS, 2.5% n-propanol and 10 mM phosphate of pH 3 is used. The procedure is applied to the determination of synthetic antioxidants (propyl gallate, tert-butylhydroquinone, 2,4,5-trihydroxybutyrophenone, nordihydroguaiaretic acid, octyl gallate, 3-tert-butyl-4-hydroxyanisole and dodecyl gallate) in sunflower, corn and olive oils. Linear calibration curves (r>0.9999) with the same sensitivities are obtained by injecting the antioxidants as micellar solutions and as oil containing microemulsions. Sensitivities are also independent from the contents and nature of the oil in the microemulsion. By injecting 20 μl microemulsion containing 5% oil, limits of detection range from 2.3 to 5.9 ng; this range corresponds to concentrations which are well below the values permitted in the US and European countries. Repeatabilities at a level of 10 μg antioxidant per gram of microemulsion range from 1.1% to 2.5%.

A model for thermal lens spectrometry (TLS) in partial optical saturation conditions which was previously applied to polar solutes in aqueous media is now checked with non-polar organic dyes and metal dithizonates in carbon tetrachloride. Some literature data are also examined. Most data can be consistently explained on the basis of the molecular structures, the sensitivity increasing with the capability of the solute to rapidly communicate the absorbed energy to the solvent molecules.

A comparative study was conducted to investigate the performance of several chemometric methods applied to the treatment of two-way kinetic-spectral data with the aim of resolving mixtures. The methods involved are non-linear least squares regression performed using the Powell algorithm, the linear and extended Kalman filter, and partial least squares regression. Both simulated and experimental data were processed. The coupling reaction of diazotized sulfanilamide with arylamines to give azodyes was monitored spectrophotometrically. Binary mixtures of the substrates with different values of the rate constant ratio and with varied degrees of spectral overlap were resolved. The effects of several influence factors have been studied using numeric simulation. The advantages and the limitations of each method have been evaluated.

In the simple case of the determination of a single parameter, the sensitivity of the signal with respect to the parameter is used to establish a sensitivity weight. It is demonstrated that the sensitivity weight is implicitly used by the least-squares method. The linearization of a model perturbates the way the sensitivity is used by the least-squares method and increases heteroscedasticity which results in an increase of the standard deviation of the evaluated parameter. However, the perturbing effects of linearization can be eliminated by multiplying the points by the reverse of the sensitivity weights of the linearized equation. The single parameter case is used to develop a sensitivity filter for the linear least-squares determination of several parameters. The two-parameter case is studied using simulated experiments. With the sensitivity filter, both the non-linear fitting of the non-linear model and the linear fittings of all the linearized equations lead to the same values of the relative standard deviations of the evaluated parameters.

In thermal lens spectrometry (TLS) the intense pump radiation can lead the chromophore to partial optical saturation conditions in which the ground state is depleted and the population of an intermediate excited state increases. A model in which the excitation process competes with both the decay processes and diffusion and convection of the species in the excited and ground states is developed. The model is used to explain the variations of the TLS/spectrophotometry sensitivity ratios found for a series of phthalein and azo dyes in aqueous media.

Owing to the thermal convection in thermal lens spectrometry the noise increases linearly with the signal. The slope, k, and the intercept of the linear noise versus signal plots depend on the nature of the solvent rather than the absorbing species. Attempts to correlate these parameters with the mechanical and thermo-optical properties of the solvent are made. When the absorbance of a blank solution increases, the limit of detection (LOD) increases proportionally to k. This decreases the differences between the LODs obtained in water and in other solvents of better thermo-optical properties, but with higher values of k.

A model for thermal lens spectrometry (TLS) in partial optical saturation conditions is developed. The model predicts that optical saturation is approached to a significant degree in many TLS experiments, and that it can be reached to a high degree with the currently available laser and optical technologies. When optical saturation is approached, the sensitivity decreases and the signal depends on the rate constant of the slowest step of the decay process. Important consequences for the future development of photothermal spectrometric techniques can be derived.

The following selected aspects of thermal lens spectrometry are briefly outlined: fundamentals, sensitivity and background noise, detection approaches, flowing samples, multi-wavelength experiments, thermal lens circular dichroism, associated colateral phenomena and physico-chemical applications and requirements of analytical procedures. The state-of-the-art and problems encountered are discussed.

A simple and compact dual-laser crossed-beam thermal lens spectrophotometer pumped with a semiconductor diode-array laser, emitting up to 100 mW of power at 818 nm, is described. The pump laser aperture was located at a very small distance from the cell wall and was not focused. The He-Ne probe beam was made to pass near the cell wall, almost grazing the wall inner surface, crossing the pump beam at right-angles with respect to the beam propagation direction, but parallel to the major axes of the narrow elliptical lobes emitted by the array. Owing the particular spatial characteristics of the pump beam, a thermal lens signal with a complex spatial distribution was obtained. Univariant and simplex optimization of the set-up were performed using CuSO 4 and phosphomolybdenum blue solutions. Limits of detection of absorbance were 3.9×10 −4 in CCl 4 , 3.4× −4 in 2-butanol, 1.1×10 −3 in ethyl acetate and 3.0×10 −3 in water.

The concept of the resolutive significance of a point in a data set with regard to a number of addressed parameters is introduced, and two algorithms able to measure it are proposed. The algorithms are validated using simulated experiments. The sum of all the pointwise resolutive significances of a data set is also proposed as a measure of the resolution of the data set. This sum correlates well with the reciprocal of the standard deviation of the fitted parameters, indicating the precision that can be expected for each parameters. Applications in experimental design, and a method for establishing the weights in the least-quarters regression analysis are discussed.

The effects of anionic, non-ionic, and cationic surfactants and thallium(I) nitrate on the paper-substrate room-temperature phosphorescence (RTP) of eleven phenothiazine derivatives are described. Enhancement factors within the range 1.5–10 are observed after addition of sodium dodecylsulfate, dodecyltrimethylammonium chloride and thallium(I) nitrate. The influence of surfactant on phosphor characteristics as well as effects of moisture are discussed. Limits of detection were in the range 0.2–2 ng. Results are also reported for p -aminobenzoic acid, carbazole and o -terphenyl.

The use of fluorescence to study physicochemical structures of alcohol/surfactant/water systems, microemulsions and reversed micelles is reviewed, and the application of these media in analytical fluorescence spectroscopy is discussed. The sodium dodecylsulfate/1-pentanol/heptane/ water system is studied by using pseudo-ternary diagrams. Wide areas of existence of thermodynamically stable and optically clear phases (Winsor IV and two liquid crystals) were found both in the absence and presence of sodium sulfate (0.2 M). The influence of the composition of media on the fluorescence characteristics of pyrene, benzo [e]pyrene, 2-naphthol and p-amino-benzoic acid is studied.