J.J. Baeza

Modelling of the elution behaviour in hybrid micellar eluents with different organic modifiers

Abstract The elution behaviour in reversed-phase liquid chromatography (RPLC), with mobile phases containing micelles, can be described according to a distribution pattern among three phases: bulk water, micellar pseudophase and stationary phase. Several mechanistic equations based on this pattern, previously used to predict the retention in hybrid mobile phases of sodium dodecyl sulphate and propanol, were checked with several organic modifiers and a group of solutes of diverse polarity. The modifiers were the alcohols 1-propanol, 1-butanol and 1-pentanol, and other organic solvents widely used in conventional RPLC such as acetonitrile and tetrahydrofuran. New models were also proposed to take into account diverse interactions of the modifiers with the micelles and the stationary phase. In spite of the differences in hydrophobicity of the three alcohols studied, the behaviour of the probe solutes in their presence was optimally described with the same equation, with relative mean fitting errors below 2%. The inclusion of the distribution of 1-butanol and 1-pentanol between bulk water and micelles in the equations produced a further increase in the accuracy. This distribution also explained the small deviations from linearity observed in the plots of 1/ k vs. micellar concentration, in the presence of pentanol. The more complex behaviour of the probe solutes in micellar eluents containing acetonitrile and tetrahydrofuran required a model with a larger number of parameters to obtain errors below 3.5%.

Comparative study of several programs used in the potentiometric evaluation of equilibrium constants including an error sensitivity analysis

Abstract The programs MINIQUAD, MINIPOT, SUPERQUAD and PHCONST are applied to the evaluation of the protonation constants of several hypothetical polyprotic substances using simulated titrations, and the precision and accuracy of the results are discussed and compared. When statistical weights are used the results are very similar, the accuracy being better with PHCONST. Error sensitivity analysis was used as a means of establishing the influence of a systematic error of an experimental variable on the accuracy of the determination. An algorithm for the accurate calculation of error sensitivities is proposed and checked. Error sensitivities can be used to make a choice between the various minimization functions or to decide if a variable must be included in the optimization process.

A model for optical saturation thermal lens spectrometry

Abstract 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.

Multi-component analysis using OPKINE, a program for the non-linear treatment of kinetic problems

The performance of OPKINE, a program for the non-linear treatment of complex kinetic problems, was studied in connection with its possible analytical applications. The Runge-Kutta-Gill integration method, a random search Monte Carlo procedure and the simplex, Davidon-Fletcher-Powell and Gauss-Newton-Marquardt optimisation methods, in their original and modified versions, are included in OPKINE. Up to 20 kinetic curves, with up to 400 points each, can be treated to evaluate up to 40 parameters. The ability of OPKINE to deal with analytical problems was checked by using simulated experiments. The analysis of complex mixtures, with similar values of the rate constants and large differences in the contributions of the analytes to the monitored signal, and in the presence of side-reactions, can be performed with good accuracy and precision.

Determination of clenbuterol in urine by azo-dye precolumn derivatization and micellar liquid chromatography

SummaryClenbuterol has been determined in urine by solidphase extraction on a C18 cartridge, diazotization of the eluate with nitrite, coupling of the diazonium ion with 1-(naphthyl)ethylenediamine, and separation of the azo dye formed by HPLC with a C18 column and a micellar mobile phase containing 0.1 M sodium dodecyl sulphate, 12%n-butanol and 0.05 M citrate buffer, pH 3. Recoveries higher than 90% were obtained by mixing the samples with a 20% 0.2 M NaOH before extraction. Limits of detection of 51 and 6.7 ng L−1 were obtained with spectrophotometric and thermal lens spectrometric detection, respectively; respective repeatabilities were 3.1% (5 μg mL−1) and 5.6% (0.16 μg mL−1).

Optical saturation, diffusion and convection effects in thermal lens spectrometry

Abstract 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.

Information content of data and variables and types of weighting in least-squares regression methods

Abstract Algorithms are given for evaluating the relative amount of useful information related to a particular parameter which is carried by individual data points and intervals of the variables. The algorithms provide an efficient means of using the information contained in a set of data. Applications to the optimization of weighting in regression methods are described. Several informational and combined informational-statistical types of weighting are studied as a means of improving the accuracy and precision of the parameters obtained by non-linear regression.

Quality control of pharmaceuticals containing clenbuterol by thermal lens spectrometry

An ultrasensitive absorptiometric procedure for the determination of clenbuterol in pharmaceutical preparations was developed. Clenbuterol was diazotized with nitrite and coupled with 1-(naphthyl)ethylenediamine, and the absorbance of the azo dye formed was measured by both spectrophotometry and ultrasensitive thermal lens spectrometry (TLS). The TLS limit of detection was 1.5 ppb, 14-fold lower than with a Hewlett-Packard diode array spectrophotometer. Thus, the TLS procedure can be advantageously applied to quality control of clenbuterol at the individual dose level and in small samples. Repeatability as relative standard deviation was 1.5% (50 ppb, n = 6).

A series expansion of the extended Debye-Hückel equation and application to linear prediction of stability constants.

The Debye-Hückel semiempirical extended equation is frequently used to calculate activity coefficients of chemical species and equilibrium constants at ionic strengths different from those used in their experimental evaluation. A series expansion of the extended Debye-Hückel equation is proposed here and checked with experimental data taken from the literature. The expansion is linear in the ionic parameters and yields a geometrical series which converges rapidly and that enables the accurate calculation of interpolated and extrapolated activity coefficients and equilibrium constants by simple and multiple linear regression without previous knowledge of the ionic parameters.