G.Ramis Ramos

Reduction of Convective Low-Frequency Noise in Thermal Lens Spectrometry

The construction and optimization of a thermal lens spectrometer with coaxial pump/probe configuration is reported. The low-frequency noise of convective origin is studied in detail, and it is demonstrated that a horizontal slit, instead of a pinhole, can be used as a simple way of reducing the noise to a negligible level without losing sensitivity.

Detection of banned drugs in sport by micellar liquid chromatography

Abstract A rapid method for the determination of banned drugs in sport (including stimulants, anabolic steroids and diuretics) based on micellar liquid chromatography is proposed. The mobile phase is an aqueous solution of a surfactant (sodium dodecyl sulphate in this instance) and the stationary phase is octadecylsilica. The retention parameters of the drugs were established and the effect of a mobile phase organic modifier on the chromatographic behaviour of the compounds was studied. Improvements in efficiency were achieved by control of the temperature of the mobile phase. This method allows the direct injection of urine samples for the determination of drugs excreted free in urine: amiphenazole, amiloride, amphetamine, clostebol, ephedrine, phenylpropanolamine, metandienone, methoxyphenamine, nandrolone and spironolactone. Limits of detection in the nanogram range and relative standard deviations of 1–11% were obtained.

Colorimetric determination of arylamines and sulphonamides by diazotization and coupling in a micellar solution

Abstract The use of a micellar solution as a means for improving the colorimetric determination of arylamines by diazotization and coupling is studied. Sodium dodecylsulphate (SDS), Triton X-100 (TX-100) and N -cetylpyridinium chloride (NCPC), together with the diazotizable substances aniline and sulphanilic acid and the coupling agents 1-naphthylamine and N -(1-naphthyl)ethylenediamine are used as model compounds. The protonation constants of the dyes follow the order SDS>H 2 O>TX-100>NCPC. In an SDS micellar solution the coupling rate increases greatly, allowing the rapid formation of the protonated dyes in an acetate buffer. Other advantages are the possibility of analysing very non-polar samples and of using a wider range of diazotizable substances and substrates. A procedure for the determination of 10–100 μg of aniline in an SDS micellar solution is proposed. The procedure is applied to the determination of sulphonamides in pharmaceutical preparations.

High-performance liquid chromatographic determination of diuretics in urine by micellar liquid chromatography

The use of micellar liquid chromatography for the determination of diuretics in urine by direct injection of the sample into the chromatographic system is discussed. The retention of the urine matrix at the beginning of the chromatograms was observed for different sodium dodecyl sulphate (SDS) mobile phases. The eluent strengths of a hybrid SDS—methanol micellar mobile phase for several diuretics were compared and related to the stationary phase/water partition coefficient with a purely micellar mobile phase. The urine band was appreciably narrower with a mobile phase of 0.05 M SDS—5% methanol (v/v) at 50°C (pH 6.9). With this mobile phase the determination of bendroflumethiazide and chlorthalidone was adequate. Acetazolamide, ethacrynic acid, furosemide, hydrochlorothiazide and probenecid were overlapped by the urine matrix, and the retention of amiloride and triamterene was too long.

Formation rates and protonation constants of azo dyes in a sodium dodecylsulphate micellar solution

The effects of a sodium dodecylsulphate micellar solution on the coupling rates of several diazotizated arylamines with N-(1-naphthyl)-ethylenediamine and the shifts in the protonation constants of the corresponding azo dyes are quantitatively studied. Aniline, o-, m- and p-aminobenzoic acids, ethyl p-aminobenzoate and several sulphadrugs are used, and the relationships among the intensity of the effects and the molecular structure of the diazonium ions and the dyes are discussed. A single simplified procedure for the determination of all the substances at pH = 1.3 +/- 0.3, where coupling is quickly completed to directly produce the protonated form of the dyes, is established and applied to the analysis of several pharmaceutical preparations.

Flow-injection spectrophotometric determination of arylamines and sulphonamides by diazotization and coupling in a micellar medium

Abstract In a sodium dodecyl sulphate (SDS) micellar solution, the rate of coupling of a diazonium ion with N-(1- naphthyl)ethylenediamine (NED) increases greatly, the protonation of the resulting azo dyes takes place at higher pH values and the dyes are more soluble. These favourable features were applied to the development of a simple flow-injection spectrophotometric procedure for the determination of diazotizable substances of pharmaceutical interest. Limits of detection in the range 0.2–0.5 μg ml−1 (signal-to-noise ratio=3), with relative standard deviation of 0.7–3% (n=3) for 5 μg ml−1 standards, were obtained.

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.

Thermal Lens Spectrometric Detection of Catecholamines after Oxidation to Aminochromes

Abstract Experimental conditions for the spectrophotometric and thermal lens spectrometry (TLS) detection of catecholamines after oxidation to aminochromes with hexacyanoferrate (III) are optimized. At the low concentrations used in TLS, and in a 0.07 M citrate buffer, catecholamine oxidation can be performed at pH 7 and is immediate, whereas a lower pH value is required in spectrophotometry to avoid aminochrome polymerisation, the oxidation reactions being much slower. Similar TLS sensitivities are obtained for all catecholamines which facilitates HPLC evaluation. Sensitivity can be enhanced using a 50% ethanol-water medium. The linear dynamic range extends over two orders of magnitude, the limit of detection being about 1 ng ml−1 with a reproducibility of 2%, indicating that TLS is adequate to detect catecholamines as aminochromes in physiological samples after HPLC separation. A chromatogram of a urine sample extract is given.

An Automatic System for Thermal Lens Spectrometry and Flowing Samples

Abstract A computer-controlled pulsed-flow system for thermal lens spectrometric detection, 1s developed. Flow pulses are synchronized with the laser pumping and data acquisition periods, measurements being made in static conditions at the end of the stopped-flow half-cycles. A comparative study of the measurement of FIA peaks in continuous and pulsed flow conditions is made. Using the pulsed-flow system, the dependence of sensitivity and noise on the flow and on the presence of turbulences or irregular pump pulses in the flow, is reduced. With a methyl red solution injected into a 1:1 ethanol-water carrier at a flow rate of 2.7 mL min−1, limits of detection and peak area repeativities were 2.1×10−8 M and 6.0%, and 6.2×10−9 M and 4.3%, for the continuous and pulsed flows, respectively. Linear dynamic range extended up to 5×10−1 M.

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.