R. M. Villanueva Camañas

Formation and instability of o-phthalaldehyde derivatives of amino acids

o-Phthalaldehyde reacts with amino acids in the presence of a thiol to give highly fluorescent 1-alkyl-thio-2-alkyl-substituted isoindoles. However, the instability of the derivatives limits the general utility of the reaction. Mechanistic descriptions of isoindole formation and degradation proposed in the last years, which permit a better understanding of the factors affecting isoindole stability, are presented. The use of alternative thiols and o-phthalaldehyde-like reagents is also reviewed.

Studies on the formation and stability of isoindoles derived from amino acids, o-phthalaldehyde and N-acetyl-L-cysteine

Abstract A kinetic-spectrophotometric study is performed on the formation and degradation of the isoindole derivatives of amino acids with o -phthalaldehyde and N -ace-tyl- l -cysteine. The experimental and structural factors which affect the formation and stability of the compounds are considered, and the results are compared with those obtained for mercaptoethanol. N -Acetyl- l -cysteine derivatives are highly stable, not requiring a strict control of the time of reaction as in the case of mercaptoethanol.

Quantitative retention—structure and retention—activity relationship studies of ionic and non-ionic catecholamines by micellar liquid chromatography

SummaryWhen ionic surfactants are used as mobile phases in micellar liquid chromatography, MLC, the retention of compounds is governed by hydrophobic and electrostatic forces. In the absence of electrostatic effects, the hydrophobicity of a compound is the predominant factor affecting its retention and its interaction with micelles. Because both interactions should be considered for ionic compounds, a novel retention model is proposed which includes the hydrophobicity of a compound and the molar fraction of its charged form. High correlations between the logarithm of the capacity factors and structural parameters were obtained for ionic compounds with different degrees of ionization. The effect of the nature and composition of the mobile phase (pH, concentration of surfactant and modifier) was studied. The modelling of the retention of compounds as a function of physico-chemical parameters and experimental variables was established by means of multivariate regression methods (MLR, PLS). In addition, a predictive model for estimating the hydrophobicity of catecholamines is proposed. Finally, quantitative retention-activity relationships in MLC were also investigated for catecholamines.

Micelle-stabilized room-temperature phosphorimetric procedure for the determination of naproxen and propranolol in pharmaceutical preparations

A direct and simple procedure for the determination of naproxen and propranolol in pharmaceutical preparations was developed. The procedure was based on the measurement of the room-temperature phosphorescence of sodium dodecyl sulfate micellar solutions of the drugs. The appropriate experimental conditions to obtain a reproducible and maximum phosphorescence signal, when sulfite is used to eliminate the oxygen from the micellar solutions, were studied. The results of the analysis of several pharmaceutical preparations were satisfactory. The presence of hydralazine in pharmaceuticals containing propranolol gave low values owing to quenching of the phosphorescence signal.

Spectrophotometric determination of N-acetylcysteine in drug formulations with o-phthalaldehyde and isoleucine

A spectrophotometric procedure is proposed for the assay of N-acetylcysteine (NAC) in the range 0.5–49 µg ml–1, based on the reaction of the thiol group with o-phthalaldehyde and isoleucine at pH 9.5 (λmax.= 335 nm). The procedure was applied successfully to the determination of NAC in several pharmaceutical formulations. The recoveries ranged from 98 to 106%, with relative standard deviations of less than 1.5%. The method is free from interferences and is simple, rapid and easy to automate.

Determination of catecholamines in urine by micellar liquid chromatography with coulometric detection

SummaryThe determination of catecholamines by HPLC with a sodium dodecyl-sulphate (SDS), micellar mobile phase on a C18 column and with coulometric detection was studied. The eluate was conditioned at +0.25 and +0.00 V, and the current at −0.16V was recorded. A previously developed model which describes the chromatographic behaviour of solutes in HPLC with hybrid, micellar mobile phases was used to optimize the SDS and ethanol concentrations. A mobile phase of 0.15M SDS in a phosphate buffer of pH 3.4 and without ethanol is recommended. The limits of detection were 0.4–0.7 ng ml−1. The procedure was applied to the determination of unconjugated L-dopa, norepinephrine and dopamine in urine. Direct injection of the urine samples gave high results but the unconjugated catecholamines could be determined with a single solid-phase extraction step on an alumina column.

Determination of Phenoxy Acid Herbicides in Drinking Waters by HPLC and Solid Phase Extraction

Abstract An HPLC procedure for determining phenoxy acid herbicides in waters is described. A LichroSpher RP select B octadecyl-silane analytical column and spectrophotometric detection at 230 nm were used. Adequate retention was achieved with a mobile phase containing MeOH/phosphate buffer 10−2 M pH 2.5/PnOH (50/42/8, v/v). The herbicides were isolated from water samples by using a single solid phase extraction procedure with C18 solid-phase columns. An enrichment factor of 500 is achieved. The coefficients of variation of the method were generally lower than 2.7% at 0.4 μg L−1 herbicide concentration levels. Recoveries ranged between 93 and 118%. The results obtained indicate that the proposed method is well suitable for monitoring phenoxy acid herbicides in compliance with the European Community standard for drinking water.

Determination of catecholamines as aminochromes by micellar liquid chromatography with thermal lens spectrophotometric detection

SummaryThe determination of catecholamines (CAs) using micellar liquid chromatography with thermal lens spectrophotometric detection has been studied. CAs are oxidized with hexacyanoferrate(III) to aminochromes which are separated with a mobile phase of 0.05 M sodium dodecyl sulphate, 7% propanol and 0.03 M citrate buffer, pH 4.8, on a partially endcapped C18 column. The aminochrome-micelles and aminochrome-stationary phase association constants are evaluated. Using the 488 nm line of an Ar+ laser with 250 mW pump power the limits of detection are about 4 ng mL−1. The technique is applied to the determination of unconjugated CAs in urine using isoproterenol as internal standard.

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.

Determination of total free amino acids with o-phthalaldehyde and N-acetyl-l-cysteine

Abstract A spectrophotometric procedure is proposed for the determination of total free amino acids after reaction with o-phthalaldehyde and N-acetyl- l -cysteine, using isoleucine as the reference. The procedure was applied to the analysis of five samples of widely different composition. Recoveries were 98–105%.