Pablo Fernández López

Sensitive determination of captopril by time-resolved chemiluminescence using the stopped-flow analysis based on potassium permanganate oxidation

The chemiluminescent behaviour of captopril when reacted with a common oxidant, potassium permanganate in different acidic media is described, using the stopped-flow technique in a continuous-flow system. A 22 bit analogue-to-digital converter that acquires analogue signals at -10 and +10V and allows the power supply to the peristaltic pump to be interrupted is used in the time-resolved chemiluminescence manifold to ensure rapid, efficient mixing of chemiluminescent reagent and sample immediately before reaching the detector; this results in high precision and detectability, particularly with fast, short-lived emissions. The optimum chemical conditions for the chemiluminescence emission were investigated. It was found that a weak CL emission was emitted during the oxidation of this drug with potassium permanganate in acidic solution. The effect of common emission enhancers such as formic acid, formaldehyde, glutaraldehyde, acetaldehyde, quinine, fluorescein, rhodamine B and rhodamine 6G was studied. The parameters selected were 4.0molL-1 sulphuric acid, 0.25mmolL-1 permanganate and 0.75molL-1 formaldehyde. Four quantitative parameters adjustable via software settings, two of them typically kinetic parameters, such as rate of the light-development reaction and rate of the light-decay reaction, and the other conventional parameters, such as maximum emission intensity and total emission area, were used to obtain linear calibration graphs with each measurement parameter. The detection limits ranged from 0.011 to 0.026μgmL-1 and R.S.D. values (n=10) of 1.21-3.93 at a 0.50μgmL-1 and 2.01-3.41 at a 1.60μgmL-1 concentration levels were obtained. The method was satisfactorily applied to the determination of captopril in pharmaceutical preparations.

Automatic chemiluminescence-based determination of carbaryl in various types of matrices.

Carbaryl, a modern pesticide widely used for both agricultural and non-agricultural purposes, was determined from the chemiluminescence produced in its reaction with Ce(IV) in a nitric acid medium containing rhodamine 6G as sensitizer, using flow-injection techniques. A straightforward automatic method based on measurements peak height and peak area, which are directly proportional to the carbaryl concentration, was thus developed. Calibration graphs are linear over the concentration range from 50 to 2000 ng mL(-1). The limit of detection, as determined according to Clayton, is 45.6 and 28.7 ng mL(-1) for peak height and peak area measurements, respectively. The relative standard deviation for 10 samples was less than 1.4% with both types of measurements. Two commercial formulations containing carbaryl were analysed using both types of measurements, which provided acceptable recovery values. Solid-phase extraction was used to concentrate and separate the analyte from the matrix. The method was successfully applied to the analysis of spiked water samples as well as in soil and grain samples. The proposed method exhibited a high selectivity no other pesticide containing the naphthalene group such as antu, napropamide or naftalam, etc., was found to interfere with the determination of carbaryl.

Direct determination of amiloride in urine using isopotential fluorimetry.

A method for the determination of amiloride at concentrations between 15 and 152 ng ml - 1 by means of matrix isopotential synchronous fluorescence spectrometry and derivative techniques is proposed. This method is useful for the determination of compounds in samples with unknown background fluorescence without the need for tedious pre-separation. As amiloride is widely used as a doping substance in sport, the method was successfully applied to the determination of amiloride in urine. To obtain maximum sensitivity and adequate selectivity, factors affecting fluorescence intensity were studied in the amiloride band centered at λ ex = 362 nm and λ em = 415 nm. As a result, the determination was performed in an ethanol–water (1 + 1, v/v) medium at pH 6.3, adjusted by using sodium citrate–citric acid (0.1 M) as buffer solution. The concentration of amiloride in urine samples can be calculated by recording its total luminescence spectrum and applying the isopotential trajectory of the urine that cuts the selected band of amiloride. The unknown analytical signal of urine is eliminated in the MISF spectrum obtained, by means of its first derivative. A calibration graph was constructed by measuring first derivative values at λ ex = 357 nm and λ em = 392 nm. Analytical parameters of the proposed method were calculated according to the error propagation theory. The sensitivity, repeatability, reproducibility and limit of determination achieved with the proposed method are adequate for the determination of amiloride in urine.

Direct determination of triamterene in urine by matrix isopotential synchronous fluorescence spectrometry

Abstract A new method for the determination of triamterene for concentrations between 10 and 1000ng ml −1 by means of matrix isopotential synchronous fluorescence spectrometry and derivative techniques is proposed. This new method is useful for the determination of compounds in samples with unknown background fluorescence, such as triamterene in urine, without the need of tedious pre-separation. The determination was performed in an aqueous medium adjusted at a pH value of 6.2, provided by adding sodium citrate-citric acid buffer solution. Since triamterene is widely used as a doping substance in sport, the method was successfully applied to the determination of triamterene in urine. Better sensitivity and repeatability are achieved for these matrices with the proposed method than with the fluorimetric ones described in the literature. An extensive statistical analysis has been developed to all calibration graphs. This treatment includes robust regression such as least median of squares which also detects outliers and leverage points. The weighted least squares regression has been applied to find the more exact straight line which fits the experimental data. The error propagation has been used for the calculation of the detection limit, determination limit and the repeatability of the method.

Direct determination of nalidixic acid in urine by matrix isopotential synchronous fluorescence spectrometry

A new method for the determination of nalidixic acid in urine is proposed for concentrations between 25 and 1000 ng ml(-1) by means of matrix isopotential synchronous fluorescence spectrometry. This new technique is useful for the determination of compounds in samples with unknown background fluorescence, such as nalidixic acid in urine, without the need for tedious preseparation. The method was performed in ethanol/water medium (80% v/v), at an apparent pH of 2.9 provided by adding sodium monochloracetate/monochloroacetic acid buffer solution. The method was successfully applied to the determination of nalidixic acid in urine. Better sensitivity and reproducibility are achieved in these matrices than with the fluorimetric methods described in the literature.

Phosphorimetric determination of dipyridamole in pharmaceutical preparations

Room temperature phosphorescence was applied to the determination of dipyridamole in pharmaceutical preparations. The response was linear in the concentration range 100–1600 ng ml - 1 . The use of phosphorescence enhancers such as thallium(I) nitrate (external heavy atom), sodium dodecyl sulfate (microemulsion stabilizer) and sodium sulfite (deoxygenation agent) was studied and optimized to obtain maximum sensitivity and adequate selectivity. The determination was performed in 0.026 M sodium dodecyl sulfate, 0.0156 M thallium nitrate and 0.02 M sodium sulfite. The pH value was 11.5, adjusted by adding sodium hydroxide. The phosphorescence was totally developed in 15 min, after that the intensity was measured at λ ex = 303 nm and λ em = 616 nm. The recovery of the method was tested on commercial formulations containing dipyridamole. The recoveries obtained were 94.67 ± 0.58% for Persantin and 96.75 ± 1.37% for Asasantin. The overall least squares regression method was applied to find the most exact straight line that fits the experimental data. The detection limit according to the error propagation theory was 16.4 ng ml - 1 . The repeatability and relative standard deviation were also determined according to this theory.