L. Lahuerta Zamora

Determination of tyrosine through a FIA-direct chemiluminescence procedure

A new FI-direct chemiluminescence method is proposed for the determination of tyrosine, based on the oxidation of the amino acid by K(3)Fe(CN)(6) in potassium hydroxide medium, at room temperature and enhanced by the presence of beta-cyclodextrin and formic acid. The dynamic range was linear over the range 1.0-10.0 mgl(-1). A large study of the influence of foreign compounds was performed, including amino acids; and, the method showed high selectivity. The reproducibility between days resulted in a rsd (in slope%) of 4.8 and the repeatability with a rsd (n=50, 10.0 mgl(-1)) of 3.1%, the LOD (s/n=3) was 50 mugl(-1) and sample throughput 98 h(-1).

Entrapment of reagents in polymeric materials. Indirect atomic absorption spectrometric determination of isoniazid by oxidation with manganese dioxide incorporated in polyester resin beads in a flow-injection system

Abstract A method for entrapment of reagents is described. The experimental procedure is based on the dispersion of the reagent (MnO2) into an unsaturated polyester solution and grinding the product to give the required particle size (300–400 μm). The reagent is packed into a bed reactor (1.5 mm i.d.) and incorporated in a flow-injection system. The procedure was used for the indirect atomic absorption spectrometric determination of isoniazid (0.5–10 μg ml−1) in solutions of pharmaceutical products, with little interference from excipients. The measurements are based on the determination of Mn2+ released on oxidation of the isoniazid. The relative standard deviation for the determination of 5 μg ml−1 of isoniazid is 2.57% (n = 40) and the sample throughput is 43 h−1.

Indirect determination of paracetamol in pharmaceutical formulations by inhibition of the system luminol–H2O2–Fe(CN)63− chemiluminescence

After a large drug scanning, the system Luminol-H2O2-Fe(CN)6(3-) is proposed for first time for the indirect determination of paracetamol. The method is based on the oxidation of paracetamol by hexacyanoferrate (III) and the subsequent inhibitory effect on the reaction between luminol and hydrogen peroxide. The procedure resulted in a linear calibration graph over the range 2.5-12.5 microg ml(-1) of paracetamol with a sample throughput of 87 samples h(-1). The influence of foreign compounds was studied and, the method was applied to determination of the drug in three different pharmaceutical formulations.

Prediction of the chemiluminescent behaviour of phenols and polyphenols

Abstract A paper from this laboratory ‘J. Anal. Chem. 73 (2001) 4301’ was recently published and dealing with the first attempt to apply molecular connectivity calculations to predict a chemical property with analytical usefulness; namely, the chemiluminescent behaviour of substances when react with common strong oxidants in liquid phase. In the present work, the usefulness of molecular topology on the search for new chemiluminescent compounds is clearly demonstrated. The proposed discriminant equation, represented a success of 92.7% in the prediction. The present paper is the further step from the cited paper; it is dealing on the application of molecular connectivity calculations (former discriminant equation ‘J. Anal. Chem. 73 (2001) 4301’) to predict the chemiluminescent behaviour of phenols and polyphenols when they react with common oxidants in liquid phase. A number of phenols and polyphenols (close to 100) were theoretically studied by means of the discriminant equation ‘J. Anal. Chem. 73 (2001) 4301’, being some of them predicted as chemiluminescent with a high probability. These theoretical predictions have been empirically checked through a continuous flow manifold. A number of 33 compounds, selected between those which chemiluminescent behaviour was predicted, were assayed. A success of 100% over the theoretical predictions was obtained.

Solid-phase reactors as high stability reagent sources in flow analysis: selective flow injection spectrophotometric determination of cysteine in pharmaceutical formulations

The flow injection spectrophotometric determination of cysteine was carried out by reaction with cobalt(II) ions entrapped in a polymeric material and filling a packed-bed reactor; the released cobalt(II) complexed with the amino acid was monitored at 360 nm. The method worked with a high repeatability, even with independent reactors, days and solutions. Selectivity of the procedure was tested with twenty different foreign compounds found in pharmaceutical formulations containing cysteine, parent amino acids included; no serious interferences were observed. The calibration graph for cysteine was linear over the range 1–90 µg ml–1 with a relative standard deviation of 0.8% at 60 µg ml–1 (n = 158). The calculated sample throughput was 90 h–1. The method was applied to determine the content of cysteine in pharmaceutical formulations.

Theoretical prediction of the photoinduced chemiluminescence of pesticides

Although it is relatively easy to find chemiluminescent (CL) molecules working on the field of direct liquid phase (especially employing strong oxidants), the molecules found as chemiluminescent are normally very weak CL compounds for developing suitable analytical CL-procedures. Therefore, it is mandatory to develop new strategies to enhance in a simple way the native chemiluminescence of such a compounds, and even to increase the number of compounds to be determined by direct chemiluminescence. Photoinduced chemiluminescence (Ph-CL) results in a simple and easily on-line accessible strategy to solve these disadvantages. In the present paper, molecular connectivity, a topological method which allows an unique mathematical characterization of molecular structures by the so-named topological descriptors and their correlation with physical, chemical and biological properties of molecules was applied to predict the Ph-CL in liquid phase. Molecular connectivity calculations and discriminant analysis was applied to 72 pesticides for which either a Ph-CL or non Ph-CL behaviour was observed in an experimental screening. The screening test is based on the on-line photodegradation of pesticides by using an automated multicommutation based flow asssembly provided with a photoreactor consisting of 150 cm x 0.8mm PTFE tubing helically coiled around a 20 W low-pressure mercury lamp. Photodegraded pesticides are detected by direct chemiluminescence of the resulting photo-fragments and their subsequent reaction with potassium permanganate in sulfuric acid medium as oxidant. The screening comprised pesticides with different molecular structures and relevant members of the most important families of pesticides were tested (oxime carbamates, sulfonylcarbamates, thiocarbamates, 1,3,5-triazines, organophosphorous, hydroxybenzonitrile, sulfonylureas, phosphonic acid derivatives, imidazolinones, carboxamides, aryloxyalkanoic acids, 1,2,4-triazinones, etc.). The theoretical predictions agree with the empirical results obtained by means of the screening test performed in the multicommutation flow-assembly.

Determination of tannic acid by direct chemiluminescence in a FIA assembly

Abstract The determination of tannic acid is performed in a FIA assembly on the basis of the analytical output obtained by oxidation of the acid. The analyte solution was daily prepared in a mixture of quinine as sensitiser and perchloric acid and it was injected into a pure water stream acting as a carrier. This solution merges with the mixture potassium permanganate in perchloric medium and the resulting chemiluminescence is monitored. The method was applied over the range 0.5–20 mg l −1 of tannic acid with a LOD 100 μg l −1 . The reproducibility was 2.1% and the sample throughput 54 h −1 . The influence of foreign substances was studied and the new method is applied to the determination of tannic acid in pharmaceutical and galenic formulations in human urine and surface waters.

Flow injection analysis–direct chemiluminescence determination of ergonovine maleate enhanced by hexadecylpyridinium chloride

A flow injection analysis–direct chemiluminescence method is proposed for the determination of ergonovine maleate, based on the oxidation of the drug by hexacyanoferrate(III) in sodium hydroxide. The chemiluminescence was enhanced by a cationic surfactant, hexadecylpyridinium chloride. The calibration graph was linear over the range from the limit of detection (70 ppt) to 1 ppm of ergonovine maleate, with an RSD (n = 50, 50 ppb) of 1.8% and a sample throughput of 118 h–1. The influence of foreign compounds was tested. The method was applied to the determination of ergonovine maleate in synthetic samples.

Continuous flow-injection-atomic absorption spectrometric method for the determination of Ondansetron

Abstract A flow-injection procedure for the indirect determination of the new drug Ondansetron is proposed. The method is based on the reaction of the drug in an oxidative solid-phase reactor included in the flow assembly. The reactor was made by lead dioxide physically entrapped by polymerization; the released lead(II) was monitored by atomic absorption spectrometry at 217.0 nm. The procedure gave a linear calibration graph up to 20 μg ml−1 of Ondansetron with a sample throughput of 338 samples h−1.

Quantitative colorimetric-imaging analysis of nickel in iron meteorites

A quantitative analytical imaging approach for determining the nickel content of metallic meteorites is proposed. The approach uses a digital image of a series of standard solutions of the nickel-dimethylglyoxime coloured chelate and a meteorite sample solution subjected to the same treatment as the nickel standards for quantitation. The image is processed with suitable software to assign a colour-dependent numerical value (analytical signal) to each standard. Such a value is directly proportional to the analyte concentration, which facilitates construction of a calibration graph where the value for the unknown sample can be interpolated to calculate the nickel content of the meteorite. The results thus obtained were validated by comparison with the official, ISO-endorsed spectrophotometric method for nickel. The proposed method is fairly simple and inexpensive; in fact, it uses a commercially available digital camera as measuring instrument and the images it provides are processed with highly user-friendly public domain software (specifically, ImageJ, developed by the National Institutes of Health and freely available for download on the Internet). In a scenario dominated by increasingly sophisticated and expensive equipment, the proposed method provides a cost-effective alternative based on simple, robust hardware that is affordable and can be readily accessed worldwide. This can be especially advantageous for countries were available resources for analytical equipment investments are scant. The proposed method is essentially an adaptation of classical chemical analysis to current, straightforward, robust, cost-effective instrumentation.