M. Hernández Córdoba

Speciation of vitamin B12 analogues by liquid chromatography with flame atomic absorption spectrometric detection

Vitamin B12 analogues and aqueous cobalt are separated by liquid chromatography using a reversed-phase column (Spherisorb ODS-2) and a mobile phase consisting of a linear gradient from a 25:75 (vv) mixture of methanol and 0.085 M phosphoric acid solution buffered at pH 5.2 with triethanolamine to a 60:40 mixture over 5 min. The vitamins are selectively detected by measuring cobalt with an atomic absorption spectrometer directly interfaced to the Chromatograph. A simple T-piece is used to compensate with air the difference between the nebulizer uptake rate and the Chromatographic flow-rate (1.5 ml min−1). Recoveries of the components from spiked preparations ranged from 90.8 to 108%. The method is applicable to the analysis of pharmaceuticals for cobalamins.

Slurry-electrothermal atomic absorption spectrometric determination of aluminium and chromium in vegetables using hydrogen peroxide as a matrix modifier

Fast heating programmes for determining aluminium and chromium in vegetables using electrothermal atomic absorption spectrometry with slurry sampling are developed using wall atomization. The build-up of carbonaceous residues inside the atomizer is avoided by adding 4% hydrogen peroxide and 1% nitric acid to the slurries instead of using an air-ashing stage. In accordance with fast-programme methodology, the conventional drying and charring steps are replaced by a modified drying stage. simplifying the heating programmes. For suspensions containing 0.1% m/V of ground vegetables, the relative standard deviation (RSD) is about +/-5% for aluminium. For chromium determination, the use of 0.5-1% m/V suspensions leads to RSDs close to +/-4%. Calibration is carried out using aqueous standards. The aluminium and chromium contents of a number of vegetable samples obtained by using the slurry approach agree with those obtained by means of a conventional procedure based on the total dissolution of the samples. The reliability of the procedures is also confirmed by analysing two certified reference materials.

Simultaneous liquid chromatographic analysis of 5-(hydroxymethyl)-2-furaldehyde and methyl anthranilate in honey

Abstract A method for the simultaneous determination of 5-(hydroxymethyl)-2-furaldehyde and methyl anthranilate in honey is reported. The analysis is carried out by reversed-phase gradient-elution high performance liquid chromatography (HPLC). The proposed method can be applied for both the routine characterization of honey based on the methyl anthranilate content and the quality control analysis based on the 5-(hydroxymethyl)-2-furaldehyde level.

Slurry procedures for the determination of cadmium and lead in cereal-based products using electrothermal atomic absorption spectrometry

Simple and rapid methods for the determination of cadmium and lead in biscuits, bread and cereal-based products using the slurry-ETAAS approach are discussed. Suspensions were prepared in a 20% v/v ethanol medium. Phosphate was used as a chemical modifier for lead determination. For cadmium determination both palladium and a copper plus ammonia mixture were used. In both cases platform atomization was used and calibration was performed using aqueous standards. Results for two reference materials confirmed the reliability of the procedures. Relative standard deviations were in the range of 2.5–6.5% for cadmium and 4.5–14% for lead. Detection limits were, respectively, 0.5 and 8 ng/g.

Flow-injection flame atomic absorption spectrometry for slurry atomization. Determination of calcium, magensium, iron, zinc and manganese in vegetables

Abstract A flame atomic absorption spectrometric procedure for determining the iron, manganese, calcium and magnesium contents of slurries prepared from vegetables is described. Samples were first submitted to a mild calcination step and then suspensions were prepared and introduced into an air-acetylene flame using a simple flow-injection manifold. The calibration of iron, zinc and manganese was carried out using either aqueous standards of a flow-injection sample-to-standard additions method applicable to suspensions. The calcium and magnesium contents were too high for the direct introduction of the slurries into the flame and so an on-line dilution system was used. Excellent agreement was found between the results of the slurry-based procedures and those obtained by an alternative method involving calcination and dissolution in acid. The reliability of the approach was confirmed by analysing two certified reference materials.

Slurry atomization of vegetables for the electrothermal atomic absorption spectrometric analysis of lead and cadmium

Abstract Slurry-ETAAS procedures for the rapid determination of lead and cadmium in vegetables are described. Fast-programme methodology, in which the drying and ashing stages are replaced by a single modified drying step, is used. Slurries are prepared in a 20% (v/v) ethanol medium using 0·1% ammonium dihydrogen-phosphate as a chemical modifier. The suspending medium also contains hydrogen peroxide to prevent the build-up of carbonaceous residue inside the atomizer. Platform atomization using 1800°C and 1700°C for lead and cadmium, respectively, permits calibration with aqueous standards. Results for two standard reference materials confirm the reliability of the procedures.

Linear flow gradients for automatic titrations

Abstract A manifold to perform automatic continuous flow titrations is presented. The system is based on the use of a variable-speed peristaltic pump which delivers the titrant at a linearly increasing flow rate and another pump working at a constant and higher speed, placed after the detector, the difference between both flow rates being compensated by the aspiration of the titrand through a T-piece. A solution containing the indicator is also propelled by the fixed speed pump. With such an arrangement, the flow rate through the spectrophotometric cell is constant and there is no drift of the baseline. The equations describing the behaviour of such a system are first obtained and then experimentally checked by using both acid-base and complexometric titrations. The experiments prove that the approach is highly reproducible (relative standard deviations (R.S.D.) below ± 1.5%), but inaccurate, since the time necessary to reach the equivalence point is longer than that predicted by theory. To avoid the need of a previous calibration, a second gradient can be performed by decreasing the variable-pump speed to zero. In this way, two equivalence points are obtained. Since both gradients are diametrically opposite, the difference in time between the two equivalence points agrees with that predicted by theory, as the errors can only cancel each other out. For acid-base and complexometric titrations the errors are below ± 2%.

Use of submicroliter-volume samples for extending the dynamic range of flow-injection flame atomic absorption spectrometry

Abstract Three computer-controlled flow-injection manifolds for extending the determination range of flame atomic absorption spectrometry are discussed. All the manifolds are based on the metering of very small sample volumes into a slowly moving carrier, the flow rate of which is gradually modified after completion of the injection. The systems use a conventional peristaltic pump equipped with small bore pump tubes and a channel to compensate for the difference between the flow delivered by the pump and the nebulizer uptake rate. The main difficulty which must be overcome is the pulsation caused by the rollers of the pump. The first manifold uses two or more metering steps, separated by an appropriate time interval and performed on the sample which is injected into the carrier propelled by the pump running at a low speed. The reproducibility is poor because the pump pulsations are periodic but highly asymmetric. The second system uses a single metering stage into the compensation channel performed while the pump is delivering the carrier at a flow rate close to the nebulizer uptake rate. Relative standard deviations ranging ± 0.6–2% are obtained but the maximum dilution degree attainable is about 2000. The third manifold is similar to the first, but a single metering stage is performed, the position of the rollers at the beginning of the metering step being controlled by means of a simple optical device. In this way, volumes as low as 0.02 μl can be metered into the system and dilution degrees of about 20,000 can be obtained. By choosing appropriate pump rates and metering times, a wide range of dilution degrees can be obtained. Sampling frequencies are in the 60–80 samples h −1 range. Calibration can be performed by using peak height or integrated absorbance. Repeatability of the integrated absorbance measurements is in the ±1.1–3.3% range.

Rapid determination of calcium, magnesium, iron and zinc in flours using flow injection flame atomic absorption spectrometry for slurry atomization

Abstract A procedure is described for the rapid determination of calcium, magnesium, iron and zine in flours obtained from several grains and legumes (wheat, whole meal, maize, barley, rice, rye, vetch, soy bean and field bean) using flame atomic absorption spectrometry. Suspensions up to 3% m/V from the ground samples are directly introduced into the spectrometer using a simple flow injection manifold. Calibration is performed using aqueous standard solutions of the determinands. The results obtained with this procedure agree with those obtained by conventional dry-ashing followed by dissolution in acids. Results for two standard reference materials are also given.

Manual and FIA methods for the determination of cadmium with malachite green and iodide

A sensitive and rapid spectrophotometric method for the determination of cadmium is described, based on the formation of a blue complex at pH 4 between the anionic iodide complex of cadmium(II) and Malachite Green; the colour is stabilized with poly(vinyl alcohol). The calibration graph for measurement at 685 nm is linear over the range 1-50 mug of cadmium per 25 ml of final solution, with a relative standard deviation of +/-1.7% for 1 mug/ml cadmium. The molar absorptivity is 6.1 x 10(4) 1.mole(-1).cm(-1). The method can be successfully adapted for FIA, the peak height being proportional to the cadmium concentration over the range 0.1-3 mug/ml; a two-channel manifold is used and an improvement in selectivity is obtained. The use of a gradient tube is demonstrated to give a good calibration for Cd(II) over the range 2 x 10(-2) -2 x 10(-6)M.