Constantinos E. Efstathiou

Flow injection chemiluminometric determination of epinephrine, norepinephrine, dopamine and L-dopa

A method is proposed for the determination of 0.0500-1.00 microgram ml-1 of epinephrine and L-dopa and 0.100-1.00 micrograms ml-1 of norepinephrine and dopamine by their chemiluminogenic oxidation with potassium permanganate in acidic medium, in the presence of formaldehyde, which greatly improves the sensitivity. Flow injection allows the measurement of 80 solutions per hour. The method was also optimized for a continuous-flow system. Comparative results from numerous organic compounds proved the necessity for electron-donating groups on the benzene ring for sensitive chemiluminescent characteristics.

Automated flow-injection spectrophotometric determination of catecholamines (epinephrine and isoproterenol) in pharmaceutical formulations based on ferrous complex formation

A novel automated flow-injection spectrophotometric method for the determination of catecholamines (epinephrine and isoproterenol) has been developed based on the formation of their coloured complexes with Fe(II) in aminoacetic-carbonate buffer pH 8.3 and measuring of the absorbance peaks at the lambda(max) of 530 nm. A fully automated FIA system controlled by home-made software (FIA-MOD) was used for optimising the chemical and manifold parameters and running of routine measurements. The calibration graph was linear in the range of 5-200 mg l(-1) for epinephrine with an RSD of 0.24% (n = 5; c = 150 mg l(-1)) and 10-300 mg(-1) for isoproterenol with an RSD of 0.13% (n = 5; c = 200 mg l(-1)). Measurement throughput was 120 h(-1) ensuring a sample throughput of 40 h(-1) analysed in triplicate. Common excipients for tablets and injections were found not interfering. The proposed method was applied for the assay of various commercial pharmaceutical formulations containing epinephrine and isoproterenol and for the content uniformity test for the isoproterenol tablets. The assay results with RSD 2-4% (n = 3) were comparable with those obtained with the official USP XXIII methods (mean difference 1.9%).

Preconcentration at a carbon-paste electrode and determination by adsorptive-stripping voltammetry of rutin and other flavonoids

Abstract The non-Faradaic preconcentration behaviour of nine flavonoids (six flavones: fisetin, galangin, morin, quercetin, rhamnetin, rutin, and three flavanones: hesperidin, hesperitin, naringin) at a carbon-paste (nujol/graphite) electrode and the factors affecting it (pH, accumulation potential, presence of various surfactants) for their subsequent differential pulse voltammetric determination are examined. All flavones tested are readily accumulated on the carbon paste electrode resulting in a considerable signal enhancement making determinations feasible down to 10−8 − 10−7 M after preconcentration for 1–4 min. Flavanones are not preconcentrated so their lower determination limits are of the order of 10-6 M. A simple voltammetric procedure for the determination of rutin in a multivitamin preparation is presented.

Comparative study of fluorescent ternary terbium complexes. Application in enzyme amplified fluorimetric immunoassay for α-fetoprotein

Abstract A systematic spectrofluorimetric study of the formation of ternary complexes of terbium ions (Tb3+) with ethylenediamine tetraacetic acid (EDTA) and a variety of bidentate organic ligands has been performed. Fourteen ligands (salicylic acid, quinolones and hydroxybenzene sulfonic acid derivatives) were examined as efficient energy donating chelators for Tb3+. The fluorescence properties of the ligands, alone as well as of the ternary complexes with Tb3+ and EDTA have been examined. Optimization studies, including the effect of pH, buffer systems and the use of EDTA analogs, were performed. A possible relationship between the structure and the ability of the ligand to act as an efficient light absorbing and energy donating chelator for Tb3+ is discussed. The phosphate ester of diflunisal, one of the most sensitive ligands for Tb3+, was used as an alkaline-phosphatase substrate for the determination of α-fetoprotein (AFP) in serum by a highly sensitive enzyme amplified lanthanide luminescence immunoassay by means of second derivative synchronous fluorescence spectroscopy as a detection technique. By using this substrate, AFP was determined in serum with a limit of detection (3 × s.d. blank) of 5 pg ml−1, coefficients of variation in the 2.4–5.3% range and mean recovery from four pooled serum samples at four concentration levels (5, 40, 100 and 250 ng ml−1) equal to (99 ± 3)%.

Kinetic identification and determination of certain carbohydrates with a periodate-sensitive perchlorate-selective electrode

Abstract The reactions between periodate and carbohydrates are easily monitored with a perchlorate-selective electrode as a periodate sensor. The relative reaction rate constant of each carbohydrate compared to glucose is introduced as a new characteristic constant, the “periodate index”, which could be related to the molecular configuration. Since appreciable differences are observed between the “periodate index” values of various carbohydrates, this constant can be used for the identification of 20-mg amounts of single pure compounds. An automatic potentiometric reaction rate method is described and shown to be simple, rapid and accurate for the determination of glucose. Amounts of glucose in the range 7–54 mg were determined in 20–140 s with precision and relative errors of about 0.4 and 0.8 %, respectively.

Comparison of chemical modifiers for the determination of gold in biological fluids by electrothermal atomic absorption spectrometry

The use of various isomorphous metals as chemical modifiers on the determination of gold has been investigated. The temperature programmes and the masses of modifiers were carefully optimized. Noble metals, copper, nickel, and rhenium increased the atomic absorption signal of gold and raised the maximum pyrolysis temperature from 700 to 1000 °C. In the presence of 20 µg of ascorbic acid this temperature was 1000 °C, whereas in combination with 1 µg of Pd it increased to 1150 °C. On the basis of platform atomization and integrated absorbance measurements, the best sensitivity for aqueous solutions was obtained with the mixed modifiers rhodium–rhenium (m0= 7.3 pg), ascorbic acid–palladium (m0= 7.1 pg) or ascorbic acid–rhodium (m0= 6.8 pg), whereas the characteristic mass in the absence of modifiers was 12.2 pg. The limit of detection was 0.23 µg l–1 in the absence of modifier, whereas it was 0.12 µg l–1 in the presence of the mixed modifier rhodium–rhenium. In the absence of chemical modifiers, a fractional order of release was observed for gold, whereas in the presence of modifiers an approximately first-order release was observed. The determination of gold in biological fluids was interference-free and complete recovery was obtained when stabilized temperature platform furnace conditions were fulfilled. Constant signals, without double peaks and shoulders, were obtained only in the presence of modifiers. It was found that gold solutions stored in the autosampler cups during measurements of the samples were stable only in the presence of 0.5 g l–1 NH4SCN or 1.0 g l–1 ascorbic acid.

Pre-concentration of indolic compounds at a carbon paste electrode and indirect determination of L-tryptophan in serum by adsorptive stripping voltammetry

The pre-concentration of indole and other indolic compounds of biochemical and pharmaceutical interest at a carbon paste electrode (Nujol/graphite) has been studied. All the compounds examined can be determined, by direct voltammetric measurements in their solutions, in the concentration range 1-8 microM. Indole, methylated indoles, harmaline and serotonin were accumulated at the electrode by a combined adsorption/extraction process. By applying the medium-exchange procedure, the accumulated compounds can be determined in the same concentration range, after a 60-s pre-concentration period, enhancing, therefore, the selectivity of the voltammetric determination. A procedure for the indirect determination of L-tryptophan in serum has been developed, which is based on these findings. L-Tryptophan was cleaved to indole by tryptophanase, and indole was subsequently determined voltammetrically after a 2-min pre-concentration period at the carbon paste electrode. From 0.3 to 1.2 micrograms of L-tryptophan in a total of 75 microliters of serum sample (20-80 microM) can be determined with an average error of ca. 0.03 microgram, whereas the recovery of added L-tryptophan in serum samples is in the range 105-115%.

Flow-injection fluorimetric determination of 1,4-benzodiazepines in pharmaceutical formulations after acid hydrolysis

A simple, rapid and fully automated flow injection method with fluorimetric detection after hydrolysis with H2SO4 in ethanolic or methanolic medium at room temperature has been developed for the determination of 1,4-benzodiazepines (oxazepam, diazepam and nitrazepam) in pharmaceutical formulations. The calibration curves are linear in the ranges (mg ml(-1)) of oxazepam (0.025-0.150), diazepam (0.010-0.125) and nitrazepam (0.010-0.150), with detection limits of 0.01, 0.005 and 0.005 mg ml(-1), respectively, and RSD (1% (n = 10). The measurement throughput is 60 h(-1) using a 200-microl sample volume obtained by the direct dissolution of formulations in alcohol.

Potentiometric determination of nicotine in tobacco products with a nicotine-sensitive liquid membrane electrode

Abstract A simple potentiometric method with standard additions is described for the rapid determination of nicotine in tobacco products. A nicotine-sensitive electrode with a liquid membrane of nicotine hydrogen tetra(m-chlorophenyl)borate dissolved in o-nitrotoluene is used. The electrode exhibits near-Nernstian response to monoprotonated nicotine cation activity from 0.08 to 10-5 M, in the pH range 4–7. Nicotine down to 2 mg per g of sample can be determined with a standard deviation of about 0.5 mg of nicotine. Comparison with an official method gave satisfactory results.

Synchronous scanning second derivative spectrofluorimetry for the simultaneous determination of diflunisal and salicylic acid added to serum and urine as ternary complexes with terbium and EDTA

Abstract By using second derivative synchronous scanning fluorescence spectrometry the simple resolution of two nonsteroidal, antiinflammatory drugs diflunisal and salicylic acid, as ternary complexes with terbium and EDTA, is accomplished. The method developed is simple, sensitive and rapid, and has been successfully applied for the determination of both compounds in 10 μl of untreated human serum and urine samples. The detection limits for diflunisal were 0.9 and 1.8 mg l −1 , and for salicylic acid 1.2 and 1.7 mg l −1 , in serum and urine, respectively. The mean analytical recoveries from serum and urine samples spiked with diflunisal (25–100 mg l −1 ) and salicylic acid (28–138 mg l −1 ) were 99 ± 8% (serum) and 102 ± 8% (urine) for diflunisal and 102 ± 9% (serum) and 95 ± 8% (urine) for salicylic acid. The precision of all determinations varied from 2.5 to 10%.