Demetrius G. Themelis

Sensitive determination of captopril by flow injection analysis with chemiluminescence detection based on the enhancement of the luminol reaction

This work reports a novel flow injection (FI) method for the determination of captopril, 1-[(2 S)-3-mercapto-2-methylpropionyl]-l-proline (CPL), based on the enhancement CPL affords on the chemiluminescence (CL) reaction between luminol and hydrogen peroxide. For this purpose alkaline luminol and hydrogen peroxide solutions were mixed online, the sample containing CPL was injected into an aqueous carrier stream, mixed with the luminol-hydrogen peroxide stream and pumped into a glass flow cell positioned in front of a photomultiplier tube (PMT). The increase in the CL intensity was recorded in the form of FI peaks, the height of which was related to the CPL mass concentration in the sample. Different chemical and instrumental parameters affecting the CL response were investigated. Under the selected conditions, the log–log calibration curve was linear in the range 5–5000 gl −1 of CPL, the limit of detection was 2 gl −1 (at the 3σ level), the R.S.D., sr was 3.1% at the 100 gl −1 level (n = 8) and the sampling rate was 180 injections h −1 . The method was applied to the determination of CPL in pharmaceutical formulations with recoveries in the range 100 ± 3%.

Selective determination of cyanides by gas diffusion-stopped flow-sequential injection analysis and an on-line standard addition approach.

A highly selective sequential injection (SI) method for the automated determination of weak-acid-dissociable cyanides is reported. The analytical procedure is based on the on-line reaction of the analyte with ninhydrin in carbonate medium to form a coloured product (lambda(max)=510 nm). Cyanides are removed from sample matrix by acidification through a gas-diffusion step incorporated in the SI manifold. The effect of instrumental and chemical variables was studied. By adopting an on-line standard addition protocol, the sensitivity of the proposed method was enhanced drastically, without affecting the determination range. The assay was validated in terms of linearity (up to 200 microg L(-1)), limit of detection (c(L)=2.5 microg L(-1)), limit of quantitation (c(Q)=7.5 microg L(-1)), precision (s(r)<2.5% at 100 microg L(-1)) and selectivity. High tolerance against critical species such as sulfides and thiocyanates was achieved. The applicability of the method was demonstrated by analyzing tap and mineral water samples at levels below the limits established by international E.U. and U.S. organizations. The percent recoveries were satisfactory in all cases, ranging between 94.2 and 103.6%.

Utilisation of kinetic-based flow injection methods for the determination of chlorine and oxychlorine species

Abstract Automated selective iodometric methods for the determination of chlorine and oxychlorine species have been developed for use in the drinking water industry. By utilising kinetic-based methods, linear ranges observed were: chlorine, 0.2–10 mg l −1 ; chlorine dioxide, 0.3–10 mg l −1 ; chlorite ion, 0.08–5 mg l −1 ; and chlorate ion, 0.08–5 mg l −1 .

Flow injection direct spectrophotometric assay for the speciation of trace chromium(III) and chromium(VI) using chromotropic acid as chromogenic reagent

A new rapid and sensitive FI assay is reported for the simultaneous direct spectrophotometric determination of trace Cr(VI) and Cr(III) in real samples. The method is based upon the reaction of Cr(VI) with chromotropic acid (CA) in highly acidic medium to form a water-soluble complex (lambda(max)=370nm). Cr(III) reacts with CA only after its on-line oxidation to Cr(VI) by alkaline KIO(4). The determination of each chromium species in the sample was achieved by absorbance differences. The calibration curves were linear over the range 3-4000mugl(-1) and 30-1200mugl(-1) for Cr(VI) and Cr(III), respectively, while the precision close to the quantitation limit was satisfactory in both cases (s(r)=3.0% for Cr(VI) and 4.0% for Cr(III) (n=10) at 10 and 50mugl(-1) level, respectively). The method developed proved to be adequately selective and sensitive (c(L)=1 and 10mugl(-1) for Cr(VI) and Cr(III), respectively). The application of the method to the analysis of water samples (tap and mineral water) gave accurate results based on recovery studies (93-106%). Analytical results of real sample analysis were in good agreement with certified values.

Hybrid sequential injection–flow injection manifold for the spectrophotometric determination of total sulfite in wines using o-phthalaldehyde and gas-diffusion

A new automated spectrophotometric method for the determination of total sulfite in white and red wines is reported. The assay is based on the reaction of o-phthalaldehyde (OPA) and ammonium chloride with the analyte in basic medium under SI conditions. Upon on-line alkalization with NaOH, a blue product is formed having an absorption maximum at 630 nm. The parameters affecting the reaction - temperature, pH, ionic strength, amount concentration and volume of OPA, amount concentration of ammonium chloride, flow rate and reaction coil length - and the gas-diffusion process - sample and HCl volumes, length of mixing coil, donor flow rate - were studied. The proposed method was validated in terms of linearity (1-40 mgL(-1), r=0.9997), limit of detection (c(L)=0.3 mgL(-1)) and quantitation (c(Q)=1.0 mgL(-1)), precision (s(r)=2.2% at 20 mgL(-1) sulfite, n=12) and selectivity. The applicability of the analytical procedure was evaluated by analyzing white and red wine samples, while the accuracy as expressed by recovery experiments ranged between 96% and 106%.

Determination of low concentrations of chlorite and chlorate ions by using a flow-injection system

Abstract A flow-injection method is reported for the determination of chlorite ion and chlorite and chlorate ions in mixtures at the submilligram per liter level in drinking water. The chlorite ion concentration is selectively determined by using its reaction with iodide ion at pH 2, which liberates iodine. Both species react with iodide ion in6 M HCl to produce iodine, the concentration of which is measured spectrophotmetricaly at 370 nm. The individual species are determined using multiplel regression. The method exhibits a linear range from 2 to 150 μM (0.1–10.1 mg l-1) for chlorite ion and from 2 to μM (0.1–8.3 mg l-1 for chlorate ion, with relative standard deviations of 0.4 and 1.2%, respectively.

Sequential injection method for the direct spectrophotometric determination of bismuth in pharmaceutical products

A spectrophotometric method is reported for the determination of bismuth in pharmaceutical products using sequential injection analysis. Methylthymol blue (MTB) was used as a color forming reagent and the absorbance of the Bi(III)-MTB complex was monitored at 548 nm. The various chemical and physical variables that affected the reaction were studied. A linear calibration graph was obtained in the range 0.0-75.0mg l -1 Bi(III) at a sampling frequency of 72 h -1 . The reagent consumption was considerably reduced compared to conventional flow injection systems, as only 150 μl of MTB were consumed per run. The precision was very satisfactory (s r = 0.5%, at 50.0 mg l -1 Bi(III), n = 12) and the limit of detection, c L , was 0.250 mg l -1 . The developed method was applied successfully to the analysis of various pharmaceutical products containing Bi(III). The relative errors e,, were < 1.5% in all cases and were evaluated by comparison of the obtained results with those found using atomic absorption spectrometry as the reference method.

Automated determination of total captopril in urine by liquid chromatography with post-column derivatization coupled to on-line solid phase extraction in a sequential injection manifold.

The present study reports a new liquid chromatographic (HPLC) method for the determination of the anti-hypertension drug captopril (CAP) in human urine. After its separation from the sample matrix in a reversed phase HPLC column, CAP reacts with the thiol-selective reagent ethyl-propiolate (EP) in a post-column configuration and the formed thioacrylate derivative is detected at 285 nm. Automated 4-fold preconcentration of the analyte prior to analysis was achieved by an on-line solid phase extraction (SPE) step using a sequential injection (SI) manifold. The Oasis HLB SPE cartridges offered quantitative recoveries and effective sample cleaning by applying a simple SPE protocol. The limits of detection and quantitation were 10 μg L(-1) and 35 μg L(-1) respectively. The percent recoveries for the analysis of human urine samples ranged between 90 and 96% and 95 and 104% using aqueous and matrix matched calibration curves respectively.

Rapid determination of methylxanthines in real samples by high-performance liquid chromatography using the new FastGradient® narrow-bore monolithic column

The present study reports one of the very first analytical applications of the new narrow-bore monolithic column, FastGradient Chromolith (50mm x 2.0mm i.d.). The three major methylxanthines (theobromine, theophylline and caffeine) were separated rapidly and determined simultaneously in various real samples. Based on the unique characteristics of this novel monolithic column the analytes were separated efficiently (R(s)>3.0) in less than 5min at a low flow rate of 0.5mLmin(-1) and using a low volume fraction of organic solvent (5% acetonitrile (ACN) in water) in the mobile phase. UV detection was carried out at 274nm. The separation was optimized in terms of mobile phase composition, flow rate and injection volume, while the method was validated for linearity, detection and quantitation limits, within and day-to-day precision, accuracy and ruggedness. Its applicability was demonstrated by analyzing a variety of real samples including beverages, soft drinks, herbal products and pharmaceuticals. Compared to a well-established monolithic (Performance Chromolith 100mm x 4.6mm i.d.) and a particulate reversed phase column (Hypersil ODS 5microm 150mm x 4.6mm i.d.), the narrow-bore FastGradient column offered satisfactory performance, faster analysis time and drastic reduction in the consumption of mobile phase and organic solvents.

Ethyl-propiolate as a novel and promising analytical reagent for the derivatization of thiols: Study of the reaction under flow conditions

The present investigation demonstrates the potentials of ethyl-propiolate (EP), a novel derivatizing reagent for thiols. To the best of our knowledge this is the first systematic study of EP in analytical chemistry. The reaction was investigated under flow conditions using sequential injection (SI) analysis and UV detection at 285 nm. The reaction kinetics was affected by parameters such as the pH, the concentration of EP and the temperature and was thoroughly examined exploiting stopped-flow experiments. Cysteine (CYS) and captopril (CAP) were selected as model thiolic compounds in terms of chemical structures. Finally, the applicability of EP as a derivatization reagent for analytical purposes was demonstrated by the development and validation of a novel automated assay for the determination of CAP in pharmaceuticals.