Mihaela Cheregi

Total antioxidant capacity of some commercial fruit juices: electrochemical and spectrophotometrical approaches.

The aim of this paper was to assess the total antioxidant capacity of some commercial fruit juices (namely citrus), spectrophotometrically and by the biamperometric method, using the redox couple DPPH· (2,2-diphenyl-1-picrylhydrazyl)/DPPH (2,2-diphenyl-1-picrylhydrazine). Trolox® was chosen as a standard antioxidant. In the case of the spectrophometric method, the absorbance decrease of the DPPH· solution was followed. For the biamperometric method, the influence of some parameters like the potential diference, ΔE, DPPH· concentration, and Trolox® concentration was investigated. The calibration graph obtained for Trolox® presents linearity between 5 and 30 µM, (y = 0.059 x + 0.0564, where y represents the value of current intensity, expressed as μA and x the value of Trolox® concentration, expressed as μM; r2 = 0.9944). The R.S.D. value for the biamperometric method was 1.29% (n = 10, c = 15 μM Trolox®). In the case of the spectrophotometric method, the calibration graph obtained for Trolox® presents linearity between 0.01 and 0.125 mM (y = -9.5789 x+1.4533, where y represents the value of absorbance and x, the value of Trolox® concentration, expressed as mM; r2 = 0.9963). The R.S.D. value for the spectrophotometric method was 2.05%. Both methods were applied to total antioxidant activity determination in real samples (natural juices and soft drinks) and the results were in good agreement.

Flow Injection Methods of Analysis for Waters. I. Inorganic Species

A comprehensive, critical, and an updated review of the applications of flow-injection analysis (FIA) techniques for the analysis of inorganic cations and anions in several types of water samples except marine waters is presented. The preconcentration of metals in water samples and automaton of FIA systems for monitoring water quality are also discussed. The review is documented with 280 references.

Flow Injection Determination Of L-Ascorbic Acid in Natural Juice with Biamperometric Detection

Abstract A flow-injection system with biamperometric detection for L-ascorbic acid (vitamin C) determination was designed. The method is based on L-ascorbic acid oxidation to dehydroascorbic acid, in acidic medium, using iodine-iodide solution as oxidizing reagent. The iodine amount consumed in the redox reaction was biaperometrically detected and it was proportional to the amount of L-ascorbic acid from the sample. The calibration graph was linear over the range of concentration 5×10−5-5×10−4 M of vitamin C. The RSD for a 2.5×10−4 M standard solution of vitamin C was 1.08% (n=10) and the throughput was 60 samples h−1. The selectivity of the proposed FIA method was verified using an ascorbate oxidase solution as carrier stream. It was found that L-ascorbic acid could be determined in natural products with a good selectivity and rapidity.

Flow Injection Methods of Analysis for Waters. II. Organic Pollutants

A comprehensive, critical and updated review for the applications of flow-injection analysis (FIA) techniques for the analysis of organic pollutants in several types of water samples, except marine waters, is presented. The methods of indirect determination of organic pollutants in water by using flow injection coupled with atomic absorption spectroscopy are also discussed.

Flow injection determination of chloride ions with spectrophotometric detection

Abstract A single-line FIA system with a stream-sample splitting device for chloride determination is presented. The analytical method is based on the Fe3+/Hg(SCN)2/Cl− system and the absorbance of the red Fe(SCN)2+ species monitored spectrophotometrically at 480 nm. Using a stream-sample splitting device, the recorded signal is composed of two merged peaks. Three calibration curves were obtained, once injecting the standard solution series, two using the maximum heights of P1 and P2 peaks and one using the height of T trough. The FIA system showed three linear responses to the concentration of chloride in the ranges 10-100 ppm (P1); 10-500 ppm (P2) and 20-1000 ppm (T), respectively. Also, it was capable of detecting chloride ions in different types of water with a throughput of 15 samples h−1 and the RSD for 240 ppm of Cl− (n=10) were 1.67% (P1); 2.38% (P2) and 1.23% (T), respectively. The interference of several ions (commonly found in water) on the FIA outputs was investigated.

Greener bioanalytical approach for LC/MS-MS assay of enalapril and enalaprilat in human plasma with total replacement of acetonitrile throughout all analytical stages.

Green bioanalytical approaches are oriented toward minimization or elimination of hazardous chemicals associated to bioanalytical applications. LC/MS-MS assay of enalapril and enalaprilat in human plasma was achieved by elimination of acetonitrile from both sample preparation and chromatographic separation stages. Protein precipitation (PP) by acetonitrile addition was replaced by liquid-liquid extraction (LLE) in 1-octanol followed by direct large volume injection of the organic layer in the chromatographic column operated under reversed phase (RP) separation mechanism. At the mean time, acetonitrile used as organic modifier in the mobile phase was successfully replaced by a mixture of propylene carbonate/ethanol (7/3, v/v). Three analytical alternatives ((I) acetonitrile PP+acetonitrile based chromatographic elution; (II) 1-octanol LLE+acetonitrile based chromatographic elution; (III) 1-octanol LLE+propylene carbonate/ethanol based chromatographic elution) were validated and the quality characteristics were compared. Comparison between these alternative analytical approaches was also based on results obtained on incurred samples taken during a bioequivalence study, through application of the Bland-Altman procedure.

Retention Studies for Large Volume Injection of Aromatic Solvents on Phenyl-Silica Based Stationary Phase in RP-LC

The use of a large volume injection of hydrophobic solvents as diluents for less hydrophobic solutes has already been proven for C18 and C8 stationary phases in reversed-phase liquid chromatography. The same possibility is investigated for a phenyl-hexyl stationary phase using aromatic solvents (benzene, toluene, ethylbenzene and propylbenzene) as diluents for several model analytes also containing aromatic rings. Both hydrophobic interaction and π-π stacking account for the competitive interaction of both the diluent and model analytes with the phenyl-hexyl phase. A linear decrease in analyte retention factor was observed with an increase of injection volume in the range of 1-100 µL. A moderate peak efficiency decrease was also observed, but peaks of model analytes remained undistorted with minimum band broadening up to 100 µL injection volume. A very small retention decrease was observed when changing the sample diluent in the homologous series: benzene, toluene, ethylbenzene and propylbenzene. The critical conditions for a successful large volume injection of analytes dissolved in studied hydrophobic solvents are for the analyte to have lower hydrophobicity and for the specified solutes to have proper solubility.

Alternative sample diluents in bioanalytical LC–MS

The problem of sample diluent in bioanalytical LC-MS is reviewed with a special focus on large-volume injections and non-miscible solvents with mobile phase components. These issues are related to the sample preparation approach, which in many instances provides the sample diluent before injecting this into the chromatographic column. The sample volume influences the quantitation limit of the chromatographic method, while its nature may influence the retention process of the injected analytes. The literature reports a few papers that are focused on alternative sample diluents in bioanalytical LC-MS that are generally non-miscible with mobile phase. The principle of this approach and some of its current bioanalytical applications from literature are discussed. However, more applications and more publications from HPLC users and vendors are expected in this field, which could prove its analytical importance and potential in bioanalysis.

Recent analytical applications of fluorinated hydrocarbon-based stationary phases in HPLC

ABSTRACT Fluorocarbon stationary phases have taken great advances in recent years in liquid chromatography. Numerous studies to elucidate the retention mechanism have been undertaken that concluded the retention is driven by a multitude of factors, due to the presence of fluorine atoms in the molecule. Many applications in this domain have been described in the literature so far. Applications for other fields have also been published in various original papers for fluorocarbon silica. GRAPHICAL ABSTRACT

Rapid voltammetric method for quinine determination in soft drinks

A novel voltammetric assay for quinine (QN) determination using an electrochemically pretreated pencil graphite electrode is described. The detection limit of QN was 2 × 10-7 M. The method possesses some obvious advantages including extreme simplicity, rapid response, and low cost.