Determination of sulphite and ascorbic acid by high-performance liquid chromatography with electrochemical detection

Comparative Studies on Carbon Paste Electrode Modified with Electroactive Polyamic Acid and Corresponding Polyimide without/with Attached Sulfonated Group for Electrochemical Sensing of Ascorbic Acid

In this study, electroactive poly (amic acid) (EPAA) and corresponding polyimide (EPI) without or with a sulfonated group (i.e., S-EPAA, and S-EPI) were prepared and applied in electrochemical sensing of ascorbic acid (AA). The electroactive polymers (EAPs) containing EPAA/EPI and S-EPAA/S-EPI were synthesized by using an amine-capped aniline trimer (ACAT) and sulfonated amine-capped aniline trimer (S-ACAT) as an electroactive segment that controlled the redox capability and influenced the degree of sensitivity of the EAPs towards AA. Characterization of the as-prepared EAPs was identified by FTIR spectra. The redox capability of the EAPs was investigated by electrochemical cyclic voltammetric studies. It should be noted that the redox capability of the EAPs was found to show the following trend: S-EPAA > S-EPI > EPAA > EPI. For the electrochemical sensing studies, a sensor constructed from an S-EPAA-modified carbon paste electrode (CPE) demonstrated 2-fold, 1.27-fold, and 1.35-fold higher electro-catalytic activity towards the oxidation of AA, compared to those constructed using a bare CPE, S-EPI-, and EPI/EPAA-modified CPE, respectively. The higher redox capability of S-EPAA-modified CPE exhibited a good electrochemical response towards AA at a low oxidative potential, with good stability and selectivity. Moreover, an electrochemical sensor constructed from S-EPAA-modified CPE was found to reveal better selectivity for a tertiary mixture of AA/DA/UA, as compared to that of EPI-modified, EPAA-modified and S-EPI-modified CPE, based on a series of differential pulse voltammograms.

The use of high-performance liquid chromatography with diode array detector for the determination of sulfide ions in human urine samples using pyrylium salts

Hydrogen sulfide is a toxic gas involved in the regulation of some essential biological processes. A novel, precise, accurate and rapid method based on high-performance liquid chromatography with diode array detection for the determination of sulfide ions in human urine sample is proposed. The method involves the derivatization of sulfide with pyrylium salts - (2,4,6-triphenylpyrylium hydrogensulfate(VI) (L1) and 4-[p-(N,N-dimethylamino)phenyl]-2,6-diphenylpyrylium chlorate(VII) (LN1). The separation occurs on InfinityLab Poroshell 120 EC C₁₈ column using acetonitrile and phosphate buffer as a mobile phase. The detectors utilized a wavelength of 371 or 580 nm. The calibration curves were linear in the range of 2-150 μmol L^-1 and 1-50 μmol L^-1 for L1 and LN1 derivatives, respectively. The samples were found to be stable from sample collection to final analysis. The method was successfully applied to samples from apparently healthy volunteers.

Crystal-defect-induced facet-dependent electrocatalytic activity of 3D gold nanoflowers for the selective nanomolar detection of ascorbic acid

Understanding and exploring the decisive factors responsible for superlative catalytic efficiency is necessary to formulate active electrode materials for improved electrocatalysis and high-throughput sensing. This research demonstrates the ability of bud-shaped gold nanoflowers (AuNFs), intermediates in the bud-to-blossom gold nanoflower synthesis, to offer remarkable electrocatalytic efficiency in the oxidation of ascorbic acid (AA) at nanomolar concentrations. Multicomponent sensing in a single potential sweep is measured using differential pulse voltammetry while the kinetic parameters are estimated using electrochemical impedance spectroscopy. The outstanding catalytic activity of bud-structured AuNF [iAuNFp(Bud)/iGCp ≅ 100] compared with other bud-to-blossom intermediate nanostructures is explained by studying their structural transitions, charge distributions, crystalline patterns, and intrinsic irregularities/defects. Detailed microscopic analysis shows that density of crystal defects, such as edges, terraces, steps, ledges, kinks, and dislocation, plays a major role in producing the high catalytic efficiency. An associated ab initio simulation provides necessary support for the projected role of different crystal facets as selective catalytic sites. Density functional theory corroborates the appearance of inter- and intra-molecular hydrogen bonding within AA molecules to control the resultant fingerprint peak potentials at variable concentrations. Bud-structured AuNF facilitates AA detection at nanomolar levels in a multicomponent pathological sample.

Detection of ascorbic acid and folic acid based on water-soluble CuInS2 quantum dots

In this paper, water-soluble CuInS(2) ternary quantum dots (QDs) modified by mercaptopropionic acid (MPA) were directly synthesized by hydrothermal method. Ascorbic acid (AA) can induce the fluorescence enhancement of MPA-capped CuInS(2) QDs and can be used for the detection of AA. Under the optimized conditions, the relationship between the fluorescence intensity of the CuInS(2) QDs and AA concentration was linear in the range of 0.25-200 μmol L(-1). Most relevant molecules and physiological ions had no effect on the detection of AA. The fluorescence intensity of CuInS(2) QDs enhanced by a certain amount of AA could be reduced in the presence of folic acid (FA) and thus can be used for the detection of FA with the linear range of 0.2-100 μmol L(-1). Compared with previous reports, the established approach utilized a simple, sensitive, and selective strategy to develop the QDs probe based on fluorescence enhancing and quenching phenomena without complicated immobilization.

Development of a quantitative method for the analysis of total l-ascorbic acid in foods by high-performance liquid chromatography

A new method has been developed for the determination of total vitamin C in foods. The method requires less time than the traditional methodologies and uses a radical oxidation of L-ascorbic acid (AA) to obtain dehydro-L-ascorbic acid (DHAA) by means of a peroxyl radical generated in situ by thermal decomposition of an azo-compound, 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH). The dehydro-L-ascorbic acid is condensed with benzene-1,2-diamine (o-phenylenediamine, OPDA) to form its highly fluorescent quinoxaline derivative, 3-(1,2-dihydroxyethyl)furo[3,4-b]quinoxaline-1-one (DFQ), which is then separated on a C(18) column eluted with a mobile phase of 80 mM phosphate buffer and methanol at pH=7.8 and detected fluorometrically at lambda(ex)=355 nm and lambda(em)=425 nm. The reaction conditions for the complete conversion of AA to DFQ were 56 degrees C, 36 min and a mumol AAPH/AA ratio of 60. The sample, extracted with an aqueous metaphosphoric acid solution, was analyzed after being filtered through a 0.45 microm membrane. The method has shown good repeatability, sensitivity and accuracy compared to the results obtained with the reference method. The response of the detection system was linear within a range of 0.5-8.0 microg/mL with a correlation coefficient of 0.9997. The limit of detection was 0.27 microg/mL and the limit of quantification was 0.83 microg/mL. The AA contents of some selected foods were analyzed.

Evaluation of the electrochemical behavior and analytical potentialities of a carbon paste electrode modified with a ruthenium (III) piperidinedithiocarbamate complex

The preparation and electrochemical characterization of a carbon paste electrode modified with bis(N,N-piperidinedithiocarbamate)-mu-tris(N,N-piperidinedithiocarbamate)diruthenium(III) complex, alpha-[Ru2(Pip)5]Cl are described. The best voltammetric response was obtained for a 10% (m/m) alpha-[Ru2(Pip)5]Cl content in the paste, potassium acid phthalate solution pH 4.0 as supporting electrolyte and scan rate of 100 mV s-1. The analytical potentialities of the electrode have been evaluated using L-ascorbic acid (vitamin C) as a probe. A sensitive linear voltammetric response for L-ascorbic acid was obtained in the concentration range 4.50-113x10(-5) mol l-1 (7.92-200 mg l-1) with a slope of 1.12x10(4) microA mol-1, and a detection limit (3sigma/slope) of 7.00x10(-6) mol l-1 using cyclic voltammetry. The concentrations of L-ascorbic acid in pharmaceutical formulations (tablets and liquid) were determined using the modified electrode and compared with an iodimetric procedure with good agreement at the 95% confidence and relative error lower than 1%.

Carbon paste electrode modified with copper (II) phosphate immobilized in a polyester resin for voltammetric determination of L-ascorbic acid in pharmaceutical formulations

A carbon paste electrode modified with copper(II) phosphate immobilized in a polyester resin (CuP-Poly) is proposed for voltammetric determination of L-ascorbic acid in pharmaceutical formulations. The modified electrode allows the detection of L-ascorbic acid at lower anodic potentials than observed at unmodified electrodes. Several parameters that can influence the voltammetric response of the proposed electrode such as carbon paste composition, pH, scan rate, and possible interference were investigated. The peak current was proportional to the concentration of ascorbic acid in the range 2.0 x 10(-5) to 3.2 x 10(-3) mol L(-1) with a detection limit of 1.0 x 10(-5) mol L(-1). The stability and repeatability of the electrode for the determination of L-ascorbic acid are also discussed. Amperometric response was also recorded for electrocatalytic oxidation of the L-ascorbic acid. Concentrations of the vitamin C in pharmaceutical formulations (tablets) measured using the modified electrode and a titrimetric method are in agreement at the 95% confidence level and within an acceptable range of error.

Determination of L-ascorbic acid and total ascorbic acid in vascular and nonvascular plants by capillary zone electrophoresis

A capillary zone electrophoresis (CZE) procedure for the determination of l-ascorbic acid (AA) and total ascorbic acid (TAA, as the sum of AA and dehydroascorbic acid) in vascular plants, lichens, bryophytes, and liverworts is described. The samples were frozen in liquid nitrogen and extracted with 2% metaphosphoric acid. To determine AA, an aliquot was directly injected in a fused-silica capillary. The determination of TAA was performed upon its reduction with dl-homocysteine at pH 7. The background electrolyte contained 60 mM sodium chloride, 60 mM sodium dihydrogen phosphate, 1 x 10(-4)% hexadimetrine bromide, and NaOH up to pH 7. The procedure was rapid and highly reproducible; the limit of detection was 0.2 microg ml-1, which corresponded to 0.4 mg/100 g of sample. The method was validated by comparison with an enzymatic assay. While the enzymatic assay failed to quantify AA in some plants owing to strong background UV absorption, CZE was successful in all the extracts assayed.