Solid-phase iodine as an oxidant in flow injection analysis: determination of ascorbic acid in pharmaceuticals and foods by background correction

A flow injection analysis (FIA)-background correction method comprising two solid-phase reactors and spectrophotometry for determination of ascorbic acid (AsA) is proposed. A polyethylene mini-column filled with solid iodine (30% m/m suspended on silica gel beads), reactor 1, and other column filled only with silica gel, reactor 2, which are then incorporated in a flow system so that solid iodine reagent in reactor 1 is affected as the sample passes through the column. The sample blank is produced by the oxidation of the AsA by iodine to form dehydroascorbic acid, insensitive to ultraviolet at 267 nm. AsA in samples is determined after injected in reactor 2; the difference in two analytical signal observed is related to amount of AsA. The linear range of the system is up to 50 μg ml⁻¹ with a detection limit of 0.08 μg ml⁻¹, R.S.D. of better than 1.0% and sampling frequency of 110 sample h⁻¹. The method is successfully applied to the determination of AsA in pharmaceuticals and foods.     

Repetitive determination of chloride using the circulation of the reagent solution in closed flow-through system

The precipitation reaction of silver chloride (AgCl) is carried out in a large amount of ammonia (NH₃). This makes possible to adopt a closed-loop flow injection (FI) system and to determine chloride repetitively. A solution of 30 mmol l⁻¹ silver nitrate and 80 mmol l⁻¹ NH₃ in a single reservoir (250 ml) is continuously circulated through the flow cell at a flow rate of 2.0 ml ml⁻¹. The chloride containing sample (100 μl) was introduced into this reagent solution by means of six-way valve. AgCl precipitates formed in the sample zone are monitored spectrophotometrically (at 500 nm) in the flow system. After passing through the flow cell, the excess NH₃ in the circulating reagent solution dissolves AgCl precipitates and the stream then returns to the reservoir. Various variables of the FI system were optimized and a study of interfering ions was also carried out. A linear calibration graph was obtained from 3.0 to 30 mg l⁻¹ chloride. Two hundred repetitive injections of 5.0 mg l⁻¹ chloride into the circulating reagent solution have shown unchanged base-line and good reproducibility. The method was successfully applied to the determination of chloride in tap, natural and the reference waters.     

Flow injection fluorometric determination of ascorbic acid using perylenebisimide-linked nitroxide

A simple and sensitive flow injection fluorometric method for the determination of ascorbic acid is described. Perylenebisimide-linked nitroxide (PBILN) is used as a fluorescent reagent, which permits the selective determination of ascorbic acid. The fluorescence of the perylenebisimide moiety in PBILN is quenched by the nitroxide moiety, which is linked to the perylenebisimide. When a stream of a solution of ascorbic acid is merged with a stream of PBILN, the ascorbic acid reacts with the nitroxide moiety of PBILN to form hydroxylamine, and the fluorescence properties of the perylenebisimide moiety are recovered. As a result, a peak-shaped fluorescence signal is produced, which can be observed by a fluorescence detector located downstream. Under optimized conditions, a good linear relationship between the concentration of ascorbic acid and peak height in the concentration range from 0.5 to 10 μmol L⁻¹ was found and the detection limit (S/N = 3) was 0.28 μmol L⁻¹. The relative standard deviation for the determination of 4.0 μmol L⁻¹ ascorbic acid samples was 1.0% (n = 5). The proposed method was applied to the determination of ascorbic acid in several soft drink beverages and the analytical results were in good agreement with those obtained using a conventional method.     

Spectrophotometric determination of ascorbic acid using copper(II)-neocuproine reagent in beverages and pharmaceuticals

The proposed method for ascorbic acid (Vitamin C) (AA) determination is based on the oxidation of AA to dehydroascorbic acid with a Cu(II)-2,9-dimethyl-1,10-phenanthroline (neocuproine (Nc)) reagent in ammonium acetate-containing medium at pH 7, where the absorbance of the formed bis(Nc)-copper(I) chelate is measured at 450 nm. This chelate was formed immediately and the apparent molar absorptivity for AA was found to be 1.60 × 10⁴ dm³ mol⁻¹ cm⁻¹. Beer's law was obeyed between 8.0 × 10⁻⁶ and 8.0 × 10⁻⁵ M concentration range. The relative standard deviation for 90 μg AA was 3%. The Cu(II)-Nc reagent is a milder and therefore more selective oxidant than the conventional Fe(III)-1,10-phenanthroline (phen) reagent used for the same assay. This feature makes the proposed method superior for real samples such as fruit juices containing weak reductants such as citrate, oxalate and tartarate that otherwise produce positive errors in the Fe(III)-phen method when equilibrium is achieved. The developed method was applied to a number of commercial fruit juices, pharmaceutical preparations containing Vitamin C, and red wine. The meta-bisulfite content of wine was removed with an anion exchanger at pH 3 prior to analysis, and a difference extractive–spectrophotometric method of AA assay in wine was developed so as to suppress the interferences caused by wine anthocyanins and polyphenols. The findings of the developed method for fruit juices and pharmaceuticals were also statistically compared with those of HPLC so as to establish it as a reliable novel method.     

Simultaneous determination of tryptophan, uric acid and ascorbic acid at iron(III) doped zeolite modified carbon paste electrode

A new chemically modified electrode is constructed based on iron(III) doped zeolite modified carbon paste electrode (Fe(3+)Y/ZCME). The electrode was evaluated as a sensor for sub-micromolar determination of tryptophan (Trp), uric acid (UA) and ascorbic acid (AA) in aqueous solutions. The measurements were carried out by application of the differential pulse voltammetry (DPV) method in phosphate buffer solution with pH 3.5. Iron(III) loaded in zeolite can increase anodic peak currents by adsorption of Trp, UA and AA on electrode surface The analytical performance was evaluated with respect to the carbon paste composition, pH of solution, accumulation time and accumulation potential. The prepared electrode shows voltammetric responses with high sensitivity and selectivity for Trp, UA and AA in optimal conditions, which makes it very suitable for simultaneous determination of these compounds. The linear calibration range for AA in the presence of 50muM UA and 50muM Trp was 0.6muM to 100muM, with a correlation coefficient of 0.9992, and a detection limit of 0.21muM (S/N=3). A linear relationship was found for UA in the range of 0.3-700muM containing 10muM AA and 50muM Trp, with a correlation coefficient of 0.9990 and a detection limit of 0.08muM. The linear calibration range for Trp in the presence of 10muM AA and 50muM UA was 0.2-150muM, with a correlation coefficient of 0.9996, and a detection limit of 0.06muM. The proposed method was successfully applied for determination Trp, UA and AA in biological systems and pharmaceutical samples.     

Spectrophotometric determination of ascorbic acid by the modified CUPRAC method with extractive separation of flavonoids-La(III) complexes

The proposed method for ascorbic acid: AA (Vitamin C) determination is based on the oxidation of AA to dehydroascorbic acid with the CUPRAC reagent of total antioxidant capacity assay, i.e., Cu(II)-neocuproine (Nc), in ammonium acetate-containing medium at pH 7, where the absorbance of the formed bis(Nc)-copper(I) chelate is measured at 450 nm. The flavonoids (essentially flavones and flavonols) normally interfering with the CUPRAC procedure were separated with preliminary extraction as their La(III) chelates into ethylacetate (EtAc). The Cu(I)-Nc chelate responsible for color development was formed immediately with AA oxidation. Beer's law was obeyed between 8.0 × 10⁻⁶ and 8.0 × 10⁻⁵ M concentration range, with the equation of the linear calibration curve: A_450 nm = 1.60 × 10⁴C (mol dm⁻³) − 0.0596. The relative standard deviation (R.S.D.) in the analysis of N = 45 synthetic mixtures containing 1.25 × 10⁻² mM AA with flavonoids was 5.3%. The Cu(II)-Nc reagent is a lower redox-potential and therefore more selective oxidant than the Fe(III)-1,10-phenanthroline reagent conventionally used for the same assay. This feature makes the proposed method superior for real samples such as fruit juices containing weak reductants such as citrate, oxalate and tartarate that may otherwise produce positive errors in the Fe(III)-phen method when equilibrium is achieved. The developed method was applied to some commercial fruit juices and pharmaceutical preparations containing Vitamin C + bioflavonoids. The findings of the developed method for fruit juices and pharmaceuticals were statistically alike with those of HPLC. The proposed spectrophotometric method was practical, low-cost, rapid, and could reliably assay AA in the presence of flavonoids without enzymatic procedures open to interferences by enzyme inhibitors.     

A simple flow injection spectrophotometric determination method for iron(III) based on O-acetylsalicylhydroxamic acid complexation

A simple and rapid flow-injection spectrophotometric method for the determination of iron(III) and total iron is proposed. The method is based on the reaction between iron(III) and O-acetylsalicylhydroxamic acid (AcSHA) in a 2 % methanol solution resulting in an intense violet complex with strong absorption at 475 nm. Optimum conditions for the determination of iron(III) and the interfering ions were tested. The relative standard deviation for the determination of 5 μg L⁻¹ iron(III) was 0.85 % (n = 10), and the limit of detection (blank signal plus three times the standard deviation of the blank) was 0.5 μg L⁻¹, both based on the injection volumes of 20 μL. The method was successfully applied in the determination of iron(III) and total iron in water and ore samples. The method was verified by analysing a certified reference material Zn/Al/Cu 43XZ3F and also by the AAS method.     

Flow injection spectrophotometric determination of iron(III) using diphenylamine-4-sulfonic acid sodium salt

A highly sensitive and very simple spectrophotometric flow-injection analysis (FIA) method for the determination of iron(III) at low concentration levels is presented. The method is based on the measurement of absorbance intensity of the red complex at 410 nm formed by iron(III) and diphenylamine-4-sulfonic acid sodium salt (DPA-4-SA). It is a simple, highly sensitive, fast, and low cost alternative method using the color developing reagent DPA-4-SA in acetate buffer at pH 5.50 and the flow-rate of 1 mL min⁻¹ with the sample throughput of 60 h⁻¹. The method provided a linear determination range between 5 μg L⁻¹ and 200 μg L⁻¹ with the detection limit (3S) of 1 μg L⁻¹ of iron(III) using the injection volume of 20 μL. FIA variables influencing the system performance were optimized. The amount of iron(III) and total iron in river and seawater samples was successfully determined. Repeatability of the measurements was satisfactory at the relative standard deviation of 3.5 % for 5 determinations of 10 μg L⁻¹ iron(III). The accuracy of the method was evaluated using the standard addition method and checked by the analysis of the certified material Std Zn/Al/Cu 43 XZ3F.     

聚中性红/纳米二氧化硅复合修饰电极直接测定抗坏血酸

用滴涂法和电化学聚合法制备了聚中性红/纳米二氧化硅修饰电极(PNR/nano-SiO2/GCE),并用循环伏安法和交流阻抗法研究了修饰电极表面的电化学行为。实验表明,该修饰电极对抗坏血酸(AA)表现出良好的电催化氧化性能,探讨了复合修饰电极协同增效作用的机理。用线性扫描伏安法研究了AA浓度与峰电流之间线性关系,在pH2.0的磷酸盐缓冲溶液中,AA氧化峰电流在1.8×10-6～5.0×10-3mol/L浓度范围内呈良好的线性关系,检出限为5.4×10-7mol/L(S/N=3)。该修饰电极制备简单,可用于药品及果蔬食品中抗坏血酸的直接测定。     

Reverse flow injection spectrophotometric determination of iron(III) using norfloxacin

A reversed flow injection colorimetric procedure for determining iron(III) at the μg level was proposed. It is based on the reaction between iron(III) with norfloxacin (NRF) in 0.07 mol l⁻¹ ammonium sulfate solution, resulting in an intense yellow complex with a suitable absorption at 435 nm. Optimum conditions for determining iron(III) were investigated by univariate method. The method involved injection of a 150 μl of 0.04% w/v colorimetric reagent solution into a merged streams of sample and/or standard solution containing iron(III) and 0.07 mol l⁻¹ ammonium sulfate in sulfuric acid (pH 3.5) solution which was then passed through a single bead string reactor. Subsequently the absorbance as peak height was monitored at 435 nm. Beer's law obeyed over the range of 0.2–1.4 μg ml⁻¹ iron(III). The method has been applied to the determination of total iron in water samples digested with HNO₃–H₂O₂ (1:9 v/v). Detection limit (3σ) was 0.01 μg ml⁻¹ the sample through of 86 h⁻¹ and the coefficient of variation of 1.77% (n=12) for 1 μg ml⁻¹ Fe(III) were achieved with the recovery of the spiked Fe(III) of 92.6–99.8%.     

11-Molybdobismuthophosphate—A new reagent for the determination of ascorbic acid in batch and sequential injection systems

The guanidinium salt of the new heteropolymolybdate 11-molybdobismuthophosphate Gua₆PBiMo₁₁O₄₀ (11-MBP) was synthesized, characterized and used as a reagent for batch spectrophotometric (SP) and sequential injection determination of ascorbic acid (AsA). When compared to other Keggin's heteropolyanions, the reduction of 11-MBP with AsA is both fast and maximal within a pH range of 1.6-2.0. The stoichiometry of the reaction was determined using molar ratio and continuous variation methods and was shown to be 1:1. The molar absorptivity of the reduced form of 11-MBP was 6.0 × 10³ L mol⁻¹ cm⁻¹ at 720 nm. The reaction is also specific for AsA. Only cysteine, hydroquinone and hydroxyacids were found to interfere with the reaction, while no interference was observed with the common reducing agents, including reducing sugars, catecholamines, nitrite, sulfite and iron(II) ions. Batch SP and sequential injection analysis (SIA) systems were developed for the determination of AsA, with calibration ranges of the SP methods at 2 × 10⁻⁶-8 × 10⁻⁵ M for a 10 mm cell and 5 × 10⁻⁷-3 × 10⁻⁵ M for a 50 mm cell and a limit of detection at 3 × 10⁻⁷ M. The linear range of the SIA method was 6 × 10⁻⁶-5 × 10⁻⁴ M, with a detection limit of 2 × 10⁻⁶ M and a sample throughput of 15 h⁻¹. The proposed methods were successfully used for the determination of AsA in both pharmaceuticals and fruit juices, and the results were consistent with those provided by the 2,6-dichlorophenolindophenol method.     

Flow injection spectrophotometric or conductometric determination of ascorbic acid in a vitamin C tablet using permanganate or ammonia

Two simple flow injection (FI) procedures for the determination of ascorbic acid content in a vitamin C tablet are proposed: spectrophotometric involving injection into a stream of acidic potassium permanganate solution and monitoring its color change due to the redox reaction; FI conductometry based on the neutralization of ascorbic acid injected into a flowing ammonia solution yielding a change in conductivity. The procedures have been applied to the analysis of locally commercial vitamin C tablet samples. A through-put of at least 90 injections h⁻¹ can be achieved. The relative standard deviation was found to be 2.5% (for a 50 mg vitamin C tablet; n=7) for both. Results obtained by either procedure agree with a standard titrimetric method.     

New method for simultaneous determination of Fe(II) and Fe(III) in water using flow injection technique

The method exploits the possibilities of flow injection gradient titration in a system of reversed flow with spectrophotometric detection. In the developed approach a small amount of titrant (EDTA) is injected into a stream of sample containing a mixture of indicators (sulfosalicylic acid and 1,10-phenanthroline). In acid environment sulfosalicylic acid forms a complex with Fe(III), whereas 1,10-phenanthroline forms a complex with Fe(II). Measurements are performed at wavelength λ = 530 nm when radiation is absorbed by both complexes. After injection EDTA replaces sulfosalicylic acid and forms with Fe(III) more stable colourless complex. As a result, a characteristic “cut off” peak is registered with a width corresponding to the Fe(III) concentration and with a height corresponding to the Fe(II) concentration. Calibration was performed by titration of four two-component standard solutions of the Fe(II)/Fe(III) concentrations established in accordance with 2² factorial plan. The method was tested with the use of synthetic samples and then it was applied to the analysis of water samples taken from artesian wells. Under optimized experimental conditions Fe(II) and Fe(III) were determined with precision less than 0.8 and 2.5% (RSD) and accuracy less than 3.2 and 5.1% (relative error) within the concentration ranges of 0.1-3.0 and 0.9-3.5 mg L⁻¹ of both analytes, respectively.     

多巴胺和抗坏血酸在十六烷基三甲基溴化铵/碳纳米管电极上的检测

研究了十六烷基三甲基溴化铵(CTMAB)/多壁碳纳米管修饰玻碳电极的制备以及多巴胺和抗坏血酸在该修饰电极上的电化学行为。在CTMAB和多壁碳纳米管的协同作用下,该修饰电极对多巴胺和抗坏血酸均具有显著的催化氧化作用,多巴胺和抗坏血酸的氧化峰电位分别为223mV和15mV,实现了在抗坏血酸共存时测定多巴胺。在pH7.0的磷酸盐缓冲溶液中,多巴胺和抗坏血酸的线性范围分别为2.0×10-6～2.0×10-3mol/L和4.0×10-5～1.0×10-2mol/L,检出限分别为6.0×10-7mol/L和1.0×10-5mol/L。     

Reverse flow injection spectrophotometric determination of iron(III) using chlortetracycline reagent

A simple reversed flow injection colourimetric procedure for determining iron(III) was proposed. It is based on the reaction between iron(III) with chlortetracycline, resulting in an intense yellow complex with a suitable absorption at 435 nm. A 200 μl chlortetracycline reagent solution was injected into the phosphate buffer stream (flow rate 2.0 ml min⁻¹) which was then merged with iron(III) standard or sample in dilute nitric acid stream (flow rate 1.5 ml min⁻¹). Optimum conditions for determining iron(III) were investigated by univariate method. Under the optimum conditions, a linear calibration graph was obtained over the range 0.5–20.0 μg ml⁻¹. The detection limit (3σ) and the quantification limit (10σ) were 0.10 and 0.82 μg ml⁻¹, respectively. The relatives standard deviation of the proposed method calculated from 12 replicate injections of 2.0 and 10.0 μg ml⁻¹ iron(III) were 0.43 and 0.59%, respectively. The sample throughput was 60 h⁻¹. The proposed method has been satisfactorily applied to the determination of iron(III) in natural waters.     

A multi-pumping flow system for chemiluminometric determination of ascorbic acid in powdered materials for preparation of fruit juices

A multi-pumping flow-based procedure with chemiluminescent detection is proposed for the determination of ascorbic acid, AA, in fruit juices (powdered form). The method relies on the inhibitory effect of AA on the oxidation of luminol by hydrogen peroxide in alkaline medium. The system comprises several discretely actuated solenoid pumps as the only active components. It handles 100 samples per hour, and requires 96 μl of sample, 42 μg of luminol and 105 μg of potassium hexacyanoferrate(III) per determination. The analytical curve is linear up to about 11 mmol l^− 1 AA, and detection limit is 0.17 mmol l^− 1 AA. The system yields precise measurements (r.s.d. < 1%; n = 11), and recovery ranges from 94% to 106%. Results are in agreement with the reference method (AOAC) at the 95% confidence level.     

Spectrophotometric method for determination of sulfide with iron(III) and nitrilotriacetic acid by flow injection

A manual colorimetric method for determination of sulfide has been adapted to flow injection, systematically optimized, and more fully characterized. Its intended application is for measurement of sodium sulfide reagent strength in pulp process streams, and sulfide contamination in effluent from Kraft pulp mills. In the flow-injection method developed, a sample solution containing sulfide is reacted with a mixture of iron(III) and nitrilotriacetic acid under ammoniacal conditions. The absorbance of the intensely-colored green product of this reaction is measured at 636 nm. Excess sulfite is present as a color stabilizer. A linear dynamic range of 20-100 ppm sulfide is readily achieved; the relative standard deviation is less than 1.2% (n = 10) throughout this range, and 0.37% (n = 10) midrange at 60 ppm. The usable dynamic range is 8-250 ppm sulfide. Long-term stability of the method is ensured by periodically performing an automatic cleaning cycle using a hydrochloric acid wash solution. This prevents tube discoloration and removes any precipitates which are formed under strongly alkaline conditions. The sample throuhput rate is at least 30/hr, given alternate acid wash cycles.     

Electrochemical determination of dopamine and ascorbic acid at a novel gold nanoparticles distributed poly(4-aminothiophenol) modified electrode

A modified electrode is fabricated by embedding gold nanoparticles into a layer of electroactive polymer, poly(4-aminothiophenol) (PAT) on the surface of glassy carbon (GC) electrode. Cyclic voltammetry (CV) is performed to deposit PAT and concomitantly deposit Au nanoparticles. Field emission transmission electron microscopic image of the modified electrode, PAT-Au_nano-ME, indicates the presence of uniformly distributed Au nanoparticles having the sizes of 8-10 nm. Electrochemical behavior of the PAT-Au_nano-ME towards detection of ascorbic acid (AA) and dopamine (DA) is studied using CV. Electrocatalytic determination of DA in the presence of fixed concentration of AA and vice versa, are studied using differential pulse voltammetry (DPV). PAT-Au_nano-ME exhibits two well defined anodic peaks at the potential of 75 and 400 mV for the oxidation of AA and DA, respectively with a potential difference of 325 mV. Further, the simultaneous determination of AA and DA is studied by varying the concentration of AA and DA. PAT-Au_nano-ME exhibits selectivity and sensitivity for the simultaneous determination of AA and DA without fouling by the oxidation products of AA or DA. PAT and Au nanoparticles provide synergic influence on the accurate electrochemical determination of AA or DA from a mixture having any one of the component (AA or DA) in excess. The practical analytical utilities of the PAT-Au_nano-ME are demonstrated by the determination of DA and AA in dopamine hydrochloride injection and human blood serum samples.     

Repetitive determination of calcium ion and regeneration of a chromogenic reagent using chlorophosphonazo III and an ion exchanger in a circulatory flow injection system.

The spectrophotometric determination of Ca2+ with chlorophosphonazo III (CPN) has been carried out by a circulatory flow injection (FI) method. A cation-exchange mini-column for the on-line regeneration of the main reagent was incorporated in this FT system, allowing a repetitive determination of Ca2+. A solution of 4.0 x 10(-5) M CPN in a 0.05 M acetate buffer (pH 5.0) in a single reservoir (50 ml) was continuously circulated at a constant flow rate of 1.5 ml min(-1). Into the stream, an aliquot (20 microl) of a sample containing Ca2+ was quickly injected by means of a 6-way valve. The complex formed was monitored spectrophotometrically at 670 nm in the flow system. Then, the stream passed through a cation-exchange column, which was introduced after the flow-through cell. A successful ligand-exchange reaction of Ca2+ between the CPN reagent and a cation exchanger, as well as a simultaneous regeneration of the free reagent took place. The stream then returned to the reservoir. The regeneration and recycling of the CPN reagent allowed as many as 300 repetitive determinations of 2.5 mg l(-1) Ca2+ solutions with the same 50 ml circulating solution.     

Flow injection spectrofluorimetric determination of iron(III) in water using salicylic acid

A simple and fast flow injection fluorescence quenching method for the determination of iron in water has been developed. Fluorimetric determination is based on the measurement of the quenching effect of iron on salicylic acid fluorescence. An emission peak of salicylic acid in aqueous solution occurs at 409 nm with excitation at 299 nm. The carrier solution used was 2 × 10⁻⁶ mol L⁻¹ salicylic acid in 0.1 mol L⁻¹ NH₄⁺/NH₃ buffer solution at pH 8.5. Linear calibration was obtained for 5–100 μg L⁻¹ iron(III) and the relative standard deviation was 1.25 % (n = 5) for a 20 μL injection volume iron(III). The limit of detection was 0.3 μg L⁻¹ and the sampling rate was 60 h⁻¹. The effect of interferences from various metals and anions commonly present in water was also studied. The method was successfully applied to the determination of low levels of iron in real samples (river, sea, and spring waters).