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.     

Chlorophyll triggered one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones via photo induced electron transfer reaction

The photocatalytic potential of chlorophyll has been investigated for the facile synthesis of dihydropyrimidinones utilizing concentrated solar irradiation towards sustainable energy solutions. This, one-pot, multicomponent Biginelli reaction, which involves a photoinduced electron transfer (PET) mechanism, affords a green and efficient approach for the transformation of the commercial aldehydes, β-keto ester and urea into valuable 3,4-dihydropyrimidin-2(1H)-ones with wide substrate scope and diversity. These improved reaction conditions allow the formation of a variety of substituted dihydropyrimidinones with high yields and purity in a short duration of time and mild reaction conditions.     

流动注射-电化学氧化法测定水体化学需氧量

采用硼掺杂金刚石(Boron-doped diamond,BDD)薄膜电极为工作电极,利用流动注射分析方法测定水体化学需氧量(COD),根据水样中有机物组分在工作电极表面氧化消耗的电量(Qoxidation)测定样品的COD值。考察了一些基本参数包括载液、工作电位、流速对检测信号的影响并选定了最佳检测条件。在最佳检测条件下,本法检测COD的线性范围为2.5~120 mg/L,检出限为1 mg/L。用该法测定化工厂和食品厂废水的COD值,相对标准偏差和回收率分别在2.4%~4.8%和96%~106%之间,且检测结果与国家标准方法(CODcr法)有良好的一致性。     

Divergence in photoinduced electron transfer (PET) reactions: a useful strategy towards identifying route-selectivity in mixed ketones

The work utilized photoinduced electron transfer (PET) reactions to identify the preferred photoreaction route in molecules having juxtaposed α,β and β,γ-enones. Such process directly converted 2-hydroxyimino derivatives of 5-benzoylbicyclo[2.2.2]octenones to the corresponding bicyclo[3.2.1]octane derivatives. First evidence of Type B rearrangement in α,β-enones having acyl substitution at Cγ-position has been depicted in this work. In rigid mixed enones, this has been found to be generally the preferred photoreaction route.     

Flow injection spectrophotometric determination of fosfestrol, following on-line thermal induced digestion and using an orthophosphate calibration graph

A novel flow injection (FI) system for the spectrophotometric determination of diethyl stilbestrol diphosphate (fosfestrol) in pharmaceutical formulations is described. On-line thermal induced digestion of the analyte by peroxodisulfate ion was performed and the orthophosphate ion generated was determined spectrophotometrically (λ_max=690 nm) using a molybdenum blue based FI approach. As the achieved conversion of the analyte was quantitative, an orthophosphate calibration graph can be used for its determination as well. The chemical and FI variables affecting the digestion were investigated. A linear calibration graph was obtained in the range 5.0×10⁻⁷-1.0×10⁻⁴ mol l⁻¹ fosfestrol. The relative standard deviation was very good (s_r=0.8% at 5.0×10⁻⁵ mol l⁻¹ fosfestrol, n=12) and the 3σ detection limit was 2.5×10⁻⁷ mol l⁻¹. The sampling rate was 60 injections h⁻¹. The average accuracies for the determination of the analyte in a pharmaceutical formulation evaluated by comparison of the results with those obtained by the supplier (Asta Medica) and the method recommended by the US Pharmacopoeia were also very good (e_r of +0.8 and −0.3%, respectively). Average recoveries of known amounts of the analyte ranging between 97.9 and 100.8% were also obtained.     

Decolorization Reactions Between Chromium (VI) and Three Colour Reagents. Determination of Chromium (VI) by Flow Injection Analysis

Abstract

Chromium (VI) can oxidize and decolor three colour reagents, i.e. 1,8-dihydroxy-2-(4′-chloro-2′-phosphonophenylazo)-7-(6″.8″-disulfonaphthylazo)-3,6-disulfonaphthalene (RI): 1,8-dihydroxy-2-(4′-chloro-2′-phosphonophenylazo)-7-(4″-sulfonamidephenylazo)-3,6-disulfonaphthalene (RII): and 1,8-dihydroxy-2-(4′-chloro-2′-phosphonophenylazo)-7-(p-hippuric acid azo)-3,6-disulfonaphthalene (RIII). Loss of absorbance of three colour reagents at maximum absorption wavelengths is proportional to the concentration of chromium (VI) in solution. We have made use of the decolorization reactions between chromium (VI) and three colour reagents to determine chromium in steel by flow injection analysis, and relative error in sample determination is less than 5.0%. Response is linear from 0.84 to 6.72 μg/mL and optimum measuring acidity is 1.3–1.7 mol/L H₂SO₄ in three systems. The effect of interference ions on determination has been studied.     

Flow injection chemiluminescence determination of the total phenolics levels in plant-derived beverages using soluble manganese(IV)

This study established a flow injection (FI) methodology for the determination of the total phenolic content in plant-derived beverages based on soluble manganese(IV) chemiluminescence (CL) detection. It was found that mixing polyphenols with acidic soluble manganese(IV) in the presence of formaldehyde evoked chemiluminescence. Based on this finding, a new FI-CL method was developed for the estimation of the total content of phenolic compounds (expressed as milligrams of gallic acid equivalent per litre of drink) in a variety of wine, tea and fruit juice samples. The proposed method is sensitive with a detection limit of 0.02 ng mL⁻¹ (gallic acid), offers a wide linear dynamic range (0.5-400 ng mL⁻¹) and high sample throughput (247 samples h⁻¹). The relative standard deviation for 15 measurements was 3.8% for 2 ng mL⁻¹ and 0.45% for 10 ng mL⁻¹ of gallic acid. Analysis of 36 different samples showed that the results obtained by the proposed FI-CL method correlate highly with those obtained by spectrophotometric methods commonly used for the evaluation of the total phenolic/antioxidant level. However, the FI-CL method was found to be far simpler, more rapid and selective, with almost no interference from non-phenolic components of the samples examined.     

Spectrophotometric determination of mercury with 5,10,15,20-tetrakis(3-chloro-4-sulfophenyl)porphine by flow injection analysis

5,10,15,20-tetrakis(3-chloro-4-sulfophenyl)porphine (m-Cl-TPPS₄) was synthesized and used for the Spectrophotometric determination of mercury by flow injection analysis. A pseudo-first-order reaction kinetic mechanism was proposed with a rate constant of 0.8 min^–1 for Hg(II) withm-Cl-TPPS₄ in the presence of 8-hydroxyquinoline in a medium of 1.0M acetic acid and sodium acetate buffer solution (pH 6.22). In the optimum conditions of reaction temperature (85 ° C), stopped-flow time (60 s) and sampling volume (100 l), the method's relative standard deviation was 0.82% (n = 12) at 5.0 g ml^–1 mercury, with a linear range of 0–12.0 g ml^–1 and an analytical frequency of 60h^–1. The detection limit (3) was 0.025 g ml^–1. Interference studies showed that most metal ions co-existing with Hg²⁺ could be tolerated at 100-fold excess levels, but Zn²⁺, Cu²⁺ and Mn²⁺ needed to be masked. The method has been applied to the analysis of water samples with satisfactory results.     

Batch and hydrodynamic monitoring of vitamin C using novel periodate selective sensors based on a newly synthesized Ni(II)-Schiff bases complexes as a neutral receptors

A highly selective membrane electrodes based on a two newly synthesized nickel (II) Schiff bases, [NiL¹] and [NiL²] where L¹ and L² are N,N/bis(salicylaldehyde)4,5-dimethyl-1,2-phenylenediamine (H₂L¹) and N,N/bis(salicylaldehyde)4,5-dichloro-1,2-phenylenediamine (H₂L²) were used as a neutral carrier ionophores for static and hydrodynamic potentiometric mode of operations for the determination of periodate. Under static mode of operation, the sensors displayed a near-Nernstian slope of −66.1 ± 0.8 and −59.9 ± 1.1 mV decade⁻¹ of activity and detection limits to 5.2 × 10⁻⁶ and 7.3 × 10⁻⁶ mol L⁻¹ for the sensors based on [NiL¹] and [NiL²], respectively. Under hydrodynamic mode of operation (FIA), the slope of the calibration plot, limit of detection, and working linear range were −71.1 mVdecade⁻¹ of activity, 7.3 × 10⁻⁶ and 1.0 × 10⁻⁵ to 1.0 × 10⁻³ mol L⁻¹, respectively. The response time of the sensors in whole concentration ranges was very short (<10 s). The response of the sensors was independent on the pH range of 3-8. A tubular version was further developed and coupled to a flow injection system for ascorbic acid (AA) determination in beverages and pharmaceutical preparations. This approach was achieved by selecting a 50-cm reactor and an overall flow of 3 mL min⁻¹, and injecting volume 100 μL of AA standards in a 1.0 × 10⁻⁴ mol L⁻¹ IO₄⁻ solution. Under these conditions, a linearity range of 2-13 μg mL⁻¹, with a slope of 4.97 mV (mg/L)⁻¹ (r² = 0.9995), detection limit 0.9 mg L⁻¹ and a reproducibility of ±1.1 mV (n = 5) was recorded. This simple and inexpensive flow injection analysis manifold, with a good potentiometric detector, enabled the analysis of ∼50 samples h⁻¹ without requiring pretreatment procedures. An average recovery of 98.8% and a mean standard deviation of 1.3% were obtained.     

Use of poly(ester-sulfonic acid) film-coated electrodes as amperometric detectors in flow injection analysis

Within the past several years significant advances have been made towards the development and incorporation of chemically modified electrodes as selective detectors for high performance liquid chromatography and flow injection analysis. In many cases the chemically modified electrode systems closely approach the “ideal” detector specifications of chemical and mechanical stability along with a significant linear response region. This paper will discuss the characterization and incorporation of ionomeric poly(ester-sulfonic acid) coated electrodes as nonaqueous electrochemical detectors. The orientation of the electrodes in the detector system as well as the increased sensitivity levels to 10⁻¹⁰ g ml⁻¹ for cationic species and 10⁻⁹ g ml⁻¹ for neutral species will be presented. Also the applicability of the ionomer coated electrodes as nonelectrolyte detectors achieved a reproducible response with detection limits to 10⁻⁶ g ml⁻¹. Overall this system performed as well as, or better than, more specialized and expensive thin layer electrochemical detectors.     

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.     

Flow injection determination of thyroxine in pharmaceutical preparations using tris(2,2′-bipyridyl)ruthenium(III)-NADH chemiluminescence detection

A flow injection (FI) method is reported for the determination of thyroxine based on its enhancement of chemiluminescence (CL) from the Ru(bpy)₃³⁺-NADH system. The calibration graph was linear over the range 2.0-10 × 10⁻⁸ mol L⁻¹ (r² = 0.9989) with relative standard deviations (R.S.D.) in the range 2.0-4.5% (n = 4). The limit of detection (3σ blank) was 1.0 × 10⁻⁹ mol L⁻¹ with sample throughput of 120 h⁻¹. The effect of some organic compounds, anions and cations were studied for l-thyroxine determination. The method was applied to pharmaceutical preparations and the results obtained were in reasonable agreement with the amount labeled. The method was statistically compared with the results obtained by RIA; no significant disagreement at 95% confidence limit was observed. A calibration graph of NADH over the range 1.3 × 10⁻⁸-1.3 × 10⁻⁶ mol L⁻¹ was also established (r² = 0.9992) with R.S.D. in the range1.0-3.5% (n = 4). The limit of detection (3σ) was 1.0 × 10⁻¹⁰ mol L⁻¹ NADH.     

Comparison of soluble manganese(IV) and acidic potassium permanganate chemiluminescence detection using flow injection and sequential injection analysis for the determination of ascorbic acid in Vitamin C tablets

The limits of detection (3s) for ascorbic acid were 5×10⁻⁸ M with acidic potassium permanganate using both flow injection analysis (FIA) and sequential injection analysis (SIA) whereas the soluble manganese(IV) afforded 1×10⁻⁸ M and 5×10⁻⁹ M for FIA and SIA, respectively. Determinations of ascorbic acid in Vitamin C tablets were achieved with minimal sample pretreatment using a standard additions calibration and gave good agreement with those of iodimetric titration.     

Flow injection determination of cationic surfactants by using N-bromosuccinimide and N-chlorosuccinimide as new oxidizing agents for luminol chemiluminescence

Two highly sensitive chemiluminescence (CL) systems are described. The method is based on the CL generated during the oxidation of luminol by N-bromosuccinimide (NBS) and N-chlorosuccinimide (NCS) in alkaline medium. The emission intensity is reduced by the presence of some surfactants at concentrations lower than critical micelle concentration (cmc).A new, simple, rapid and selective flow injection CL method for the determination of cationic surfactants such as dodecyltrimethylammonium bromide (DTAB), cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC) is proposed. Their determinations are based on the reducing effect on the emission intensity of NBS-luminol and NCS-luminol chemiluminescent reactions. The effect of analytical and flow injection analysis (FIA) variables on these CL systems and on the determination of the cationic surfactants are discussed. The optimum parameters for the determination of cationic surfactants were studied and were found to be the following: luminol, 1×10⁻⁶ M; NBS and NCS both, 5×10⁻² M; NaOH, 5×10⁻² M and flow rate, 3.5 ml min⁻¹.     

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.     

Investigation on sensing mechanism of a fluorescent probe for TNP detection in aqueous solution

In this paper, the synthesis and spectral properties of a fluorescent probe Nph-An for TNP detection has been introduced and its sensing mechanism has been studied with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. For TNP detection in aqueous solution the probe shows high sensitivity, selectivity and efficiency. The theoretical results demonstrate that the excited-state intermolecular hydrogen bonding (HB) plays an important role for the TNP detection. In the excited state, the intermolecular HB between Nph-An and TNP is strengthened, and thus strongly facilitates the nonirradiative PET process to induce the fluorescence quenching. Whereas, the probe Nph-An emits strong fluorescence because of the intramolecular charge transfer (ICT) properties, which is confirmed by the solvatochromism effect. The calculated absorption spectra of Nph-An and Nph-An + TNP agree well with the experimental values and theoretical analysis provides a detailed and clear explanation of the sensing mechanism.     

Rapid determination of chromium(VI) in electroplating waste water by use of a spectrophotometric flow injection system

A new rapid and sensitive FI method is reported for spectrophotometric determination of trace chromium(VI) in electroplating waste water. The method is based on the reaction of Cr(VI) with sodium diphenylamine sulfonate (DPH) in acidic medium to form a purple complex (lambda(max) = 550 nm). Under the optimized conditions, the calibration curve is linear in the range 0.04-3.8 microg ml(-1) at a sampling rate of 30 h(-1). The detection limit of the method is 0.0217 microg ml(-1), and the relative standard deviation is 1.1% for eight determinations of 2 microg ml(-1) Cr(VI). The proposed method was applied to the determination of chromium in electroplating waste water with satisfactory results.     

多巴胺在聚钙羧酸膜修饰碳糊电极上的电催化及与尿酸的同时测定

采用循环伏安法(CV)制备了聚钙羧酸(PCCA)膜修饰的碳糊电极(CPE)。考察了电极对多巴胺(DA)、尿酸(UA)的电氧化催化性能。结果显示,聚钙羧酸膜修饰碳糊电极(PCCA/CPE)对DA有良好的电催化效果,DA呈现出一对准可逆的氧化还原峰,氧化峰电流与DA浓度在3.0×10-7～1.0×10-4mol/L范围内呈线性关系,检出限为1×10-7mol/L(S/N=3)。使用微分脉冲伏安法(DPV),DA和UA在PCCA/CPE上的氧化峰能完全分离(ΔEp=192 mV),且峰电流与浓度均呈现良好的线性关系,可实现对DA和UA的同时测定。实验还进行了实际样品测定。     

Flow injection fluorimetric determination of chromium(VI) in electroplating baths by luminescence quenching of tris(2,2'-bipyridyl) ruthenium(II)

A sensitive and selective luminescence quenching method is developed and used for manual and flow injection analysis (FIA) of chromium(VI) by reaction with [Ru(bpy)₃]²⁺. The emission peak of ruthenium(II) at 595 nm is linearly decreased as a function of Cr(VI) concentration. This permits determination of chromium(VI) ion over the concentration range 0.1-20 μg ml⁻¹ with a detection limit of 33 ng ml⁻¹. The quenching process is due to an electron transfer from the luminescent [Ru(bpy)₃]²⁺ complex ion to Cr(VI) resulting in the formation of the non-luminescent [Ru(bpy)₃]³⁺ complex ion. Selectivity for Cr(VI) over many anions and transition, alkali and alkaline earth metal cations is demonstrated. High concentration levels of sulphate, chloride, borate, acetate, phosphate, nitrate, cyanide, Pb²⁺, Zn²⁺, Hg²⁺, Cu²⁺, Cd²⁺, Ni²⁺ and Mn²⁺ ions are tolerated. The effects of solution pH and [Ru(bpy)₃]²⁺ reagent concentration are examined and the reaction conditions are optimized. Validation of the method according to the quality assurance standards show suitability of the proposed method for use in the quality control assessment of Cr(VI) in complex matrices without prior treatment. The method is successfully applied to determine chromium(VI) in electroplating baths using flow injection analysis. Results with a mean standard deviation of ±0.6% are obtained which compare fairly well with data obtained using atomic absorption spectrometry.     

Optimization of a flow injection analysis system for tartaric acid determination in wines

The objective of this work is the development and optimization of a method for tartaric acid analysis in wines that does not require any sample pre-treatment and with adequate accuracy. A flow injection analysis manifold with three channels, using a dialysis unit to eliminate sample matrix interferences and to accomplish on-line dilution, is proposed for the spectrophotometrical determination of tartaric acid in wines making use of its reaction with vanadate. The proposed method is fast, accurate, simple, economic and does not require any sample pre-treatment. Preliminary studies using factorial designs were performed to determine which operational parameters should be included in the optimization stage. The optimization was performed using a modified simplex algorithm with a response function that included sensitivity, deviation from linearity at low concentrations and residence time, used as an inverse measure of sampling rate. The most relevant analytical parameters of the method are presented, including a comparison between the results provided by the proposed method and by an alternative procedure in the analysis of a set of wine samples from Portugal, with tartaric acid values in the range 0.5–4 g l⁻¹.