Interfacing in-line gas-diffusion separation with optrode sorptive preconcentration exploiting multisyringe flow injection analysis

An automatic multisyringe flow injection analysis (MSFIA) system coupling a flow-through optical fiber diffuse reflectance sensor with in-line gas-diffusion (GD) separation is proposed for the isolation, preconcentration and determination of traces of volatile and gas-evolving compounds in samples containing suspended solids, with no need for any preliminary batch sample treatment. The flowing methodology overcomes the lost of sensitivity of the in-line separation technique, when performed in a uni-directional continuous-flow mode, through the implementation of disk-based solid-phase extraction schemes. The high selectivity and sensitivity, the low reagent consumption and the miniaturization of the whole assembly are the outstanding features of the automated set-up. The proposed combination of techniques for separation, flow analysis, preconcentration and detection was applied satisfactorily to sulfide determination in environmental complex matrixes. The method based on multicommutation flow analysis involves the stripping of the analyte as hydrogen sulfide from the donor channel of the GD-module into an alkaline receiver segment, whereupon the enriched plug merges with well-defined zones of the chormogenic reagents (viz., N,N-dimethyl-p-phenylenediamine (DMPD) and Fe(III)). The in-line generated methylene blue dye is subsequently delivered downstream to the dedicated optrode cell furnished with a C₁₈ disk, while recording continuously the diffuse reflectance spectrum of the pre-concentrated compound. This procedure provides a linear working range of 20-500 μg l⁻¹ sulfide with a relative standard deviation of 2.2% (n = 10) at the 200 μg l⁻¹ level, and a detection limit of 1.3 μg l⁻¹.     

Spectrophotometric flow injection monitoring of sulfide during sugar fermentation

A spectrophotometric flow injection procedure involving N,N-dimethyl-p-phenylenediamine (DMPD) is applied to the sulfide monitoring of a sugar fermentation by Saccharomyces cerevisiae under laboratory conditions. The gaseous chemical species evolving from the fermentative process, mainly CO₂, are trapped allowing a cleaned sample aliquot to be collected and introduced into the flow injection analyzer. Measurement rate, signal repeatability, detection limit and reagent consumption per measurement were estimated as 150 h⁻¹, 0.36% (n = 20), 0.014 mg L⁻¹ S and 120 μg DMPD, respectively. The main characteristics of the monitoring record are discussed. The strategy is worthwhile for selecting yeast strain, increasing the industrial ethanol production and improving the quality of wines.     

A study on the reaction between N-substituted p-phenylenediamines and ozone: experimental results and theoretical aspects in relation to their antiozonant activity

The following p-phenylenediamines (PPD): N,N,N^′,N^′-tetramethyl-p-phenylenediamine (TMPPD), N,N^′-dimethylbutyl-p-phenylenediamine (6PPD), N,N^′-diaryl-p-phenylenediamine (DPPD), tris-(N-dimethylpentyl-p-phenylenediamine)-N^′,N^′,N^′-1,3,5-triazine (6PPDTZ), have been oxidized under the action of O₃ in diluted solutions. In all cases the radical cation or semiquinone radical was the first derivative formed by monoelectronic oxidation of the substrate. The radical cation has been studied by electronic spectroscopy and the electronic spectral changes of all mentioned PPD has been followed as function of the ozonation time. The results have been discussed in the frame of the antiozonant properties of these PPD which are used as antiozonant agents in diene rubber protection. It is shown that the antiozonant activity of each PPD considered correlates with the free enthalpy of formation of the respective radical cation. The lowest is the free energy of formation of a PPD radical cation and the highest is the antiozonant activity in a diene rubber compound.     

The use of 2,2′-thiodiethanol in the fluorimetric determination of secondary amines with o-phthalaldehyde and 2-mercaptoethanol

The fluorimetric determination of secondary amines based on conversion with sodium hypochlorite to primary amines and the o-phthalaldehydemercaptoethanol (OPA) reagent is discussed. Interference by excess hypochlorite (e.g., oxidation of OPA or conversion of primary amines to chloramines) were suppressed by 2,2′-thiodiethanol. Two spectrophotofluorimetric method are reported, one for aromatic and the other for aliphatic secondary amines. These methods permitted determinations at nanomolar levels; relative standard deviations were 4% for 5 nmol of N-methylaniline and 13% for 5 nmol of sarcosine.     

Enantioselective addition of aryllithium reagents to aromatic imines mediated by 1,2-diamine ligands

A variety of optically enriched amines have been obtained by addition of aryllithium reagents to aromatic imines using N,N′-tetramethylcyclohexane-1,2-diamine as chiral ligands. Enantiomeric excesses up to 90% could be obtained.     

Spectrophotometric determination of aromatic primary amines and nitrite by flow injection analysis

Aromatic primary amines are determined by injection into dilute hydrochloric acid carrier which merges sequentially with 4-N-methylaminophenol and dichromate. The purple-red color formed by oxidative coupling of amines with 4-N-methylaminophenol is measured at 530 nm. In contrast to the manual procedure, the flow-injection procedure avoided errors arising from the instability of the coupling intermediate, oxidation of the amine, and too great an excess of the oxidant. The method improves the selectivity for certain amines in the presence of those which are sterically hindered or have an electron-deficient aromatic nucleus. Nitrite is determined by diazotization of sulfanilamide and quantifying the residual sulfanilamide by oxidative coupling. The sample thourghout for the assay of amines (0.05–20 μg ml^?1 NH₂-N) and nitrite (1–10 μg ml^?1 NO₂^--N) was 120 h^?1. A system for the consecutive determination of aromatic primary amines and nitrite is decribed.     

Novel chiral tetraaza ligands: synthesis and application in asymmetric transfer hydrogenation of ketones

Novel chiral tetraaza ligands, N¹,N²-bis(2-(piperidin-1-yl)benzylidene)cyclohexane-1,2-diamine 1 and N¹,N²-bis(2-(piperidin-1-yl)benzyl)cyclohexane-1,2-diamine 2, have been synthesized and fully characterized by analytical and spectroscopic methods. The structure of (R,R)-1 has been established by X-ray crystallography. Asymmetric transfer hydrogenation of aromatic ketones with the catalysts prepared in situ from [IrHCl₂(COD)]₂ and the chiral tetraaza ligands in 2-propanol gave the corresponding optically active secondary alcohols in high conversions and good ees (up to 91%) under mild reaction conditions.     

Spectrofluorimetric determination of aliphatic amines using a new fluorigenic reagent: 2,6-dimethylquinoline-4-(N-succinimidyl)-formate

A new amino fluorescence probe, 2,6-dimethylquinoline-4-(N-succinimidyl) formate (DMQF-OSu) has been synthesized. Based on the selective reaction of DMQF-OSu with primary and secondary aliphatic amines to yield strong fluorescence, a new spectrofluorimetric method for the determination of total aliphatic amines has been developed. At λ_ex/λ_em=324.4/416 nm, the linear calibration range was 6×10⁻⁸-6×10⁻⁶ mol l⁻¹ with the detection limit (3σ) of 1.94×10⁻¹⁰ mol l⁻¹ for the determination of aliphatic amines in weak basic media. The proposed method has been applied to the determination of aliphatic amines in tap water and lake water with the recoveries of 99-104%. Compared with the reported methods, the method presented here is rapid, simple, sensitive and feasible.     

Open tubular CEC with novel block copolymer coatings for separation of aromatic amines

A new type of block copolymer, which was synthesized by styrene and maleic anhydride, P(MAn‐alt‐St)₁₁₉‐b‐PSt₅₅₈, was successfully developed as the capillary coatings in open tubular CEC. It is interesting that the covalently bonded coatings are porous and the coatings can play the role of surfactants in the separation of aromatic amines when the buffer solution was composed of 30.0 mM ammonium acetate at pH 7.7 with 20% THF. Thus, successful baseline separation of five kinds of aromatic amines has been achieved. As validated by both artificially prepared solutions of aromatic amines and four real samples of commercially available permanent hair dyes, this proposed method was successfully applicable to the quantitative analysis of p‐phenylenediamine (PPD) and o‐phenylenediamine (OPD) ingredients in these commercial products, with a linear range between 8.3 μM and 6.0 mM, correlation coefficient above 0.990 and recovery between 83.5 and 110.9%. The detection limit obtained from calculations based on signal‐to‐noise ratio (S/N=3) was 4.2 μM for PPD and 6.0 μM for OPD, respectively. Furthermore, the role of the surfactants played by the block copolymer coatings has been primarily explored.     

Development and validation of a hydrophilic interaction liquid chromatographic method for determination of aromatic amines in environmental water

A simple, precise, and accurate hydrophilic interaction liquid chromatographic (HILIC) method has been developed for the determination of five aromatic amines in environmental water samples. Chromatography was carried out on a bare silica column, using a mixture of acetonitrile and a buffer of NaH₂PO₄–H₃PO₄ (pH 1.5, containing 10 mM NaH₂PO₄) (85:15, v/v) as a mobile phase at a flow rate of 1 mL min⁻¹. Aromatic amines were detected by UV absorbance at 254 nm. The linear range of amines was good (r² > 0.998) and limit of detection (LOD) within 0.02–0.2 mg L⁻¹ (S/N = 3). The retention mechanism for the analytes under the optimum conditions was determined to be a combination of adsorption, partition and ionic interactions. The proposed method was applied to the environmental water samples. Aromatic amines were isolated from aqueous samples using solid-phase extraction (SPE) with Oasis HLB cartridges. Recoveries of greater than 75% with precision (RSD) less than 12% were obtained at amine concentrations of 5–50 μg L⁻¹ from 100 mL river water and influents from a wastewater treatment plant (WWTP). The present HILIC technique proved to be a viable method for the analysis of aromatic amines in the environmental water samples.     

Simultaneous determination of N-acetyl-p-aminophenol and p-aminophenol with poly(3,4-ethylenedioxythiophene) modified glassy carbon electrode

A sensitive and selective method was developed for the determination of N-acetyl-p-aminophenol (APAP) and p-aminophenol (PAP) using poly(3,4-ethylenedioxythiophene) (PEDOT)-modified glassy carbon electrode (GCE). Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical reaction of APAP and PAP at the modified electrode. Both APAP and PAP showed quasireversible redox reactions with formal potentials of 367 mV and 101 mV (vs. Ag/AgCl), respectively, in phosphate buffer solution of pH 7.0. The significant peak potential difference (266 mV) between APAP and PAP enabled the simultaneous determination both species based on differential pulse voltammetry. The voltammetric responses gave linear ranges of 1.0 × 10⁻⁶-1.0 × 10⁻⁴ mol L⁻¹ and 4.0 × 10⁻⁶-3.2 × 10⁻⁴ mol L⁻¹, with detection limits of 4.0 × 10⁻⁷ mol L⁻¹ and 1.2 × 10⁻⁶ mol L⁻¹ for APAP and PAP, respectively. The method was successfully applied for the determination of APAP and PAP in pharmaceutical formulations and biological samples.     

Synthesis of chiral vicinal C2 symmetric and unsymmetric bis(sulfonamide) ligands based on trans-1,2-cyclohexanediamine by aminolysis of N-tosylaziridines

The ring opening of N-tosylaziridines with aliphatic amines can be efficiently catalyzed by lithium perchlorate to provide derivatives of the trans-1,2-diamine in high yields. The reaction was used in desymmetrization of several cyclic N-tosylaziridines using chiral amines. Using this strategy, an efficient synthesis of chiral vicinal C₂ symmetric bis(sulfonamide) and unsymmetrical bis(sulfonamide) ligands based on trans-1,2-cyclohexanediamine was developed.     

Development of an optical fibre reflectance sensor for p-aminophenol detection based on immobilised bis-8-hydroxyquinoline

2,2′-(1,4-Phenylenedivinylene)bis-8-hydroxyquinoline (PBHQ), a highly sensitive reagent used for the colorimetric determination of p-aminophenol (PAP), was successfully immobilised on XAD-7 and coupled with optical fibres to investigate a sensor-based approach for determining p-aminophenol. The solid-state sensor is based on the reaction of PAP with PBHQ in presence of an oxidant to produce an indophenol dye. The reflectance measurements were carried out at a wavelength of 647 nm since it yielded the largest divergence different in reflectance spectra before and after reaction with the analyte. The linear dynamic range of PAP was found within the concentration range of 0.1-2.18 mg l⁻¹ with its LOD of 0.02 mg l⁻¹. The sensor response from different probes (n = 7) gave a R.S.D. of 4.4% at 1.09 mg l⁻¹ PAP concentration. The response time of the optical one-shot sensor was 5 min for a stable solution. As this PAP sensor is irreversible, a fresh sensor has to be used for each measurement. All the experimental parameters were optimized for the determination of PAP. Using the optical sensing probe, PAP in pharmaceutical wastewater and paracetamol was determined. The effect of potential interferences such as inorganic and organic compounds was also evaluated. Potential on-site determination of PAP with such sensors can indirectly aid detection of organo-phosphorus nerve agents and pesticides in the field by inhibition of acetylcholine esterase-catalyzed hydrolysis of p-aminophenyl acetate to p-aminophenol.     

Application of sequential injection-monosegmented flow analysis (SI-MSFA) to spectrophotometric determination of sulfide in simulated waters samples

This paper describes the coupling of sequential injection with monosegmented flow analysis (SI-MSFA) for determination of sulfide at typical concentrations in wastewaters. The method was based on the reaction of sulfide with 19 mmol l⁻¹ Fe³⁺ and 3.63 mmol l⁻¹N,N-dimethyl-p-phenylene diamine hydrochloride in medium of 1.1 mol l⁻¹ HCl, forming the dye methylene blue. The analytical curves were constructed by in-line dilution of a single stock standard solution. The robustness of the proposed method was checked constructing analytical curves in different working days and comparing the slopes, which had a relative standard deviation of 5.2% (n=5) for a concentration range between 0.17 and 1.0 mg l⁻¹ S²⁻. The analytical throughput was 38 samples per h and the limit of detection was 0.040 mg l⁻¹. The feasibility of the SI-MSFA approach to perform standard additions for S²⁻ determination was also described. Simulated samples spiked with known amounts of sulfide were analyzed by the proposed method, presenting recoveries between 70 and 115%. These results demonstrate the feasibility of the SI-MSFA method to perform in situ analysis of S²⁻ in automatic monitoring stations.     

Synthesis, crystal structure and antibacterial activity of a group of mononuclear manganese(II) Schiff base complexes

Five mononuclear complexes of manganese(II) of a group of the general formula, [MnL(NCS)₂] where the Schiff base L = N,N′-bis[(pyridin-2-yl)ethylidene]ethane-1,2-diamine (L¹), (1); N,N′-bis[(pyridin-2-yl)benzylidene]ethane-1,2-diamine (L²), (2); N,N′-bis[(pyridin-2-yl)methylidene]propane-1,2-diamine (L³), (3); N,N′-bis[(pyridin-2-yl)ethylidene]propane-1,2-diamine (L⁴), (4) and N,N′-bis[(pyridin-2-yl)benzylidene]propane-1,2-diamine (L⁵), (5) have been prepared. The syntheses have been achieved by reacting manganese chloride with the corresponding tetradentate Schiff bases in presence of thiocyanate in the molar ratio of 1:1:2. The complexes have been characterized by IR spectroscopy, elemental analysis and other physicochemical studies, including crystal structure determination of 1, 2 and 4. Structural studies reveal that the complexes 1, 2 and 4 adopt highly distorted octahedral geometry. The antibacterial activity of all the complexes and their respective Schiff bases has been tested against Gram(+) and Gram(−) bacteria.     

Photovoltammetric behavior and photoelectrochemical determination of p-phenylenediamine on CdS quantum dots and graphene hybrid film

A photoelectroactive film composed of CdS quantum dots and graphene sheets (GS) was coated on F-doped SnO₂ (FTO) conducting glass for studying the electrochemical response of p-phenylenediamine (PPD) under photoirradiation. The result indicated that the cyclic voltammogram of PPD on CdS–GS hybrid film became sigmoidal in shape after exposed under visible light, due to the photoelectrocatalytic reaction. Such a photovoltammetric response was used to rapidly optimize the photoelectrocatalytic activity of hybrid films composed of different ratios of CdS to GS toward PPD. The influences of scan rate and pH on the photovoltammetric behavior of PPD on CdS–GS film revealed that although the controlled step for electrochemical process was not changed under photoirradiation, more electrons than protons might participate the photoelectrocatalytic process. Furthermore, the photoelectroactive CdS–GS hybrid film was explored for PPD determination based on the photocurrent response of film toward PPD. Under optimal conditions, the photocurrent signal on CdS–GS film was linearly proportional to the concentration of PPD ranging from 1.0 × 10⁻⁷ to 3.0 × 10⁻⁶ mol L⁻¹, with a detection limit (3S/N) of 4.3 × 10⁻⁸ mol L⁻¹. Our work based on CdS–GS hybrid film not only demonstrated a new facile photovoltammetric way to study the photoinduced electron transfer process of PPD, but also developed a sensitive photoelectrochemical strategy for PPD determination.     

New Determination Scheme of p‐Aminophenol by MnO2 Modified Electrode Coupled with Flow Injection Analysis

New determination scheme of p‐aminophenol by using MnO₂ as a preoxidant is demonstrated in this work. In the flow injection system, the p‐aminophenol is oxidized to quinoneimine by MnO₂ at up‐stream, which can be detected at a suitable reductive potential. After optimization, the linear range of PAP is started from 1 μM to 30 μM (R²=0.999), the estimated detection limit (S/N=3) is 0.28 μM. Two real samples are studied and excellent recoveries are achieved by using standard addition method.     

Spectrophotometric determination of aromatic amines by reaction with p-benzoquinone

A comprehensive study is made of the application of the p-benzoquinone spectrophotometric technique to the aromatic amines. The technique involves a reaction with excess p-benzoquinone; the colored products display maximum absorption at 490–510 nm (varying according to the type of the amine used) and E_{1 cm}^1% in the range of 90–380. On the basis of an IR investigation, ethyl alcohol has been selected as a suitable solvent medium for aromatic amine determination. Electron-donating groups react faster and give more intense color than do electron-withdrawing groups. Results with an average recovery of 95% and mean standard deviation of 3.4% are obtained with seven aromatic amines.     

Exploiting Sulphide Generation and Gas Diffusion Separation in a Flow System for Indirect Sulphite Determination in Wines and Fruit Juices

A flow system for sulphite determination in wines and fruit juice samples has been developed exploiting H₂S generation and gas diffusion separation. The composed sandwich gas diffusion cell was coupled in the flow manifold to permit sulphite reduction to H₂S and H₂S diffusion through a microporous PTFE membrane to produce methylene blue resulting from the reaction with N,N-dimethylphenylene diamine (DMPD) and Fe³⁺. The flow parameters were optimized, and the best results were obtained by using NaOH solution for H₂S collection. Interferences of H₂S, CO₂ and ascorbic acid were evaluated. Limits of detection (3σ) for the free and total sulfite were calculated at 0.12 and 0.25 mg L⁻¹, respectively. The results obtained by the proposed process were in agreement with the reference methods for a 95% confidence level.     

Microvolume turbidimetry for rapid and sensitive determination of the acid labile sulfide fraction in waters after headspace single-drop microextraction with in situ generation of volatile hydrogen sulfide

In this work, we demonstrate the feasibility of applying headspace single-drop microextraction with in-drop precipitation for the quantitative determination of the acid labile sulfide fraction (H₂S, HS⁻, and S²⁻ (free sulfide), amorphous FeS and some metal sulfide complexes-clusters as ZnS) in aqueous samples by microvolume turbidimetry. The methodology lies in the in situ hydrogen sulfide generation and subsequent sequestration into an alkaline microdrop containing ZnO₂²⁻ and exposed to the headspace above the stirred aqueous sample. The ZnS formed in the drop was then determined by microvolume turbidimetry. The optimum experimental conditions of the proposed method were: 2 μL of a microdrop containing 750 mg L⁻¹ Zn(II) in 1 mol L⁻¹ NaOH exposed to the headspace of a 20-mL aqueous sample stirred at 1600 rpm during 80 s after derivatization with 1 mL of 6 mol L⁻¹ HCl. An enrichment factor of 1710 was achieved in only 80 s. The calibration graph was linear in the range of 5-100 μg L⁻¹ with a detection limit of 0.5 μg L⁻¹. The repeatability, expressed as relative standard deviation, was 5.8% (N = 9). Finally, the proposed methodology was successfully applied to the determination of the acid labile sulfide fraction in different natural water samples.