Constantina N. Konidari

Analytical methods to determine phosphonic and amino acid group-containing pesticides

A comprehensive view on the possibilities of the most recently developed chromatographic methods and emerging techniques in the analysis of pesticides glyphosate, glufosinate, bialaphos and their metabolites is presented. The state-of-the-art of the individual pre-treatment steps (extraction, pre-concentration, clean-up, separation, quantification) of the employed analytical methods for this group of chemicals is reviewed. The advantages and drawbacks of the described analytical methods are discussed and the present status and future trends are outlined.

Ion chromatographic method for the simultaneous determination of nitrite and nitrate by post-column indirect fluorescence detection.

This short paper highlights the suitability of ion chromatography with post-column indirect fluorescence detection to determine simultaneously nitrite and nitrate based on the quenching of tryptophan native fluorescence. The method uses an enhanced fluorescence mobile phase containing tryptophan and detects the suppression of fluorescence of the mobile phase due to the elution of the target ions. The phenomenon of fluorescence quenching of tryptophan is highly induced by the presence of phosphate ions. The quenched fluorescence intensity exhibits concentration dependence in the range 1-25 mg/l and 3-65 mg/l for nitrite and nitrate, respectively. The relative standard deviation for five replicates of a standard solution containing a mixture of 5 mg/l of nitrite and 10 mg/l of nitrate lies around 2.8%. This simple coupling technique results in a relatively sensitive, fast, and accurate method, allowing for both qualitative and quantitative analysis of nitrite and nitrate. The method can easily be implemented to real samples such as foodstuffs, fertilizers and soils and is proven to be precise and accurate when compared with reference methods.

Gas chromatographic method for the sensitive determination of 2,5-hexanedione using electron capture and mass-selective detection

Hexanedione (2,5HD) is the principle metabolite of n-hexane and methyl ethyl ketone in human urine. In this paper, a rapid and highly sensitive method for the determination of urinary 2,5HD by gas chromatography-electron capture detection and gas chromatography-mass selective detection is described. The method comprises, optionally, acid hydrolysis before the analyte is being derivitized with 2,4,6-trichlorophenyl hydrazine (TCPH) towards the formation of a chlorinated hydrazone derivative for 2,5HD analysis. The reaction conditions such as temperature, time, reagent concentration and pH were optimized properly. The detection limit of the method — defined as signal to noise ratio = 3 — is as low as 0.4 and 1.5M using the ECD and MSD (SIM mode), respectively. The linearity of the detection systems extends up to 50M of 2,5HD with correlation coefficients better than 0.9995. The within-day and between-day reproducibility for a urine sample of 10M in 2,5HD were 2.6 and 3.2%, respectively. Spiked pooled samples with various concentrations in the range 5-10M were processed following the proposed procedure. The recovery was satisfactory enough ranging from 92 to 104%. The method was applied to the analysis of urine samples received from people working in a chemical laboratory and the results revealed variable, yet low concentrations of 2,5HD in the hydrolyzed samples.

Kinetic and mechanistic study of the reaction of 2,6-dichlorophenol-indophenol and cysteine

In this work the reaction of cysteine (H(2)Q) with 2,6-dichlorophenolindophenol (D) is studied kinetically in the pH range 2.5-9.0. Taking into consideration the distribution diagrams for the species H(3)Q(+), H(2)Q, HQ(-), Q(2-) for cysteine and DH(+)(2), DH, D(-) for 2,6-dichlorophenolindophenol the reaction rate constants k(i) for all possible combinations of the reacting species were determined. The maximum reactivity appears at pH 6.88 with an overall reaction constant k = 306 1.mole(-1).sec(-1) at 22 degrees . The effect of the concentrations of the reagents and the ionic strength on the reaction rate is also given. From Arrhenius plots an activation energy E(a) = 8.1 kcal/mole was calculated. Working curves for the determination of cysteine in aqueous solutions are also presented applying the reaction rate method. Finally the paper includes important analytical information for the calculation of the errors due to interference of cysteine by the kinetic determination of ascorbic acid, using its reaction with 2,6-dichlorophenolindophenol.

KINETIC AND MECHANISTIC STUDY OF THE REDUCTION OF 2,6-DICHLOROPHENOL INDOPHENOL BY DITHIONITES

Abstract The reaction of 2,6-dichlorophenolindophenol (DCPI) and dithionites is studied kinetically by applying the stopped-flow technique. Reaction rate constants are given for the pH range 1.30–6.80. The reaction was found to follow first-order kinetics with respect to each of the reactants. For pH 3.97, 5.10 and 6.80, the second-order reaction rate constant was determined by applying four different technique. Mean values of k = 172±5, 200±2 and 276±4 l mol −1 s −1 are given for pH 3.97, 5.10 and 6.80, respectively. A mechanism is proposed for the reaction, which suggests partial reactions of all possible species of DCPI and dithionites at any pH. An equation for the calculation of k at any pH is derived, which gives k as a function of [H + ], the partial reaction rate constants and the dissociation constants of DCPI and H 2 S 2 O 4 . Values of reaction rate constants of all possible partial reactions are also presented.

Interference in the kinetic determination of ascorbic acid by sulphide and sulphite

The reduction of 2,6-dichlorophenolindophenol (DCPI) by sulphides and sulphites has been studied kinetically by the stopped-flow technique. The reaction is first-order with respect to each of the reactants. From the distribution diagrams for the species DH(+)(2), DH and D(-) for DCPI and H(2)Q, HQ(-) and Q(2-) for sulphides or sulphites, a mechanism is proposed which suggests partial reactions of all possible combinations of the reacting species at any pH. An equation for calculation of the second-order reaction rate constants k at any pH is derived, which gives k as a function of [H(+)], the partial reaction rate constants and the dissociation constants of DCPI and H(2)S or H(2)SO(3). Values of the overall reaction rate constants over a wide pH-range have been determined, together with values of k for all possible partial reactions. For particular pH-values the second-order reaction rate constant was determined by four different methods. Mean values of k = 251 +/- 1 and 240 +/- 1 l.mole(-1).sec(-1) were obtained for pH 3.15 and 4.17, respectively, for the DCPI-Na(2)S reaction and k = 137 +/- 1, 127 +/- 1 and 136 +/- 1 l.mole(-1).sec(-1) for pH 2.02, 4.25 and 5.10, respectively, for the DCPI-Na(2)SO(3) reaction. From the slopes of the linear Arrhenius plots activation energies of 6.6 +/- 0.2 and 4.0 +/- 0.1 kcal/mole for the DCPI-Na(2)S and DCPI-Na(2)SO(3) reactions, respectively were calculated. The effect of ionic strength on the reactions supports the proposed mechanism.

Decomposition of 2,6-dichlorophenolindophenol in strongly acidic solutions: a potential kinetic method for the determination of pH

The problem of pH measurements at extreme pH values is well known. Higher pH values are obtained at pH values of below 1 because of the so-called ‘acid error’. The acid error depends on various parameters and it is not always reproducible. In order to determine accurate pH values at high acidities, a kinetic study of the decomposition of 2,6-dichlorophenolindophenol (DCPl) in strongly acidic solutions was investigated by applying the stopped-flow technique. The DCPl is unstable at lower pH values (below 2) and the reaction rate constant for its decomposition in the pH range 0–2 is dependent on the pH. A good correlation between pH and the observed reaction rate constant of the decomposition of DCPl (kob) was found for low pH values. The reaction rate constant, k1, was calculated by three different approaches for the evaluation of the experimental data. The weighted mean value of k1 was found to be (77 ± 1)× 10–3 l mol–1 S–1, with confidence limits of 95%. A knowledge of k1 is also important in analysis as DCPl is the main reagent for the determination of assorbic acid in acidic solutions.

KINETIC STUDY OF THE FAST STEP OF THE ALKALINE HYDROLYSIS OF P-CHLORANIL USING STOPPED FLOW TECHNIQUE

The alkaline hydrolysis of p-chloranil or 2,3,5,6-tetrachloro-1,4-benzoquinone (C6Cl4O2, Q) was studied, using stopped flow spectrophotometry and Electron Spin Resonance techniques (E.S.R.). In the present study it was shown for the first time, that a free radical is produced chemically and that it can account for the propagation of the reaction. It was found that in alkaline conditions chloranil in a “Michael” fashion undergoes 1,2 addition being hydrolyzed and in turn produces a chloranil free radical (Q•) The hydrolysis then proceeds via a number of intermediates yielded by this radical and a number of different products is formed. The formation of these products, both quantitatively and qualitatively has a strong dependence on the concentration of the OH− species and chloranil. The various possible routes of the hydrolysis are studied either spectrophotometrically or by E.S.R. Two different intermediates are observed absorbing at 426 nm and at 540 nm, respectively. Each species was formed and destroyed within 10 s to 30 min depending on the exact conditions. The reaction rate constants for the formation and the decay of the intermediates was estimated using the Guggenheim method. At both wavelengths the rate constants seem to have a complex relation to the concentration of the anion.