M.I. Karayannis

Micelle mediated methodology for the determination of free and bound iron in wines by flame atomic absorption spectrometry

A methodology utilizing the cloud point phenomenon for the determination of free and tannin-bound iron in wines is presented. The method employs precipitation of the tannins and other phenolic and insoluble compounds in the micelles of a non-ionic surfactant mixture (TX-100 and TX-45) upon increase of the solution temperature, which are subsequently separated from the initial solution by centrifugation. The surfactant-rich-phase containing the tannins and the insoluble iron fraction is directly aspirated into the nebulizer of a flame atomic absorption spectrometer after its uptake with a methanolic solution of HNO3. The supernatant is submitted to the same cloud point extraction procedure for the determination of free iron species in the presence of a chelating agent, ammonium pyrrolidine dithiocarbamate (APDC), in order to form water-insoluble complexes with free iron. The complexes are extracted in the micelles and directly analyzed after they are diluted in a methanolic solution as described above. The total content of iron was also determined by conventional methods for comparison, yielding satisfactory results. The calibration graph was rectilinear up to 0.35 mg l −1 Fe, with detection limits of 0.02 mg l −1 with a relative standard deviation of 2.4%. The method was successfully applied to red and white wines.

Simultaneous spectrophotometric determination of nitrite and nitrate by flow-injection analysis

An automatic direct spectrophotometric method for the simultaneous determination of nitrite and nitrate by flow-injection analysis has been developed. Nitrite reacts with 3-nitroaniline in the presence of hydrochloric acid (0.96-1.8 M HCl or pH 0.5-0.7) to form a diazonium cation, which is subsequently coupled with N-(1-naphthyl)-ethylenediamine dihydrochloride to form a stable purple azo dye, the absorbance of which is measured at 535 nm. Nitrate is reduced on-line to nitrite in a copper-coated cadmium column which is then treated with azo dye reagent and the absorbance due to the sum of nitrite and nitrate is measured; nitrate is determined from the difference in absorbance values. A copper column incorporated into the reaction manifold before the copperised cadmium column not only improves the long-term accuracy, but also extends the life time of the copperised cadmium column. Various analytical parameters, such as effect of acidity (pH), flow rate, sample size, dispersion coefficient, time, temperature, reagent concentration and interfering species, were studied. The calibration graphs were rectilinear for 0.1-3.5 mug ml(-1) of NO(3) and 10 ng ml(-1)-2.2mug ml(-1) of NO(2). The method is successfully applied to some food samples (meat, flour and cheese), environmental waters (inland and surface), beer and soil samples. Up to 30 samples can be analysed per hour with a relative precision of approximately 0.1-2%.

An integrated gas chromatographic method towards the simultaneous determination of phosphoric and amino acid group containing pesticides

SummaryAmpropylfos, glyphosate, glufosinate and bialaphos are water soluble pesticides that contain phosphoric and amino acid groups in their molecules. Their polar nature and high water solubility make extraction difficult and limit the options for using the various derivatization techniques often employed for gas chromatographic analysis. A multiresidue method is proposed that allows the simultaneous determination of six analytes (pesticides and some of their metabolites) in environmental waters down to sub-μb L−1 levels. A mixture of trifluoroacetic anhydride and trifluoroethanol converts them in a single-step reaction to derivatives that are sufficiently volatile for gas chromatographic analysis. Electron capture and mass-selective detection are used for quantitation purposes, and their analytical characteristics are presented. Under the chromatographic conditions employed, acceptable separation is attained for the derivatized analytes, with both glufosinate and bialaphos emerging as double peaks. Recoveries of spiked environmental water samples range from 88 to 106%. The mean relative standard deviation of the method does not exceed 3.0%, whereas, depending on the detection system, the limits of detection for the analytes are in the range of 0.09–65 μg L−1, the upper value being the bialaphos detection limit.

Determination of glycerol in alcoholic beverages using packed bed reactors with immobilized glycerol dehydrogenase and an amperometric FIA system

A flow injection analysis system incorporating amperometric detection and enzyme reactor for glycerol determination in alcoholic beverages is described. The reactor is based on the glycerol dehydrogenase system, and the enzyme was immobilized through chemical modification on several supporting materials such as aminopropyl and isothiocyanate controlled pore glass, aminopolystyrene resin and m-aminobenzyloxymethyl cellulose. NADH, the product of the enzymatic reaction, was monitored amperometrically with a three-electrode wall-jet type flow through cell, at + 0.5 V vs. Ag/AgCl. The method was evaluated in the presence or absence of potassium and the following linear dynamic ranges were found: 2 x 10(-5) -2 x 10(-4) mol l(-1) and 4 x 10(-5) -4 x 10(-4) mol l(-1), respectively. The interference effects of various compounds were also studied. The relative standard deviation was found to be better than 1.0% (n = 6). The reactors are stable for over a period of 3 months and after about 2500 injections. Under optimum working conditions the sampling frequency was 30 samples h(-1). The successive application of the method was confirmed by comparison with a reference method. The mean relative error is 2.2% and the recovery 95-102%.

Comparison and evaluation of cloud point extraction and low-temperature directed crystallization as preconcentration tools for the determination of trace elements in environmental samples

A comparative study between cloud point extraction (CPE) and low-temperature directed crystallization (LTDC) is presented. Trace elements (Cd, Pb, Cr, Cu, Zn, Ni and Fe) were preconcentrated by both methods from model and natural water samples and the results were evaluated with respect to extraction efficiency, accuracy, precision, sample throughput and interferences. Flame atomic absorption spectrometry (FAAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) were used for the final measurements. The results indicate that these extraction and preconcentration procedures ensure the required accuracy and precision for the reliable identification and quantification of trace elements in natural waters. Drawbacks of each method identified can further assist the analyst towards a better application of each method depending on the target species, the detector employed and the application intended (routine analysis, trace analysis, speciation analysis, etc.).

Spectrophotometric kinetic determination of copper(II) trace amounts based on its catalytic effect on the reaction of the reduced 2,6-dichlorophenolindophenol and hydrogen peroxide

A catalytic spectrophotometric method is presented for the determination of Cu(II), based on the oxidation of the leucocompound of the 2.6-dichlorophenolindophenol (DCPI)(r), in the presence of hydrogen peroxide and the catalytic effect of Cu(II) on this reaction in ammonia-ammonium chloride buffer solution of pH 10.5. The above reaction is followed spectrophotometrically at 562 nm. The study was carried out with a filter spectrophotometer equipped with a fiber optic and an immersed type optical cell of 1 cm. The optimum operating conditions regarding concentration of the reagents involved, pH and temperature were established. The interference effect of several metallic species was also investigated. It was found that the proposed method shows fairly good selectivity and sensitivity, simplicity and rapidity compared to other kinetic methods. The working curve of the recommended reaction-rate method is linear in the concentration range 5-300 ng/ml. The relative standard deviation for a standard solution of 30 ng/ml Cu(II) is better than 2.5%. The method was applied successfully on the determination of Cu(II) in a wide variety of real samples such as alloys, pharmaceuticals, foodstuffs and environmental samples. The results were compared to those received with official methods. Good agreement was attained.

Application of two-way decomposition methods in the simultaneous determination of cobalt, nickel and iron by coupling stopped-flow techniques and CCD detection

The simultaneous kinetic determination of cobalt, nickel, and iron with principal component regression (PCR) and partial least squares (PLS) methods is discussed. PCR and PLS extract information from kinetic differences between the complexes of these metals after their reaction with a common ligand. The coupling of stopped-flow (SF) with CCD detection resulted in massive data quantities making the use of data-reduction techniques not only interesting, but also necessary. Initial rates of the reactions were calculated using an algorithm, whose description is given somewhere else [Anal. Chim. Acta 423 (2000) 277]. Both methods were successfully applied to the simultaneous determination of these metals in alloys, and a comparative study of the results was undertaken.

Kinetic spectrophotometric assay of sulfonamides by use of the griess reactionand a stopped-flow procedure

Abstract The time interval between two preset levels of the output signal is used as a measure of the sulfonamide concentration. Nitrite and N -(1-naphthyl)ethylenediamine, are used as reagents, and 0.5–5 × 10 −5 M of a sulphonamide can be determined. The method is applied to urine.

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.