Kinetic Determinations and Some Kinetic Aspects of Analytical Chemistry

A Reaction-Based Optical Fingerprinting Strategy for the Recognition of Fat-Soluble Samples: Discrimination of Motor Oils

Optical "fingerprints" are widely used for chemometrics-assisted recognition of samples of different types. An emerging trend in this area is the transition from obtaining "static" spectral data to reactions analyzed over time. Indicator reactions are usually carried out in aqueous solutions; in this study, we developed reactions that proceed in an organic solvent, thereby making it possible to recognize fat-soluble samples. In this capacity, we used 5W40, 10W40, and 5W30 motor oils from four manufacturers, with six samples in total. The procedure involved mixing a dye, sample, and reagents (HNO3, HCl, or tert-butyl hydroperoxide) in an ethanolic solution in a 96-well plate and measuring absorbance or near-infrared fluorescence intensity every several minutes for 20-55 min. The obtained photographic images were processed by linear discriminant analysis (LDA) and the k-nearest neighbors algorithm (kNN). Discrimination accuracy was evaluated by a validation procedure. A reaction of oxidation of a dye by nitric acid allowed us to recognize all six samples with 100% accuracy for LDA. Merging of data from the four reactions that did not provide complete discrimination ensured an accuracy of 93% for kNN. The newly developed indicator systems have good prospects for the discrimination of other fat-soluble samples. Overall, the results confirm the viability of the kinetics-based discrimination strategy.

Estimation of doses absorbed by potato tubers under electron beam or X-ray irradiation using an optical fingerprinting strategy

High-energy electron beam and X-ray processing of foods can be used for extending their storage life and for combating pests and pathogens. Several instrumental techniques are used to estimate irradiation doses in foods, but these methods are complex and laborious, require expensive equipment, and do not always allow to determine low doses. This study was aimed at developing simple methods for detecting irradiation in potato tubers and for dose estimation. We used a "fingerprinting" strategy that does not involve quantitation of any compound; instead, the rate of indicator reactions involving carbocyanine dyes is measured. The dye content was monitored by its near-infrared fluorescence intensity and visible-light absorption. Potatoes not subjected to treatment and those irradiated with different doses (10, 100, 1000, 5000, or 10,000 Gray) could be distinguished by linear discriminant analysis. Thus, the order of magnitude of the absorbed dose can be estimated with 89% ± 3% accuracy for a mixture of tubers of two potato varieties irradiated with an electron beam or with 95% ± 8% accuracy for one variety irradiated with an X-ray source.

Copper (II)-Catalyzed Oxidation of Ascorbic Acid: Ionic Strength Effect and Analytical Use in Aqueous Solution

Copper is an important metal both in living organisms and in the industrial activity of humans, it is also a distributed water pollutant and a toxic agent capable of inducing acute and chronic health disorders. There are several fluorescent chemosensors for copper (II) determination in solutions; however, they are often difficult to synthesize and solvent-sensitive, requiring a non-aqueous medium. The present paper improves the known analytical technique for copper (II) ions, where the linear dependence between the ascorbic acid oxidation rate constant and copper (II) concentration is used. The limits of detection and quantification of the copper (II) analysis kinetic method are determined to be 82 nM and 275 nM, respectively. In addition, the selectivity of the chosen indicator reaction is shown: Cu2+ cations can be quantified in the presence of the 5–20 fold excess of Co2+, Ni2+, and Zn2+ ions. The La3+, Ce3+, and UO22+ ions also do not catalyze the ascorbic acid oxidation reaction. The effect of the concentration of the common background electrolytes is studied, the anomalous influence for chloride-containing salts is observed and discussed.

A New Kinetic Spectrophotometric Method for the Quantitation of Amorolfine

Amorolfine (AOF) is a compound with fungicide activity based on the dual inhibition of growth of the fungal cell membrane, the biosynthesis and accumulation of sterols, and the reduction of ergosterol. In this work a sensitive kinetic and spectrophotometric method for the AOF quantitation based on the AOF oxidation by means of KMnO4 at 30 min (fixed time), pH alkaline, and ionic strength controlled was developed. Measurements of changes in absorbance at 610 nm were used as criterion of the oxidation progress. In order to maximize the sensitivity, different experimental reaction parameters were carefully studied via factorial screening and optimized by multivariate method. The linearity, intraday, and interday assay precision and accuracy were determined. The absorbance-concentration plot corresponding to tap water spiked samples was rectilinear, over the range of 7.56 × 10-6-3.22 × 10-5 mol L-1, with detection and quantitation limits of 2.49 × 10-6 mol L-1 and 7.56 × 10-6 mol L-1, respectively. The proposed method was successfully validated for the application of the determination of the drug in the spiked tap water samples and the percentage recoveries were 94.0-105.0%. The method is simple and does not require expensive instruments or complicated extraction steps of the reaction product.

Best Combination of Promoter and Micellar Catalyst for Room Temperature Rapid Conversion of D-Lyxose to D-Lyxonic Acid in Aqueous Medium

The kinetic study of catalytic oxidation of D-lyxose by hexavalent chromium has been investigated spectrophotometrically under pseudo first order condition at temperature 313 K. The rate of oxidation of D-lyxose is very slow. Picolinic acid (PA), 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen) are used as promoter to accelerate the rate of the reaction. Phen acts as the most effective promoter in aqueous medium. The rate of the reaction is also increased in presence of nonionic surfactant Triton-X-100 (TX-100) and anionic surfactant sodium dodecyl sulphate (SDS). They are used as catalyst in this reaction. Thus the observed micellar effects have been explained by considering the hydrophobic and electrostatic interactions between the reactants and surfactants in terms of the proposed mechanism. However, the combination of promoter and surfactants produces a better result. The maximum rate enhancement is obtained in presence of the combination of phen and TX-100.

Kinetic spectrophotometric method for determination of amlodipine besylate in its pharmaceutical tablets

A simple and sensitive kinetic spectrophotometric method has been developed and validated for determination of amlodipine besylate (AML). The method was based on the condensation reaction of AML with 7-chloro-4-nitro-2,1,3-benzoxadiazole in an alkaline buffer (pH 8.6) producing a highly colored product. The color development was monitored spectrophometrically at the maximum absorption λmax 470 nm. The factors affecting the reaction were studied and the conditions were optimized. The stoichiometry of the reaction was determined, and the reaction pathway was postulated. Moreover, both the activation energy and the specific rate constant (at 70 °C) of the reaction were found to be 6.74 kcal mole-1 and 3.58 s-1, respectively. The initial rate and fixed time methods were utilized for constructing the calibration graphs for the determination of AML concentration. Under the optimum reaction conditions, the limits of detection and quantification were 0.35 and 1.05 μg/mL, respectively. The precision of the method was satisfactory; the relative standard deviations were 0.85-1.76%. The proposed method was successfully applied to the analysis of AML in its pure form and tablets with good accuracy; the recovery percentages ranged from 99.55±1.69% to 100.65±1.48%. The results were compared with that of the reported method.

Kinetic spectrophotometric determination of certain cephalosporins using oxidized quercetin reagent

A simple, precise and accurate kinetic spectrophotometric method for determination of cefoperazone sodium, cefazolin sodium and ceftriaxone sodium in bulk and in pharmaceutical formulations has been developed. The method is based upon a kinetic investigation of the reaction of the drug with oxidized quercetin reagent at room temperature for a fixed time of 30 min. The decrease in absorbance after the addition of the drug was measured at 510 nm. The absorbance concentration plot was rectilinear over the range 80-400 microg mL(-1) for all studied drugs. The concentration of the studied drugs was calculated using the corresponding calibration equation for the fixed time method. The determination of the studied drugs by initial rate, variable time and rate-constant methods was feasible with the calibration equations obtained but the fixed time method has been found to be more applicable. The analytical performance of the method, in terms of accuracy and precision, was statistically validated; the results were satisfactory. The method has been successfully applied to the determination of the studied drugs in commercial pharmaceutical formulations. Statistical comparison of the results with a well established reported method showed excellent agreement and proved that there is no significant difference in the accuracy and precision.

Kinetic spectrophotometric determination of certain cephalosporins in pharmaceutical formulations

A simple, reliable, and sensitive kinetic spectrophotometric method was developed for determination of eight cephalosporin antibiotics, namely, Cefotaxime sodium, Cephapirin sodium, Cephradine dihydrate, Cephalexin monohydrate, Ceftazidime pentahydrate, Cefazoline sodium, Ceftriaxone sodium, and Cefuroxime sodium. The method depends on oxidation of each of studied drugs with alkaline potassium permanganate. The reaction is followed spectrophotometrically by measuring the rate of change of absorbance at 610 nm. The initial rate and fixed time (at 3 minutes) methods are utilized for construction of calibration graphs to determine the concentration of the studied drugs. The calibration graphs are linear in the concentration ranges 5-15 mug mL(-1) and 5-25 mug mL(-1) using the initial rate and fixed time methods, respectively. The results are validated statistically and checked through recovery studies. The method has been successfully applied for the determination of the studied cephalosporins in commercial dosage forms. Statistical comparisons of the results with the reference methods show the excellent agreement and indicate no significant difference in accuracy and precision.

Exploiting kinetic spectrophotometric determination of captopril, an angiotensin-converting enzyme inhibitor, in a multi-pumping flow system

The implementation of a differential kinetic spectrophotometric method for the determination of angiotensin-converting-enzyme inhibitors in pharmaceutical formulations is described. The determination method was based on the monitoring (350 nm) of the reaction between captopril and iodate, in the presence of iodide, versus time and was fully automated by exploiting the multi-pumping flow concept. The developed multi-pumping flow system included four discretely actuated solenoid micro-pumps as unique flow manifold active components. The automatic control of the solenoid micro-pumps, under time-based and pulse-counting routines, allowed the implementation of a reliable and versatile analytical determination, with the additional advantage of permitting a runtime access to important analytical parameters, such as flow rate, sample insertion and reagent addition synchronisation, facilitating this way the establishment of an approach for kinetic measurements, directly due to the efficient solution handling and accurate timing control. A linear range of determination was verified for captopril concentrations between 10.0 and 60.0 microgmL(-1) with a sample throughput of about 100 determinations per hour. The results were in agreement with those obtained by the reference procedure with relative deviations between 1.81 and 4.48%.

Kinetic method for determination of ascorbic acid on flow injection system by using its catalytic effect on the complexation reaction of an ultra sensitive colorimetric reagent of porphyrin with Cu(II)

A kinetic method performed on a flow injection system is described for the determination of ascorbic acid by using its catalytic effect on the complexation reaction of Cu(II) with 5,10,15,20-tetrakis(4-N-trimethyl-aminophenyl)porphyrin. The characteristic spectrum of porphyrin (Soret band), which shows intense absorption around 400 nm (epsilon>2.0 x 10(5) cm(-1)M(-1)), was used first time for determining ascorbic acid. By incorporating the complexation reaction into a flow injection system, ascorbic acid could be determined either over a broad dynamic range of 0.1-1000 microg/ml or at a trace level below 5 ng/ml. Good repeatability was also achieved by testing a working standard of 0.1 microg/ml with 10 injections at a throughput of 35 h(-1), obtaining a relative standard deviation of 0.11%. Substances like amino acids, vitamins, sugars, organic acids and metal ions, showed no or little interference even present at high concentrations. The method was validated in the determination of ascorbic acid contents of some commercially available soft drinks by comparison with the official 2,6-dichloroindophenol method with reasonable agreement.

Continuous flow thermal cycler microchip for DNA cycle sequencing

We report here on the use of a polymer-based continuous flow thermal cycler (CFTC) microchip for Sanger cycle sequencing using dye terminator chemistry. The CFTC chip consisted of a 20-loop spiral microfluidic channel hot-embossed into polycarbonate (PC) that had three well-defined temperature zones poised at 95, 55, and 60 degrees C for denaturation, renaturation, and DNA extension, respectively. The sequencing cocktail was hydrodynamically pumped through the microreactor channel at different linear velocities ranging from 1 to 12 mm/s. At a linear velocity of 4 mm/s resulting in a 36-s extension time, a read length of >600 bp could be obtained in a total reaction time of 14.6 min. Further increases in the flow rate resulted in a reduction in the total reaction time but also produced a decrease in the sequencing read length. The CFTC chip could be reused for subsequent sequencing runs (>30) with negligible amounts of carryover contamination or degradation in the sequencing read length. The CFTC microchip was subsequently coupled to a solid-phase reversible immobilization (SPRI) microchip made from PC for purification of the DNA sequencing ladders (i.e., removal of excess dye-labeled dideoxynucleotides, DNA template, and salts) prior to gel electrophoresis. Coupling of the CFTC chip to the SPRI microchip showed read lengths similar to that obtained from benchtop instruments but did not require manual manipulation of the cycle sequencing reactions following amplification.

Kinetic spectrophotometric method for o-phenylenediamine in the presence of gold(III)

From the color developing reactions of o-phenylenediamine oxidizing agent and gold(III), the kinetic reactions between both of them in aquaeous solutions were studied using spectrophotometric and differential method. Light absorbances in the visible spectral range are measured as a function of mole fractions of phenylenediamine at a fixed gold(III) concentration and as a function of mole fraction of gold(III) at a fixed o-phenylenediamine concentration at periodic time internal. In the differential method, which was suggested by van't Hoff, one deals with the actual rates of reactions as determined by measuring the slopes of absorbance-time curves. Optimum condition of the reaction were established as pH 6 at lambda=466 nm and room temperature. When the oxidation of o-phenylenediamine by gold(III) was investigated, it was observed that the following rate formula and rate constant were found: v=k[Au+3]1/2[o-phenylenediamine]1/2, k=2.33x10(-2) s-1.

Simultaneous spectrophotometric determination of Hg2+, Pb2+, and Cu2+ by Zn(dmit) using the multivariate calibration method

This work investigates the applicability of the anionic complex [NBu4]2[Zn(1,3-dithiol-2-tione-4,5-dithiolate)2] on the simultaneous spectrophotometric determination of transition metals. Principal Component Regression was used in the data analysis. This method was used to determine Hg2+, Cu2+ and Pb2+ in water solution, at t = 0, 20 and 60 min of reaction. The root mean square errors (rms) for Hg2+, Cu2+, Pb2+ were 1.92 x 10(-7), 1.89 x 10(-7) and 9.35 x 10(-8) for t = 0 min, 1.79 x 10(-7), 2.1 x 10(-7) and 2.22 x 10(-7) for t = 20 min and 1.22 x 10(-7), 1.31 x 10(-7) and 1.10 x 10(-7) for t = 60 min, respectively. Results showed the viability of the method in determining concentrations of metallic ions in aqueous medium with high spectral overlap. It is an efficient alternative method for exploring the kinetic behavior of multicomponents systems of interest in physicochemical research.

Copper in the silk formation process ofBombyx morisilkworm

Evidence is presented here that cupric ions play a part in the natural spinning of Bombyx mori silk. Proton induced X-ray emission studies revealed that the copper content increased from the posterior part to the anterior part of silk gland, and then further increased in the silk fiber. Spectrophotometric analysis demonstrated that cupric ions formed coordination complexes with silk fibroin chains while Raman spectroscopy indicated that they induced a conformation transition from random coil/helix to beta-sheet. Taken together these findings indicate that copper could play a role in the natural spinning process in silkworms.

Quantification of acetaminophen by oxidation with tyrosinase in the presence of Besthorn's hydrazone

A method for the quantitative determination of acetaminophen by measuring the kinetics of its oxidation by the enzyme tyrosinase in the presence of the nucleophilic agent 3-methyl-2-benzothiazolone hydrazone (Besthorn's hydrazone) is described. The hydrazone traps the o-quinone product of the catalytic activity, 4-acetamido-o-benzoquinone, thus increasing the level of the catecholic intermediate, 3'-hydroxyacetaminophen, in the pathway, as was shown by liquid chromatography. This reduces the time necessary for the steady state to be reached and increases the steady state oxidation rate, reaching activations of 124 times at an initial concentration of 3mM hydrazone. Despite being rapid and simple the method is thorough and provides a linear range of quantification of the drug from 10 nmol/ml to 1.4 micromol/ml. The assay can be performed in continuous or discontinuous form, meaning that it can be used with the immobilized enzyme for the design of specific biosensors.

A Continuous-Flow System for High-Precision Kinetics Using Small Volumes

A generally applicable continuous-flow kinetic analysis system that gives data of a precision high enough to measure small kinetic isotope effects for enzymatic and nonenzymatic reactions is described. It employs commercially available components that are readily assembled into an apparatus that is easy to use. It operates under laminar flow conditions, which requires that the time between the initiation of the reaction in the mixer and the observation be long enough that molecular diffusion can effect a symmetrization of the concentration profile that results from a thin plug of reagents introduced at the mixer. The analysis of a second-order irreversible reaction under pseudo-first-order conditions is presented. The Yersinia pestis protein tyrosine phosphatase catalyzed hydrolysis of p-nitrophenyl phosphate is characterized with the system, and a proton inventory on kcat is presented.

Kinetic spectrophotometric determination of nizatidine and ranitidine in pharmaceutical preparations

A new simple and sensitive kinetic spectrophotometric method is described for analysis of nizatidine (I) and ranitidine (II). The method involves the reaction of the drugs with alkaline potassium permanganate, whereby a green color peaking at 610 nm is produced. The reaction is monitored spectrophotometrically by measuring the rate of change of absorbance of the resulting manganate species at 610 nm. Calibration graphs are linear over the concentration range 0.8-4.0 microg/ml and the precision (% RSD 1.80, 1.53 for I and II, respectively) is quite acceptable. The method is satisfactorily applied for direct analysis of pharmaceutical preparations containing I and II. A proposal of the reaction pathway is postulated.

Matrix-assisted laser desorption/ionization mass spectrometry for locating abasic sites and determining the rates of enzymatic hydrolysis of model oligodeoxynucleotides

A method using a combination of exonuclease enzymatic digestion and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry was developed to locate model abasic sites in a series of model 21-base oligodeoxynucleotides in which a nucleobase was replaced by a hydrogen atom. The exonuclease digestion rate, with either snake venom phosphodiesterase (SVP) or bovine spleen phosphodiesterase (BSP), clearly slows as the digestion approaches the abasic sites and stops as it reaches it. An oligodeoxynucleotide containing an abasic site in which OH replaces the nucleobase shows similar results. MALDI mass spectra taken at appropriate times during the course of hydrolysis are the basis for rate measurements, and the kinetics also reveal the location of the abasic site. A mathematical treatment of the time-dependent MALDI data was implemented to obtain rate curves and rate constants for the enzymatic digestion of both modified and unmodified oligodeoxynucleotides.

Nicotinamide adenine dinucleotide species in the horseradish peroxidase-oxidase oscillator

NADH chemistry ancillary to the oscillatory peroxidase-oxidase (PO) reaction has been reexamined. Previously, (NAD)2 has been thought of as a terminal, inert product of the PO reaction. We now show that (NAD)2 is a central reactant in this system. Although we found traces of the dimer after several hours of the PO reaction, no accumulation of the dimer occurred, regardless of the reaction time or the number of oscillations. (NAD)2 can convert horseradish peroxidase (HRP) compound I (CpI) to compound II (CpII) with apparent rate constant (2.7 +/- 0.2) x 105 M-1.s-1 and CpII to HRP at 1 x 105 M-1.s-1. Moreover, a reduction of HRP compound III (CpIII) to CpI by (NAD)2 occurs with a rate constant faster than 5 x 106 M-1.s-1. The (NAD)2 reduction of CpIII provides an alternative to the reduction by NAD radical suggested by Yokota and Yamazaki. HRP catalyzes oxidation of alpha-NADH, not only the beta anomer as previously assumed. Rate constants of alpha- and beta-NADH reactions with CpI are (7.4 +/- 0.4) x 105 M-1.s-1, and (1.7 +/- 0.2) x 105 M-1.s-1, and with CpII are estimated as 5 x 104 M-1.s-1, and 4 x 104 M-1.s-1. Apparent rate constants of reduction of methylene blue (MB) to leuco-methylene blue (MBH) are 3.8 x 104 M-1.s-1 for NADH and 6.4 x 104 M-1.s-1 for NAD dimer, (NAD)2, while reoxidation of MBH proceeds at (2.1 +/- 0.2) x 103 M-1.s-1 All the rates were measured in 0.1 M acetate buffer, pH 5.1.

Estimating realistic confidence intervals for the activation energy determined from thermoanalytical measurements

A statistical procedure is proposed for estimating realistic confidence intervals for the activation energy determined by using an advanced isoconversional method. Nine sets of five thermogravimetric measurements have been produced for the process of gassification of ammonium nitrate at five different heating rates. Independent estimates of the confidence intervals for the activation energy have been obtained from these data sets. Agreement with these independent estimates demonstrates that the proposed statistical procedure is capable of adequately estimating the actual uncertainty in the activation energy determined from a small number of measurements. The resulting averaged relative errors in the activation energy were found to be 26, 21, and 17% for three, four, and five heating rate estimates, respectively.

Correction of temperature variations in kinetic-based determinations by use of pruning computational neural networks in conjunction with genetic algorithms

The joint use of genetic algorithms and pruning computational neural networks is shown to be an effective means for selecting the number of inputs required to correct temperature variations in kinetic-based determinations. The genetic algorithm uses a pruning procedure based on Bayesian regularization and is highly efficient as a feature selector; it provides quite good results in the generalization process without the need to use a validation set. The fitness function is defined as the sum of two subfunctions: one controls the learning ability of the network and the other its complexity. The training, pruning, and generalization processes were initially tested with simulated data in order to acquire preliminary information for the ensuing work with real data. The performance of the proposed method was assessed by applying it to the determination of the amino acid L-glycine by its classical spectrophotometric reaction with ninhydrin. A straightforward network topology including temperature as input (40+T:2:1 with 19 connections after the pruning process) was used to estimate the L-glycine concentration from kinetic curves affected by temperature variations over the range 60-75 degrees C, using kinetic data acquired up to only 1.5 half-lives. The trained network estimates this concentration with a standard error of prediction for the testing set of ca. 8%, which is much smaller than those provided by a classical parametric method such as nonlinear regression (even if kinetic data acquired at longer half-lives are used). Finally, a kinetic interpretation of the pruning process is provided in order to better demonstrate its potential for kinetic analysis.