Formation of two reaction zones in flow-injection systems for kinetic determinations of cobalt and nickel

The injection of a large sample volume (ca. 1 ml) into a single-channel flow-injection system was studied with a dye (to examine physical dispersion) and with chemical systems having easily-controlled reaction rates (to examine chemical kinetics). With the dye, the response curve has a central plateau caused by non-mixing of carrier and sample. When a chemical reaction takes place, two peaks are obtained with a central minimum corresponding to little or no mixing and reaction. Comparison of these two types of response provides relationships of analytical interest between response parameters and variables in te flow-injection system. The configuration is used for individual kinetic determinations of cobalt and nickel (2.5–30 μg ml^?1 based on the rate of their complex formation with 2-hydroxybenzaldehyde thiosemicarbazone. Sample injection rates were 15 h^?1 for cobalt and 40 h^?1 for nickel. Differential kinetic determinations of cobalt and nickel in mixtures are based on the increment in peak height (or area) between the two peaks obtained for each injection; sample throughput is 7 h^?.     

早期生物矿化过程中紫外吸收动力学曲线上的负峰现象

近年来, Jens等[1]利用紫外光度法测定生物矿化溶液的吸光度(即混浊度)的变化, 实时地记录生物矿物形成过程的信息, 从而研究其矿化规律. 实验发现, 胶原/羟基磷灰石矿化的紫外吸收动力学曲线并不是胶原和磷酸钙沉淀混浊度的简单迭加, 而是一条平滑的阶梯形曲线, 它反映了有机相和无机相相互作用的过程特征. 在采用紫外光度法研究生物矿化过程中首次发现, 阶梯形矿化曲线上还有更精细的变化: 在一定条件下, 矿化曲线起峰时出现一个负峰. 本文还研究了磷酸钙浓度、胶原浓度及入射光波长对该负峰峰位和峰值的影响. 研究该负峰与系统参数的相关性, 对于了解早期生物矿化的机理和优化新型骨及牙齿等组织工程框架材料的制备工艺[2]都具有重要的意义.     

Application of a kinetic energy partitioning scheme for ab initio molecular dynamics to reactions associated with ionization in water tetramers

We analyze the short-time dynamics of 'cyclic' and 'branched' water tetramers after an ionization event, with the aid of a scheme that partitions the kinetic energy of a solute plus solvent system into separate solute and solvent (or bath) contributions, using instantaneous internal coordinates and atomic velocities. The analysis supports the partitioning of the tetrameric systems into two subsystems, a 'reactive trimer' and a 'solvent' molecule. The partitioned kinetic energy exhibits two features, a broad peak assigned to the interaction between the two sub-systems and a sharper peak arising from the proton transfer that occurs upon ionization. It is found that the stability of the hydroxyl radical formed upon ionization is sensitive to the configuration of the water molecules around the ionized water at the moment of the ionization event.     

Capillary electrophoresis: An integrated system with a unique split-flow sample introduction mechanism

The performance of an integrated capillary electrophoresis system with a novel split-flow sample injection mechanism and special high sensitivity UV absorbance detector is described. Sample introduction into the capillary is accomplished with a standard HPLC-type microliter syringe. The injected sample is divided proportionally between the separation capillary and an adjustable splitvent. The volume of sample introduced into the capillary can be manipulated by varying the length or the i.d. of the splitvent tubing; or the volume of sample injected. Data are presented showing reproducibility of retention time, peak height, and peak area; minimum detectability; and operation at short UV wavelengths.     

Determination of rate constants for heterogeneous mass transfer kinetics in liquid chromatography

The heterogeneity of the stationary phase surface leads to undesired peak broadening and tailing in chromatography. In order to properly characterize the interactions between the solute molecules and the surface of the stationary phase, the determination of the equilibrium isotherm is usually necessary by overloading the column. In this study we show that measurements made in the linear range of the isotherm can also be utilized to estimate the heterogeneity of the stationary phase. When one studies the peak shape obtained after injecting a small (analytical) amount of sample, the peak shape parameters can be correlated with the kinetic rate constants. The peak of a mildly polar solute, phenol, was measured in reversed-phase HPLC under various conditions and kinetic information was obtained regarding the heterogeneity of interactions.     

Simultaneous determination of iron(II), iron(III), and titanium(IV) by flow injection analysis using kinetic spectrophotometry with Tiron

Summary Two flow injection analysis systems have been worked out for the simultaneous determination of Fe(III), Fe(II), and Ti(IV) based on the kinetic spectrophotometry with Tiron. The first system uses a silver reductor column and a single detector with two flow cells aligned in the same optical path to yield two peaks corresponding to (a) Ti(IV)-Tiron and (b) Ti(IV) plus total iron(III)-Tiron complexes. An another sample injection without the silver column yields a single peak which corresponds to Ti(IV) plus Fe(III)-Tiron complexes. With the two sample aliquot injections the system permits simultaneous determinations with throughput of 30 samples/h in the g to several tens g range of each species. The second system is a multidetection system with or without the silver reductor column using the same spectrophotometry with Tiron, in which the entrapment of the sample plug into a closed system allows its repetitive passage through a single detector. With the advantage of much simpler instrumentation, the system permits 6 samples/h to be analyzed for the three metal species with somewhat lower precisions than the first system.     

Time effects on the rheological behavior of hydrophobically modified polyacrylamide aqueous solutions mixed with sodium dodecyl sulfate (SDS)

The transient response of hydrophobically modified polyacrylamides (HMPAM) aqueous solutions with sodium dodecyl sulfate (SDS) during step shear rate experiments was evaluated. The experimental protocol involved employing first a low shear rate for a specific time, followed by an abrupt increase in shear rate (applied for the same time) and then finally reducing the shear rate to the initial value and recording the sample relaxation with time. While the second step was characterized by a general anti-thixotropic behavior, a large viscosity peak was observed at the third step at very short times followed by a thixotropic relaxation of the viscosity value. This peak was interpreted as a consequence of the formation of a transient network during the relaxation process. A kinetic model was used to better understand and to describe the thixotropy of HMPAM solutions in the presence of SDS. The kinetic constants obtained from the model decrease in value when the shear rate applied during the second step increases, reflecting the increase in recovery time needed for the system to relax back to a lower strain rate. As expected, as the degree of hydrophobic association increases or the degree of fluid structuring increases, the kinetic constant values strongly decrease.     

Kinetic energy release in ionic fragmentations: Use as an ion structure probe

The kinetic energy released in unimolecular reactions, as measured from the width of the corresponding metastable peak, shows only a small dependence on such parameters as source temperature, ion-source residence time and ion acceleration voltage. Similarly, fragmenting ions generated from different members of an homologous series of molecular ions have been found to release the same kinetic energy and hence do not exhibit a degrees-of-freedom effect analogous to that for metastable abundances. In general, molecular ions formed by electron-impact have been found to release slightly less kinetic energy on fragmentation than the corresponding ions formed via a fragmentation sequence. These observations suggest that kinetic energy release is a useful method of structural characterization of metastable ions; while increase in the average internal energies of the ions sample lead to larger energy releases, this effect is usually small. The use of a very narrow energy resolving (β) slit and a procedure in which the metastable peak width is extrapolated to zero slit width has been found to improve the accuracy of measurement of the kinetic energy release, particularly when the metastable and main beam peak widths are of comparable magnitude.     

Pseudo-phase changes in cupric sulfate pentahydrate during dehydration

A study of the dehydration kinetics of a model system, cupric sulfate pentahydrate, was made using the temperature programmed desorption technique. The three decomposition steps from the pentahydrate to the anhydrous salt were clearly resolved into distinct spectral peaks even at sample heating rates in excess of 10 K. min^?1. A linear relationship between spectral peak temperature and square root of heating rate was observed for each dehydration step. The kinetic data revealed different activation enthalpies and entropies for each dehydration sequence for heating rates above and below approximately 8 K min^?1. These latter findings are interpreted in terms of crystalline to amorphous pseudo-phase changes in the solid hydrate during decomposition which become apparent only at fast sample heating rates. Enthalpy and entropy changes associated with these structural alterations are evaluated. The results also help to clarify earlier work on the dehydration mechanism in calcium phosphates.     

Numerical analysis of an electrokinetic double-focusing injection technique for microchip CE

The injection techniques in electrophoresis microchips play an important role in the sample-handling process, whose characteristics determine the separation performance achieved, and the shape of a sample plug delivered into the separation channel has a great impact on the high-quality separation performance as well. This paper describes a numerical investigation of different electrokinetic injection techniques to deliver a sample plug within electrophoresis microchips. A novel double-focusing injection system is designed and fabricated, which involves four accessory arm channels in which symmetrical focusing potentials are loaded to form a unique parallel electric field distribution in the intersection of injection channel and separation channel. The parallel electric field effectuates virtual walls to confine the spreading of a sample plug at the intersection and prevents sample leakage into separation channel during the dispensing step. The key features of this technique over other injection techniques are the abilities to generate regular and nondistorted shape of sample plugs and deliver the variable-volume sample plugs by electrokinetic focusing. The detection peak in the proposed injection system is uniform regardless of the position of the detection probe in the separation channel, and the peak resolution is greatly enhanced. Finally, the double-focusing injection technique shows the flexibility in detection position and ensures improved signal sensitivity with good peak resolution due to the delivered high-quality sample plug.     

Influence of micelles on the electrochemical behaviors of catechol and hydroquinone and their simultaneous determination

A simple and highly selective electrochemical method for the simultaneous determination of CAT and HQ at a glassy carbon electrode in micellar solutions has been developed. The electrochemical behaviors of CAT and HQ in aqueous CPB and SDBS micellar solutions have been studied by cyclic voltammetry. The oxidation peak potentials shift negatively, the reduction peak potentials shift positively, and the peak currents increase in the presence of CPB for both CAT and HQ. However, the oxidation peak potentials shift positively, the reduction peak potentials shift negatively, and the peak currents decrease in the presence of SDBS for both CAT and HQ. The electrochemical kinetic parameters for CAT and HQ in aqueous CPB and SDBS micellar solutions were also determined by chronocoulometry (CC) and chronoamperometry (CA). The cyclic and pulse differential voltammetric behaviors of the system consisting of CAT coexisting with HQ were also investigated in this work. It was found that the oxidation peak potential waves of CAT and HQ were separated by 100 mV in the presence of CPB in 0.10 M PBS (pH 6.8). Therefore, CAT and HQ can be determined simultaneously in such a system. This simple method was applied to the simultaneous determination of HQ and CAT in a household tap water sample and it exhibited high selectivity.     

Kinetic approach to partially overlapped thermal decomposition processes

Practical usefulness of the kinetic deconvolution for partially overlapped thermal decomposition processes of solids was examined by applying to the co-precipitated basic zinc carbonate and zinc carbonate. Comparing with the experimental deconvolutions by thermoanalytical techniques and mathematical deconvolutions using different statistical fitting functions, performance of the kinetic deconvolution based on an accumulative kinetic equation for the independent processes overlapped partially was evaluated in views of the peak deconvolution and kinetic evaluation. Two-independent kinetic processes of thermal decompositions of basic zinc carbonate and zinc carbonate were successfully deconvoluted by means of the thermoanalytical measurements in flowing CO₂ and by applying sample controlled thermal analysis (SCTA). The deconvolutions by the mathematical curve fittings using different fitting functions and subsequent formal kinetic analysis provide acceptable values of the mass-loss fractions and apparent activation energies of the respective reaction processes, but the estimated kinetic model function changes depending on the fitting functions employed for the peak deconvolution. The mass-loss fractions and apparent kinetic parameters of the respective reaction processes can be optimized simultaneously by the kinetic deconvolution based on the kinetic equation through nonlinear least square analysis, where all the parameters indicated acceptable correspondences to those estimated through the experimental and mathematical deconvolutions. As long as the reaction processes overlapped are independent kinetically, the simple and rapid procedure of kinetic deconvolution is useful as a tool for characterizing the partially overlapped kinetic processes of the thermal decomposition of solids.     

Resolution of chemical species in flow injection analysis by using self-modeling curve resolution

Mixtures can be resolved into their components by separation, spectrometric and kinetic (chemical) procedures, which have varying degrees of efficiency. Chemometric methods of data processing have proved useful for increasing resolution, especially in spectral and chromatographic procedures. This study shows that chemometric methods are also useful for kinetic flow-injection methods based on chemical reactions combined with multichannel spectral detection, where neither the reaction nor the detector provides adequate resolution. The problem of background absorbance of a sample in a flow-injection system with an ultraviolet/visible photodiode array detector is solved by applying a self-modeling curve resolution method. The example used is a mixture of methyl violet and phenolphthalein.     

Sampling system for mass spectrometric monitoring of a stream of reaction liquid

Reaction liquid is continuously circulated between the reactor and an automated Bendix liquid injection valve. A well defined volume of liquid is injected into a hot zone. The liquid injected is then vaporized and the resulting sample gas is swept with a stream of helium into a series of dilution chambers. The analyte concentration-time profile at the exit of the last chamber was mathematically modelled. It was found that the degree of symmetry of the profile increases when the number of chambers increases. The design and performance of the system are discussed together with results of a kinetic study of a chemical reaction.     

Thermoluminescence and chemical characterization of natural calcite collected from Kodwa mines

This paper reports the thermoluminescence (TL) and chemical characterization of natural calcite collected from Kodwa mines of the C.G. Basin. The sample was irradiated with a 10-Gy dose from an Sr-90 beta source. The heating rate used for TL measurements was 6.7 °/s. The sample had a good TL peak at approximately 127 °C and the corresponding kinetic parameter were calculated. The effect of annealing temperature is also described for natural calcite. The photoluminescence excitation spectrum at ~254 nm and the corresponding emission spectrum peak at 395 nm are reported. Inductively coupled plasma–activated emission spectroscopy (ICP–AES) was conducted to determine the percentages of elements in the calcite mineral.     

On the potential of two- and multi-dimensional separation systems

A probability model of zone overlapping in n-dimensional (n-D) separation systems has been developed. The probability that all sample components are separated is given as a function of the number of components, m, and of the peak capacity, n_c, of the n-D system. Application to 1-D separations provides the same expression as that previously obtained with a more rigorous peak overlapping model, in the limit of large m and n_c and of low saturation of the separation space. The major result is that the probability of total resolution of the sample decreases exponentially with the square of the number of sample components and with the reciprocal of the peak capacity, whatever the dimension of the separation system. In addition, a simple general relationship is obtained between this probability and the probability to separate one or a few components of interest from all other sample components. It is found that, for a given number of components and a given peak capacity, these probabilities slightly depend on the dimension of the separation system, which indicates that the peak capacity is not a fully universal index of characterization of the resolving power. The peak capacity required to separate all sample components at a given probability level increases with the square of the number of components. Accordingly, the individual peak capacity per dimension does not increase as fast as m when the dimension of the system exceeds 2.Dedicated to Professor Dr. h.c. mult. J.F.K. Huber on the occasion of his 70th birthday     

生物质焦油热裂解动力学参数实验研究

基于进样过程中焦油蒸发的不稳定性以及反应温度的测定和控制难度,设计了移动床焦油进样系统,使用样品槽携带焦油进入具有线性温度分布的炉膛空间,以样品槽的运动进样代替焦油流动进样,以获得稳定的焦油蒸发进样和更容易控制的反应温度。在此基础上,研究了不同进样速率下的焦油热裂解及其动力学特性。结果表明,在573~1 123 K,焦油的裂解率随温度的升高而加速升高,并在1 123 K时达到58%的裂解率;焦油热裂解遵循一级动力学反应,其平均活化能为39.5 kJ/mol,指前因子为1.58 min^-1。     

Kinetics in continuous flow sample processing Chemical Contributions to Dispersion in Flow-Injection Techniques

A numerical model based on a basic dispersion equation modified by simultaneous consideration of the chemical effect introduced by a chemical reaction taking place as the sample plug disperses has been tested. Computer analysis and simulation (by use of an alternating direction implicit finite difference approximation) by solving the laminar dispersion equation and the rate expression simultaneously, have been evaluated with experimental studies of a reaction of moderate speed (oxidation of ascorbic acid by dichromate ion). Theoretically generated signal profiles indicated the need for three correction factors representative of the deviation of the experimental system from ideality. The rate coefficient in the chemical kinetic term (if pseudo-first-order is assumed) appears to change in an oscillating manner reflecting the complexity of the overall kinetic process. A straight-line relationship was found between the time at peak maximum and the rate coefficient in computer-simulated curves. Such a correlation provided a way to estimate a value for the rate coefficient of the L-ascorbic acid/dichromate reaction which agreed with the one obtained from batch kinetic determinations. In some of the cases studied, the unbalanced evolution of dispersion forces resulted in chemically induced doubled-humped peaks.     

Biasing effect of non-conductive solids in x-ray photoelectron spectra

The effect of biasing the sample holder was investigated in a range of ?300 to +300 V, in order to study the surface charging of non-conductive samples. Shifts in kinetic energies of photoelectron and peak broadening were compared to the dielectric constants and the volume resistivity of the solid.