Kinetic studies on the mechanism of atomization in electrothermal atomic absorption spectrometry with and without chemical modifiers

Over the past decade a few methods for determining kinetic data for atom formation from the absorption signal in electrothermal atomic absorption spectrometry (ETAAS) have been developed in the author's laboratory. These approaches include improvement to the Smets method, and development of new methods for simultaneous determination of kinetic order and activation energy for atom formation at increasing or constant temperatures. The steady-state approximation and first-order kinetic assumption for atom formation have been avoided during their derivation. One of the most distinct features of these new methods is their suitability for quantitative determination of the kinetic order for atom formation from absorption signals under normal analytical conditions, even for atomization processes with fractional reaction orders and/or with multiple mechanisms. The application of the developed methods to the study of the mechanism of atomization in the graphite-furnace atomizer, with and without chemical modifiers, is reviewed with emphasis on research work in the authors' laboratory.     

Kinetic studies on the mechanism of atomization in electrothermal atomic absorption spectrometry with and without chemical modifiers

Over the past decade a few methods for determining kinetic data for atom formation from the absorption signal in electrothermal atomic absorption spectrometry (ETAAS) have been developed in the author’s laboratory. These approaches include improvement to the Smets method, and development of new methods for simultaneous determination of kinetic order and activation energy for atom formation at increasing or constant temperatures. The steady-state approximation and first-order kinetic assumption for atom formation have been avoided during their derivation. One of the most distinct features of these new methods is their suitability for quantitative determination of the kinetic order for atom formation from absorption signals under normal analytical conditions, even for atomization processes with fractional reaction orders and/or with multiple mechanisms. The application of the developed methods to the study of the mechanism of atomization in the graphite-furnace atomizer, with and without chemical modifiers, is reviewed with emphasis on research work in the authors’ laboratory.     

Some difficult problems still existing in the preparation and certification of CRMs

Differences between particle size measurements of CRMs by various methods are discussed and the importance of the reliability of such data for proper estimation of the homogeneity of the material is emphasized. On the basis of a very simple model, the dependence of the Ingamells’ sampling constant on the average mass of a single particle of the material is derived, and theoretical predictions are compared with the experimental results. Various approaches to the certification of the candidate RMs are briefly reviewed. The merits of the approach being used in this laboratory to evaluate data obtained in the interlaboratory comparison, and to assign certified and information values, is discussed. The conclusions are supported by results obtained for selected trace elements by use of “definitive” (primary) and “very accurate” methods. Some observations on the unusual resistance of some biological materials to wet ashing and the resulting possibility of making analytical errors are mentioned.     

Studies on poly(aquahydroxychromium diphenylphosphinate)

The novel inorganic polymer poly(aquahydroxychromium diphenylphosphinate) was investigated to obtain information concerning its degree of polydispersity and properties when dissolved in organic solvents. Viscometric, infrared absorption spectroscopic, and electrical conductimetric techniques were employed to ascertain the characteristics of the polymer dissolved in benzene, chlorobenzene, and chloroform. Gel permeation chromatography was used to determine the degree of heterogeneity of the polymer. Results indicate that the polymer, as prepared, is quite polydisperse. The polymer main chains do not undergo radical reactions with the solvents. High values of the Huggins interaction constant K′ suggest that polymer–solvent association reactions involving chain-endgroups are probable. Such reactions are also indicated by association constant values and by the shapes of phoreograms obtained from electrical conductance data. Both “local” and “bulk” solvent properties, such as the dielectric constant, appear to influence the characteristics of the polymer when dissolved in the different solvents.     

Alternative mechanisms of electrophilic substitution in azole series

Data on three mechanisms of aromatic electrophilic substitution in azole series have been presented. First of them is similar to an ordinary addition–elimination mechanism including the formation of a cationic σ-complex (Wheland intermediate). The second mechanism is realized according to the elimination–addition scheme and includes a protonation or a formation of a complex on the “pyridine” N atom, proton abstraction from the C atom adjacent to the above N atom, and addition of a cationoid reagent to the ylide (carbene) formed. The third mechanism proposed by the present authors can be realized for azoles having three and two N atoms of the pyridine type. It does not require the preliminary N-protonation or complex formation and occurs due to a strong electron withdrawing effect of several “pyridine” atoms, resulting in the CH-deprotonation followed by the interaction of the carbanion formed with a cation under mild conditions.     

Reference material “total protein in human urine”

Two batches of a reference material (RM) “total protein in human urine” were prepared for quality assurance in the determination of human urinary proteins. The material was prepared by freeze-drying of a pooled urine sample obtained from healthy volunteers. Protein concentrations in both batches were within physiological values, close to their upper limit. Interlaboratory comparisons conducted in 26 laboratories using the standard operation procedures elaborated for six of the most frequently employed methods (biuret, Bradford, Exton, Lowry, Pesce, and Watanabe) revealed that the results obtained by the individual methods were significantly different due to various reaction mechanisms involved. It was therefore necessary to certify the mean values and their uncertainties individually for each method. These operationally defined certified or information values are valid only when the prescribed standard operation procedures are strictly followed.     

A general approach for atom-type assignment and the interconversion of molecular structure files

In molecular modeling projects which require use of several different computer programs, one encounters problems in sharing data between programs. One difficult problem is the conversion of atom types from one program's definition to another. A second problem is the conversion of a polymer, such as a protein or polynucleotide molecule, from a “general” program, which understands molecules as a collection of atoms, to a “polymer” program, which understands molecules as a collection of molecular fragments stored in some library. We describe here a new method by which atom types are deduced from the environment of each atom. We use the Daylight Chemical Information Systems library of programs to deduce the atom types based only on the atomic symbol, connectivity and formal charge of each atom in the molecule. We also describe a method by which the polypeptide nature and sequence of a molecule can be deduced from minimal information about the atoms in the molecule. We have written a computer program which demonstrates this method. This program deduces atom types for AMBER, GRIN/GRID, CHARMm, and ALOGP. It will also produce input files for the AMBER/PREP fragment library preparation program.     

Theoretical and experimental values of the spectroscopic constant relative to the Hg 253.7 nm line at different temperatures.

The fundamental parameter (cm²/atom) relating the absorbance and the number of atoms, called “atomic absorptivity” (C.Th.J. Alkemade et al. Metal Vapours in Flames, Pergamon, New York, 1982) and herewith referred to as the “spectroscopic constant” k, can be either theoretically calculated or derived from experimental calibration curves. The comparison between these two values is useful because the theory can be refined and the operating conditions chosen with more confidence. One of the most important parameters needed is the variation of k with temperature, since a good control of the atomizer temperature is not easy due to difficulties in calibrating the sensor. In fact, if k is constant, there is no need to control the temperature, while if its variation is known, one can calculate the error caused by such inaccuracy. In this paper, some experimental and theoretical data for Hg, in the temperature interval 300–2400 K, are discussed.     

Cinétique de dégradation thermique de polymeres lineaires

Statistical methods are applied to the study of thermal degradation kinetics of polymers with an initial “most probable” molecular weight distribution. The parameters required by this theory are the degree of polymerization of the largest molecule which is volatile in the experimental conditions, and the number average degree of polymerization of the polymer sample before degradation. Theoretical results are compared with experimental data obtained by isothermal thermogravimetric analysis of synthesized “model” polycondensates. The observed agreement justifies the hypothesis on polymer degradation mechanism and allows a direct analytical application of isothermal thermogravimetry.     

Data quality in tissue analysis using desorption electrospray ionization

There has been a recent surge in applications of mass spectrometry (MS) to tissue analysis, particularly lipid-based tissue imaging using ambient ionization techniques. This recent growth highlights the need to examine the effects of sample handling, storage conditions, and experimental protocols on the quality of the data obtained. Variables such as time before freezing after organ removal, storage time at −80 °C, time stored at room temperature, heating, and freeze/thaw cycles were investigated for their effect on the data quality obtained in desorption electrospray ionization (DESI)-MS using mouse brain. In addition, analytical variables such as tissue thickness, drying times, and instrumental conditions were also examined for their impact on DESI-MS data. While no immediate changes were noted in the DESI-MS lipid profiles of the mouse brain tissue after spending 1 h at room temperature when compared to being frozen immediately following removal, minor changes were noted between the tissue samples after 7 months of storage at −80 °C. In tissue sections stored at room temperature, degradation was noted in 24 h by the appearance of fatty acid dimers, which are indicative of high fatty acid concentrations, while in contrast, those sections stored at −80 °C for 7 months showed no significant degradation. Tissue sections were also subjected to up to six freeze/thaw cycles and showed increasing degradation following each cycle. In addition, tissue pieces were subjected to 50 °C temperatures and analyzed at specific time points. In as little as 2 h, degradation was observed in the form of increased fatty acid dimer formation, indicating that enzymatic processes forming free fatty acids were still active in the tissue. We have associated these dimers with high concentrations of free fatty acids present in the tissue during DESI-MS experiments. Analytical variables such as tissue thickness and time left to dry under nitrogen were also investigated, with no change in the resulting profiles at thickness from 10 to 25 μm and with optimal signal obtained after just 20 min in the dessicator. Experimental conditions such as source parameters, spray solvents, and sample surfaces are all shown to impact the quality of the data. Inter-section (relative standard deviation (%RSD), 0.44–7.2%) and intra-sample (%RSD, 4.0–8.0%) reproducibility data show the high quality information DESI-MS provides. Overall, the many variables investigated here showed DESI-MS to be a robust technique, with sample storage conditions having the most effect on the data obtained, and with unacceptable sample degradation occurring during room temperature storage.     

Some particular aspects of the low-temperature oxidation of methane initiated by nitric acid

During methane oxidation initiated by nitric acid or nitrogen dioxide, it is possible to observe, under certain conditions of temperature, initiator content, and flow rates, the formation of an adiabatic “temperature peak.” Because the maximum temperature of the peak does not exceed the initial temperature of the reactor by more than 220°C, the recorded temperature-increase curves were used for a kinetic interpretation. The obtained kinetic parameters (overall n and E) agree with the values reported in the literature for isothermal methane oxidation and suggest that the overall mechanism does not change in the conditions of “temperature peak.”     

Temperature- and Concentration-dependency of the Electrical Resistivity of Molten Ag-Mg-, Cu-Sb-, and Zn-Sn-alloys

Abstract

The electrical resistivity was measured in the liquid systems Ag-Mg, Cu-Sb, and Zn-Sn. With the eutectic system Zn-Sn the resistivity shows a convex smooth run between the values of the pure components. With the systems Cu-Sb and Ag-Mg the resistivity shows maxima at the concentrations of the solid intermetallic compounds. The relative superelevation of these maxima decreases with rising temperature, corresponding to the decay of “compound-molecules” in the melt.

With the system Zn-Sn the small maxima found by Roll and Motz could not be confirmed. Also no influence of the magnetic susceptibility on the run of resistivity vs. concentration could be detected during the measurements in a rotating field apparatus.

A convincing consistency of the measured values of the electrical resistivity in the system silver-magnesium with those values, which were obtained by calculation from X-ray data is shown.     

An Example of Information Exchange Between a Liquid Chromatographic System And a Robotic Sample Preparation System

Abstract

Analytical laboratories are looking for the “Total Solution” today. Ideally, the “Total Solution” is the automation of all steps after receiving a sample for analysis including generation of the final report. While that goal is not yet fully realized, efforts to attain it are well underway. Robotic sample preparation systems have rapidly evolved to become common tools in many analytical laboratories which use them along with standard analytical instruments such as chromatographs and spectrophotometers. Physically coupling the two types of systems can offer advantages – the full extent of which depends upon the degree of sophistication in the communication between them. The goal of a “Total Solution” demands real-time decision-making capability based on the information associated with each sample. This information includes everything from its origin to just-completed chromatographic results as the sample travels through the lab. Hence, the level of information exchange between preparation and analysis systems will determine how fully the potential power of such a coupling is utilized.     

Laser microprobe mass spectrometry of quaternary phosphonium salts: Direct versus matrix-assisted laser desorption

The use of laser microprobe mass spectrometry (LMMS) for the structural characterization of thermolabile quaternary phosphonium salts has been evaluated. A comparison has been made between LM mass spectra obtained by direct analysis of “neat” organic salts and the corresponding “matrix-assisted” LM mass spectra. Main limitations of LMMS for the direct analysis of neat organic salts (i.e., no matrix) result from (1) formation of artifact ions that originate from thermal degradation and surface recombination reactions and (2) poor shot-to-shot reproducibility of the spectra. Dilution of the organic salts in a suitable, UV-absorbing matrix (e.g., nicotinic acid) significantly enhances the quality of the LM mass spectra. Improvements are: (1) an increase of the ion yield of preformed cations, (2) reduction or elimination of thermal decomposition and other deleterious surface reactions, and (3) a much better shot-to-shot spectral reproducibility. An interesting analytical feature is that these LM mass spectra, which contain only a few matrix peaks, can be obtained for subnanogram amounts of sample. The results also show that triphenylphosphonium salts with polycyclic aromatic substituents can be used as “molecular thermometers” to probe both the temperatures experienced by the sample molecules during the laser-induced desorption ionization process and the internal energies of the desorbed ion species. In this way, quaternary phosphonium salts can be used for evaluating whether improvements have been achieved by applying different sample treatments. Comparison of four different matrices (i.e., nicotinic acid, ammonium chloride, glycerol, and 3-nitrobenzylalcohol) indicates that the effectiveness of a matrix to reduce thermal degradation and to decrease the internal energies of the ions depends on the UV-absorption characteristics and the volatilization/sublimation temperature of the matrix material.     

A mechanistic approach on the self-organization of the two-component thermoreversible hydrogel of riboflavin and melamine

The riboflavin (R) and melamine (M) supramolecular complex in the mole ratio of 3:1 (RM31) produces a thermoreversible gel in aqueous medium. The gelation mechanism has been elucidated from morphological investigations using optical, electron, and atomic force microscopy together with time-dependent circular dichroism (CD) and photoluminescence (PL) spectroscopy. Optical microscopy indicates spherulitic morphology at lower gelation temperature (相似文献   

Estimation of uncertainties in clinical analysis

 Owing to the importance of clinical analysis for human health, it is necessary to have reliable analytical information. Considering that the reliability of analytical information is a complex function of uncertainties of the sample, method, instrumentation, and data processing, it should be observed that the maximum reliability of analytical information is obtained by minimization of uncertainty values. Applying this concept to clinical analysis, the role of spectrometric and electrometric methods is highlighted.     

Solvent effect on stabilization energy: An approach based on density functional reactivity theory

In the present article a formalism and the corresponding computational method is developed to take care of the variation of stabilization energy with solvent polarity in the process of adduct formation. For this purpose, a simple but physically insightful definition of “net desolvation energy” is proposed keeping in mind the sequence of events taking place in the process of adduct formation in a solvent. The approach used here is based on density functional reactivity theory (DFRT) and the representative samples chosen are adduct formation between (a) methyltrioxorhenium (MTO) and pyridine and (b) (azidomethyl)benzene and methylpropiolate. The generated data in case (a) is correlated with already known experimental parameter that is, formation constant (K_f). The observed trends claim that with the increase in solvent polarity interaction (or stabilization) energy becomes less negative which means that on increasing the solvent polarity the chances of adduct formation are less. This is further supported by calculating hardness values of adducts in different solvents which goes on decreasing with the increase in solvent polarity. Here, the computed data show that on increasing the polarity (i.e., dielectric constant) of the solvent, the “net desolvation energy” increases. Finally, when “net desolvation energy” is added to the stabilization energy obtained from DFRT the predicted trends are achieved.     

Determination of silicon in high-silicon electrical steel by ICP-AES with on-line sample electrolytic dissolution.

A flow-injection procedure combining electrolytic sample decomposition and inductively coupled plasma atomic emission spectrometry (ICP-AES) is proposed in order to rapidly determine the content of silicon in high-silicon electrical steel. This system is characterized by sample decomposition through electrolysis directly coupled to ICP-AES. A steel sample is dissolved by electrolysis using a 6 mol L(-1) HCl solution as an electrolyte with a flow rate of 5 mL min(-1); the electrolyte containing a dissolved sample is subsequently introduced into ICP-AES via a nebulizer. The effects of the electrolysis current and the temperature on the decomposition of the sample were studied. Samples were electrolyzed under the condition of a 1.5 A constant current, at room temperature (25 degrees C) to avoid the hydrolysis of silicon to precipitate. Comparing the analytical results of steel samples obtained by this analytical system with those obtained by the gravimetric method, determined values agreed well quantitatively. The RSD of silicon at approximately 3% was 0.3% (n = 6).     

γ-radiation-induced crosslinking of polystyrene

The γ-radiation-induced crosslinking of polystyrene was studied at 30–100°C in vacuo with a dose rate of 6.35 × 10⁵ rad/hr. The amount of hydrogen formation increased with increasing irradiation time, and the rate of the formation decreased with the time. The results were well described by the zero-order formation kinetics with respect to the hydrogen concentration combined with the first-order disappearance. The apparent rate constant for the formation of hydrogen increased somewhat with rising irradiation temperature, and the one for the disappearance was little affected by the temperature. The gel fraction increased with the time by the irradiation beyond the critical time for incipient gel formation, and the rate of gel formation decreased with the time. The gel formation was retarded by rising irradiation temperature, and only a little gel fraction was observed at 100°C. The G values for the crosslinking and main-chain scission were obtained from the gel data by using the Charlesby–Pinner equation. On the basis of these results, the mechanism of the γ-radiation-induced crosslinking of polystyrene was discussed.     

Biocatalytic synthesis of polymers. II. Preparation of [AA–BB]x polyesters by porcine pancreatic lipase catalyzed transesterification in anhydrous,low polarity organic solvents

Enzyme-catalyzed preparation of polymers offers several potentially valuable advantages over the usual polymerization procedures. (1) Such polymerizations may allow the polymer to retain functionality that would be destroyed under normal polymerization conditions. (2) The selectivity provided by enzyme catalysts may permit polymers, including optically active polymers, to be prepared that are either not accessible or accessible only with difficulty by other methods. (3) The characteristics of the enzyme and the mild polymerization conditions may permit formation of polymers having highly regular sizes and backbone structures. This report describes the first successful use of an enzyme-catalyzed polycondensation to prepare a chiral (AA–BB)_x polyesters of more than a few repeat units. Polymerization of bis(2,2,2-trichloroethyl) alkanedioates (BB) with diols (AA) using the enzyme porcine pancreatic lipase (PPL) as a catalyst is detailed. The polycondensations were carried out at ambient temperature in anhydrous, low polarity organic solvents such as ether, THF, and methylene chloride. End group analysis by NMR provided M_n values of 1300–8200 daltons while GPC provided M_w values of 2800–14900 daltons for the polymers. Based on proton NMR spectra obtained during the polymerization, relatively rapid formation of an AA–BB “dimer” and an AA–BB–AA “trimer,” slower formation of a BB–AA–BB “trimer,” and subsequent condensation of these to give higher polymers are suggested to be components of the polymerization mechanism.