Analysis of organosilicone copolymers by gradient polymer elution chromatography with evaporative light scattering detection

Organosilicone copolymers have found numerous applications in the cosmetics, detergent and coating industries. Coupling a polar polymer (like polyglycols) to a non-polar silicone gives anchorage and emulsification capabilities to the polymer. When coupling a silicone polymer to a polyglycol, the copolymer formed differs from the starting polymers by a single bond which is often difficult to evidence using spectroscopic techniques such as NMR or infrared, especially when the polymers have a high molecular mass. Gradient polymer elution chromatography (GPEC) coupled to an evaporative light scattering detector was developed for the characterization of copolymers based on their chemical composition distribution. Different block and graft polyglycol-silicone copolymers were successfully characterized by GPEC and residual homopolymers have been easily quantified.     

Separation of poly-3-hydroxyvalerate-co-3-hydroxybutyrate through gradient polymer elution chromatography

Polyhydroxyalkanoates are biodegradable polyesters produced by bacteria that can have a wide distribution in molecular weight, composition of monomers, and functionalities. This large distribution often leads to unpredictable physical properties making commercial applications challenging. To improve polymer homogeneity and obtain samples with a clear set of physical characteristics, poly-3-hydroxyvalerate-co-3-hydroxybutyrate copolymers were fractionated using gradient polymer elution chromatography (GPEC) as opposed to extensively used bulk fractionation. Separation was achieved using a reversed-phase column with chloroform and ethanol as the solvent and non-solvent, respectively. A separation was also conducted on a normal-phase column to compare elution patterns between columns of varied polarity. The fractions were analyzed using Size Exclusion Chromatography (SEC) and NMR to determine the percentage of 3-hydroxyvalerate in the copolymer as well as its molecular weight. It was found that as the percentage of "good" solvent was increased in the mobile phase, the polymers eluted with decreasing percentage of 3-hydroxyvalerate and increasing molecular weight which indicates the importance of precipitation/redissolution in the separation. The elution pattern of the polymer remained unchanged when using both a normal- and reversed-phase column which also illustrates the dominance of precipitation/redissolution in GPEC of polyhydroxyalkanoates. As such, GPEC is shown to be an excellent choice to provide polyhydroxyalkanoate samples with a narrower distribution in composition than the original bulk copolymer sample.     

Evaluation of "labile" metal in sediments by anodic stripping voltammetry

A procedure for determining the "labile" metal content of contaminated sediments (in different chemical environments) has been critically examined. The sediments were extracted overnight with different chemical solutions and the suspensions were analysed by differential pulse anodic stripping voltammetry. The extractants used have been recommended for soil/sediment speciation schemes, and by examination of the suspensions directly in the ASV cell, errors due to re-adsorption of released metal ion were minimized. The existence of different chemical forms of metal was signified by changes in peak shape and position or by the appearance of additional peaks. With complexing agents present the peak size was pH-dependent. The limitations of the ASV/suspension analysis technique have been carefully evaluated by using ten different extractants and seventeen sediments. The "lability" results obtained have been compared with the values obtained from a cation-exchanger transfer procedure. For characterizing the lability behaviour of the metal contents of sediments, preliminary extraction into a minimum of four base solutions is advisable, e.g., 0.02M nitric acid (low-pH labile); hydroxylamine in acetic acid (reducing conditions); acetic acid/acetate buffer (weakly sorbed and carbonate-bound) and 0.05M calcium chloride (exchangeable fraction at natural system pH), where the terms in parentheses describe the character of the fraction. The results are critically compared with those obtained by atomic-absorption analysis of the extracts and with those obtained by an earlier ion-exchange fractionation scheme. The advantages and limitations of the ASY systems are discussed.     

On the use of the Edgeworth-Cramér series to obtain diffusion coefficients from taylor dispersion peaks

Three methods of determining the parameters required for the calculation of diffusion coefficients in Taylor dispersion experiments from quasi-Gaussian output signals have been examined, the height-area method, moment calculation by direct integration, and moment calculation from a nonlinear least squares fit to a truncated Edgeworth-Cramér series.The method using moments determined by integration has been found to be unsatisfactory, as it yields diffusion coefficients very heavily dependent on the choice made for the width of the peak base. Those derived from the height-area method can deviate systematically and significantly from those determined from the variance obtained from the least squares series fit where, as is usual, the peak varies from a true Gaussian. The series fit method, within our reproducibility of 1–2%, yields values consistent with accurate and precise literature data, and is recommended for precise work.     

A qualitative study to the influence of molar mass on retention in gradient polymer elution chromatography (GPEC)

Gradient Polymer Elution Chromatography (GPEC) appears to be a powerful method for the determination of chemical composition distributions (CCD) of high molar mass copolymers. In order to perform these experiments, molar mass dependence of the retention time must be minimized. It is shown that variation in solvent and non-solvent strength is a powerful method for adjusting the molar mass dependence. There appears to be a relation between molar mass dependence of retention behavior in GPEC on one hand and solubility under critical solvent conditions on the other hand. This phenomenon can be explained qualitatively from the important precipitation/redissolution mechanism in GPEC.     

Evaluation of the use of consensus values in proficiency testing programmes

Proficiency testing (PT) is an essential tool used by laboratory accreditation bodies to assess the competency of laboratories. Because of limited resources of PT providers or for other reasons, the assigned reference value used in the calculation of z-score values has usually been derived from some sort of consensus value obtained by central tendency estimators such as the arithmetic mean or robust mean. However, if the assigned reference value deviates significantly from the ‘true value’ of the analyte in the test material, laboratories’ performance will be evaluated incorrectly. This paper evaluates the use of consensus values in proficiency testing programmes using the Monte Carlo simulation technique. The results indicated that the deviation of the assigned value from the true value could be as large as 40%, depending on the parameters of the proficiency testing programmes under investigation such as sample homogeneity, number of participant laboratories, concentration level, method precision and laboratory bias. To study how these parameters affect the degree of discrepancy between the consensus value and the true value, a fractional factorial design was also applied. The findings indicate that the number of participating laboratories and the distribution of laboratory bias were the prime two factors affecting the deviation of the consensus value from the true value.     

Decoding complex multicomponent chromatograms by fourier analysis

Summary The present work discusses the many attributes—classified as observable, intrinsic or hidden—which can be conceived for any complex multicomponent chromatogram. Discussion ensues on how to decode such chromatograms, i.e. determining the intrinsic and/or hidden attributes from those which can be observed. There are two main steps. The first is based on Fourier Analysis (FA) and determines the intrinsic attributes: i.e., the number of single components which can be detected; their distribution over the available chromatographic space and peak capacity. The second evaluates the hidden attributes: i.e., the effects of incomplete separation, the number of peaks created by one or more single components as well as their degree of purity. The hidden attributes can be obtained by applying the theory of Statistical Degree of peak Overlapping (SDO) and the paper goes into the extent to which the SDO step depends on the FA results. In addition, the role Exponential distribution plays as a point of reference for the distribution of both single component peak position interdistances and peak heights is discussed. Finally, a simplified graphical FA procedure is presented and the main achievements in this field are reviewed. Presented at the 21^st ISC held in Stuttgart, Germany, 15^th–20^th September, 1996     

Quadrupole moment function and absolute infrared quadrupolar intensities for N2

High level ab initio methods have been used to calculate values of the quadrupole moment of the ground X (1)Sigmag+ state of N2 on a dense radial mesh spanning the interval of 0.8-12.1 a.u. Detailed convergence tests indicate that the resulting equilibrium values of the quadrupole moment Theta(e)=-1.1273 a.u. and its first radial derivative dTheta(R)/dR/e=0.9604 a.u. have absolute uncertainties of 0.3% and 0.8%, respectively, and are more accurate than the best experimental values of these quantities. The calculated quadrupole moment function, together with a recently reported accurate analytic empirical potential energy function [Le Roy et al., J. Chem. Phys. 125, 164310 (2006)], is used to generate values of the radial matrix elements determining the absolute intensities of infrared vibration-rotation transitions of ground-state N2, which take full account of vibration-rotation interactions. These results should improve the reliability of the interpretations of N2 contributions to infrared atmospheric spectra.     

Measurement of the electronic transition dipole moment by Autler-Townes splitting: Comparison of three- and four-level excitation schemes for the Na2 A 1Sigma(u)+ - X 1Sigma(g)+ system

We present a fundamentally new approach for measuring the transition dipole moment of molecular transitions, which combines the benefits of quantum interference effects, such as the Autler-Townes splitting, with the familiar R-centroid approximation. This method is superior to other experimental methods for determining the absolute value of the R-dependent electronic transition dipole moment function mu(e)(R), since it requires only an accurate measurement of the coupling laser electric field amplitude and the determination of the Rabi frequency from an Autler-Townes split fluorescence spectral line. We illustrate this method by measuring the transition dipole moment matrix element for the Na2 A 1Sigma(u)+ (v' = 25, J' = 20e)-X 1Sigma(g)+ (v" = 38, J" = 21e) rovibronic transition and compare our experimental results with our ab initio calculations. We have compared the three-level (cascade) and four-level (extended Lambda) excitation schemes and found that the latter is preferable in this case for two reasons. First, this excitation scheme takes advantage of the fact that the coupling field lower level is outside the thermal population range. As a result vibrational levels with larger wave function amplitudes at the outer turning point of vibration lead to larger transition dipole moment matrix elements and Rabi frequencies than those accessible from the equilibrium internuclear distance of the thermal population distribution. Second, the coupling laser can be "tuned" to different rovibronic transitions in order to determine the internuclear distance dependence of the electronic transition dipole moment function in the region of the R-centroid of each coupling laser transition. Thus the internuclear distance dependence of the transition moment function mu(e)(R) can be determined at several very different values of the R centroid. The measured transition dipole moment matrix element for the Na2 A 1Sigma(u)+ (v' = 25, J' = 20e)-X 1Sigma(g)+ (v" = 38, J" = 21e) transition is 5.5+/-0.2 D compared to our ab initio value of 5.9 D. By using the R-centroid approximation for this transition the corresponding experimental electronic transition dipole moment is 9.72 D at Rc = 4.81 A, in good agreement with our ab initio value of 10.55 D.     

Comparison of multianalyte proficiency test results by sum of ranking differences, principal component analysis, and hierarchical cluster analysis

Sum of ranking differences (SRD) was applied for comparing multianalyte results obtained by several analytical methods used in one or in different laboratories, i.e., for ranking the overall performances of the methods (or laboratories) in simultaneous determination of the same set of analytes. The data sets for testing of the SRD applicability contained the results reported during one of the proficiency tests (PTs) organized by EU Reference Laboratory for Polycyclic Aromatic Hydrocarbons (EU-RL-PAH). In this way, the SRD was also tested as a discriminant method alternative to existing average performance scores used to compare mutlianalyte PT results. SRD should be used along with the z scores—the most commonly used PT performance statistics. SRD was further developed to handle the same rankings (ties) among laboratories. Two benchmark concentration series were selected as reference: (a) the assigned PAH concentrations (determined precisely beforehand by the EU-RL-PAH) and (b) the averages of all individual PAH concentrations determined by each laboratory. Ranking relative to the assigned values and also to the average (or median) values pointed to the laboratories with the most extreme results, as well as revealed groups of laboratories with similar overall performances. SRD reveals differences between methods or laboratories even if classical test(s) cannot. The ranking was validated using comparison of ranks by random numbers (a randomization test) and using seven folds cross-validation, which highlighted the similarities among the (methods used in) laboratories. Principal component analysis and hierarchical cluster analysis justified the findings based on SRD ranking/grouping. If the PAH-concentrations are row-scaled, (i.e., z scores are analyzed as input for ranking) SRD can still be used for checking the normality of errors. Moreover, cross-validation of SRD on z scores groups the laboratories similarly. The SRD technique is general in nature, i.e., it can be applied to any experimental problem in which multianalyte results obtained either by several analytical procedures, analysts, instruments, or laboratories need to be compared.

Determination of the chemical potential using energy-biased sampling

An energy-biased method to evaluate ensemble averages requiring test-particle insertion is presented. The method is based on biasing the sampling within the subdomains of the test-particle configurational space with energies smaller than a given value freely assigned. These energy wells are located via unbiased random insertion over the whole configurational space and are sampled using the so-called Hit-and-Run algorithm, which uniformly samples compact regions of any shape immersed in a space of arbitrary dimensions. Because the bias is defined in terms of the energy landscape it can be exactly corrected to obtain the unbiased distribution. The test-particle energy distribution is then combined with the Bennett relation for the evaluation of the chemical potential. We apply this protocol to a system with relatively small probability of low-energy test-particle insertion, liquid argon at high density and low temperature, and show that the energy-biased Bennett method is around five times more efficient than the standard Bennett method. A similar performance gain is observed in the reconstruction of the energy distribution.     

线性非理想条件下液相色谱柱末端峰形规律

 色谱流出曲线的二阶中心矩μ２和三阶中心矩μ３以及描述峰形非对称程度的偏态系数∑ｋ＝μ３／μ１．５２是反映色谱峰形的重要参数。从液相色谱过程动力学方程出发，运用电子计算机证明了在线性非理想条件下高效液相色谱体系中不同保留值组分在柱末端峰形的分布基本一致的结论。     

线性非理想条件下液相色谱柱末端峰形规律

色谱流出曲线的二阶中心矩μ２和三阶中心矩μ３以及描述峰形非对称程度的偏态系数∑ｋ＝μ３／μ１．５２是反映色谱峰形的重要参数。从液相色谱过程动力学方程出发，运用电子计算机证明了在线性非理想条件下高效液相色谱体系中不同保留值组分在柱末端峰形的分布基本一致的结论。     

Comparison of shelf-life appraisal techniques for the estimation of the limiting date of stability for agrifood reference materials

Reference materials play a major role for the verification of the quality control of biological and chemical analysis. Due to the poor availability of certified reference materials, external reference materials (ERM) are intensively developing in complement and as a consequence of proficiency testing schemes (PTS). However, the use of any reference material must be time-limited, whereas the assigned reference value may change with time and it is under the responsibility of the material producer to indicate the date until this can be used for calibration purpose with some guarantee. This article compares different methods used to determine the limiting date of stability (LDS) in food and food-related reference materials, using some specific analytes (moisture, protein and oil contents) as stability-monitoring analytes. LDS is defined as the moment when the assigned values of one or several analytes can no longer be guaranteed by the producer. For most of the ERMs used as examples (i.e., rapeseed, bread wheat, durum wheat) the calculated periods of stability were longer than the present storage times used in BIPEA. However, for sunflower, the rapid degradation of the monitoring analytes indicates a shorter time than forecast.     

The infrared fundamental intensities and polar tensor of CH3NC

The molecular force field and polar tensor of methyl isocyanide have been determined from its gas phase vibrational frequencies and infrared intensities. Quantum chemical results from MP2(FC), B3LYP and quadratic configuration interaction calculation including single and double substitutions procedures using a 6-311 + +G(3d,3p) basis set have been used to determine the signs of the dipole moment derivatives with respect to the normal coordinates as well as estimate individual fundamental intensities of the overlapped v1-v5 and v3-v6 band systems. Principal component graphical representations of the A1 and E symmetry polar tensor elements were useful in determining preferred sets of tensor elements. The mean dipole moment derivative (GAPT charge) of the methyl carbon in CH3NC, 0.347 e, is between the corresponding values in CH3CN, 0.110 e, and CH3F, 0.541 e. The mean dipole moment derivatives obtained here indicate the correct 1s methyl carbon ionization energy as 293.35 eV which is 0.98 eV higher than the corresponding ionization energy of the terminal atom.     

Contact values of the particle-particle and wall-particle correlation functions in a hard-sphere polydisperse fluid

The contact values g(sigma,sigma') of the radial distribution functions of a fluid of (additive) hard spheres with a given size distribution f(sigma) are considered. A "universality" assumption is introduced, according to which, at a given packing fraction eta,g(sigma,sigma')=G(z(sigma,sigma')), where G is a common function independent of the number of components (either finite or infinite) and z(sigma,sigma')=[2sigmasigma'/(sigma+sigma')]mu2/mu3 is a dimensionless parameter, mu n being the nth moment of the diameter distribution. A cubic form proposal for the z dependence of G is made and known exact consistency conditions for the point particle and equal size limits, as well as between two different routes to compute the pressure of the system in the presence of a hard wall, are used to express Gz in terms of the radial distribution at contact of the one-component system. For polydisperse systems we compare the contact values of the wall-particle correlation function and the compressibility factor with those obtained from recent Monte Carlo simulations.     

“Age” determination of irradiated materials utilizing inductively coupled plasma mass spectrometric (ICP-MS) detection

A gas pressurized extraction chromatography (GPEC) system has been developed to perform elemental separations on radioactive samples to determine total and isotopic compositions of Cs and Ba from an irradiated salt sample, fuel sample and two sealed radiation sources. The GPEC system employs compressed nitrogen to move liquid through the system, compared to gravity or pumped liquids that are typically used for separations. A commercially available Sr-Resin™ was used to perform the separation for the above mentioned analytes. A 1% acetic acid solution was determined to be the best extractant for Ba. A flow rate of 0.1 mL/min was determined to be optimal for the separation of Ba. Complete recovery of the Cs and Ba was achieved, within the systematic uncertainties of the experiments.     

Transport coefficients and orientational distributions of spheroidal particles with magnetic moment normal to the particle axis (Analysis for an applied magnetic field normal to the shear plane)

We have investigated the influence of the magnetic field strength, shear rate, and rotational Brownian motion on transport coefficients such as viscosity and diffusion coefficient, and also on the orientational distributions of rodlike particles of a dilute colloidal dispersion. The rodlike particle is modeled as a magnetic spheroidal particle which has a magnetic moment normal to the particle axis; such a particle may typically be a hematite particle. In the present study, an external magnetic field is applied in the direction normal to the shear plane of a simple shear flow. The basic equation of the orientational distribution function has been derived from the balance of torques and solved numerically. The results obtained here are summarized as follows. Although the orientational distribution function shows a sharp peak in the shear flow direction for a very strong magnetic field, such a peak is not restricted to the field direction alone, but continues in every direction of the shear plane. This is due to the characteristic particle motion that the particle can rotate around the axis of the magnetic moment in the shear plane, although the magnetic moment nearly points to the magnetic field direction. This particle motion in the shear plane causes negative values of the viscosity due to the magnetic field. The viscosity decreases, attains a minimum value, and then converges to zero as the field strength increases. Additionally, the diffusion coefficient is significantly influenced by such characteristic particle motion in the shear plane for a strong magnetic field.     

An analytical solution of a newly proposed peak broadening equation and extension of polydispersities to higher molecular weight averages in size exclusion chromatography (SEC)

Summary Herein is reported an analytical solution to the peak broadening or peak dispersion/flattening equation based on the recently proposed Instrumental Spreading Shape Function and its application to correction for imperfect resolution (inadequate peak separation and/or excessive peak broadening) for higher molecular weight averages. The relationship of these higher MW averages with the familiar Weight Average and number average molecular weights is also discussed. Criteria for perfect resolution are specified and a true molecular weight calibration curve is accordingly defined.