Chromatography of Suspensions – An Experimental and Theoretical Investigation of Axial Dispersion Phenomena

Abstract

Herein is reported an experimental and theoretical investigation of axial dispersion phenomena in the chromatography of spherical suspensions in the submicron range. Peak separation and broadening were measured for a number of particle suspensions (polystyrene, polyvinylacetate, styrene-acrylic acid copolymer, butadiene-acrylonitrile copolymer latices and silica particles) using different column combinations containing porous inorganic packing materials (Fractosils, Bioglass, Corning Glass) and over a wide range of carrier fluid (water containing 1 g/l Aerosol O.T. and 1 g/l potassium nitrate) flowrates. Peak separation was virtually independent of carrier fluid flowrate while peak broadening increased significantly. Analytical solutions of the integral equation which describes axial dispersion in the chromatography of suspensions have been found for a general detector which includes light scattering, refractometer and viscosity-concentration detectors. These solutions were used to obtain dispersion corrections for various particle diameter averages (number, surface, weight and volume). Corrections to number and surface average particle diameters for axial dispersion are excessive with a light scattering detector. These large corrections are related to the fact that in the Rayleigh scattering regime, the extinction coefficient is proportional to diameter to the fourth power. The refractometer gives reasonable dispersion corrections for all particle diameter averages. The theoretical equations derived herein are equally applicable to hydrodynamic chromatography (HDC) and capillary particle chromatography (CPC).     

Chromatography of Suspensions - Analytical Corrections for Axial Dispersion

Abstract

An analytical method is proposed to correct the signal from a turbidity detector for axial dispersion in the chromatography of particle suspensions. An earlier analytical method due to Hamielec and Singh [1] is limited to light scattering in the Rayleigh regime. The solution reported herein is derived using the more general Mie light scattering theory and enables the correction of various moments of the particle diameter frequency distribution function. The axial dispersion phenomenon is described by the well known integral equation where the instrumental spreading function is assumed Gaussian. The solution is applicable for both linear and nonlinear particle diameter-retention volume calibration curves.     

Chromatography of Suspensions. An Absolute Particle Size Detector Based on Turbidity-Spectra Analysis

Abstract

Herein is reported the theoretical basis for an absolute particle size detector based on turbidity-spectra analysis for the chromatography of spherical suspensions in the submicron range. A suitable detector for this purpose would be a variable wavelength spectrophotometer with very rapid scan in the wavelength range, 200–900 nm. It is proposed that such a detector be used with a liquid chromatograph which separates a spherical particle suspension according to size. An absolute detector of this kind would, to a large extent, overcome limitations of the chromatographic technique such as relatively poor resolution, flow rate variations and the lack of a particle diameter-retention volume calibration curve.     

Chromatography of Suspensions,An Absolute Particle Size Detector Based on Turbidity-spectra Analysis - a Simulation Study

Abstract

In the chromatography of particle suspensions the use of a variable wavelength spectrophotometer can provide turbidity spectra continuously with retention volume. As suggested by Hamielec [1] this device could form the basis for an absolute particle size detector provided the size distribution in the detector cell could be represented by a known distribution function. In this work it is shown that the two parameter log-normal distribution function is an adequate representation of the detector cell contents. Further a computational scheme is outlined whereby the distribution parameters and the number of particles in the detector cell are calculated as a function of retention volume. It is then possible to obtain size distribution information of the original suspension.     

Size Exclusion Chromatography (SEC) of Complex Polymers-Methods of Correction for Imperfect Resolution

Abstract

Herein is reported a review and extension of recently published methods of correction for imperfect resolution (inadequate peak separation and or excessive peak broadening) for the SEC of linear homo and uniform copolymers and complex polymers such as polymers with long chain branching and nonuniform copolymers. Emphasis Is placed on analytical correction methods and absolute detector systems such as the low angle laser light scattering photometer and viscometer in series with a mass detector. Analytical correction methods and absolute detector systems play a vital role in the SEC characterization of complex polymers.     

Aging of Pressure Sensitive Adhesives. II: Use of Multidetector Sec

Abstract

The use of multiple detector size exclusion chromatography is described for the study of aging of pressure sensitive adhesive films based on styrene-isoprene-styrene block copolymers. In this preliminary investigation of thin films, room temperature oxidation resulted in the formation of an easily detected carbonyl chromophore. While the chromophore concentration was monitored with a UV detector on the chromatograph, the molecular weight distribution was measured with a differential refractometer. Several important general implications of this combination of detectors in SEC are described.     

Reductive Mode Thin-Layer Amperometric Detector for Liquid Chromatography

Abstract

An electrochemical detector for liquid chromatography is described based on the use of a mercury film electrode in a thin-layer cell with the auxiliary electrode placed across the channel from the working electrode. Considerations relevant to optimizing performance of the detector are described including removal of dissolved oxygen from the sample and mobile phase. Detection limits are reported for a number of reducible organic compounds of interest in environmental and biomedical research. Detection limits at the picomole level are readily achieved for easily reduced nitro compounds. Typical applications are illustrated for the determination of parathion in field runoff water and chloramphenicol in human plasma.     

On-line measurement of molecular weight and radius of gyration of polystyrene in a good solvent and in a theta solvent measured with a two-angle light scattering detector

On-line two-angle (15° and 90°) light scattering measurements with a gel permeation chromatography for linear and branched polystyrene in tetrahydrofuran (a good solvent) and in trans-decalin (a theta solvent) were made and compared with data from a multi-angle light scattering detector and literature values. Theoretically, weight-average molecular weight and the radius of gyration R_g can be determined accurately in the range where R_g²k² is less than 1.2 (rod)∼1.7 (random coil); here, k is the absolute value of the scattering vector for a right angle detector with the Berry square root method. Molecular weight dependence of the radius of gyration obtained from the two-angle light scattering detector for linear and branched polystyrenes under different thermodynamic conditions were measured and found to be almost the same as values measured with a multi-angle light scattering detector and literature values in the appropriate range of molecular weight.     

Measurement of Phenols in Automobile Exhaust

Abstract

Methods were investigated to measure exhaust emission rates of individual low-molecular-weight phenols from gasoline and diesel vehicles driven on a chassis dynamometer using the FTP driving cycle. An extraction procedure to isolate the phenols from exhaust scrubs was evaluated. Separation of individual phenols was done by gas chromatography, with detection by a flame ionization detector. Phenol identifications were confirmed by mass spectroscopy. Results of a limited survey of gasoline and diesel vehicles are reported.     

THE DECONVULATION OF ABSORPTION SPECTRA OF GREEN PLANT MATERIALS—IMPROVED CORRECTIONS FOR THE SIEVE EFFECT

The extinction (absorbance, optical density) of a suspension of algal cells or chloroplasts, etc. can be an a linear function of the absorption coefficient of the cell contents, even when the effects of scattering are negligible. The deviation from linearity is Duysens' sieve effect. The heights of absorption maxima of suspensions are depressed as compared with those of the “particle” contents. Duysens proposed an expression for correcting for this distortion; that expression proves to be a special approximate case of a more general one. This paper proposes an improved method of correction which utilizes this general expression, conventional expressions for particle absorption based on the Lorentz-Mie and Aden and Kerker equations for scattering and absorbing, and iterative methods for numerically deducing particle absorption coefficients from suspension parameters. The procedure is illustrated by applying it to previous experimental spectra of Chlorellu cells and spinach chloroplasts.     

Liquid Chromatography with Fluorescence Detection in the Analysis of Biological Fluids

ABSTRACT

Liquid chromatography with fluorescence detection is well suited to the analysis of biological fluids, as it combines both selectivity and sensitivity. The determinations are not limited to fluorescent compounds, as non-fluorescent substances can be converted to fluorescent derivatives by appropriate reactions. As a consequence of progress in methodology and of the development of new reagents, a great number of biological substances and drugs can now be successfully analyzed by this technique. Reliable automated procedures using pre-column derivatization are available, in particular for the analysis of amino acids and amines. In addition, systems using short columns, reduced particle size of the stationary phase and ultramicro detector cells represent a promising approach to the analysis of very small volumes of sample.     

General model for precalculation of the retention indices of isoalkanes separated by gas or liquid chromatography

A general model consisting of two terms is proposed for precalculation of retention indices of isoalkanes separated by gas chromatography (GC) on a squalane column and by high-performance liquid chromatography (HPLC) on a column containing a bonded octadecyl phase. The first term (referred to as extensive) includes parameters which have the greatest correlation with the experimental value of the index; the second term (referred to as intenzive) includes parameters which can modify the value of the roughly calculated index in the direction of the experimental value. The equations derived have correlation coefficients better than 0.99. The maximum discrepancy between the experimental and calculated retention index for twenty C₆C₈ isoalcanes was less than ± 1 index unit in gas chromatography and less than 4 index units in HPLC. The retention indices of 23 other C₇C₉ isoalkanes, calculated in the extrapolation region of the equations, were sufficiently accurate.     

Gas-Liquid Chromatographic Separation of Partially and Fully Methylated Methyl Glucopyranosides

Abstract

Carbowax 20M treated with 2-nitroterephthalic acid used as the liquid phase for gas-liquid chromatography gives sharply resolved peaks for several partially and fully methylated methyl glucopyranosides. The detector response of the standard compounds shows that the absence of a C₆ methoxyl group lowers the response of a flame ionization detector.     

Numerical approximation of a two-dimensional model of non-isothermal reactive liquid chromatography involving slow rates of adsorption–desorption kinetics

Abstract

A two-dimensional general rate model of non-isothermal reactive column chromatography is formulated considering homogenous and heterogeneous reaction rates, slow rates of adsorption–desorption kinetics, and enthalpies of adsorption and reaction. The model is expressed by a system of six nonlinear partial differential equations (PDEs) coupled with algebraic expressions for the adsorption and reaction rates. The nonlinearity of adsorption isotherm and reaction term hinders the derivation of analytical solutions. For that reason, a flux-limiting high-resolution finite volume scheme is suggested to numerically approximate the model equations. The effects of several kinetic and thermodynamic parameters are rigorously analyzed on the reactant conversion and components separation.     

Numerical solution of nonlinear and non-isothermal general rate model of reactive liquid chromatography

Abstract

A nonlinear general rate model (GRM) of liquid chromatography is formulated to analyze the influence of temperature variations on the dynamics of multi-component mixtures in a thermally insulated liquid chromatographic reactor. The mathematical model is formed by a system of nonlinear convection–diffusion reaction partial differential equations (PDEs) coupled with nonlinear algebraic equations for reactions and isotherms. The model equations are solved numerically by applying a semi-discrete high-resolution finite volume scheme (HR-FVS). Several numerical case studies are conducted for two different types of reactions to demonstrate the influence of heat transfer on the retention time, separation, and reaction. It was found that the enthalpies of adsorption and reaction significantly influence the reactor performance. The ratio of density time heat capacity of solid and liquid phases significantly influences the magnitude and velocity of concentration and thermal waves. The results obtained could be very helpful for further developments in non-isothermal reactive chromatography and provide a deeper insight into the sensitivity of chromatographic reactor operating under non-isothermal conditions.     

Alpha-particle energy distributions from thin sources measured with small solid angles

The energy distribution of the alpha particles emitted from a source is in general complex. Only under particular circumstances, as in the case of very thin sources measured at large distances from the detector, can the energy distribution be approximated theoretically. In this work, we used the well-known code SRIM to simulate the interaction of alpha particles within a thin radioactive source and within the entrance window of a typical Si semiconductor detector. We considered several thin alpha particle sources measured at a large source-to-detector distance (small solid angle), in order to compare the distributions obtained by simulation with those determined by the theoretical model applied to this case. The study was performed for a variety of realistic alpha particle sources: UF₄, UO₂, U₃O₈, Gd₂O₃, and BaSO₄, considering as alpha emitters ²³⁵U, ²³³U, ¹⁴⁸Gd and ²²⁶Ra. For all these cases, we obtained the distributions due to the source and due to the entrance window of the detector, and also the final distribution given by the convolution of these two distributions. All the energy distributions obtained by simulation showed, in general, good agreement with the predictions given by the theoretical model, which includes corrections for Bohr straggling.     

A Spectrophotometric Detector for High-Performance Liquid Chromatography of Inorganic Polyphosphates

Abstract

A spectrophotometric detector for high-performance liquid chromatography of inorganic polyphosphates such as orthophosphate, diphosphate and triphosphate is described. This method is based on the employment of an air-segmented technique (AutoAnalyzer II, Technicon) as a detector, in combination with a post-column reaction using a molybdenum reagent.     

Determination of the Monodisperse Mark-Houwink Equation of High-Vinyl Polybutadiene by Gel Chromatography and Light Scattering

Abstract

Gel chromatographic, light scattering, viscosimetric and osmotic pressure measurements have been made for high vinyl polybutadiene fractions and whole polymer. Three alternative methods for deducing a monodisperse Mark-Houwink equation from gel chromatographic, light scattering and viscosimetric data of samples with unequal distribution widths are suggested. The monodisperse Mark-Houwink equations obtained by these methods for high vinyl poly(butadiene) are all alike.     

Dual Electrochemical Detection of Biogenic Amine Metabolites in Micro High-Performance Liquid Chromatography

Abstract

The dual electrochemical detectors for ordinary and micro high-performance liquid chromatography were briefly reviewed.

The electrochemical behaviors of biogenic amine metabolites were studied by cyclic semi-differential and semi-integral voltammetry with a glassy carbon working electrode. It was found that the electrochemical reactions of many biogenic amine metabolites are quasi-reversible. The dual electrochemical detector based on thin-layer electrolytic cell with two working electrodes (anode and cathode) in series configuration was tested for selective detection of biogenic amine metabolites on their electrochemical quasi-reversibility. The detector was successfully utilized for the simultaneous determination of 3, 4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindole-3-acetic acid in human urine directly injected by micro high-performance liquid chromatography.     

Evaluation of a Commercial Differential Viscometer as a GPC Detector and Its Application to Polymer Characterization

Abstract

When used as a GPC detector, Viscotek's differential viscometer (DV) measures specific viscosities at each elution volume across the chromatogram. With the addition of a concentration detector, intrinsic viscosities may be calculated. As a result, true molecular weights can be calculated via the universal calibration method.

It was found that true molecular weights and branching analysis obtained using DV for acrylate polymers initiated by VAZO and benzoyl peroxide show excellent agreement with those from low angle laser light scattering (LALLS) measurement. Moreover, comparison of intrinsic viscosities for different polymers at the same molecular weights can be made from the DV technique. In general, linear polymers will have a higher IV than branches ones and the concept has been verified for acrylate polymers in this work.

A comparison between DV and LALLS in terms of capabilities, ease of use, and maintenance is also included.