Charge density profiling of circulating human low-density lipoprotein particles by capillary zone electrophoresis

Capillary zone electrophoresis (CZE) has been utilized to profile the low-density (LDL) particles in human blood serum in this study. A 5 mM sodium phosphate buffer, pH 7.40, was chosen as the most suitable CE buffer and an extensive ultrafiltration (UF) procedure was applied to purify the LDL sample. Two LDL particle species, LDL with lower mobility and LDL- with higher mobility were observed. The electropherograms were highly reproducible with good precision of effective mobilities, corrected peak areas (CPAs) and CPA ratio of LDL-/LDL. LDL particles shown on the electropherogram were also characterized by several procedures. The applications of Sigma HDL cholesterol reagent and CE on-line 2-propanol precipitation indicated that the two particle species shown in the electropherogram belong to LDL. The LDL particles were found to associate with the buoyant LDL fraction and the LDL- particles associate with the dense LDL fraction. This study utilizes CZE for the profiling of LDL isoforms and provides a new analytical method for the resolution of LDL subspecies. It demonstrates a high-mobility LDL particle which circulates in healthy subjects and diminishes in atherosclerotic patients. Diminution of the high-mobility LDL subspecies may be linked to minimal formation of arterial plaque in atherosclerotic patients.     

Capillary zone electrophoresis of sub-microm-sized particles in electrolyte solutions of various ionic strengths: size-dependent electrophoretic migration and separation efficiency

To gain insight into the mechanisms of size-dependent separation of microparticles in capillary zone electrophoresis (CZE), sulfated polystyrene latex microspheres of 139, 189, 268, and 381 nm radius were subjected to CZE in Tris-borate buffers of various ionic strengths ranging from 0.0003 to 0.005, at electric field strengths of 100-500 V cm(-1). Size-dependent electrophoretic migration of polystyrene particles in CZE was shown to be an explicit function of kappaR, where kappa(-1) and rare the thickness of electric double layer (which can be derived from the ionic strength of the buffer) and particle radius, respectively. Particle mobility depends on kappaR in a manner consistent with that expected from the Overbeek-Booth electrokinetic theory, though a charged hairy layer on the surface of polystyrene latex particles complicates the quantitative prediction and optimization of size-dependent separation of such particles in CZE. However, the Overbeek-Booth theory remains a useful general guide for size-dependent separation of microparticles in CZE. In accordance with it, it could be shown that, for a given pair of polystyrene particles of different sizes, there exists an ionic strength which provides the optimal separation selectivity. Peak spreading was promoted by both an increasing electric field strength and a decreasing ionic strength. When the capillary is efficiently thermostated, the electrophoretic heterogeneity of polystyrene microspheres appears to be the major contributor to peak spreading. Yet, at both elevated electric field strengths (500 V/cm) and the highest ionic strength used (0.005), thermal effects in a capillary appear to contribute significantly to peak spreading or can even dominate it.     

A new approach to the studies of submicron particle suspensions based on the effect of pressure in capillary zone electrophoresis

A new approach based on the effect of pressure in CZE is suggested for acceleration of particle migration in electrophoretic runs resulting in reduction of the analysis time. It provides conditions for studying fast processes in suspensions. The effect of pressure on the migration of silica spheres with average diameters of 100, 150, and 390 nm was studied by CZE at an applied voltage of 25 kV. The particle hydrodynamic behavior was also investigated under the same capillary dimensions and BGE composition. The total particle mobility (excluding the average flow rate) was found to increase with increasing the pressure applied and particle size. The particle migration mechanism explaining the effect of pressure on particle velocity was shown to be almost the same as in wide‐bore hydrodynamic chromatography. It is based on changing radial distribution of particle concentration along the capillary cross section depending on particle diffusivity. On the basis of this mechanism appearance of a zone of negatively charged particles in electropherograms ahead of the marker peak can be explained.     

MS2 and Qβ bacteriophages reveal the contribution of surface hydrophobicity on the mobility of non‐enveloped icosahedral viruses in SDS‐based capillary zone electrophoresis

SDS is commonly employed as BGE additive in CZE analysis of non‐enveloped icosahedral viruses. But the way by which SDS interacts with the surface of such viruses remains to date poorly known, making complicate to understand their behavior during a run. In this article, two related bacteriophages, MS2 and Qβ, are used as model to investigate the migration mechanism of non‐enveloped icosahedral viruses in SDS‐based CZE. Both phages are characterized by similar size and surface charge but significantly different surface hydrophobicity (Qβ > MS2, where ‘>’ means ‘more hydrophobic than’). By comparing their electrophoretic mobility in the presence or not of SDS on both sides of the CMC, we show that surface hydrophobicity of phages is a key factor influencing their mobility and that SDS‐virus association is driven by hydrophobic interactions at the surface of virions. The CZE analyses of heated MS2 particles, which over‐express hydrophobic domains at their surface, confirm this finding. The correlations between the present results and others from the literature suggest that the proposed mechanism might not be exclusive to the bacteriophages examined here.     

A Critical Assessment of Particle Temperature Distributions During Plasma Spraying: Numerical Studies for YSZ

A three-dimensional computational model is used to simulate the in-flight particle melting behavior during plasma spraying process. The stochastic model is used for the particle size distribution. The particles surface temperature distributions at various spray distances have been presented. The results show that the surface temperature distribution varies with the spray distance. Single peak to double peaks and back to single peak has been observed in the simulations and also in the experiment. The effects of particle size and its distribution and plasma composition on the pattern shift have been investigated. Understanding the pattern shift may enable the design of a good control indicator to determine the particle melting status.     

Electrophoretic mobility of latex particles: effect of particle size and surface structure

The influence of particle size on the electrophoretic mobility of negatively charged latex particles was examined by a comparison between theory and experiment. Theoretical values for the dependence of the mobility on electrolyte concentration were calculated by a modified White–O’Brian model (Hidalgo-Alvarez et al., Adv. Coll. Interf. Sci. 67 (1996) 1) which enables the consistent calculation of the zeta (ζ) potential. For three polystyrene latexes of different size but similar surface charge density the measured mobilities increased with increasing radius for the electrolyte range under consideration. The theoretical calcalations resulted in a qualitatively correct prediction of the experimental data. The experimental comparison of the mobilities of hydrophobic and hydrophilic particles of similar size and surface charge density lead to the conclusion that hydrophilic surfaces lower the electrophoretic mobility. The same theoretical model was able to describe correctly this observed behavior by assuming a greater distance of the plane of shear. The effect of a spatial distribution of the charges was examined by characterizing an electrosterically stabilized latex. Contrary to all standard latices with surface charges this latex didn't show any mobility maximum as a function of electrolyte concentration.     

Overcharging in Colloids: Beyond the Poisson–Boltzmann Approach

A broad range of manufactured products and biological fluids are colloids. The ability to understand and control the processes (of scientific, technological and industrial interest) in which such colloids are involved relies upon a precise knowledge of the electrical double layer. The traditional approach to describing this ion cloud around colloidal particles has been the Gouy–Chapman model, developed on the basis of the Poisson–Boltzmann equation. Since the early 1980s, however, more sophisticated theoretical treatments have revealed both quantitative and qualitative deficiencies in the Poisson–Boltzmann theory, particularly at high ionic strengths and/or high surface charge densities. This review deals with these novel approaches, which are mostly computer simulations and approximate integral equation theories based on the so‐called primitive model. Special attention is paid to phenomena that cannot be accounted for by the classic theory as a result of neglecting ion size correlations, such as overcharging, namely, the counterion concentration in the immediate neighborhood of the surface is so large that the particle surface is overcompensated. Other illustrative examples are the nonmonotonic behavior of the electrostatic potential and attractive interactions between equally charged surfaces. These predictions are certainly remarkable and, on paper, they can have an effect on experimentally measurable quantities (for instance, electrophoretic mobility). Even so, these new approaches have scarcely been applied in practice. Thus a critical survey on the relevance of ion size correlation in real systems is also included. Overcharging of macroions can also be brought about by adsorption of oppositely charged polyelectrolytes. Noteworthy examples and theoretical approaches for them are also briefly reviewed.     

Nonlinear electrophoresis of dielectric particles in Newtonian fluids

In classical electrokinetics, the electrophoretic velocity of a dielectric particle is a linear function of the applied electric field. Theoretical studies have predicted the onset of nonlinear electrophoresis at high electric fields because of the nonuniform surface conduction over the curved particle. However, experimental studies have been left behind and are insufficient for a fundamental understanding of the parametric effects on nonlinear electrophoresis. We present in this work a systematic experimental study of the effects of buffer concentration, particle size, and particle zeta potential on the electrophoretic velocity of polystyrene particles in a straight rectangular microchannel for electric fields of up to 3 kV/cm. The measured nonlinear electrophoretic particle velocity is found to exhibit a 2(±0.5)-order dependence on the applied electric field, which appears to be within the theoretically predicted 3- and 3/2-order dependences for low and high electric fields, respectively. Moreover, the obtained nonlinear electrophoretic particle mobility increases with decreasing buffer concentration (for the same particle) and particle size (for particles with similar zeta potentials) or increasing particle zeta potential (for particles with similar sizes). These observations are all consistent with the theoretical predictions for high electric fields.     

CE characterization of semiconductor nanocrystals encapsulated with amorphous silicium dioxide

The electrophoretic mobility of silica-encapsulated semiconductor nanocrystals (quantum dots) dependent on the pH and the ionic strength of the separation electrolyte has been determined by CE. Having shown the viability of the approach, the electrophoretic mobility mu of the nanoparticles investigated is calculated for varied zeta potential zeta, particle radius r, and ionic strength I employing an approximate analytical expression presented by Ohshima (J. Colloid Interface Sci. 2001, 239, 587-590). The comparison of calculated with measured data shows that the experimental observations exactly follow what would be expected from theory. Within the parameter range investigated at fixed zeta and I there is an increase in mu with r which is a nonlinear function. This dependence of mu on size parameters can be used for the size-dependent separation of particles. Modeling of mu as function of I and zeta makes it possible to calculate the size distribution of nanoparticles from electrophoretic data (using the peak shape of the particle zone in the electropherogram) without the need for calibration provided that zeta is known with adequate accuracy. Comparison of size distributions calculated via the presented method with size histograms determined from transmission electron microscopy (TEM) micrographs reveals that there is an excellent matching of the size distribution curves obtained with the two independent methods. A comparison of calculated with measured distributions of the electrophoretic mobility showed that the observed broad bands in CE studies of colloidal nanoparticles are mainly due to electrophoretic heterogeneity resulting from the particle size distribution.     

Effect of a surface charge on the halfwidth and peak position of cluster plasmons in colloidal metal particles

The optical response of colloidal particles depends on a variety of properties of the cluster, e.g., shape, size, size distribution and particle material. Since particles often are charged, also the surface charge may be a parameter which influences their optical properties. In this paper the effect of a surface charge on optical properties of spherical colloidal particles is studied and its magnitude is estimated by extended computations for silver clusters with surface plasmon in aqueous suspension. Two models are presented and discussed. The first model is based on the electrodynamic solution by Bohren and Hunt (Can. J. Phys. 55, 1930 (1977)), where a surface conductivity _S for a free surface charge yield an additional contribution _S to the dielectric constant of the particle material. In the second model, the surface charge contributes to the number density of free electrons in the cluster. Both models lead to a shift of the cluster plasmon peak, while an increase of the plasmon halfwidth could not be derived. The effect is quite small and limited on very small clusters.PACS 61.46+w 73.20.Mf 78.20.Dj     

Electrophoresis and electric conduction in a salt-free suspension of charged particles

The electrophoresis and electric conduction of a suspension of charged spherical particles in a salt-free solution are analyzed by using a unit cell model. The linearized Poisson-Boltzmann equation (valid for the cases of relatively low surface charge density or high volume fraction of the particles) and Laplace equation are solved for the equilibrium electric potential profile and its perturbation caused by the imposed electric field, respectively, in the fluid containing the counterions only around the particle, and the ionic continuity equation and modified Stokes equations are solved for the electrochemical potential energy and fluid flow fields, respectively. Explicit analytical formulas for the electrophoretic mobility of the particles and effective electric conductivity of the suspension are obtained, and the particle interaction effects on these transport properties are significant and interesting. The scaled zeta potential, electrophoretic mobility, and effective electric conductivity increase monotonically with an increase in the scaled surface charge density of the particles and in general decrease with an increase in the particle volume fraction, keeping each other parameter unchanged. Under the Debye-Hückel approximation, the dependence of the electrophoretic mobility normalized with the surface charge density on the ratio of the particle radius to the Debye screening length and particle volume fraction in a salt-free suspension is same as that in a salt-containing suspension, but the variation of the effective electric conductivity with the particle volume fraction in a salt-free suspension is found to be quite different from that in a suspension containing added electrolyte.     

Adhesion as an interplay between particle size and surface roughness

Surface roughness plays an important role in the adhesion of small particles. In this paper we have investigated adhesion as a geometrical effect taking into account both the particle size and the size of the surface features. Adhesion is studied using blunt model particles on surfaces up to 10 nm root-mean-square (RMS) roughness. Measurements with particles both smaller and larger than surface features are presented. Results indicate different behavior in these areas. Adhesion of particles smaller than or similar in size to the asperities depend mainly on the size and shape of the asperities and only weakly on the size of the particle. For large particles also the particle size has a significant effect on the adhesion. A new model, which takes the relative size of particles and asperities into account, is also derived and compared to the experimental data. The proposed model predicts adhesion well over a wide range of particle/asperity length scales.     

Analysis of ion-association reaction in aqueous solution and its utilization by capillary zone electrophoresis.

Electrophoretic migration of analytes in capillary zone electrophoresis (CZE) reflects the dissolved status of analytes in solution, and the electrophoretic mobility is controlled to develop the resolution among analytes by adding a "modifier" to the migrating solution. Such addition of modifier is essentially the utilization of molecular interactions. Precise measurement of electrophoretic mobility by CZE allows analyzing molecular interactions, and CZE apparatus is very useful for physicochemical measurements. This review focuses on the advantages on using CZE to analyze equilibrium reaction; the capillary electrophoretic method and mathematical analyses that apply acid dissociation and complex formation reactions are also validated. Ion association reactions are deeply related to analytical chemistry and separation science, and CZE has been used for the investigation of ion-ion interactions. Various types of interactions have been clarified through the CZE measurements: contributions of hydrophobicity, probability, and aromatic-aromatic interaction were quantitatively evaluated. Ion association reaction in aqueous solution also elucidates the stepwise reactions of liquid-liquid distribution of ion associates. Development and applications of ion association reaction in CZE analysis are also introduced.     

Hydrothermal growth of mesoporous SBA-15 silica in the presence of PVP-stabilized Pt nanoparticles: synthesis, characterization, and catalytic properties

A novel high surface area heterogeneous catalyst based on solution phase colloidal nanoparticle chemistry has been developed. Monodisperse platinum nanoparticles of 1.7-7.1 nm have been synthesized by alcohol reduction methods and incorporated into mesoporous SBA-15 silica during hydrothermal synthesis. Characterization of the Pt/SBA-15 catalysts suggests that Pt particles are located within the surfactant micelles during silica formation leading to their dispersion throughout the silica structure. After removal of the templating polymer from the nanoparticle surface, Pt particle sizes were determined from monolayer gas adsorption measurements. Infrared studies of CO adsorption revealed that CO exclusively adsorbs to atop sites and red-shifts as the particle size decreases suggesting surface roughness increases with decreasing particle size. Ethylene hydrogenation rates were invariant with particle size and consistent with a clean Pt surface. Ethane hydrogenolysis displayed significant structure sensitivity over the size range of 1-7 nm, while the apparent activation energy increased linearly up to a Pt particle size of approximately 4 nm and then remained constant. The observed rate dependence with particle size is attributed to a higher reactivity of coordinatively unsaturated surface atoms in small particles compared to low-index surface atoms prevalent in large particles. The most reactive of these unsaturated surface atoms are responsible for ethane decomposition to surface carbon. The ability to design catalytic structures with tunable properties by rational synthetic methods is a major advance in the field of catalyst synthesis and for the development of accurate structure-function relationships in heterogeneous reaction kinetics.     

Electroacoustics of particles dispersed in polymer gel

This study examines the electroacoustics of particles dispersed in polymer hydrogels, with the particle size either less than or greater than the gel mesh size. When the particles are smaller than the gel mesh size, their acoustic vibration is resisted by only the background water medium, and the measured dynamic electrophoretic mobility, μ(d) (obtained in terms of colloid vibration current, CVI), is the same as that in water. For the case of particles larger than the gel mesh size, μ(d) is decreased due to trapping, and the net decrease depends on the viscoelastic properties of the gel. The gel mesh size was varied by varying its cross-link density, with the latter being characterized as the storage modulus, G'. The dependence of mobility on G', for systems of a given particle size, and on particle size, for gels of a given G', are investigated. The measured mobility remains constant as G' is increased (i.e., mesh size is decreased) up to a value of approximately 300 Pa, beyond which it decreases. In the second set of measurements, the trapped particle size was increased in a gel medium of constant mesh size, with G' being approximately 100 Pa. In this case, the measured μ(d) is found to be effectively constant over the particle size range studied (14-120 nm); that is, it is independent of the degree of trapping as expressed by the ratio of the particle size to the mesh size.     

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.     

Initial stages of soot formation in thermal pyrolysis of acetylene. II. A model for the incipience and growth of soot particles

A model is developed which describes simultaneously occurring processes of the initial hydrocarbon pyrolysis, nucleation, surface growth, and coagulation of soot particles. The model permits one to find the size distribution of the primary soot particles up to size 30–40 nm using a relatively small set of equations. The computed time dependence of soot particle concentration agrees satisfactorily with available experimental data. The existence of two limiting stages of the soot formation is revealed.     

Electrokinetics of charged spherical colloidal particles taking into account the effect of ion size constraints

The electrokinetic properties of suspended spherical particles are examined using a modified standard electrokinetic model, which takes into account the finite ion size and considers that the minimum approach distance of ions to the particle surface need not be equal to their effective radius in the bulk solution. We calculate the conductivity increment and the electrophoretic mobility and present a detailed interpretation of the obtained results, based on the analysis of the equilibrium and field-induced ion concentrations, as well as the convective fluid flow in the neighborhood of the particle surface. We show that when charge reversal takes place, the sign of the concentration polarization remains unchanged while the sign of the electrophoretic mobility only changes under favorable circumstances.     

"Depth-profiling" and quantitative characterization of the size, composition, shape, density, and morphology of fine particles with SPLAT, a single-particle mass spectrometer

A significant fraction of atmospheric particles are composed of inorganic substances that are mixed or coated with organic compounds. The properties and behavior of these particles depend on the internal composition and arrangement of the specific constituents in each particle. It is important to know which constituent is on the surface and whether it covers the particle surface partially or entirely. We demonstrate here an instrument consisting of an ultrasensitive single-particle mass spectrometer coupled with a differential mobility analyzer to quantitatively measure in real time individual particle composition, size, density, and shape and to determine which substance is on the surface and whether it entirely covers the particle. For this study, we use NaCl particles completely coated with liquid dioctyl phthalate to generate spherical particles, and NaCl particles partially coated with pyrene, a solid poly aromatic hydrocarbon, to produce aspherical particles with pyrene nodules and an exposed NaCl core. We show that the behavior of the mass spectral intensities as a function of laser fluence yields information that can be used to determine the morphological distribution of individual particle constituents.     

Potential of capillary zone electrophoresis for estimation of humate acid-base properties

Capillary zone electrophoresis (CZE) has been applied for fractionation and characterization of soil-derived humic acids (HAs). Humic acids from soddy-podzolic (HA(s)) and chernozem (HA(ch)) soils were studied as well as hydrophobic high-molecular-weight (HMW) and hydrophilic low-molecular-weight (LMW) HA(s) fractions obtained by salting-out with ammonium sulfate at a saturation of 0-40% and >70%, respectively. The possibility of CZE partial fractionation of HAs has been demonstrated. The shape of "humic hump" was shown to depend on the pH of running electrolyte. Almost the whole peak overlapping occurred if alkaline solutions were used for fractionation, but the peak resolution was improved at pH 5-7. Under appropriate fractionation conditions (pH 7), at least three humic acid subfractions with different electrophoretic mobilities were distinguished in the electropherograms of initial HA and HA(s) fractions. Such a high peak resolution has never been achieved for humic acids before. The presence of three subfractions in the HA is in agreement with gel-filtration analysis and was confirmed by comparison of the electrophoretic behavior of HA(s) with those of its HMW (hydrophobic) and the LMW (hydrophilic) fractions. The potentiometric titration of HA and its fractions was performed and the pK(a) of the functional groups were calculated. An attempt was made for the first time to relate the variation of electrophoretic mobility values with acid-base properties of humic acids. It was shown that changes in the humate charge resulting from the variation of the ionization degree of its functional groups as a function of pH can be estimated on the basis of electrophoretic mobility values. Potential of CZE in estimation of HA isoelectric point was demonstrated. The pH value corresponding to the lowest absolute electrophoretic mobility value of about 20 x 10(-5) cm(2) V(-1) s(-1) can be used for approximate estimation of HA isoelectric point. The data were discussed and agreement with the random coil structural model has been shown.