Capillary zone electrophoresis of some extremely weak bases in acetonitrile

In water capillary zone electrophoresis cannot be used to investigate basic compounds, which are so weak that their pK(a,HB+) values are less than zero. In acetonitrile the basic strength of such compounds is increased by many orders of magnitude. Accordingly, several extremely weak bases are protonated at low pH in acetonitrile, thus, allowing their investigation by CZE. In this work the CZE separation of thioacetamide, acetamide, thiourea, benzamide, and 4-nitrobenzamide as well as that of N-methylformamide, N,N-dimethylformamide, formamide, and dimethyl sulfoxide is demonstrated in acetonitrile using 10 mmol/L perchloric acid as an electrolyte. The effect of BGE additives, like water and acetic acid, on the CZE performance is discussed. The problem of finding a suitable electroosmotic flow marker at low pH in acetonitrile is addressed, and nitromethane is shown to be a proper marker compound under such extreme conditions. This work demonstrates how organic solvents can enlarge the field of application of CZE.     

Capillary electrophoresis separation in the presence of an immiscible boundary for droplet analysis

This paper demonstrates the ability to use capillary electrophoresis (CE) separation coupled with laser-induced fluorescence for analyzing the contents of single femtoliter-volume aqueous droplets. A single droplet was formed using a T-channel (3 microm wide by 3 microm tall) connected to microinjectors, and then the droplet was fluidically moved to an immiscible boundary that isolates the CE channel (50 microm wide by 50 microm tall) from the droplet generation region. Fusion of the aqueous droplet with the immiscible boundary effectively injects the droplet content into the separation channel. In addition to injecting the contents of droplets, we found aqueous samples can be introduced directly into the separation channel by reversibly penetrating and resealing the immiscible partition. Because droplet generation in channels requires hydrophobic surfaces, we have also investigated the advantages to using all hydrophobic channels versus channel systems with patterned hydrophobic and hydrophilic regions. To fabricate devices with patterned surface chemistry, we have developed a simple strategy based on differential wetting to deposit selectively a hydrophilic polymer (poly(styrenesulfonate)) onto desired regions of the microfluidic chip. Finally, we applied our device to the separation of a simple mixture of fluorescein-labeled amino acids contained within a approximately 10-fL droplet.     

Role of charge suppression and ionic strength in free zone electrophoresis of proteins.

The free zone electrophoretic mobility of proteins can be predicted from the protein's amino acid content by applying a model based on the Debye-Hückle-Henry theory and Henderson-Hasselbalch equation. Calculated mobilities are always greater than actual mobility but a pH-independent proportionality (described by the constant FZ) is found between the two. Thus, determination of a protein's mobility at one pH allows, with the use of the model and FZ, calculation of its mobility at other pH conditions. This leads directly to optimum conditions for the electrophoretic resolution of proteins in capillary zone electrophoresis. The fundamental nature of FZ is examined and found to be a function of a proteins molecular weight, charge, and solution ionic strength. This work aids in explaining the form of previously proposed empirically based equations for peptide and protein mobility.     

Prediction of electrophoretic mobilities. 3. Effect of ionic strength in capillary zone electrophoresis

Plots of mobility versus the square root of ionic strength (I(1/2)) do not show the linear behavior predicted by Kohlrausch's law. Classical electrolyte theory states that such deviations are to be expected due to the finite size of the ions. This paper uses the Pitts equation to account for the effect of ionic size on the ionic strength dependence of mobilities in CZE. Experimental mobilities for carboxylates, phenols, and sulfonates of -1 to -6 charge in aqueous buffers ranging from 0.001 to 0.1 M ionic strength were described by μ(-) = μ(0) - Az (I(1/2)/(1 + 2.4I(1/2))), where the constant in the denominator is empirically determined. Infinite dilution mobilities (μ(0)) determined by extrapolation of mobility data to zero ionic strength based on this expression yielded excellent agreement (100.3 ± 3.3%) with literature values for 14 compounds in a variety of buffers. The Pitts equation provides a reasonable estimate of the constant A for solutes up to a charge of -5. However, this constant also depends on temperature and the nature of the buffer counterion, presumably due to ion association. Thus it is most appropriate to determine the constant A empirically for a given buffer system.     

The principle cause for lower plate numbers in capillary zone electrophoresis with most organic solvents.

With longitudinal diffusion as an unavoidable source of peak broadening, the peak efficiency (expressed by the plate number, N) in capillary zone electrophoresis depends on the ratio of electrophoretic mobility, mu, and tracer- or self-diffusion coefficient, D. Both parameters are functions of the ionic strength of the electrolyte solution. According to theory, the mobility is decreased with increasing ionic strength by the relaxation effect (depending on the relative permittivity) and the electrophoretic effect (depending on the relative permittivity and the viscosity of the solvent), whereas the diffusion coefficient is decreased only by the relaxation effect. This allows the theoretical predictions that the plate number, which is proportional to the ratio mu/D, decreases with increasing ionic strength and that the magnitude of this reduction depends on the solvent. Taking the values for relative permittivity and viscosity allows forecasting that, in general, water as a solvent exhibits the smallest lowering of the plate number, as compared to organic solvents. The theoretical predictions are confirmed by the data for the ratio calculated from measured mobilities and diffusion coefficients for iodide as the analyte ion in water, methanol, and acetonitrile with ionic strength of the background electrolyte varying between 0.005 and 0.080 mol L(-1). Whereas the experimentally observed plate number per volt is reduced from its "ultimate value" of about 20 (analyte charge number z = 1, zero ionic strength) in water by only 10%, the decrease at the same ionic strength in methanol and acetonitrile reaches 25 to 30%. Thus, the maximum plate number should read Nmax approximately equals 13 zU (with U being the effective voltage) for these solvents with ionic strengths normally applied in capillary electrophoresis. This reduction is not stemming from inappropriate experimental conditions, but has fundamental physicochemical causes.     

Oxidation of aluminum in the presence of nanodiamond additives

The study of the kinetics of aluminum oxidation process in the presence of modified nanodiamonds and diamond-containing soot (DND-TAN and DCS-boron, respectively) shows their impact on the growth and quality of anodic films: the film thickness and the current efficiency have increased by 13 % at 2 A/dm² and using DCS-boron; the number of pores has been reduced two-fold using nanodiamond additives; no pinholing has been detected; micro-hardness of the oxide coating has increased 1.5 to 1.6 times (up to 1020 kg/mm²).     

Size-dependent electrophoretic migration and separation of liposomes by capillary zone electrophoresis in electrolyte solutions of various ionic strengths

The size-dependent electrophoretic migration and separation of liposomes was demonstrated and studied in capillary zone electrophoresis (CZE). The liposomes were extruded and nonextruded preparations consisting of phosphatidylcholine/phosphatidylglycerol/cholesterol in various ratios and ranging from 125 to 488 nm in mean diameter. When liposomes of identical surface charge density were subjected to CZE in Tris-HCl (pH 8) buffers of various ionic strengths (0.001-0.027), they migrated in order of their size. Size-dependent electrophoretic migration and separation of liposomes in CZE can be enhanced or brought about by decreasing the ionic strength of the buffer. It was shown that size-dependent migration is primarily a function of kappaR, where kappa(-1) is the thickness of the electric double layer (which can be derived from the ionic strength, I, of the buffer) and R, the liposome radius. Liposome mobility depends on kappaR and surface charge density in a manner consistent with that expected from the Overbeek-Booth electrokinetic theory. Thus, the relaxation effect appears to be the physical mechanism underlying the size-dependent electrophoretic separation of liposomes.     

Experimental characterization of end-column electrochemical detection in conjunction with nonaqueous capillary electrophoresis

An end-column electrochemical detector arrangement for capillary electrophoresis (CE) based on a 75-microm-i.d. capillary and a 25-microm microdisk electrode is characterized. The investigations were carried out using a nonaqueous (acetonitrile-based) buffer and ferrocene model compounds which offer high reliability for voltammetric measurements. The positioning of the microdisk electrode relative to the capillary outlet is the most important parameter for optimization of detection performance as it determines the characteristics of mass transport toward the electrode and the effect of ohmic potential drop resulting from the electrophoretic current on the actual detection potential. On the basis of spatially resolved studies, it was concluded that for the detection system used the microdisk electrode should be placed in a central position relative to the capillary outlet at a distance within the range of 75-100 microm. The presence of a high-voltage electric field had no negative effect on baseline noise, which was demonstrated by comparison of capillary flow injection based on gravity flow and CE experiments. Even a faster stabilization of the baseline was observed by increasing the separation voltage.     

One-pot synthesis of supported-nanoparticle materials in ionic liquid solvents

Highly porous supported-nanoparticle materials were synthesized by a rational method involving the encapsulation of poly(vinylpyrrolidone) (PVP)-stabilized Au nanoparticles into titania xerogels employing room temperature ionic liquids (1-butyl-3-methylimidazolium hexaflurophosphate, [BMIM]PF₆) as a medium followed by solvent extraction of the ionic liquid and calcination of the materials. The materials were thoroughly characterized by TEM, nitrogen adsorption-desorption isotherms, and XRD. After calcinations at 350 °C, the Au-titania system resulted in the formation of a highly mesoporous materials with BET surface areas of 200 m²/g and average pore sizes of 3-5 nm. These materials can find potential applications in catalysis and photocatalysis.     

Automatic potentiometric determination of uranium in the presence of organic solvents

Redox potentiometric titration of small amounts of uranium in mixed systems aqueous H₃PO₄ solutionorganic solvent using an automatic titrator was studied. The accuracy of the U determination strongly depends on the kind and concentration of the solvent. In determination of 0.1–0.2 mg of U in mixed phosphoric acid-organic solvent systems using ammonium vanadate as titrant, the determination accuracy decreases in the order acetone ? acetonitrile > nitromethane > ethanol. In the presence of acetone, 0.1 mg of U can be determined with high accuracy, which was impossible when using aqueous H₃PO₄ solutions. With all the organic components tested, the metrological characteristics are improved with an increase in the organic solvent content from 10 to 37 vol %.