Negative Voltage Polarity Research Articles

Rapid analysis of alkylphosphonate drugs by capillary zone electrophoresis using indirect ultraviolet detection

A rapid capillary electrophoretic method for the analysis of three alkylphosphonate drugs (i.e. fosfomycin disodium (FOS), clodronate disodium (CLO) and alendronate sodium (ALN)) was developed by using multiple probe BGE and indirect UV detection. BGE containing 30 mM benzoic acid, 5 mM salicylic acid and 0.5 mM CTAB (pH 3.8), temperature of 30 degrees C, applied voltage of -30 kV and detection at 220 nm provided baseline separation of all analytes (resolution (R)>2.2) in 3.2 min. EOF reversal by addition of CTAB and negative voltage polarity leading to the co-EOF flow and short analysis time. Two probe BGE greatly improved peak symmetry. The method showed good linearity (r(2)>0.999 in ranges of 20-1000 microg/mL for FOS, 100-1000 microg/mL for CLO and 100-750 microg/mL for ALN) repeatablitiy (RSD<2.15%), recovery (99.3-101.1%) and sensitivity (LOD<50 microg/mL). Freshly prepared BGE and sample solutions are essential for the method precision and accuracy. This new method can be utilized for routine analysis of FOS, CLO and ALN in dosage forms because of its efficiency, reliability, speed and simplicity.

Scanning tunneling microscopy on unpinnedGaN(11¯00)surfaces: Invisibility of valence-band states

We investigated the origins of the tunnel current in scanning tunneling microscopy (STM) and spectroscopy experiments on $\text{GaN}(1\overline{1}00)$ surfaces. By calculating the tunnel currents in the presence of a tip-induced band bending for unpinned $n$-type $\text{GaN}(1\overline{1}00)$ surfaces, we demonstrate that only conduction-band states are observed at positive and negative voltage polarities independent of the doping concentration. Valence-band states remain undetectable because tunneling out of the electron-accumulation zone in conduction-band states dominates by four orders of magnitude. As a result band-gap sizes cannot be determined by STM on unpinned $\text{GaN}(1\overline{1}00)$ surfaces. Appropriate band-edge positions and gap sizes can be determined on pinned surfaces.

Single‐run separation of cationic, anionic, and polyanionic compounds by CE‐ESI‐MS

A method for a single-run separation of cationic, anionic, and polyanionic compounds by CE hyphenated to ESI MS (CE-ESI-MS) is described. One of the main issues for coupling CE to MS with an ESI source consists in maintaining an electric contact for the electrophoretic separation. This condition is only performed if a liquid flow arising from the separation capillary is directed to the needle, making it coupling-compatible. This latter situation is incompatible with the separations of polyanionic compounds of higher electrophoretic mobility (in absolute value) than the electroosmotic mobility, performed in bare fused-silica capillaries under a negative polarity. In this study, several alternative approaches were evaluated to circumvent this difficulty, and applied to the setup of the CE-MS separation of a mixture containing both cationic and polyanionic compounds, which are synthesis intermediates of contrast agents for medical imaging. Eventually, the detection of the cationic and anionic compounds in a single run could be obtained by either using neutrally coated polymethylsiloxane (DB-1) capillaries and simultaneously applying a negative voltage polarity and a pressure allowing to compensate for the residual cathodic EOF or by dynamically modifying the inner wall of a bare fused-silica capillary with a polycationic polymer (hexadimethrine bromide) and using it afterwards under negative voltage polarity.

Particle precipitation efficiency in an electrostatic precipitator

In this paper, results of the measurements of the size distribution of seed particles after their precipitation in a wire-plate-type ESP with seven wire electrodes are presented. The cigarette smoke was used as a source of seed particles. The particles of sizes from 0.5 to 8 μm were counted in the 0.5 μm-wide ranges at the ESP outlet using a particle size-meter of the laser light reflection type. The particle precipitation efficiency for each size fraction was determined for various primary flow velocities at positive and negative voltage polarity. Due to the size-meter measuring range limited to the particles larger than 0.5 μm, the present measurements do not show the precipitation of the particles of sizes smaller than 0.5 μm.

Speciation of arsenic and selenium by capillary electrophoresis

Capillary electrophoresis (CE) with direct UV detection, both on-capillary and with a high-sensitivity detection cell (HSDC), were used for the simultaneous determination of arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), selenate [Se(VI)], selenite [Se(IV)], selenocystine (SeC), selenomethionine (SeM) and selenocystamine (SeCM). These anionic and cationic species were separated with negative separation voltage polarity in a capillary coated with poly(diallydimethylammonium chloride) (PDDAC). The background electrolyte (BGE) providing optimal separation was 15 mM phosphate buffer at pH 10.6. Arsenic and selenium species were detected at 195 and 200 nm, respectively. The limit of detection (LOD) values with on-capillary detection (75 μm i.d.) for the individual analytes were 1.02, 1.50, 1.08, 1.35, 19.5, 0.36, 0.32, 1.11 and 1.47 mg/L, respectively (expressed as arsenic or selenium). The method precision for peak area was from 2.1 to 3.4% relative standard deviation. HSDC was applied to increase the detection sensitivity and gave LOD improvement factors from 4.7 to 8.2, yielding LODs for the individual target analytes ranging from 0.049 to 2.38 mg/L. When the normal sample stacking mode was employed using a sample plug up to 4.6% of capillary volume, both the cationic and anionic analytes were stacked simultaneously with LOD improvement factors of 2.6–4.5 yielding LODs for the individual analytes ranging from 0.11 to 7.42 mg/L. The method was applied for the speciation of arsenic in sediment and determination of SeM in a selenium nutrition supplement. The achieved separation selectivity also gives the method a more general application potential for simultaneous arsenic and selenium speciation when hyphenated with inductively coupled plasma MS.

Speciation of arsenic and selenium by capillary electrophoresis

Capillary electrophoresis (CE) with direct UV detection, both on-capillary and with a high-sensitivity detection cell (HSDC), were used for the simultaneous determination of arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), selenate [Se(VI)], selenite [Se(IV)], selenocystine (SeC), selenomethionine (SeM) and selenocystamine (SeCM). These anionic and cationic species were separated with negative separation voltage polarity in a capillary coated with poly(diallydimethylammonium chloride) (PDDAC). The background electrolyte (BGE) providing optimal separation was 15 mM phosphate buffer at pH 10.6. Arsenic and selenium species were detected at 195 and 200 nm, respectively. The limit of detection (LOD) values with on-capillary detection (75 μm i.d.) for the individual analytes were 1.02, 1.50, 1.08, 1.35, 19.5, 0.36, 0.32, 1.11 and 1.47 mg/L, respectively (expressed as arsenic or selenium). The method precision for peak area was from 2.1 to 3.4% relative standard deviation. HSDC was applied to increase the detection sensitivity and gave LOD improvement factors from 4.7 to 8.2, yielding LODs for the individual target analytes ranging from 0.049 to 2.38 mg/L. When the normal sample stacking mode was employed using a sample plug up to 4.6% of capillary volume, both the cationic and anionic analytes were stacked simultaneously with LOD improvement factors of 2.6–4.5 yielding LODs for the individual analytes ranging from 0.11 to 7.42 mg/L. The method was applied for the speciation of arsenic in sediment and determination of SeM in a selenium nutrition supplement. The achieved separation selectivity also gives the method a more general application potential for simultaneous arsenic and selenium speciation when hyphenated with inductively coupled plasma MS.

Measurement of the flow velocity field in multi-field wire-plate electrostatic precipitator

In this paper results of Particle Image Velocimetry (PIV) measurements of the partiele flow velocity fields in a wire-to-plate type electrostatic precipitator (ESP) are presented. One or two grounded stainless-steel plate electrodes and seven wire electrodes in the middle of the ESP height were used. The measurements were carried out in the plane placed perpendicularly to the wire electrodes (for the ESP version with two plate electrodes) and in four planes placed parallel to the wire electrodes (for the ESP version with one plate electrode). Either negative or positive voltage polarity was applied to the wire electrodes. The main gas flow velocity was 0.14 m/s. The obtained results showed strong influence of the electrohydrodynamic (EHD) forces on the flow patterns, which exhibited strong upstream and downstream vortexes. The experiment confirmed that the flow patterns in the ESP have 3-dimensional character.

Laser flow visualization and velocity fields by particle image velocimetry in an electrostatic precipitator model

Although improving electrostatic precipitator (ESP) collection of fine particles (micron and submicron sizes) remains of interest, it is not yet clear whether the turbulent flow patterns caused by the presence of electric field and charge in ESPs advance or deteriorate fine particle precipitation process. In this paper, results of the laser flow visualization and Particle Image Velocimetry (PIV) measurements of the particle flow velocity fields in a wire-to-plate type ESP model with seven wire electrodes are presented. Both experiments were carried out for negative and positive polarity of the wire electrodes. The laser flow visualization and PIV measurements clearly confirmed formation of the secondary flow (velocity of several tens of cm/s) in the ESP model, which interacts with the primary flow. The particle flow pattern changes caused by the strong interaction between the primary and secondary flows are more pronounced for higher operating voltages (higher electrohydrodynamic number NEHD) and lower primary flow velocities (lower Reynolds number Re). The particle flow patterns for the positive voltage polarity of the wire electrodes are more stable and regular than those for the negative voltage polarity due to the nonuniformity of the negative corona along the wire electrodes (tufts).

Constant voltage stress induced degradation in HfO2/SiO2 gate dielectric stacks

Defect generation in HfO2/SiO2 gate dielectric stacks under constant voltage stress is investigated. It is found that the stress induced electrical degradation in HfO2/SiO2 stacks is different than in the SiO2 layer. The variation of the gate leakage current with different polarities shows different degradation characteristics after stress. Positive charge generation is also observed under both negative and positive gate voltage polarities. These degradation phenomena are explained by the composite effect of three components: neutral trap generation, electron trapping, and positive charge generation in the gate stacks.

Effects of Applied Electric Fields on Drop—Interface and Drop—Drop Coalescence*

ABSTRACT In this work, coalescence of a single organic or aqueous drop with its homophase at a horizontal liquid interface was investigated under applied electric fields. The coalescence time was found to decrease for aqueous drops as the applied voltage was increased, regardless of the polarity of the voltage. For organic drops, the coalescence time increased with increasing applied voltage of positive polarity and decreased with increasing applied voltage of negative polarity. Under an electric field, the coalescence time of aqueous drops decreases due to polarization of both the drop and the flat interface. The dependency of organic drop-interface coalescence on the polarity of the electric field may be a result of the negatively charged organic surface in the aqueous phase. Due to the formation of a double layer, organic drops are subjected to an electrostatic force under an electric field, which, depending on the field polarity, can be attractive or repulsive. Pair-drop coalescence of aqueous drops in the organic phase was also studied. Aqueous drop-drop coalescence is facilitated by polarization and drop deformation under applied electric fields. Without applied electric fields, drop deformation increases the drainage time of the liquid film between two approaching drops. Therefore, a decrease in the interfacial tension, which causes drop deformation, accelerates drop-drop coalescence under an electric field and inhibits drop coalescence in the absence of an electric field. *The submitted manuscript has been authored by a contractor of the U.S. Government under contract DE-AC05-00OR22725. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.

A polarized infrared spectroscopic study on electric-field-induced layer rotation of ferroelectric liquid crystal mixtures with ultrashort pitch

In the present study we found by observing with a polarizing microscope that the liquid crystal layers of ferroelectric liquid crystal (FLC) mixtures with ultrashort pitch (FLCUSP) rotate in the smectic-C* (Sm-C*) phase under the application of a simple rectangular pulse wave with a voltage of positive or negative polarity. In order to investigate the mechanism of the electric-field-induced layer rotation of FLCUSP, polarization angle-dependent infrared spectra were measured before and after the layer rotation. The polarization angle-dependence of infrared spectra of FLCUSP suggests that the layers of FLCUSP rotate by about 40°. This result is in good agreement with the observation by the polarizing microscope. It was also found from the polarization angle-dependence that the CF3 and CO groups of the chiral dopants are subjected to the hindered rotation around the molecular long axis. It is very likely that both the CF3 and CO groups are responsible for the large polarization. The obtained polarized infrared spectra are complex particularly in the 1290–1200 cm−1 region where a band due to the CF3 stretching mode is expected to appear. Thus, we used two-dimensional (2D) correlation spectroscopy and curve fitting to analyze the complex spectra. The 2D correlation analysis reveals that before the layer rotation the directions of the CF3 groups in the chiral dopants with respect to the molecular long axis under the applied dc voltage are different from those in the absence of the electric field and that the directions of the CF3 groups in the absence of the electric field change significantly before and after the layer rotation.

Identification of heparin oligosaccharides by direct coupling of capillary electrophoresis/Ionspray-Mass spectrometry

A direct coupling between capillary electrophoresis (CE) and ionspray (IS) mass spectrometry (MS) has been optimized to identify oligosaccharides obtained by enzymatic digestion of heparin. The separation electrolyte was made compatible with the requirements of a direct coupling using electrolytes made of ammonium acetate buffer, pH 3.5 and 9.2. The different parameters of the CE/IS interface were optimized using a standard mixture of disaccharides: flow rate and composition of the sheath liquid, flow rate of the sheath gas and position of the capillary in the needle. Different combinations of positive or negative CE voltage polarity and positive or negative MS ionization modes were investigated. They allowed for detection orders to be easily reversed and for complementary structural information to be gathered. Finally, the optimized methodology was applied to the separation and characterization of porcine mucosa heparin depolymerized by heparinases II and III.

Identification of heparin oligosaccharides by direct coupling of capillary electrophoresis/ionspray-mass spectrometry.

A direct coupling between capillary electrophoresis (CE) and ionspray (IS) mass spectrometry (MS) has been optimized to identify oligosaccharides obtained by enzymatic digestion of heparin. The separation electrolyte was made compatible with the requirements of a direct coupling using electrolytes made of ammonium acetate buffer, pH 3.5 and 9.2. The different parameters of the CE/IS interface were optimized using a standard mixture of disaccharides: flow rate and composition of the sheath liquid, flow rate of the sheath gas and position of the capillary in the needle. Different combinations of positive or negative CE voltage polarity and positive or negative MS ionization modes were investigated. They allowed for detection orders to be easily reversed and for complementary structural information to be gathered. Finally, the optimized methodology was applied to the separation and characterization of porcine mucosa heparin depolymerized by heparinases II and III.

Jet and drops formation in electrohydrodynamic spraying of liquids. A systematic approach

The surface of the liquid issuing from a capillary subjected to the electric stresses deforms to an elongated jet which disrupts to a series of droplets. In the paper the detailed experimental studies of jet and drop formation from liquid issuing from a capillary maintained at high voltage of positive or negative polarity have been reported. The forms of the jet and the way it disrupts into droplets have been classified into the modes of spraying. A definition of the mode of spraying and the criteria for the mode classification, based on the geometrical form of the meniscus, jet or drop formation are given.

Determination of lactic acid and poly(lactic acid)s in a dermatological formulation by capillary electrophoresis.

A stability-indicating capillary electrophoresis (CE) method was developed to determine lactic acid at a level of 0.9% m/m in a dermatological formulation. This level includes up to 34% present in the form of poly(lactic acid)s. Current methods which involve treatment of samples with sodium hydroxide to hydrolyse the various lactic acid oligomers to free lactic acid cannot strictly be regarded as true measures of stability. The harsh hydrolysis conditions used can also lead to the generation of many degradation peaks from excipients such as natural extracts which can interfere with the detection of lactic acid in HPLC and CE methods. Application of indirect UV detection using 4-methoxybenzoic acid (p-anisate) as the background electrolyte and negative voltage polarity (detector towards anode) allows the direct quantification of the total available lactic acid from the monomer and the predominant linear oligomers in the raw material, i.e., the dimer (lactoyllactate) and the trimer. Tetradecyltrimethylammonium bromide was used as the electroosmotic flow modifier, lactic acid, lithium salt, as a calibration standard and butyric acid as an internal standard. A typical RSD for standard response using migration time-corrected peak area ratios was 1.17% (n = 9). Duplicate analysis of total lactic acid from two similar product placebos spiked with raw material gave average recoveries of 101.0 and 99.6%. The method, when applied to fully and partially hydrolysed raw material, showed good agreement (99.6%) with a standard titration assay for raw materials and was successfully applied to 5 month storage samples of a product. Advantages of the method include speed, simplicity, low consumption of reagents and no organic solvents.

Influence of discharge production conditions, gas pressure, current intensity and voltage type, on SF6 dissociation under point–plane corona discharges

The study of the formation of Sulfur Hexafluoride (SF6) dissociation products under point to plane corona discharges was carried out at PSF6=300 kPa using different discharges production conditions (50 Hz ac voltage, dc negative polarity voltage, mean discharge current intensity Ī varying between 2 and 45 μA for dc negative polarity voltage), for two plane electrode materials (aluminum and stainless steel), and moisture levels (200 and 2000 ppmv H2O). The stable gaseous by-products formed (SO2F2, SOF4, SOF2, and S2F10) were assayed by gas-phase chromatography. The results indicate an important effect of the metal constituting the plane electrode and of the moisture conditions whatever the SF6 pressure (100–300 kPa), discharges intensity (Ī) and voltage type studied. An effect of the increase of SF6 pressure up to 300 kPa was mainly observed for S2F10 and corresponds to a greater formation of this compound with PSF6. The influence of the mean discharge current intensity on SF6 by-product formation carried out for a transported charge of 1 C showed that for Ī≤10 μA, the effect varies according to the compound considered and depends on the water content of the SF6 and/or on the plane electrode material, whereas for Ī≳10 μA, the levels of the four compound studied hardly vary with the current. Comparison of results obtained under ac and dc voltage for a cumulated charge of between 0.5 and 11 C showed that (SO2F2+SOF4) and SOF2 were formed in larger quantities with ac than with dc, unlike S2F10 for which the opposite effect was observed.

Control of edge electric field by limiter biasing and electron beam injection in HYBTOK-II tokamak

Experimental research on the modification of the edge radial electric field by limiter biasing and electron beam injection from a hot cathode has been performed in a small research tokamak, HYBTOK-II. Control of the amplitude and/or the spatial profile of the radial electric field might enable us to improve the plasma confinement properties. In HYBTOK-II a bias voltage of negative or positive polarity with respect to the wall is applied to the poloidal circular limiters. In the positive biasing the plasma potential in the edge region rises in proportion to the applied bias voltage. Penetration of the radial electric field into the core region is about 2 cm, which corresponds roughly to the ion poloidal Larmor radius in the edge. The radial electric field of about 25 V/cm was generated at the edge region by the positive biasing of 50 V, and substantial suppression of the electrostatic fluctuations was observed in both the edge and scrape-off regions. By reducing the plasma potential and making the radial electric field weak in the case of the negative biasing, the electrostatic fluctuation increases significantly.

Photogeneration and transport of carriers in atactic polystyrene

We have studied the photoconductivity of atactic polystyrene at room temperature under both positive and negative polarity (of the unilluminated electrode). There is a maximum in the response under 5.2-eV uv illumination. If this illumination is sufficiently intense, the total current may have positive polarity even with negative voltage polarity. This is due to the fact that the photocurrent is always positive, due to the diffusion of holes. This may be understood in terms of the Onsager model of geminate generation and recombination. The microscopic phenomena underlying the behavior are discussed as is the applicability of the Onsager model to other polymers.

Sparking Potentials of and Similarity Law in Nitrous Oxide Between Coaxial Concentric Cylinders

The dc sparking potential of nitrous oxide is measured between concentric coaxial cylindrical electrodes over the gas pressure range 2 X 1O-2-81O torr for both positive and negative voltage polarities of the inner electrode. An intersection in the sparking potential and pressure curves is observed at high pressures when the sparking potential for the negative inner electrode changes from being lower to higher than the corresponding positive breakdown at a fixed gas pressure. The similarity law is found to hold in this gas only for a limited range of gas pressures. The sparking potential values are also calculated by means of an empirical general discharge concept and compared with the measured values. Reasonable agreement is obtained.

Electrical breakdown of a point-plane gap in high vacuum and with variation of pressure in the range 10−7−10−2 Torr of air, nitrogen, helium, sulphur hexafluoride, and argon

The dc breakdown potential of a point-plane electrode configuration is measured in high vacuum (∼10−7 Torr) using the positive and the negative voltage polarity of the point electrode as a function of gap separation. Air, nitrogen, helium, sulphur hexafluoride, and argon are used in turn to alter the electrode coverage by adsorption of various gases in high vacuum. It has been found that helium gives the highest breakdown voltage at a given gap length, in high vacuum. It has also been found that very high sparking potential values are obtained using positive-point-negative-plane electrodes (90 kV at 0.3 mm gap length, helium at 3×10−7 Torr). The effect of ac (50 Hz) glow discharge conditioning on the dielectric strength of the gap is investigated and found to give considerable improvement in the voltage that the gap can withstand before a vacuum breakdown occurs. The effect of introducing various gases in the pressure range 10−7−10−2 Torr on the breakdown potential of point-plane gaps is investigated. Maxima are observed in the breakdown voltage and pressure curves in the range 10−4−10−3 Torr. Helium and nitrogen give the highest breakdown voltage of about 90 kV for a gap length of 0.2 mm at about 10−4 Torr. The observed improvements in the breakdown potential that the gap can withstand with certain gases are attributed to the increase in the work function of the combined metal-gas system.