Relative magnitudes of the effects of electrostatic image and thermophoretic forces on particles in the respiratory tract

The National Radiological Protection Board's Advisory Group on Non-ionising Radiation has recommended research into the deposition, in the lung, of charged particles in the size range 0.005-1 microm. In vivo measurements of the temperature distribution in the respiratory tract have been used to estimate the temperature gradients in the generations up to the segmental bronchus. These gradients define the thermophoretic velocities, which oppose deposition during inhalation and assist it during exhalation. The thermophoretic forces are effective over a longer range than those due to the electrostatic image of a single charge; and, at distances greater than a few microns from the airway wall, the thermophoretic velocities of 0.02 and 0.1 microm particles are greater than those due to electrostatic drift. It is concluded that models describing the effects of electric charge on the deposition of particles with diameters of order 0.1 microm need to take account of the thermal conditions in the respiratory tract.     

Comparison between an unipolar corona charger and a polonium-based bipolar neutralizer for the analysis of nanosized particles and biopolymers

In this work we present results on the charging efficiency of nanoparticles by means of a corona based unipolar charging unit. This device was designed to replace a Po210 bipolar charger unit in a commercial electrospray aerosol generator (TSI Mod 3480). The charging efficiency has been investigated for negative and positive charged particles of various chemical composition in the size range between 5 and 18 nm. The corona current has been found to be the most influential operation parameter on the charging efficiency. With a positive electrospray droplet charge and a negatively-biased corona needle, a rapidly decreasing yield of singly positively charged aerosol particles with increasing corona current was found. An increasing yield of negatively charged particles was observed with increasing current of the corona process. Providing appropriate corona settings nanoparticles with charge levels similar to these obtained with a Po210 charger were found. At optimal corona settings the yield of singly charged particles was found to be two to four times higher for negative and positive particles compared to bipolar charging. This gain in the charging efficiency increases directly the sensitivity of analysis and enhances all measurement and manipulation processes of airborne nanoparticles for which electrical charging is required.     

Dual needle corona discharge to generate stable bipolar ion for neutralizing electrosprayed nanoparticles

The performances of dual needle corona discharge (NCD) as bipolar ion source to neutralize the electrospray (ES) particles were characterized and optimized. The NCD was constructed from a tungsten needle and grounded mesh electrode in the needle-to-plane configuration. The dual NCD created a bipolar ion environment by mixing the balanced concentration of positive and negative ions produced by each NCD. The dual NCD was placed in an electrospray aerosol generator (EAG) apparatus to reduce the charge state of the ES particles. Polystyrene latex (PSL) suspensions having the particle size range of 96–256 nm were used as the precursor solution for the electrospray process. Some characterizations to the NCD were carried out to obtain optimum operating voltage and air flow rate. The size distribution and charge fraction of the electrospray PSL (ES-PSL) particles exiting the EAG were also investigated. The result showed the dual NCD could generate stable bipolar ions by mixing positive and negative ions with balanced concentration. The bipolar ions from the dual NCD were capable of neutralizing and reducing the charge state of highly charged ES-PSL particles larger than 100 nm. The EAG, equipped with the dual NCD, could generate ES-PSL particles with stable concentration and consistent size distribution. The charge fraction calculation of the ES-PSL particles showed that more than 80% of the particles exiting the EAG were positively charged.     

Ion recombination on dust particles in a dense gas plasma excited by a hard ionizing factor

A theoretical model is proposed that describes the main kinetic mechanisms operating in a dust plasma formed upon the bulk ionization of a dense gas by a hard factor. An expression is derived for the ion density distribution around a charged dust particle. It is shown that ions in the plasma recombine with charged dust particles according to the Langevin mechanism. A condition is established under which the dust particles are bearing all the negative charge in the plasma, compensated by the positive charge of ion clouds. A new criterion for manifestation of the collective properties in the dust plasma is formulated.     

Effects of Powder Velocity and Contact Materials on Tribocharging of Polymer Powders for Powder Coating Applications

Electrostatic powder deposition using corona charging is widely used in a plethora of industrial applications. Disadvantages of this technique are back corona onset and the Faraday penetration limitation. Another method to charge powders is to use tribochargers. Tribocharging depends upon the work function difference between the contacting materials and generates bipolarly charged particles. In this study, acrylic and epoxy powders were fluidized and charged by passing through stainless steel, copper, aluminum, and polycarbonate static mixers, respectively. The particle velocity and powder flow rate were varied to determine their effect on the net charge-to-mass ratio (Q/M) acquired by the powders. The Q/M increased rapidly with velocities between 1.5 to 2.5 m/s and stabilized for higher velocities but decreased with increasing powder flow rate at a constant velocity. The net positive or negative charge on each powder was determined to be dependant on the charger material. The use of an aluminum charger (net negative charge) in combination with a PTFE finger nozzle (net positive charge) resulted in a net powder Q/M of - 0.05 μC/g. The generation of an ion-free powder cloud with high bipolar charge but overall charge density of almost zero is anticipated to provide a better coverage of recessed areas.     

A note on the charging of aerosols by overhead line corona

Elevated concentrations of corona ions have been measured in the vicinity of high voltage overhead lines. It is shown that the integrated ion exposure of aerosols in the corona plume is of the order of (x/u)n(x) where n(x) is the ion concentration at distance x downwind from the line and u is the wind speed. Estimated ion exposures are of order 10(11) m(-3) x s, less then 1% of the 10(13) m(-3) x s needed to cause saturation charging of 20 nm aerosols. It is suggested that it is not valid to postulate that AC corona is a health hazard as a consequence of its charging aerosols in the size range 20-125 nm.     

Analysis of megadalton ions using cryodetection MALDI time-of-flight mass spectrometry

Presented are initial results from the first commercially available matrix-assisted laser desorption/ionization time-of-flight mass spectrometer specifically designed for the sensitive detection of very high mass ions (macromizer, Comet AG). This new instrument utilizes a 16-element superconducting tunnel junction detector coupled with a fully adjustable gimbal-mounted ion source/focusing region that allows unparalleled sensitivity for detection of singly charged high molecular weight ions. Using this new technology, the singly charged ions in the megadalton region are detected from immunoglobulin M and von Willebrand factor proteins. This detector technology also measures the kinetic energy of the particles impacting the detector, which can be correlated to the charge of the particles. Immunoglobulin G and streptavidin were used to demonstrate the ability of the macromizer instrument to detect high-mass ions and to discern the charge state of the ions.     

Corona discharge in charge reduction electrospray mass spectrometry

Corona discharge is applied to charge reduction electrospray mass spectrometry for the analysis of complex mixtures of biological molecules. Recent work has described a method of charge reduction (reducing the charge states of analyte ions generated by the electrospray process) employing the radioactive isotope 210Po to produce neutralizing species. A variation to this approach is presented, in which charge neutralization is mediated by ions produced in a corona discharge. Varying the corona discharge voltage controls the current and the degree of charge reduction, providing predominantly singly charged ions that are detected by a commercial electrospray time-of-flight mass spectrometer. This technique provides charge reduction for the simplification of ESI spectra, without need for any radioactive material.     

Electrospray ionization high-resolution ion mobility spectrometry-mass spectrometry

A hybrid atmospheric pressure ion mobility spectrometer is described which exhibits resolving power approaching the diffusion limit for singly and multiply charged ions (over 200 for the most favorable case). Using an electrospray ionization source and a downstream quadrupole mass spectrometer with electron multiplier as detector, this ESI-IMS-MS instrument demonstrates the potential of IMS for rapid analytical separations with a resolving power similar to liquid chromatography. The first measurements of gas-phase mobility spectra of mass-identified multiply charged ions migrating at atmospheric pressure are reported. These spectra confirm that collision cross sections are strongly affected by charge state. Baseline separations of multiply charged states of cytochrome c and ubiquitin demonstrate the improved resolving power of this instrument compared with previous atmospheric pressure ion mobility spectrometers. The effects of electric potential, initial pulse duration, ion-molecule reactions, ion desolvation, Coulombic repulsion, electric field homogeneity, ion collection, and charge on the resolving power of this ion mobility spectrometer are discussed.     

Charge reduction electrospray mass spectrometry

A new mass spectrometric technique, charge reduction electrospray mass spectrometry (CREMS), allowing the analysis of complex mixtures of biological molecules is described. The charge state of ions produced by electrospray ionization may be reduced in a controlled manner to yield predominantly singly charged ions through reactions with bipolar (i.e., both positively and negatively charged) ions generated using a 210Po alpha particle source. The electrospray-generated multiply charged ions undergo charge reduction in a "neutralization chamber" positioned before the entrance nozzle to the mass spectrometer. The ions are detected using a commercial orthogonal electrospray time-of-flight mass spectrometer, although the neutralization chamber can be adapted to virtually any mass analyzer. The CREMS results obtained exhibit a signal intensity drop-off with increasing oligonucleotide size similar to that observed with matrix-assisted laser desorption/ionization mass spectrometry. Proton-transfer reactions were found to be responsible for reducing charge on proteins and oligonucleotides in both positive and negative ion mode.     

Corona Generation and Deposition of Metal Nanoparticles on Conductive Surfaces and Their Effects on the Substrate Surface Texture and Chemistry

A corona discharge ion bombardment technique was used successfully to generate gold particles of submicron diameters. In a negative corona discharge, the glow region contains electrons, negative ions, and positive ions. Positive ions collided with the negative corona tip electrode, causing it to sputter and emit fine particles of the electrode material. These nanoparticles were deposited on grounded metal substrates or thin mica sheets supported by grounded metal substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the size distribution and deposition pattern of the metal nanoparticles. The diameter of these nanoparticles was dependent upon the material of the electrode and ranged from 20 to 450 nm for gold and from 15 to 240 nm for tungsten. The nanoparticles were deposited on aluminum, mica, and carbon steel test panels for different amounts of time. The electrochemical response of the carbon steel panels exposed to aerated salt solution was measured by direct current (DC) polarization technique before and after the gold nanoparticles were deposited. This technique was employed to determine the changes in the surface chemistry because of the presence of gold nanoparticles, and it proved to be a sensible method for detecting the presence of fine layers of nanoparticles on the metallic substrate. The presence of the gold nanoparticles increased the electrochemical potential E_corr from -0.640 V to -0.211 V, compared with the value for a noncorrosive surface, like that of pure gold, which is 0 V.     

Kinetic processes in the plasma formed in combustion of hydrocarbon fuels

An analysis of the basic kinetic processes responsible for the formation of ions, electrons, charged and neutral carbon clusters and particles of nanometer size in the combustion of hydrocarbon fuels has been made. It has been shown that the formation of a polydisperse ensemble of positively and negatively charged particles is mainly caused by the ion adhesion to primary particles and secondarily formed particles and also by particle coagulation. Account must be taken not only of the Coulomb interaction but also of the van der Waals and polarization interaction between particles. The distinstice features of the deposition of polar molecules on charged particles have been considered.     

Noise characteristics of stacked CMOS active pixel sensor for charged particles

The noise characteristics of a stacked CMOS active pixel sensor (SCAPS) for incident charged particles have been analyzed under 4.5 keV Si⁺ ion irradiation. The source of SCAPS dark current was found to change from thermal to electron leakage with decreasing device temperature. Leakage current at charge integration part in a pixel has been reduced to 0.1 electrons s⁻¹ at 77 K. The incident ion signals are computed by subtracting reset frame values from each frame using a non-destructive readout operation. With increase of irradiated ions, the dominant noise source changed from read noise, and shot noise from the incident ions, to signal frame fixed-pattern noise from variations in sensitivity between pixels. Pixel read noise is equivalent to ten incident ions. The charge of an incident ion is converted to 1.5 electrons in the pixel capacitor. Shot noise corresponds to the statistical fluctuation of incident ions. Signal frame fixed-pattern noise is 0.7% of the signal. By comparing full well conditions to noise floor, a dynamic range of 80 dB is achieved. SCPAS is useful as a two-dimensional detector for microanalyses such as stigmatic secondary ion mass spectrometry.     

Increasing the negative charge of a macroanion in the gas phase via sequential charge inversion reactions

Protonated and deprotonated biological molecules in the gas phase play an important role in life sciences research. The structural information accessible from the ions is highly dependent upon their charge states. Therefore, it is desirable to develop means for increasing absolute charge states, particularly for ionization methods, such as MALDI, that yield relatively low charge ions. The work presented here demonstrates the formation of a doubly deprotonated polypeptide or oligonucleotide ion (dianion) from a singly deprotonated analogue via two sequential ion/ion proton-transfer reactions involving charge inversion. The high exoergicity and the large cross section arising from the long-range attractive Coulomb potential of ion/ion reactions make this process plausible. In this example, an overall efficiency of conversion of singly charged ions to doubly charged ions of roughly 8% for polypeptide was noted while lower efficiency (roughly 2%) observed with an oligonucleotide is likely due to a greater degree of neutralization. No other approach to increasing the net negative charge of an anion in the gas phase has as yet been reported.     

Micromachined Bradbury-Nielsen gates

Bradbury-Nielsen gates (BNGs) are a standard way for gating or steering beams of charged particles in ion mobility spectrometry and time-of-flight mass spectrometry. They consist of a pair of interleaved electrodes that when at the same potential allow ions to pass through the electrodes undeflected and, when a voltage is applied, cause the ions to be deflected from their propagation axis. Previous efforts to construct such devices have relied on mechanical assembly by winding wires across an aperture. We describe a micromachining method for making monolithic BNGs using deep reactive ion etching of silicon-on-insulator wafers. This method enables the creation of electrodes with spacings ranging from 25 to 100 microm with a thickness of 20 microm, covering a 5 mm by 5 mm active area. We characterize the performance of these micromachined BNGs by ion imaging in a pseudorandom time-of-flight mass spectrometer.     

Electrically charged filter materials

Filters made from ordinary textile fibres are too coarse to remove micrometre-sized dust particles, which are responsible for much respiratory disease. If, however, an electric charge is applied to the fibres, a filter can provide a very good respiratory protection. Fibres can be charged by triboelectric exchange, by corona or by induction; and charge levels can give rise to electric fields in the interstices of the filter, approaching the breakdown field of the air. Some of the effect of the charge is lost as the filters become loaded with dust, but charge stability during storage is high, with shelf lives of years being attainable. The author discusses the capture of particles by electric forces and electrically charged filter materials and their production methods. The measurement of electric charge on filters, charge stability during storage, filter performance when loaded with dust and the advantages and disadvantages of such filters, are also discussed     

Chemical noise reduction via mass spectrometry and ion/ion charge inversion: amino acids

Charge inversion ion/ion reactions can provide a significant reduction in chemical noise associated with mass spectra derived from complex mixtures for species composed of both acidic and basic sites, provided the ions derived from the matrix largely undergo neutralization. Amino acids constitute an important class of amphoteric compounds that undergo relatively efficient charge inversion. Precipitated plasma constitutes a relatively complex biological matrix that yields detectable signals at essentially every mass-to-charge value over a wide range. This chemical noise can be dramatically reduced using multiply charged reagent ions that can invert the charge of species amenable to the transfer of multiple charges upon a single interaction and by detecting product ions of opposite polarity. The principle is illustrated here with amino acids present in precipitated plasma subjected to ionization in the positive mode, reaction with anions derived from negative nanoelectrospray ionization of poly (amido amine) dendrimer generation 3.5, and mass analysis in the negative ion mode.     

Development of a Compact Electrostatic Nanoparticle Sampler for Offline Aerosol Characterization

A compact electrostatic nanoparticle sampler has been developed to support the offline analysis of nanoparticles via electron microscopy. The basic operational principle of the sampler is to electrically charge particles by mixing nanoparticles and unipolar ions produced by DC corona discharge, and electrostatically collecting charged particles. A parametric study was first performed to identify the optimal operating condition of the sampler: a total flow rate (i.e., the sum of the particle and ion carrier flow rates) of 1.0 lpm, an aerosol/ion carrier flow rate ratio of 1.0, and a collection voltage of 4.5 kV. Under the above condition, the sampler achieved a collection efficiency of more than 90 % for particles ranging from 50 to 500 nm. The effect of particle material on the sampler’s performance was also studied. The prototype had lower collection efficiencies for oleic acid particles than for sodium chloride particles in the size range from 50 to 150 nm, while achieving a comparable efficiency in the size range large than 150 nm. Effects of particle diameter, particle material, and total flow rate on the sampler’s collection efficiency are explained by the particle charging data, i.e., charging efficiencies and average charges per particle.     

Mass spectrometry of acoustically levitated droplets

Containerless sample handling techniques such as acoustic levitation offer potential advantages for mass spectrometry, by eliminating surfaces where undesired adsorption/desorption processes can occur. In addition, they provide a unique opportunity to study fundamental aspects of the ionization process as well as phenomena occurring at the air-droplet interface. Realizing these advantages is contingent, however, upon being able to effectively interface levitated droplets with a mass spectrometer, a challenging task that is addressed in this report. We have employed a newly developed charge and matrix-assisted laser desorption/ionization (CALDI) technique to obtain mass spectra from a 5-microL acoustically levitated droplet containing peptides and an ionic matrix. A four-ring electrostatic lens is used in conjunction with a corona needle to produce bursts of corona ions and to direct those ions toward the droplet, resulting in droplet charging. Analyte ions are produced from the droplet by a 337-nm laser pulse and detected by an atmospheric sampling mass spectrometer. The ion generation and extraction cycle is repeated at 20 Hz, the maximum operating frequency of the laser employed. It is shown in delayed ion extraction experiments that both positive and negative ions are produced, behavior similar to that observed for atmospheric pressure matrix-assisted laser absorption/ionization. No ion signal is observed in the absence of droplet charging. It is likely, although not yet proven, that the role of the droplet charging is to increase the strength of the electric field at the surface of the droplet, reducing charge recombination after ion desorption.     

用于微机电系统的SiO2/Si3N4驻极体的制备及电荷稳定性

驻极体微型发电机是近期提出的微电子机械系统开发中的一个新领域,驻极体电荷稳定性则是影响驻极体微型发电机性能的关键.用等离子体增强化学气相沉积(PECVD)方法制备SiO2/ Si3N4双层膜,采用电晕充电和热极化方法对材料进行注极形成驻极体,探讨了器件加工工艺及存储环境对双层膜驻极体电荷稳定性的影响.结果表明,电晕充电后SiO2/ Si3N4双层膜的电荷存储稳定性明显优于SiO2单层膜;传统的电晕注极方法仅适用于大面积驻极体的制备,但对微米量级的材料表面不适用;微器件制备的工艺流程对驻极体电荷稳定性有显著影响,但存储环境对热极化驻极体电荷稳定性的影响很小.