Electrospray Aerosol Generator Research Articles

Enhancing electrospray ionization efficiency forparticle transmission through an aerodynamic lens stack.

This work investigates the performance of the electrospray aerosol generator at the European X-ray Free Electron Laser (EuXFEL). This generator is, together with an aerodynamic lens stack that transports the particles into the X-ray interaction vacuum chamber, the method of choice to deliver particles for single-particle coherent diffractive imaging (SPI) experiments at the EuXFEL. For these experiments to be successful, it is necessary to achieve high transmission of particles from solution into the vacuum interaction region. Particle transmission is highly dependent on efficient neutralization of the charged aerosol generated by the electrospray mechanism as well as the geometry in the vicinity of the Taylor cone. We report absolute particle transmission values for different neutralizers and geometries while keeping the conditions suitable for SPI experiments. Our findings reveal that a vacuum ultraviolet ionizer demonstrates a transmission efficiency approximately seven times greater than the soft X-ray ionizer used previously. Combined with an optimized orifice size on the counter electrode, we achieve >40% particle transmission from solution into the X-ray interaction region. These findings offer valuable insights for optimizing electrospray aerosol generator configurations and data rates for SPI experiments.

Experimental study of turbulent transport of nanoparticles in convective turbulence

We perform an experimental study of transport of nanoparticles in convective turbulence with the Rayleigh number ∼108 in the air flow. We measure a temperature field in many locations by a temperature probe equipped with 11 E-thermocouples. Nanoparticles of the size ∼70 nm in diameter are produced by an advanced electrospray aerosol generator. To determine the number density of nanoparticles, we use a condensation particle counter. The joint action of turbulent effects and the large-scale circulations in convective turbulence, which are important in the core flow, and molecular effects, which are essential near the boundaries of the chamber, results in an effective accumulation of nanoparticles at the cold wall of the chamber. The turbulent effects are characterized by turbulent diffusion and turbulent thermal diffusion of nanoparticles, while the molecular effects are described by the Brownian diffusion and thermophoresis, as well as the adhesion of nanoparticles at the cold wall of the chamber. In different experiments in convective turbulence in a chamber with the temperature difference ΔT between the bottom and top walls varying between ΔT=29 and ΔT=61 K, we find that the mean number density of nanoparticles decreases exponentially in time. For instance, the characteristic decay time of the mean number density of nanoparticles varies from 12.8 min for ΔT=61 K to 24 min for ΔT=29 K. For better understanding of experimental results, we perform one-dimensional mean-field numerical simulations of the evolution of the mean number density of nanoparticles for conditions pertinent to the laboratory experiments. The obtained numerical results are in agreement with the experimental results.

Optimizing singly-charged electrosprayed particle throughput of an electrospray aerosol generator utilizing a corona-based charger

Optimizing singly-charged electrosprayed particle throughput of an electrospray aerosol generator utilizing a corona-based charger

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.

Evaluation of the NanoAerosol Generator by Kanomax FMT Inc. in Aerosolization of Size Standard Nanoparticle and Protein

A new nebulizer called NanoAerosol Generator (NAG) by Kanomax FMT, Inc. was evaluated experimentally in their performance in aerosolizing sub-100 nm particles and sub-10 nm protein molecules which cannot be aerosolized with conventional compressed-air jet nebulizers due to the interference by a number of residue particles. For the challenge particles, 29 nm-polystyrene latex (PSL) particles and bovine serum albumin (BSA) protein molecules were used as sub-100 nm and sub-10 nm test materials, respectively. For both particles, the aerosolization by the NAG was successful with a clear separation between the peak of the challenge particles and the peak of residual particles in the size distribution spectra. The size distributions of aerosolized 29 nm-PSL particles were compared between the NAG and an electrospray aerosol generator (EAG) with liquid samples of the same particle concentration. The EAG showed clearer separation of the PSL peak from the residual peak; on the other hand, the peak height was taller for the NAG by a factor of 6 than for the EAG, which indicated the generation of airborne particles from a liquid suspension/solution of the same concentration was more efficient for the NAG than for the EAG.

Non-invasive aerosol delivery and transport of gold nanoparticles to the brain

Targeted delivery of nanoscale carriers containing packaged payloads to the central nervous system has potential use in many diagnostic and therapeutic applications. Moreover, understanding of the bio-interactions of the engineered nanoparticles used for tissue-specific delivery by non-invasive delivery approaches are also of paramount interest. Here, we have examined this issue systematically in a relatively simple invertebrate model using insects. We synthesized 5 nm, positively charged gold nanoparticles (AuNPs) and targeted their delivery using the electrospray aerosol generator. Our results revealed that after the exposure of synthesized aerosol to the insect antenna, AuNPs reached the brain within an hour. Nanoparticle accumulation in the brain increased linearly with the exposure time. Notably, electrophysiological recordings from neurons in the insect brain several hours after exposure did not show any significant alterations in their spontaneous and odor-evoked spiking properties. Taken together, our findings reveal that aerosolized delivery of nanoparticles can be an effective non-invasive approach for delivering nanoparticles to the brain, and also presents an approach to monitor the short-term nano-biointeractions.

An electrospray aerosol generator with X-ray photoionizer for particle charge reduction

A prototype single-capillary electrospray (ES) aerosol generator having a soft X-ray photoionizer for charge reduction has been constructed and its performance under various configurations has been investigated in this study. The prototype essentially consists of spray and charge reduction chambers having an orifice disk plate as the partition. Two soft X-ray irradiation directions (i.e., 90° and 180° relative to the spray direction) and two disk plates (with the orifice diameters of 0.25in. and 1.25in.) were tested for the prototype configuration. In the investigation, the spray current as a function of ES voltage was recorded. It is shown that the presence of soft X-ray irradiation in the close proximity of the electrospray process resulted in the increase of applied voltage needed to operate the spray while the spray current at the cone-jet mode remained unchanged. The fluorescence analysis was applied in this study to quantify the transmission efficiency and the charge fractions of particles exiting from the prototype generator. A maximal 47% transmission efficiency could be achieved by the prototype having the setup of 180° irradiation and use of 0.25in. D-orifice plate, and operated at carrier gas flow rate of 8.0lpm. Under the above setup and operation the charge fraction of particles with the sizes ranging from 30 to 300nm was further characterized. It is found that the measured charge fractions of particles exiting from the generator is in good agreement with the calculated Fuchs bipolar charge factions for the cases having the particle sizes less than 110nm. The deviation from the Fuchs charge fraction increases as the particle size increases. It may be because (1) more ions were needed to reduce the charge level on particles of large sizes; (2) spatial non-uniformity of aerosol concentration and ions in charge reduction chamber; and (3) because of the direct photoionization of particles under the X-ray irradiation. At last, the size measurement of ES particles via a scanning mobility particle sizer (SMPS) shows the good quality of monodispersity for particles generated by the prototype when operated at the cone-jet mode.

Enhancement of Surfactants in Nanoparticles Produced by an Electrospray Aerosol Generator

Electrospray aerosol generators (EAGs) disperse conducting solutions into air, promptly neutralize the particles to remove the excess charge, and evaporate the residual solvent with a dry air flow. For solutions containing multiple solutes, the particles may become enhanced in the more surface-active solutes. The extent of the enhancement was estimated for nanoparticles electrosprayed from a solution containing NaCl and surfactant sodium dodecyl sulfate (SDS) mixed in a 9:1 weight ratio. A tandem particle mobility analyzer was used to quantify the hygroscopic growth factor (GF). The relative fractions of NaCl and SDS in the particles were estimated from the measured GFs assuming that NaCl and SDS take up water independently of each other. The nanoparticles were considerably enhanced in SDS relative to the starting solution, with the NaCl:SDS weight ratio increasing with the distance from the EAG electrified capillary tip to the neutralizer, and reaching ∼1:1 at the longest distances probed. The enhancemen...

A New Electrospray Aerosol Generator with High Particle Transmission Efficiency

A new single-capillary electrospray (ES) aerosol generator has been developed for monodisperse particle production with maximal transmission efficiency. The new generator consists of both a spray chamber in a point-to-orifice-plate configuration and a charge reduction chamber that can hold up to 4 Nuclespot ionizers (Model P-2042, NRD Inc.). The 2 chambers are partitioned by an orifice plate. To optimize the particle transmission efficiency of the prototype, a systematic study was performed on the generator by varying the system setup and operation. Two key dimensions of the generator setup, the orifice diameter and the distance from the capillary tip to the orifice plate, were varied. Fluorescence analysis was applied to characterize the loss of ES-generated particles at different locations of the prototype. It was found that particle loss in the generator could be reduced by either increasing the orifice diameter or decreasing the distance between the capillary tip and the orifice plate. Increasing either the total radioactivity of the ionizers or the flowrate of the particle carrier gas also further decreased the particle loss in the system. The maximum particle transmission efficiency of 88.0% was obtained with the spray chamber fully opened to the charge reduction chamber, the capillary tip at the same level as the orifice plate, and 4 bipolar ionizers installed.

Design, demonstration and performance of a versatile electrospray aerosol generator fornanomaterial research and applications

Carbon nanotubes are difficult to aerosolize in a controlled manner. We present a methodfor generating aerosols not only of carbon nanotubes, but also of many referenceand proprietary materials including quantum dots, diesel particulate matter,urban dust, and their mixtures, using electrospraying. This method can be usedas a teaching tool, or as the starting point for advanced research, or to delivernanomaterials in animal exposure studies. This electrospray system generates180 µg of nanotubesper m3 of carrier gas, and thus aerosolizes an occupationally relevant mass concentrationof nanotubes. The efficiency achievable for single-walled carbon nanotubes is9.4%. This system is simple and quick to construct using ordinary lab techniquesand affordable materials. Since it is easy to replace soiled parts with clean ones,experiments on different types of nanomaterial can be performed back to backwithout contamination from previous experiments. In this paper, the design,fabrication, operation and characterization of our versatile electrospray method arepresented. Also, the morphological changes that carbon nanotubes undergo asthey make the transition from dry powders to aerosol particles are presented.

Measurement of Retention Efficiency of Filters against Nanoparticles in Liquids using an Aerosolization Technique

An aerosolization technique has been developed to measure liquid-borne nanoparticles down to 30 nm and applied to evaluate retention efficiencies of liquid filters. This technique involves dispersing nanoparticle suspensions into air-borne form and measuring the size and concentration by a differential mobility analyzer coupled to a condensation particle counter. Polystyrene latex particles larger and smaller than 70 nm in diameter were dispersed by a constant output atomizer, COA, and an electrospray aerosol generator, ES, respectively, to avoid the interference from residue particles. With the ES, residue particles can be controlled to be less than 10 nm, allowing latex particles as small as 30 nm to be clearly distinguished from the size distribution measurements. Calibrations with 30, 50, 125, and 200 nm latex particles showed that liquid-borne and air-borne particle concentrations are proportionally related. This provides an effective way to quantify liquid-borne particles as small as 30 nm, which cannot be analyzed by state-of-the-art liquid particle counters. An application of this technique is to evaluate the nanoparticle retention performance of liquid filters. Both 200 and 400 nm rated Nuclepore filters were challenged with latex particles of different sizes, and retention efficiency as a function of particle size was determined by comparing the particle concentrations upstream and downstream of the tested filters. The results are comparable with the nominal pore size stated by the manufacturer if sieving is the dominant filtration mechanism and demonstrate the feasibility of using the aerosolization technique to evaluate the retention efficiency of filters against nanoparticles in liquids.

Calibration of a Condensation Particle Counter Using a NIST Traceable Method

This work presents a calibration of a commercial condensation particle counter using National Institute of Standards and Technology (NIST) traceable methods. By the nature of the metrology involved, this work also compares the measurement results of three independent techniques for measuring aerosol concentration: continuous flow condensation particle counter (CPC); aerosol electrometer (AE); and the aerosol concentration derived from microscopic particle counting. Because of the transient nature of aerosol, there are no concentration artifact standards such as exist for particle diameter standards. We employ a mobility classifier to produce a nearly monodisperse, 80 nm, polystyrene latex aerosol. The test aerosol is used as a challenge for the CPC and the AE, and is subsequently filter sampled for electron microscopy. Our test stand design incorporates a continuous CPC aerosol concentration monitor to verify the aerosol stability. The CPC determines particle concentration by single particle counting at a constant sample flow rate. The AE has been calibrated to a NIST traceable current standard. The subsequent aerosol concentration measurement is obtained by determining the electrical current produced by a charged aerosol transported to the detector by a controlled aerosol flow rate. We have NIST traceability for flow rates for all methods and a methodology to calibrate the AE to NIST traceable electrical standards. The latter provides a calibration and a determination of the uncertainty in the aerosol derived current measurement. A bias in the measurements due to multiple charged particles was observed and overcome by using an electrospray aerosol generator to produce the challenge particles. This generator was able to produce aerosol concentrations over the range of 100 particles/cm 3 to 15 000 particles/cm 3 with lower number of dimer particles (≈1%). In our work, independent measurement of aerosol concentration is obtained by quantitatively collecting samples of the airborne polystyrene latex spheres on a small pore filter material and determining the number of particles collected by electron microscopy. Electron micrograph images obtained using a field-emission scanning electron microscope are analyzed using particle counting. We found the relative uncertainty in the aerosol electrometer measurements to be in excess of 100% for particle concentrations of approximately 120 particles/cm 3 and approximately 5% for concentrations above 6000 particles/cm 3 . The uncertainty found by the microscopy method was approximately 3%.

Real-time detection of airborne viruses on a mass-sensitive device.

We present real-time detection of airborne Vaccinia viruses using quartz crystal microbalance (QCM) in an integrated manner. Vaccinia viruses were aerosolized and neutralized using an electrospray aerosol generator, transported into the QCM chamber, and captured by a QCM crystal. The capture of the viruses on the QCM crystal resulted in frequency shifts proportional to the number of viruses. The capture rate varied linearly with the concentration of initial virus suspensions (8.5x10(8)-8.5x10(10) particlesml) at flow rates of 2.0 and 1.1 lmin. This work demonstrates the general potential of mass sensitive detection of nanoscale biological entities in air.

Collection efficiency of fiber filters operating on the removal of nano-sized aerosol particles: I—Homogeneous fibers

The collection efficiency of a fiber filter of polyester operating in the removal of nano-sized aerosols was studied here. The polyester filter utilized had 0.882 in porosity, 4.5 mm in thickness, and fiber mean diameter of 16.0 μm. A TSI Electrospray Aerosol Generator, model 3480, was used to generate monodispersed nanometric particles, with sizes varying from 8.5 to 94.8 nm. The nanoparticles were generated from solutions of NaCl which were atomized as droplets, followed by drying. The filtration device was built in acrylic with a filtration area of 0.4 m 2. The filter efficiencies were obtained by counting the particles before and after the filter, using a TSI Condensation Particle Counter, model 3007. The tests were accomplished at gas superficial velocities varying from 0.03 to 0.25 m/s. The results show the efficiency decreasing with increasing particle size, indicating the predominance of the diffusional mechanism in the range studied. Also, the filter efficiency decreased with filtration velocity. The results were compared to well known theoretical predictions based on the classical collection mechanisms: diffusional and direct interception. The comparison of the calculated collector efficiency with the experimental results showed that the predictions underestimate the results. A correction was proposed to the existing correlation for direct interception, where an ‘effective’ diameter accounting for the Brownian motion of the particle while in the vicinity of the collector surface was accounted for. The proposed modification included no fitting parameter. The results showed a considerable improvement in the prediction correlation.

Collection efficiency of fiber filters operating on the removal of nano-sized aerosol particles: II. Heterogeneous fibers

The collection efficiency of a fiber filter of cellulose (HEPA) operating in the removal of nano-sized aerosols was studied here. The filter utilized had 0.920 in porosity, 0.4 mm in thickness, and fiber median diameter of 0.45 μm. A TSI electrospray aerosol generator, model 3480, was used to generate monodispersed nanometric particles, with sizes varying from 8.5 to 94.8 nm. The nanoparticles were generated from solutions of NaCl which were atomized as droplets, followed by drying. The filtration device was built in acrylic with a filtration area of 0.4 m 2. The filter efficiencies were obtained by counting the particles before and after the filter, using a TSI condensation particle counter, model 3007. The tests were accomplished at gas superficial velocities varying from 0.03 to 0.25 m/s. The results show that the efficiency decrease with increasing particle size, indicating the predominance of the diffusional mechanism in the range studied. Also, the filter efficiency decreased with filtration velocity. The results were compared to theoretical predictions from the literature based on the classical collection mechanisms: diffusional and direct interception. The comparison of the calculated collector efficiency with the experimental results showed that the predictions underestimate the results. A correction was proposed to the existing correlation by defining the filter bed as composed of a number of filters in series, each with the fiber diameter and porosity derived from the fiber size distribution. The proposed modification included no fitting parameter. The results showed a considerable improvement in the prediction correlation.

Effects of charge and size on condensation of supersaturated water vapor on nanoparticles of SiO2

The effects of size and charge on the condensation of a supersaturated water vapor on monodisperse nanoparticles of SiO(2) were investigated in a flow cloud chamber. The dependences of the critical supersaturation S(cr) on particle size at diameters of 10, 12, and 15 nm as well as on charge and charge polarity are determined experimentally. A novel electrospray aerosol generator was developed to generate a high concentration of SiO(2) nanoparticles of less than 10 nm by electrospraying silicon tetraethoxide (STE) ethanol solution followed by the thermal decomposition of STE. The effects of liquid flow rate, liquid concentration, flow rate of carrier gas, and liquid conductivity on the particle size distribution and concentration were examined. For charged particles, the nucleation occurs at a critical supersaturation S(cr) lower than that on neutral particles, and the charge effect fades away as particle size increases. The charge effect is stronger than the theoretical predictions. In addition, a sign preference is detected, i.e., water vapor condenses more readily on negatively charged particle, a trend consistent with those observed on ions. However, both effects of charge and charge polarity on S(cr) are stronger than that predicted by Volmer's theory for ion-induced nucleation.

Size measurements of gasborne poly(amidoamine) (PAMAM) dendrimers using a differential mobility analyzer (DMA)

Poly(amidoamine) (PAMAM) dendrimers generations 2 through 6 were aerosolized by an electrospray aerosol generator and their electrical mobility spectra observed using a differential mobility analyzer (DMA). We succeeded in measuring the electrical mobilities of the monomer and dimer ions of these dendrimers. The experimental results suggested that PAMAM dendrimers are useful as standard particles in the range of a few nanometers.

Measurement of 100 nm and 60 nm Particle Standards by Differential Mobility Analysis.

The peak particle size and expanded uncertainties (95 % confidence interval) for two new particle calibration standards are measured as 101.8 nm ± 1.1 nm and 60.39 nm ± 0.63 nm. The particle samples are polystyrene spheres suspended in filtered, deionized water at a mass fraction of about 0.5 %. The size distribution measurements of aerosolized particles are made using a differential mobility analyzer (DMA) system calibrated using SRM® 1963 (100.7 nm polystyrene spheres). An electrospray aerosol generator was used for generating the 60 nm aerosol to almost eliminate the generation of multiply charged dimers and trimers and to minimize the effect of non-volatile contaminants increasing the particle size. The testing for the homogeneity of the samples and for the presence of multimers using dynamic light scattering is described. The use of the transfer function integral in the calibration of the DMA is shown to reduce the uncertainty in the measurement of the peak particle size compared to the approach based on the peak in the concentration vs. voltage distribution. A modified aerosol/sheath inlet, recirculating sheath flow, a high ratio of sheath flow to the aerosol flow, and accurate pressure, temperature, and voltage measurements have increased the resolution and accuracy of the measurements. A significant consideration in the uncertainty analysis was the correlation between the slip correction of the calibration particle and the measured particle. Including the correlation reduced the expanded uncertainty from approximately 1.8 % of the particle size to about 1.0 %. The effect of non-volatile contaminants in the polystyrene suspensions on the peak particle size and the uncertainty in the size is determined. The full size distributions for both the 60 nm and 100 nm spheres are tabulated and selected mean sizes including the number mean diameter and the dynamic light scattering mean diameter are computed. The use of these particles for calibrating DMAs and for making deposition standards to be used with surface scanning inspection systems is discussed.

Capture of airborne nanoparticles in swirling flows using non-uniform electrostatic fields for bio-sensor applications

Collection of biological particles is the first and critical step for any biological agent detection system. Towards our goal of capturing and detecting airborne biological entities in real time, here we investigate on the design of an electrostatic particle capture system. We report on the capture of airborne 100 nm diameter polystyrene nanoparticles as a model system, in swirling flows under non-uniform electrostatic fields with an electrospray aerosol generator and a homemade particle collector. The particle collector has five positive electrodes on the bottom and one large grounded electrode on the top. The nanoparticles coming into the collector were slowed down during their swirling and stayed in the collector long before leaving the collector. Silicon chips were placed on the bottom electrodes and the electrostatically captured particles were counted as a function of flow rates, electrode positions, bias voltages, and capture times by epifluorescent images and scanning electron micrographs (SEMs). Particles captured in the electrode at the center of the collector were much less than those on the surrounding four electrodes and 10–25% of the particles with negative charges entering the collector were captured on the bottom electrodes at a flow rate of 1.1 l/min and an applied potential of 2 kV. Particle capture increased with decreasing flow rates. We also simulated flow and electrical fields separately, and found the positional trends to be in good agreement with the measurements. This collector is well adaptable to integration with micro resonator devices and can be used for real-time monitoring of bioaerosols.

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.