Simultaneous Elemental Composition and Size Distributions of Submicron Particles in Real Time Using Laser Atomization Ionization Mass Spectrometry

Composition and size of individual submicron particles have been measured using a laser atomization ionization mass spectrometry technique, the Particle Blaster. Individual particles are quantitatively converted to atomic cations, providing information on both their complete elemental composition and particle size. Measured average atomic ratios for 100 nm particles of sodium chloride is 1.12 +- 0.36 (Cl:Na), for 50 nm particles of silica is 1.93 +- 0.52 (O:Si), and for 64 nm polystyrene latex spheres (PSL) is 1.13 +- 0.19 (H:C), in excellent agreement with the empirical formulae. Calculated particle sizes agree well with electrostatic classifier or TEM measurements in the size range of 17-900 nm diameter for particles of sodium chloride, silicon, and PSL. Size distributions are also obtain able, giving narrower distributions than are measured with an electrostatic classifier, for particles of alumina, silica, sodium chloride, and PSL spheres. Comparison with TEM data shows comparable primary particle sizes, but numerous particle aggregates are detected by the Particle Blaster which are unreported by the TEM measurements.     

An Electrostatic Lens for Focusing Charged Particles in a Mass Spectrometer

The ability to transmit particles into the ablation region of an aerosol mass spectrometer determines in part the lower size limit for particles that can be analyzed. A large fraction of small particles (< 100 nm) are lost due to processes such as Brownian diffusion that broaden the particle beam. In this work, electrostatic focusing is used to overcome the limits of aerodynamic focusing in the analysis of nanometer-sized particles by aerosol mass spectrometry. A simple tube lens is used to focus charged particles into the ablation laser beam path. The diameter of the focused beam is smaller than the fundamental aerodynamic limit imposed by Brownian motion. Measured enhancements of the hit rate for particles between 21 and 33 nm diameter are between 3 and 6. These values are lower limits for the true enhancements. The lens is also energy selective and can be used to select the mass (size) of the particles being analyzed.     

Performance of a Single Ultrafine Particle Mass Spectrometer

We report on the performance of a rapid single particle mass spectrometer (RSMS-II), designed to obtain the size and composition of individual ultrafine particles. Particles are sized aerodynamically at the inlet using a dynamic focusing mechanism to transmit particles to the source region of a time-of-flight mass spectrometer. Since the target particles are too small to be detected optically, an excimer laser is pulsed at high frequency so that data are acquired only when a particle coincides with a laser pulse within the source region. The instrument is tested with sodium chloride and oleic acid particle standards of various sizes and the hit rate efficiency is monitored as the normalized number of particle hits per unit time. The hit rate efficiency depends on the particle flux through the active region of the laser beam, in addition to the particle size and composition, and may thus be used to determine the relative transmission efficiency and size selectivity of the inlet.     

Understanding the Interaction of an Intense Laser Pulse with Nanoparticles: Application to the Quantification of Single Particle Mass Spectrometry

Understanding the characteristic behavior of ions produced from the interaction of a high energy laser pulse with nanoparticles is essential for quantitative determination of composition and size of nanoparticles from single particle mass spectrometry (SPMS). We employed a one-dimensional hydrodynamic model, where the laser field is coupled to the non-equilibrium time-dependent plasma hydrodynamics of the heated particles. We focus on regimes of laser width from 0.01 ns to 10 ns (532 nm wavelength, 100 mJ/pulse) and particle size (20–400 nm in diameter) most relevant to commonly used SPMS, and determine the properties of ions generated during the interaction with a strong laser pulse. We compare the simulation results with experiments conducted on aluminum nanoparticles.

The laser-particle interaction is separated into a “soft heating” regime followed by a hydrodynamic expansion. Simulation results showed that the ablation/ionization is effectively complete well before the laser ever reaches its peak intensity. As the pulse width decreased for a given pulse energy, the kinetic energy of ions increased, suggesting that too short a pulse laser (i.e., high laser intensity) would be undesirable because higher energetic ions lead to lower detection efficiency in the SPMS. Results also show that for particle sizes in the range of 100 nm ～ 400 nm, as particle size increased, the kinetic energy of ions produced from the particle increased with a power law relationship, consistent with experiment. Lastly our simulations indicated that ions from the surface of the particle are of higher energy, and therefore have lower detection efficiency.     

Comparison of Two Aerodynamic Lenses as an Inlet for a Single Particle Laser Ablation Mass Spectrometer

By means of a newly designed portable aerosol mass spectrometer SPLAT (Single Particle Laser Ablation Time-of-flight mass spectrometer) for the analysis of single atmospheric aerosol particles we investigated the system performance in dependency on two different aerodynamic lenses (Liu and Schreiner type) capable of focusing particles with diameters ranging from 80 nm to 800 nm and 300 nm to 3000 nm, respectively. By using the pressure regulated Schreiner lens, the instrument is independent of variations in atmospheric pressure which would lead to changing dynamical properties of the aerosol particles. Active pressure control inside the inlet system facilitates airborne measurements without complicated corrections. With the Liu setup no pressure regulation was used. Here the overall efficiency of our instrument was 7% while with the Schreiner setup 2% was achieved. The Liu lens setup is optimal for measuring submicron particles at low particle concentrations. To detect supermicron particles the Schreiner lens setup is favored. Together with these experiments we present key details of the SPLAT setup and its characterization. Our instrument is able to measure simultaneously the size and the chemical composition of individual aerosol particles larger than 300 nm in diameter. It uses forward scattered light of single aerosol particles at two positions to determine their vacuum aerodynamic diameter from the flight time between the two lasers. Chemical analysis of the particles is done by laser ablation mass spectrometry utilizing a bipolar time-of-flight mass spectrometer.     

A High Resolution Study of the Effect of Morphology On the Mass Spectra of Single PSL Particles with Na-containing Layers and Nodules

The interpretation and quantification of measurements of particle composition by laser ablation based single particle mass spectrometry is complex. Among the most difficult systems to quantify are internally mixed particles containing alkali metals and organics. The alkali atoms in such particles tend to suppress the formation of other ions sometimes to below the detection limit. Here we present a study of the behavior of single particle mass spectral peak intensities as a function of the amount of the sodium containing compounds deposited on the surface of 240 nm polystyrene latex (PSL) spheres. We generate three morphologically distinct and well defined coating types: uniform layers, cubic nodules and rounded nodules, and measure the individual particle mass spectra as a function of the vacuum aerodynamic diameter with nanometer resolution. The data show that the probability of detecting the PSL spheres depends on the amount of the alkali metal on the PSL sphere surface, its morphological distribution and the ablation laser power. The data suggest that PSL spheres with localized Na-containing nodules are easier to detect than those which are completely encapsulated. We show, for example, that at low laser power, PSL particles that are completely encapsulated with Na-containing compounds, whose weight fraction is close to 50%, cannot be detected, while 35% of PSL spheres with same amount of coating can be detected if coating is localized in nodules on a fraction of the particle surface.     

Performance Evaluation of an Improved Particle Size Magnifier (PSM) for Single Nanoparticle Detection

Several modifications of the particle size magnifier (PSM) developed by Okuyama et al. have been introduced recently for detection of particles at diameters of 1 nm and below. However, their evaluation has been incomplete. Here we provide the first direct measurements of counting efficiencies near unity below 2 nm. We use the modified PSM described by Sgro and Fernández de la Mora, which separates thermally the PSM's original vapor generator from the water-cooled growth chamber by means of a narrow and short T where turbulent mixing with the aerosol takes place. The counting efficiency is seen to depend greatly on the aerosol flow, the amount of vapor, and temperature. With ethylene glycol vapor, under optimal conditions, the counting efficiency is 100% down to 1.6 nm (actual diameter of 1.2 nm), and negative particles are more easily activated than positive particles. The improved PSM is applied to the measurement of gold nanoparticle size distributions, and the results show it is a powerful aerosol detector for nanoparticles.     

COUNTING EFFICIENCY OF THE API AEROSIZER

The Aerosizer (Amherst Process Instruments, Inc. Hadley MA) is a time-of-flight instrument frequently used to measure the size distribution of an aerosol. However, if the Aerosizer’s counting efficiency, defined as the number of particles counted divided by the total number entering the instrument, is not 100% or varies with particle size, the resulting size distribution will be inaccurate.Experiments were conducted to determine the effect of particle diameter, particle concentration, photomultiplier tube (PMT) voltage, and model type on the Aerosizer’s counting efficiency. To calculate counting efficiency, the number of particles between 0.3 and 10 μm recorded by the Aerosizer was divided by the number of particles of the same size collected on each stage of a cascade impactor.Particle diameter, aerosol concentration, Aerosizer model, PMT voltage, and the diameter interaction terms influenced counting efficiency. Counting efficiencies were less than 1% for particles smaller than 0.45 μm, and more than 100% for particles larger than 7 μm. Increasing the PMT voltage increased the counting efficiency for the smaller particles, but also created false, larger particles. Counting efficiency decreased as count rate increased for count rates greater than 20,000 particles per second. The Aerosizer LD counted particles more efficiently than the Aerosizer Mach 2 because of improved laser and optics systems. Four regression models that relate counting efficiency to the salient operating parameters were developed, one for each combination of Aerosizer model and photomultiplier tube voltage studied.     

Continuous Measurements of Ambient Particle Deposition in Human Subjects

The total deposition fraction (TDF) of fine and ultrafine aerosols was measured in a group of six healthy adults exposed to polydisperse ambient aerosols in Boston. Fifteen repeated inhalation-exhalation cycles were conducted during a given exposure session. Deposition efficiency for particles with aerodynamic diameter ranging from 63.5 to 2045 nm was determined using the average concentration of inhaled and exhaled particles measured during these cycles. Deposition efficiencies ranged from 7.3±18.7%(240-275 nm) to 98.6±28.1%(1545-2045 nm). Subjects exhibited similar deposition patterns with minimum efficiencies between 200-400 nm. Results from ANOVA and mixed-model regression analyses showed significant differences (p < 0.05) in particle deposition efficiency by particle size as well as among the subjects. Deposition efficiencies varied most among the subjects for particles between 100 and 1000 nm in size. A comparison with the ICRP model showed good agreement, with best agreement for male subjects and particle sizes <400 nm.     

Removing 20 nm PSL Particles Using a Supersonic Nano-particle Beam

Abstract

Cryogenic particle beam is an effective means of removing nano-sized contaminant particles from a substrate. Based on the previous finding that a smaller bullet size and a higher velocity are more effective for removing contaminant particles with a higher adhesion pressure, a new technique of generating bullet particle beam with novel properties required for removing polystyrene latex (PSL) particles—smaller particle size moving at a higher velocity — was developed, using Ar/He mixture gas and a Laval nozzle of special design. Particles of 10 nm size range — smaller by a factor of 100 than the conventional argon aerosols — were successfully generated. Particles generated at optimum pressure and temperature conditions could perfectly remove PSL particles down to 20 nm. Unlike the case of ceramic particles, cleaning performance was found to be dependent on the direction of wafer surface during drying.     

Achieving Size Independent Hit-Rate in Single Particle Mass Spectrometry

Recent improvements in single particle mass spectrometers make it possible to optically detect, size, and characterize the compositions of individual particles with diameters larger than a micron and smaller than 100 nm. In these instruments, two stages of optical detection are used to generate a precisely timed trigger pulse that is used to fire the ion generation laser or lasers. However, experience shows that the wide particle size range results in significant differences in laser trigger timing between small and large particles. If not treated these differences produce an instrument with size dependent hit-rate. In this case the operator is forced to optimize the instrument for the desired size range, while contending with a significantly lower hit-rate for other particle sizes. This article presents an analysis of the phenomenon and demonstrates that the dependence of laser trigger timing on particle size stems from the differences in the particle position within the detection laser beam at the instant of detection. We demonstrate that it is possible to compensate for these differences by generating, for each particle, a laser trigger delay coefficient that is a function of particle's time of flight, i.e., its vacuum aerodynamic size. The study also shows that a single function can be used to eliminate the size bias for particles with a wide range of densities.     

Development of laser-induced breakdown spectroscopy (LIBS) with timed ablation to improve detection efficiency

A laser-induced breakdown spectrometer (LIBS) was developed for determining the elemental composition of individual airborne particles. The system employs two lasers focused on a narrow beam of particles. A continuous wave laser placed upstream scatters light from particles, while a pulse laser downstream ablates the particles. The scattered light from the upstream laser is used to trigger the downstream pulse laser, resulting in more accurate hitting of the particles than a free-firing laser system without the triggering signal (i.e., constant pulse laser firing). Various laboratory-generated aerosols (NaCl, MgCl₂, KCl, and CaCl₂) were used to evaluate the newly developed LIBS system. Particles were tightly focused into a center line with a sheath air focusing system using an optimum aerosol-to-sheath air velocity ratio. The locations of both the scattering laser and pulse laser beams were precisely controlled by a motorized X-Y stage controller. Data showed that for the LIBS with the triggering system, the hitting efficiency (%) of particles (200–600 nm) significantly increased (e.g., 350 nm particles had more than 26 times higher hitting efficiency at 1,000 particles/cm³), and much lower limits of particle size (～200 nm) and number concentration (<100 particles/cm³) were achieved compared to the free-firing laser condition. Additionally, the hitting rate (hits/min) significantly increased with the triggering system. Our results suggest that the LIBS with the triggering system can be useful for real-time detection of elements of particles existing at low number concentrations (e.g., atmospheric particles) and for the determination of the variation of elemental composition among particles.

© 2017 American Association for Aerosol Research     

Preparation of temperature‐ and pH‐sensitive,stimuli‐responsive poly(N‐isopropylacrylamide‐co‐methacrylic acid) nanoparticles

The effects of the monomer ratio, surfactant, and crosslinker contents on the particle size and phase‐transition behavior of the copolymer poly(N‐isopropylacrylamide‐co‐methacrylic acid) (PNIPAAm–MAA) were investigated with Fourier transform infrared, differential scanning calorimetry, and dynamic laser scattering techniques. In addition to the thermoresponsive property of poly(N‐isopropylacrylamide), ionized methacrylic acid groups brought pH sensitivity to the PNIPAAm–MAA copolymer particles. The polymer particle size varied with the amounts of the monomer ratio, surfactant, and crosslinker. As the monomer ratio and crosslinker content increased and the amount of the surfactants decreased, the particle size increased. The influence of the crosslinker content on the particle size was less significant than the effect of the monomer ratio and surfactants. When the temperature increased, the particles tended to shrink and decreased in size to near or below 100 nm. Particle sizes at 20°C decreased to less than 100 nm with increased surfactant content. The control of the particle size within the 100‐nm range makes PNIPAAm–MAA copolymer particles useful for biomedical and heavy‐metal‐ion adsorption applications. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Characterization of an Automated,Water-Based Expansion Condensation Nucleus Counter for Ultrafine Particles

The design and experimental characterization of a condensation nucleus counter (CNC) is presented. The counter produces supersaturation by means of fast volume-controlled adiabatic expansion. The aerosol number concentration is derived from observing scattered laser light in the forward direction under a solid angle between 1.1° and 4.4° over the full annular sector. The number concentration is derived by application of Mie theory from the characteristic pattern in the temporal evolution of the detected signal during the droplet growth process. The equation for calculation of the aerosol number density by this method is presented. Theoretical considerations for the smallest aerosol particles that can be activated indicate a lower size cut-off between 2.5 and 3.0 nm. Model calculations of the expected Mie scatter signal during expansion agree very well with the experimental observations. The Expansion-CNC can be operated fully automated under computer (PC) control in 10-second sample cycles. For characterization it is compared with a TSI 3025A Ultrafine-CPC (TSI UCPC) for measurements of monodisperse sodium chloride and sulfuric acid aerosol particles, indicating good agreement between the two counters down to particle sizes as low as 3.5 nm under laboratory conditions. In addition, ambient aerosol measurements in urban air show excellent agreement with simultaneous TSI UCPC measurements for particle number concentrations ranging from roughly 50 cm^{? 3} to 130000 cm^{? 3}.     

Study of a Magnetic Filter System for the Characterization of Particle Magnetic Property

A magnetic filter system has been constructed, and its performance has been investigated, to measure the magnetic property of monodisperse γ -Fe ₂ O ₃ particles in the size range from 100 to 300 nm. In the system, SS 430 screens are placed in the magnetic filter element and exposed to a strong external magnetic field generated by an electric coil. The high magnetic field gradient resulted from magnetized fine wires enhances the collection of magnetic particles in addition to the particle collection via the diffusion mechanism. The particle concentrations at the upstream and downstream of the magnetic filter element were measured by an Ultrafine Condensation Particle Counter (UCPC, TSI model 3025A). Particle penetration obtained in the experiment is a function of particle size, particle magnetic property, and wire magnetization. To retrieve the magnetic property of characterized particles from the measured penetration data, a numerical model was further developed using the finite element package COMSOL Multiphysics 3.5. In this modeling, a single mesh screen is assumed to be represented by unit cells. The flow, the magnetic fields, and particle trajectory were solved in a unit cell. The relationship between particle penetration and magnetic property can then be obtained via this model for the given particle size, aerosol flowrate, and external magnetic field strength. The numerical model was first validated by comparing the experimental penetration with the simulation results for the case of 100, 150, and 250 nm γ -Fe ₂ O ₃ particles having the magnetic susceptibility characterized by Vibrating Sample Magnetometer (VSM). The magnetic susceptibilities of other sizes from 100 to 300 nm were then derived from this model according to the measured penetration data.     

Influence of Particle Shape on Filtration Processes

The influence of particle shape on filtration processes was investigated. Two types of particles, including spherical polystyrene latex (PSL) and iron oxide, and perfect cubes of magnesium oxide, were examined. It was found that the removal efficiency of spherical particles on fibrous filters is very similar for corresponding sizes within the range of 50–300 nm, regardless of the fact that the densities of PSL and iron oxide differ by a factor of five. On the other hand, the removal efficiency of magnesium oxide cubic particles was measured, and found to be much lower than the removal efficiency for the aerodynamically similar spheres. Such disparity was ascribed to the different nature of the motion of the spherical and cubic particles along the fiber surface, following the initial collision. After touching the fiber surface and before coming to rest, the spherical particles could either slide or roll compared to the cubic ones, which could either slide or tumble. During tumbling, the area of contact between the particle and the fiber changes significantly, thus affecting the bounce probability, whilst for the spheres, the area of contact remains the same for any point of the particle trajectory. The extra probability of particle bounce by the cubes was derived from the experimental data. The particle kinetic energy was proposed to be responsible for the difference in removal efficiency of particles with alternative shapes, if all other process parameters remain the same. The increase in kinetic energy is shown to favor the increase of the bounce probability.     

Wavelength-Resolved UV Photoelectric Charging Dynamics of Nanoparticles: Comparison of Spheres and Aggregates

The objective of this work is to understand the charging dynamics of metal nanoparticles under wavelength-selected UV irradiation, with a particular focus on the effect of particle structure on the quantum yield. We employed an ion mobility analysis technique to measure the size-resolved single charging efficiency of structure-controlled silver nanoparticles (spheres vs. aggregates) in the mobility diameter (D _m) range of 10 ～ 100 nm. We found that the measured particle charging efficiency follows D ² _m dependence for both spherical and aggregate particles. Based on the measured charging efficiency and calculated particle photon absorption cross section, we are also able to determine the mobility size dependence of photoelectric quantum yield for both spheres and aggregates. The quantum yield of spheres is a constant for larger particles (50 nm or larger) but significantly enhanced as particle size decreases. The quantum yield of aggregates is shown to be particle structure dependent and does not behave as a simple summation of individual primary particles. The aggregate particles have higher quantum yield compared with spheres of the same mobility size but is offset by the lower photon absorption cross section, and thus overall charging efficiency of aggregates is lower than spheres of the same mobility size.

© 2013 American Association for Aerosol Research     

Extending the Capabilities of Single Particle Mass Spectrometry: II. Measurements of Aerosol Particle Density without DMA

Particle density is an important and useful property that is difficult to measure because it usually requires two separate instruments to measure two particle attributes. As density measurements are often performed on size-classified particles, they are hampered by low particle numbers, and hence poor temporal resolution. We present here a new method for measuring particle densities using our single particle mass spectrometer, SPLAT. This method takes advantage of the fact that the detection efficiency in our single particle mass spectrometer drops off very rapidly as the particle size decreases below 100 nm creating a distinct sharp feature on the small particle side of the vacuum aerodynamic size distribution. Thus, the two quantities needed to determine particle density, the particle diameter and vacuum aerodynamic diameter, are known. We first test this method on particles of known compositions and densities to find that the densities it yields are accurate. We then apply the method to obtain the densities of particles that were characterized during instrument field deployments. We illustrate how the method can also be used to measure the density of chemically resolved particles. In addition, we present a new method to characterize the instrument detection efficiency as a function of particle size that relies on measuring the mobility and vacuum aerodynamic size distributions of polydisperse spherical particles of known density. We show that a new aerodynamic lens used in SPLAT II improves instrument performance, making it possible to detect 83 nm particles with 50% efficiency.     

An electrostatic sensor for the continuous monitoring of particulate air pollution

We developed and evaluated a particulate air pollution sensor for continuous monitoring of size resolved particle number, based on unipolar corona charging and electrostatic detection of charged aerosol particles. The sensor was evaluated experimentally using combustion aerosol with particle sizes in the range between approximately 50 nm and several microns, and particle number concentrations larger than 10¹⁰ particles/m³. Test results were very promising. It was demonstrated that the sensor can be used in detecting particle number concentrations in the range of about 2.02×10¹¹ and 1.03×10¹² particles/m³ with a response of approximately 100 ms. Good agreement was found between the developed sensor and a commercially available laser particle counter in measuring ambient PM along a roadside with heavy traffic for about 2 h. The developed sensor proved particularly useful for measuring and detecting particulate air pollution, for number concentration of particles in the range of 10⁸ to 10¹² particles/m³.     

Picobubble Enhanced Fine Coal Flotation

Abstract

Froth flotation is widely used in the coal industry to clean ?28 mesh fine coal. A successful recovery of particles by flotation depends on efficient particle‐bubble collision and attachment with minimal subsequent particle detachment from bubble. Flotation is effective in a narrow size range beyond which the flotation efficiency drops drastically. It is now known that the low flotation recovery of particles in the finest size fractions is mainly due to a low probability of bubble‐particle collision while the main reason for poor coarse particle flotation recovery is the high probability of detachment. A fundamental analysis has shown that use of picobubbles can significantly improve the flotation recovery of particles in a wide range of size by increasing the probability of collision and attachment and reducing the probability of detachment.

A specially designed column with a picobubble generator has been developed for enhanced recovery of fine coal particles. Picobubbles were produced based on the hydrodynamic cavitation principle. They are characterized by a size distribution that is mostly below 1 µm and adhere preferentially to the hydrophobic surfaces. The presence of picobubbles increases the probability of collision and attachment and decreases the probability of detachment, thus enhancing flotation recovery. Experimental results with the Coalberg seam coal in West Virginia, U.S.A. have shown that the use of picobubbles in a 2″ column flotation increased fine coal recovery by 10–30%, depending on the feed rate, collector dosage, and other flotation conditions. Picobubbles also acted as a secondary collector and reduced the collector dosage by one third to one half.