SPLAT II: An Aircraft Compatible,Ultra-Sensitive,High Precision Instrument for In-Situ Characterization of the Size and Composition of Fine and Ultrafine Particles

This article describes SPLAT II, a new aircraft compatible single particle mass spectrometer that provides significantly improved performance when compared with SPLAT, or any other existing single particle mass spectrometer. SPLAT II detects and characterizes 100% of spherical particles and ～30% of aspherical particles with diameters between 125 nm and 600 nm. It also brings significant increase in temporal resolution, sizing over 500 particles per second, while characterizing the composition of up to 100 of them. The increase in sensitivity to small particles makes it possible, under most conditions, to use a differential mobility analyzer upfront SPLAT II in order to simultaneously measure in addition to individual particle size and composition, a number of other particle attributes, such as density or effective density, dynamic shape factor, fractal dimension, and even hygroscopicity. SPLAT II provides sizing precision on the order of a monolayer, and makes it possible to distinguish between spherical and aspherical particles. SPLAT II uses a two-step, two-laser process to generate ions. The mass spectra of the semivolatile fraction are generated by ionization in the gas phase, reducing fragmentation and yielding highly reproducible mass spectra, while the mass spectra of the refractory fraction are simultaneously generated by ablation. The instrument control board generates size-dependent delays for laser triggers to eliminate a size-dependent hit-rate and mass spectra are recorded with 14-bit resolution. Data analysis is facilitated by SpectraMiner and ClusterSculptor; two visually driven software packages specifically designed to analyze the large datasets that this instrument produces.     

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.     

Nanoparticle Detection by Aerosol Mass Spectrometry

An aerodynamic lens system with efficient transmission of particles in the 10-300 nm size range is used to study the efficiency of nanoparticle detection by laser ablation mass spectrometry with 193 nm and 266 nm radiation. Ideally, all particles in the beam path when the laser fires should be detected. However, the probability of particle detection is much less than 1 and dependent upon the particle type, defined by particle size and chemical composition, and the ablation conditions, defined by the laser wavelength and irradiance. Particles above 100 nm can be ablated and detected with near unit efficiency. Below 100 nm, the detection probability decreases with decreasing particle size and salt particles (sodium chloride, potassium chloride) are detected with higher efficiency than organic particles (oleic acid, 3-nitrobenzyl alcohol). The results are discussed in relation to the mechanism of laser ablation and the instrumental requirements for particle detection.     

In-line dynamic control monitoring of fluids for space systems

An electro-optical instrument is described which will measure particle size and concentration in hydraulic systems. The system utilizes a HeNe laser source and a linear array of 50 photodiode detectors. Shadows are formed by the particles contained in the fluid as they pass through the laser beam and these shadows are magnified and projected onto the photodiode array. Particle size is determined by the number of photodiodes shadowed. The instrument operates “on-line” and is capable of counting approximately 10,000 particles/sec with the size range of 5 to 100 μm. The instrument will be interfaced with a computer for data collection, manipulation, and displaying the results.     

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.     

激光散射法快速测定催化剂粒度分布

采用激光粒度仪快速分析催化剂的粒度,利用激光光束对样品池内的样品颗粒进行衍射,散射光的角度与颗粒的粒度成反比,散射光的强度与颗粒的浓度成正比,所有测量由计算机分析测量并计算。该方法分析速度极快,测量范围广,操作简单、快捷,可以很好的满足对各种催化剂、粉尘的粒度分析的需要。     

The Detection Efficiency of the Single Particle Soot Photometer

A single particle soot photometer (SP2) uses an intense laser to heat individual aerosol particles of refractory black carbon (rBC) to vaporization, causing them to emit detectable amounts of thermal radiation that are used to quantify rBC mass. This approach is well suited for the detection of the majority of rBC mass loading in the ambient atmosphere, which occurs primarily in the accumulation mode (～ 1–300 fg-rBC/particle). In addition to operator choices about instrument parameters, SP2 detection of rBC number and/or mass can be limited by the physical process inherent in the SP2 detection technique — namely at small rBC mass or low laser intensities, particles fail to heat to vaporization, a requirement for proper detection. In this study, the SP2's ability to correctly detect and count individual flame-generated soot particles was measured at different laser intensities for different rBC particle masses. The flame-generated soot aerosol used for testing was optionally prepared with coatings of organic and non-organic material and/or thermally denuded. These data are used to identify a minimum laser intensity for accurate detection at sea level of total rBC mass in the accumulation mode (300 nW/(220-nm PSL)), a minimum rBC mass (～ 0.7-fg rBC-mass corresponding to 90 nm volume-equivalent diameter) for near-unity number detection efficiency with a typical operating laser intensity (450 nW/(220-nm PSL)), and a methodology using observed color temperature to recognize laser intensity insufficient for accurate rBC mass detection. Additionally, methods for measurement of laser intensity using either laboratory or ambient aerosol are presented.     

A Fast Scanning Mobility Particle Spectrometer for Monitoring Transient Particle Size Distributions

The Scanning Mobility Particle Spectrometer (SMPS) is a key tool for measuring particle size distribution. The application of the instrument to obtain size distributions throughout a wide range of particle sizes for transient systems, such as motor vehicle emissions, has been limited by the time resolution of the SMPS. In this paper, we present a fast-SMPS (f-SMPS) that utilizes a Radial Differential Mobility Analyzer (rDMA) and a Wixing Condensation Particle Counter (mCPC). The combination of these two components allows for the acquisition of particle size distributions on the time scale of several seconds. The Instrument has an operating range of 5–98 nm and can obtain particle size distributions at rates of up to 0.4 Hz. This paper presents the initial construction and calibration of the instrument followed by its application to several sampling scenarios. Samples from the on-road testing of a heavy-duty diesel (HDD) vehicle demonstrate the utility of this instrument for momtor vehicle emissions measurements as size distributions can now be associated with discrete events taking piace during vehicle onroad operation. For instance, these data indicate the presence of a number peak at 15 nm during transient vehicle operation. Previous work indicates that these particles are associated with the loss of engine lubricating oil.     

Performance Comparison of Scanning Electrical Mobility Spectrometers

Scanning electrical mobility spectrometers (SEMS) are commonly used for near real-time ultrafine particle size distribution measurements. Analysis of SEMS measurements to calculate particle size distributions requires detailed understanding of instrument characteristics and operation. Varying instrument designs are used in the different commercial SEMS systems, and data analysis with these instruments requires accurate knowledge of their relative performance. In this study, an experimental approach to evaluate and reconcile differences between different SEMS instruments is established. This approach is used to characterize the relative performance of two SEMS systems—TSI's SMPS 3936-L22 and MSP's WPS XP1000—for particle sizes in the range of 20 to 300 nm. In these tests, the instruments were operated under a low flowrate condition with aerosol and sheath air flows of 0.3 and 3 LPM, respectively. Measurements show that the particle sizing characteristics of the instruments are very consistent with each other over the entire range of particle sizes studied. Particle number characteristics are dependent on the treatment of particle losses in the system and accounting of non-idealities of transfer function. The number concentrations reported by two instruments are generally consistent with each other and with an upstream reference counter for particle sizes larger than ～ 90 nm. For smaller particles, the low flowrate operation of the two systems results in significant penetration losses. A net particle detection efficiency (NPDE) factor for the two systems was determined from experiments with monodisperse aerosol. This factor is seen to be effective in characterizing and reconciling measurements made with these two SEMS instruments.     

Coincidence in Time-of-Flight Aerosol Spectrometers: Phantom Particle Creation

When using time-of-flight aerosol spectrometers, particle size measurement is based upon a particle's transit time between two laser beams. The particle's transit time is assumed to be the time difference between the two pulses of light that are produced as the particle passes through the two laser beams. Particle coincidence, which occurs when a second particle crosses the first laser beam before the first particle crosses the second laser beam, has a complex effect upon the measured size distribution. As a result of coincidence, time-of-flight aerosol spectrometers can replace real particles of one size with spurious, or phantom, particles of a different size in the measured distribution. When partial detection of a particle occurs, i.e., only one pulse from a particle is detected, another particle producing a pulse that occurs while the timer is open can cause the recording of a randomly sized phantom particle. The creation of these phantom particles, which we termed “open-timer” phantom particles, has been investigated theoretically and experimentally in a commercially available time-of-flight aerosol spectrometer. The number of these open-timer phantom particles was found to increase with particle size and aerosol concentration. In addition, the instrument's detection logic affects the number and size of the phantom particles. These are most apparent in the tails and minima of the measured distribution. In order to minimize phantom particle creation, the concentration of partially detected particles must be minimized. Strategies to reduce phantom particle concentration involve reducing the concentration of small particles near the optical detection threshold of the spectrometer.     

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.     

Extrapolation of single particle soot photometer incandescent signal data

The Single Particle Soot Photometer (SP2) is an instrument for quantifying the refractory black carbon (rBC) mass of individual aerosol particles. It heats the particle’s rBC component to vaporization and quantifies the resulting visible thermal radiation to infer rBC mass. For purely technical reasons, SP2s are unable to quantify rBC mass beyond an easily adjustable limit due to eventual saturation of the electronics that record the visible light signals. Here, we evaluate an extrapolation algorithm to estimate rBC masses exceeding this upper limit in an SP2. The algorithm is based on identifying the crossing points of linear fits to unsaturated data, and using the duration of the saturated data to constrain potential errors. We find that extrapolation performance is quite insensitive to instrument parameters including laser intensity, rate of data acquisition, and particle speed through the laser. However, this approach increases uncertainty on the detection limit of the instrument, and is hence only useful in unknown aerosols for very limited extrapolation to approximately a factor of 1.5 increase in the upper mass range, corresponding to a 15% increase in the upper diameter limit. This increased range small enough that early identification of meaningful saturation during measurement campaigns remains the only tenable approach to robustly characterizing rBC mass size distributions and, in some cases, rBC mass concentrations.     

燃烧法和固相法所制备的红褐色陶瓷颜料ZnFe1.2Cr0.8O4的研磨效果研究

本文以燃烧法和固相法分别制备的红褐色陶瓷颜料ZnFe_1.2Cr_0.8O₄(燃烧法：C-ZnF;固相法：S-ZnF)为研究对象,采用正交试验的方法对颜料颗粒的研磨最优参数进行分析探讨。使用激光粒度仪表征研磨前后颜料颗粒的粒度及其分布,通过极差分析法来分析研磨的最优参数。结果表明,上述两种颜料颗粒的最优研磨条件均为：添加5wt%的分散剂WF211,研磨时间为100min,研磨转速为2000rpm。在相同的研磨条件下,对比固相法制备的颜料颗粒S-ZnF,燃烧法制备的颜料颗粒C-ZnF可以得到颗粒粒度细且分布窄的颜料颗粒产品。另外,采用一种粒数衡算模型(PBM)来模拟颜料颗粒的研磨过程破碎行为,计算颜料颗粒在研磨过程中的选择函数矩阵。通过模拟分析表明,颜料颗粒C-ZnF的研磨效果要优于颜料颗粒S-ZnF。     

Characterization of rubber particle size distribution of high-impact polystyrene using low-angle laser light scattering

A new application of low-angle laser light scattering has led to a new instrument capable of characterizing the rubber particle size distribution of high-impact polystyrene (HIPS) containing particles as small as 0.1 μ. Rubber particle size distributions of several HIPS resins have been characterized, and the particle size ranking of resins using light scattering parallels the ranking of resins using photomicroscopy. Several solvents have been employed to suspend the HIPS rubber particles for the scattering determination. Swelling of the rubber phase has been found to be relatively insensitive to variations in rubber phase crosslinking when methyl ethyl ketone is used to suspend the rubber particles. Particle swelling in methyl ethyl ketone does not detract from the usefulness of the light scattering method for HIPS rubber particle size characterization.     

The segregation of particulate materials. A review

An improved device has been developed for measuring particulate stack emissions. Known as the LTV monitor, the new instrument counts both number and size of particles, at multiple points across stack gas without the need for sample extraction or probe positioning inside the stack. It combines laser illumination, TV imaging, and signal-processing electronics to determine particle emissions in the range of 0.2 – 10 μm, In both laboratory and field tests the LTV monitor showed good agreement with EPA filter trains for measuring mass load, and it also measured particle size distribution. The instrument is portable for on-line industrial applications.     

A Novel Optical Instrument for Estimating Size Segregated Aerosol Mass Concentration in Real Time

A novel optical instrument has been developed that estimates size segregated aerosol mass concentration (i.e., PM ₁₀ , PM ₄ , PM _2.5 , and PM ₁ ) over a wide concentration range (0.001–150 mg/m ³ ) in real time. This instrument combines photometric measurement of the particle cloud and optical sizing of single particles in a single optical system. The photometric signal is calibrated to approximate the PM _2.5 fraction of the particulate mass, the size range over which the photometric signal is most sensitive. The electrical pulse heights generated by light scattering from particles larger than 1 micron are calibrated to approximate the aerodynamic diameter of an aerosol of given physical properties, from which the aerosol mass distribution can be inferred. By combining the photometric and optical pulse measurements, this instrument can estimate aerosol mass concentrations higher than typical single particle counting instruments while providing size information and more accurate mass concentration information than traditional photometers. Experiments have shown that this instrument can be calibrated to measure aerosols with very different properties and yet achieve reasonable accuracy.     

Shape distinction of particulate materials by laser diffraction pattern analysis

A pattern of a diffraction image depends on the particle shape, while the size of the pattern depends on the sectional area of the particle. In this work, the method to extract differences from the diffraction patterns due to different shapes of non-spherical particles was studied conceptually. In this respect, a radial segment (wedge) photo-detector was assumed as a detector. Diffraction patterns and intensity patterns detected by the radial segment detector were calculated for many kinds of two-dimensional shapes, corresponding to the projections of particles, as a circle, ellipses, triangles, quadrangles, other anonymous shapes, also shapes extracted from real phytoplanktons. From these detected light intensity patterns, we extracted (or define) two indexes: “circular index” and “peak number.” It was shown that various shapes can be distinguished by means of two-dimensional mapping with these parameters. In addition, an applicability of a concentric detector was examined to estimate the particle size when the particle is non-spherical but is a single particle in the measurement. As a result, it was found that the circle equivalent diameter determined with usual scheme agreed well with the sectional area equivalent diameter of the original particle even in any cases of non-spherical samples. From these results, it was shown that the particle size and shape in wide range can be distinguished from the three-dimensional mapping with “circular index”, “peak number” and “particle size”.     

Design of an instrument for real-time detection of bioaerosols using simultaneous measurement of particle aerodynamic size and intrinsic fluorescence

A prototype instrument has been constructed to measure individual airborne particles based on their aerodynamic size and their intrinsic fluorescence at selected excitation and emission wavelength bands. The instrument combines features of an aerodynamic particle sizing device with capabilities similar to those of a liquid flow cytometer. The goal of the instrument is to provide real-time data indicative of particle characteristics, and it is especially targeted to respond to bioaerosols from 0.5 to 10 micrometers (aerodynamic diameter) with intrinsic fluorescence exited at a wavelength of 325 nm and emitting from 420 to 580 nm. This size range covers individual airborne bacteria and bacteria clusters, and the fluorescence sensitivity is selected for biological molecules commonly found in cellular systems, for example, reduced nicotinamide adenine dinucleotide phosphate [NAD(P)H] and riboflavin. Initial tests with nebulised Bacillus subtilis var. niger (BG, ATCC 9372) spores have shown that, for both individual spores and spore clumps, a low level of fluorescence is detected from 17% of the particles. This detection percentage is on the same order as previous experiments that have measured viability of about 12% for mechanically dispersed BG spores (Ho and Fisher (1993) Defense Research Establishment Suffield Memorandum 1421) and suggests a need for further investigation into the possible relationship between the detected fluorescence and viability of bacterial spores.     

A pragmatic approach to the particle sizing of submicrometre emulsions using a laser nephelometer

The use of a commercial laser nephelometer to measure the mean particle size of submicrometre emulsions is described. The instrument is sufficiently sensitive to measure particles smaller than 600 nm. The method is comparative, and two standard materials of known size that are physically and chemically similar to the test emulsion must be used. The method is rapid and inexpensive, and may be used as a routine, in-house quality control procedure for monitoring emulsion production processes such as homogenization. However, it is only suitable if the average particle size of the system is smaller than 600 nm and, probably, with a relatively narrow size distribution.