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.     

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.     

Differential Electrical Mobility Analysis: A Theoretical Study

ABSTRACT

The subject of this work is the theoretical investigation of slowly scanning differential mobility analyzers (DMAs) which are, e.g., utilized to determine DMA transfer functions and to measure particle mobility distributions. A model to describe such systems is introduced and applied to investigate three different regimes of input mobility distributions: 1) a mobility distribution much narrower than the DMA transfer function, 2) a mobility distribution of about the same width as the DMA transfer function, and 3) a mobility distribution much wider than the DMA transfer function. Cases 1) and 2) are relevant for DMA transfer function measurements utilizing tandem differential mobility analyzer (TDMA) systems. For either regime, it is not possible to determine DMA transfer functions directly from the concentration distributions measured at the outlet of a DMA. For these cases, a deconvolution procedure is needed. Therefore, an iterative deconvolution procedure was developed. Determining DMA transfer functions utilizing the developed deconvolution procedure, different shapes of transfer function (triangular, Gaussian) are discussed. Case 3) is relevant for particle size distribution measurements. Here, the mobility distribution upstream of the DMA can be obtained by dividing the concentration distribution measured downstream of a DMA by the DMA transfer function area. The DMA transfer function area is influenced by diffusional losses inside the DMA, and therefore is size-dependent. Neglecting this size dependence results in an underprediction of particle number concentrations in the ultrafine particle size range.     

扫描电镜能谱仪在涂料中的应用研究

随着涂料工业的发展,对涂料分析的要求也越来越高,扫描电镜能谱仪(SEM/EDS)是涂料微区分析的重要手段之一,它在分析样品表面形貌的同时还可以分析某一微区的元素组成,二者结合可以在涂料颜填料质量控制、颜填料分析、涂层分析、涂料全剖析等方面发挥重要的作用,为改进涂料工艺、提升产品质量提供强有力的技术支持。文中结合SEM/EDS在各个方面的应用实例,对SEM/EDS在涂料中的应用展开系统的讨论。     

An Asymptotic Analysis of Differential Electrical Mobility Classifiers

An asymptotic analysis of balanced flow operations of differential mobility analyzers (DMAs) and a new class of instruments that includes opposed migration aerosol classifiers (OMACs) and inclined grid mobility analyzers (IGMAs) provides new insights into the similarities and differences between the devices. The characteristic scalings of different instruments found from minimal models are shown to relate the resolving powers, dynamic ranges, and efficiencies of most such devices. The resolving powers of all of the instruments in the nondiffusive regime of high voltage classifications, , is determined by the ratio of the flow rate of the separation gas (sheath or crossflow) to that of the aerosol. At low voltage, when diffusion degrades the classification, the OMAC and the IGMA share an factor advantage in dynamic range of mobilities over the DMA, although the OMAC also suffers greater losses because diffusion immediately deposits particles onto its porous electrodes. On the basis of this analysis, a single master operating diagram is proposed for DMAs, OMACs, and IGMAs. Analysis of this operating diagram and its consequences for the design of differential electrical mobility classifiers suggests that OMACs and IGMAs also have advantages over DMAs in design flexibility and miniaturization. Most importantly, OMACs and IGMAs may outperform DMAs for the currently difficult classification of particles with diameters less than 10 nm. On the other hand, DMAs are more amenable to voltage scanning-mode operation to enable accelerated size distribution measurements, whereas it is most convenient to operate OMACs and IGMAs in voltage stepping-mode operation.     

Inter-Comparison of a Fast Mobility Particle Sizer and a Scanning Mobility Particle Sizer Incorporating an Ultrafine Water-Based Condensation Particle Counter

An Ultrafine Water-based Condensation Particle Counter (UWCPC), a Scanning Mobility Particle Sizer (SMPS) incorporating an UWCPC, and a Fast Mobility Particle Sizer (FMPS) were deployed to determine the number and size distribution of ultrafine particles. Comparisons of particle number concentrations measured by the UWCPC, SMPS, and FMPS were conducted to evaluate the performance of the two particle sizers using ambient particles as well as lab generated artificial particles. The SMPS number concentration was substantially lower than the FMPS (FMPS/SMPS = 1.56) measurements mainly due to the diffusion losses of particles in the SMPS. The diffusion loss corrected SMPS (C-SMPS) number concentration was on average ～ 15% higher than the FMPS data (FMPS/C-SMPS = 0.87). Good correlation between the C-SMPS and FMPS was also observed for the total particle number concentrations in the size range 6 nm to 100 nm measured at a road-side urban site (r² = 0.91). However, the particle size distribution measured by the C-SMPS was quite different from the size distribution measured by the FMPS. An empirical correction factor for each size bin was obtained by comparing the FMPS data to size-segregated UWCPC number concentrations for atmospheric particles. The application of the correction factor to the FMPS data (C-FMPS) greatly improved the agreement of the C-SMPS and C-FMPS size distributions. The agreement of the total particle concentrations also improved to well within 10% (C-FMPS/C-SMPS = 0.95).     

Fast Mixing Condensation Nucleus Counter: Application to Rapid Scanning Differential Mobility Analyzer Measurements

Condensation nucleus counters (CNCs) exhibit slower time response than expected due to mixing effects within the detector.This mixing produces an exponential distribution of delay times with a characteristic mixing time m that ranges from 0.1 s to 0.9 s for commonly used instruments and limits their usefulness for measuring rapidly changing aerosols. Moreover, when used as detectors in the scanning electrical mobility spectrometer (SEMS; also known as scanning mobility particle sizer, SMPS), CNCs limit the speed with which size distribution measurements can be made. In order to overcome this limitation, a new, fast-response mixing CNC (MCNC) has been developed and characterized. The time response of this new detector and TSI Models 3025 and 3010 CNCs has been measured using a spark source to generate an aerosol pulse. The mixing induced response smearing of this new detector, m , of this instrument is 0.058 s, which is significantly shorter than either of the other instruments tested. Its lower detection limit is about 5 nm diameter. The high aerosol flow rate of the MCNC (0.65 l min -1 ), fast time response, and low detection limit make it an ideal detector for SEMS/SMPS measurements. Using this MCNC as a detector for the SEMS, size distribution measurements over the 5 nm to 140 nm range have been made in 3 s with minimal distortion. The size distribution of a coagulation aerosol was effectively recovered by deconvolution with scans as short as 1 s. Uncertainties in the 1 s scans result, in part, from electronics problems in the scanning DMA.     

Causes of Concentration Differences Between a Scanning Mobility Particle Sizer and a Condensation Particle Counter

Accurate aerosol concentration measurement is important in many applications of aerosol science. Here we compare aerosol concentration measurements of classified NaCl aerosol in the size range of 20 to 80 nm (diameter) between a scanning mobility particle sizer (SMPS) and a condensation particle counter (CPC). The SMPS systematically measured higher concentrations than the CPC, with the difference increasing with decreasing particle size. Experiments suggest several causes for the discrepancy. First, the factory calibration of the SMPS impactor flow was incorrect for the study site at 780 mbar. Second, the neutralizer used in the SMPS was inefficient in bringing the classified aerosol to charge equilibrium, and third, there were significant losses of charged aerosol within the CPC. The comparisons were improved with proper impactor flow calibration and proper charge neutralization of the classified aerosol before measurement by the SMPS and CPC. The results of this study point to the importance of proper conditioning of aerosol below about 100 nm for measurement with the SMPS and condensation-based particle counters.     

Electrospraying Large Sparingly Soluble Tetraalkylammonium Ions as Monomobile Standards of Moderately Low Electrical Mobility

A procedure is described to electrospray the relatively large ions tetrahexadecylammonium (C₁₆H₃₃)₄N⁺ and tetraoctadecylammonium (C₁₈H₃₇)₄N⁺ from their commercially available bromide salts. Their low solubility and the corresponding low conductivity of their alcohol solutions complicates their electrospraying by the convenient method of applying the high voltage to the solution reservoir (rather than to the electrospraying tip). If the capillary bore is widened to reduce the reservoir-tip electrical resistance, the solvent evaporates at its tip, leading to solute precipitation and spray destabilization. These difficulties are overcome via conventional nanospray: we use a wide (～100 μm ID) capillary whose tip is pulled in a flame down to <20 μm OD. Mobility spectra result where the dominant and most mobile peaks are (C₁₆H₃₃)₄N⁺ or (C₁₈H₃₇)₄N⁺. Their electrical mobilities in air at room temperature and pressure are 0.63 and 0.60 cm²/V/s (±1%).

Copyright 2015 American Association for Aerosol Research     

The Scan Time Effect on the Particle Size Distribution Measurement in the Scanning Mobility Particle Sizer System

The scan time effect in the scanning mobility particle sizer was confirmed. The magnitude of this effect was shown in a typical situation. The cause of this scan time effect is the mixing process described by Russell et al. (1995). In that case, the result obtained at the longer scan time is the more accurate one.     

High Throughput Screening of Low Temperature SCR and SCD De-NOx Catalysts in Scanning Mass Spectrometer

High-throughput synthesis and screening methods have been developed for the discovery of highly active lead compounds for the selective catalytic reduction as well as direct decomposition of NO in the temperature range 200–300 °C. The discovery libraries for primary screening consisted of 16 × 16 catalyst arrays on 4in. square quartz wafers. Catalysts were prepared by robotic liquid dispensing techniques and screened for catalytic activity in Symyx' scanning mass spectrometer. The scanning mass spectrometer is a fast serial screening tool that uses flat wafer catalyst surfaces, local laser heating, a scanning/sniffing nozzle and a quadrupolar mass spectrometer to compare relative catalytic activities. The feed consisted of NO/NH₃ mixtures with optional O₂ cofeed and Kr as the internal standard in Ar carrier gas. QMS detection allowed for tracking of H₂O, N₂, NO, O₂, N₂O and Kr. Screening protocols for catalytic materials encompassed metal precursors and carriers for supported vanadia systems, extensive doping of V₂O₅/TiO₂, and broad screening of mixed redox metal oxides and supported base and noble metal systems. More than 500 samples could be screened in a single day. Active hits (high NO consumption accompanied by corresponding N₂ production) identified in discovery libraries were re-synthesized as focus libraries for lead confirmation and further optimization. These libraries used shallower compositional gradients, for example 56 points (compositions) per ternary, with four 56-point ternaries per 4in. wafer. Broad screening ternaries were generally 8 or 15 points. The focus libraries more clearly reveal the trends and provide guidelines for secondary screening and scale-up. High conversions were achieved in scanning mass spectrometer so the scalability risk is small for the short contact time reactions.     

756型与751型分光光度计相关性的验证实验

通过对756型和751型两种分光光度计所测数据的相关性进行比对,验证新用仪器的可靠性,发挥新用仪器的准确性、精确性和便利性,以便更好的为环境管理服务.     

Single-Particle Fluorescence Spectrometer for Ambient Aerosols

A fluorescence particle spectrometer (FPS) for real-time measurement of the fluorescence spectra of aerosol particles in the size range 1-10 w m diameter is reported. The prototype FPS has a sufficiently high sample rate (from 5 to 28 l/min for 3.5 w m to 11 w m diameter particles) to measure aerosol within buildings at practical rates (from 1 up to 600 particle fluorescence spectra per minute). Previously reported bioaerosol prototype detectors for measurement of single particle spectra (Pan et al., Opt. Lett ., 24, 116-118 (1999); Hill et al., Field Anal. Chem. Tech ., 3, 221-239 (1999)) were unable to sample the ambient environment; air containing particles had to be forced under pressure into a sample cell. In addition, sample rates were so small (less than 0.01 l/min) as to be impractical for most applications. The present design overcomes these deficiencies by the use of an airtight cell that highly concentrates micrometer-sized particles. A virtual impactor first concentrates aerosol particles, which are then drawn under negative pressure through an aerodynamic focusing nozzle in the inlet of the instrument, through the sample region, providing further concentration. The rate of particle spectra measured by the FPS increases significantly when the particle inlet is within a few meters of some common sources of indoor biological particles, e.g., a person coughing, sneezing, or rubbing his skin, or the presence of a dog. The spectra obtained have a variety of spectral shapes. The FPS may be useful in a variety of areas, e.g., in studying and monitoring airborne particles that cause diseases or allergies.     

质谱仪常见故障分析

介绍了VG PrimaδB型质谱仪的电源系统、真空系统、通讯系统的结构构成及其在质谱工作中的作用。分析质谱仪常见故障,并提出科学有效的处理方法。     

Stochastic Modeling of a New Spectrometer

A new spectrometer for classifying aerosol particles according to specific masses is being considered (Ehara et al. 1995). The spectrometer consists of concentric cylinders which rotate. The instrument is designed so that an electric field is established between the cylinders. Thus, aerosol particles injected into the spectrometer are subjected to a centrifugal force and an electric force. Depending on the balance between these two forces, as well as Brownian motion, charged particles either pass through the space between the cylinders or stick to either cylinder wall. Particles which pass through are detected. Given the rotation rate, voltage drop and physical dimensions of the device, we calculate the probability of detection in terms of particle density, diameter and charge. This is the transfer function. In this work, the focus is on situations where Brownian motion is significant. To solve for the transfer function, the trajectory of a particle in the spectrometer is modeled with a stochastic differential equation. Laminar flow is assumed. Further, attention is restricted to spherical particles with uniform density. The equation is solved using both numerical and Monte Carlo methods. The agreement between methods is excellent.     

利用红外光谱技术分析水泥性能

１引言 １８００年,Ｗ．Ｈｅｒｓｃｈｅｌ发现了电磁波的红外光部分,即波长为０．７５～１０００μｍ范围内的电磁波。当物质被连续的红外光照射后,红外光的某些频率会被物质吸收,由此得到红外吸收光谱。通常把红外光谱按其波长范围分为三个区域,如表１所示。每一区域的红外光谱被物质吸收后会使构成物质的各种粒子产生微观运动。物质的红外光谱主要是指中红外区的光谱,因为在中红外区的某些频率被物质吸收后会引起分子中振动能级的变化,从而产生与分子结构有密切关系的分子振动光谱。所以红外光谱分析是分析物质结构的有效工具。 研…