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.     

Variations in the Size and Chemical Composition of Nitrate-Containing Particles in Riverside,CA

High time resolution measurements of nitrate-containing particles were made in Riverside, CA using an automated particle nitrate monitor and an aerosol time-of-flight mass spectrometer. The automated particle nitrate monitor provides quantitative data on the concentration of total particle-bound nitrate with a temporal resolution of 10 min. The aerosol time-of-flight mass spectrometer provides continuous data on aerodynamic size and single particle chemical composition. Data sets acquired with the two instruments are compared for a two-day intensive sampling period in August 1997 as part of the 1997 Southern California Ozone Study-North American Research Strategy for Tropospheric Ozone (SCOS97-NARSTO). Temporal variations in the number of nitrate-containing particles observed by the mass spectrometry system track (R² 0.73) the nitrate mass concentrations measured by the automated particle nitrate monitor. Both systems detected four periods of elevated nitrate concentrations of several hours duration. For these periods, the nitrate mass concentrations as measured by the automated particle nitrate monitor were similar, ranging from 11 to 19 mu g m3. However, the particle size and single particle composition of nitrate-containing particles as measured by the aerosol time-of-flight mass spectrometer were distinctly different. Specifically, the nitrate maxima observed in the midmorning hours were characterized by supermicrometer nitrate particles associated with either ammonium and organic species or sodium. The afternoon maxima were characterized by submicrometer ammonium nitrate particles, most of which contained organic material.     

THE ON-LINE CHEMICAL ANALYSIS OF SINGLE PARTICLES USING AEROSOL BEAMS AND TIME OF FLIGHT MASS SPECTROMETRY

This paper describes an on-line instrument, capable of measuring the size and chemical composition of single, aerosol particles. Possible applications include monitoring aerosol reactors and studying atmospheric chemistry. The main conclusion is that a working prototype has been built and tested. It uses a three stage vacuum system to generate an aerosol beam with a low divergence angle and a high transmittance. The pressure is reduced sufficiently to allow the application of a time-of-flight mass analyzer. The aerosol beam is probed in the analysis section by the focused beam of a low-power helium-neon laser. Every particle crossing the laser beam scatters light, which is detected by two photomultiplier tubes, mounted at angles of 45 and 90°. The signal is stored when both detectors produce a pulse simultaneously, and this event triggers the chemical analysis cycle. A pulsed Nd: YAG laser vaporizes the particle and generates ions, which are next analyzed by a time-of-flight mass spectrometer. In this way combined information on the size and the composition of the particle is obtained.     

A New Time-of-Flight Aerosol Mass Spectrometer (TOF-AMS)—Instrument Description and First Field Deployment

We report the development and first field deployment of a new version of the Aerosol Mass Spectrometer (AMS), which is capable of measuring non-refractory aerosol mass concentrations, chemically speciated mass distributions and single particle information. The instrument was constructed by interfacing the well-characterized Aerodyne AMS vacuum system, particle focusing, sizing, and evaporation/ionization components, with a compact TOFWERK orthogonal acceleration reflectron time-of-flight mass spectrometer. In this time-of-flight aerosol mass spectrometer (TOF-AMS) aerosol particles are focused by an aerodynamic lens assembly as a narrow beam into the vacuum chamber. Non-refractory particle components flash-vaporize after impaction onto the vaporizer and are ionized by electron impact. The ions are continuously guided into the source region of the time-of-flight mass spectrometer, where ions are extracted into the TOF section at a repetition rate of 83.3 kHz. Each extraction generates a complete mass spectrum, which is processed by a fast (sampling rate 1 Gs/s) data acquisition board and a PC. Particle size information is obtained by chopping the particle beam followed by time-resolved detection of the particle evaporation events. Due to the capability of the time-of-flight mass spectrometer of measuring complete mass spectra for every extraction, complete single particle mass spectra can be collected. This mode provides quantitative information on single particle composition. The TOF-AMS allows a direct measurement of internal and external mixture of non-refractory particle components as well as sensitive ensemble average particle composition and chemically resolved size distribution measurements. Here we describe for the first time the TOF-AMS and its operation as well as results from its first field deployment during the PM _2.5 Technology Assessment and Characterization Study—New York (PMTACS-NY) Winter Intensive in January 2004 in Queens, New York. These results show the capability of the TOF-AMS to measure quantitative aerosol composition and chemically resolved size distributions of the ambient aerosol. In addition it is shown that the single particle information collected with the instrument gives direct information about internal and external mixture of particle components.     

HIGH-SPEED PARTICLE BEAM GENERATION: SIMPLE FOCUSING MECHANISMS

Modern chemical characterization instruments employ an aerosol inlet that transmits atmospheric aerosols to the low pressure source region of a time-of-flight mass spectrometer, where particles are ablated and ionized using high energy irradiation. The ions when analyzed in the mass spectrometer yield information about the elemental composition of airborne aerosols. Often, the rate at which particles are analyzed is limited by the transmission rate of the inlet used. Depending on their size, particles are lost during sampling usually due to inertial effects or diffusion. Often simple capillaries and conical nozzles are used as primary focusing elements in the formation of high-speed particle beams. Due to the basic nature of the focusing mechanism, such elements transmit particles efficiently over a narrow size range. This size range strongly depends on the nozzle geometry and operating conditions. In this work, numerical techniques are used to (a) simulate fluid and particle transport in axi-symmetric nozzles, (b) help understand and identify the mechanisms by which particle beams are formed in capillaries and conical nozzles, and (c) illustrate the contrasting nature of the beams thus formed. Particle focusing is also simulated in some typical inlets to validate the predictions and illustrate the merits and drawbacks of each design.     

Development of portable aerosol mobility spectrometer for personal and mobile aerosol measurement

We describe development of a portable aerosol mobility spectrometer (PAMS) for size distribution measurement of submicrometer aerosol. The spectrometer is designed for use in personal or mobile aerosol characterization studies and measures approximately 22.5×22.5×15 cm and weighs about 4.5 kg including the battery. PAMS uses electrical mobility technique to measure number-weighted particle size distribution of aerosol in the 10–855 nm range. Aerosol particles are electrically charged using a dual-corona bipolar corona charger, followed by classification in a cylindrical miniature differential mobility analyzer. A condensation particle counter is used to detect and count particles. The mobility classifier was operated at an aerosol flow rate of 0.05 L/min, and at two different user-selectable sheath flows of 0.2 L/min (for wider size range 15–855 nm) and 0.4 L/min (for higher size resolution over the size range of 10.6–436 nm). The instrument was operated in voltage stepping mode to retrieve the size distribution in approximately 1–2 min. Sizing accuracy and resolution were probed and found to be within the 25% limit of NIOSH criterion for direct-reading instruments. Comparison of size distribution measurements from PAMS and other commercial mobility spectrometers showed good agreement. The instrument offers unique measurement capability for on-person or mobile size distribution measurement of ultrafine and nanoparticle aerosol.     

Calibration Correction of an Active Scattering Spectrometer Probe to Account for Refractive Index of Stratospheric Aerosols: Comparison of Results with Inertial Impaction

Real-time particle size spectra are being acquired on our research aircraft with relative ease and speed by techniques that make use of the real-time interaction of laser light with aerosols and cloud droplets. The results are, however, sometimes ambiguous, because the optical “signatures” of the particles depend on their refractive indices in addition to physical dimensions. The calibration supplied by the manufacturer is based on instrument response to a specific test aerosol, e.g., latex spheres (refractive index = 1.59). Such a calibration is strictly valid only for sample aerosols of refractive index and shape similar to the test aerosol. Whenever the sample aerosol differs from the test aerosol, a calibration correction is in order. Of concern here is the use of an active scattering spectrometer probe (ASAS-X), to measure sulfuric acid aerosols on high-flying U-2 and ER-2 research aircraft. Correcting the calibration of the ASAS-X for dilute sulfuric acid droplets (refractive index = 1.44) that predominate the stratospheric aerosol changes the inferred sizes by up to 32% per size interval from that determined from the nominal calibration. This results in an average increase in particle surface area and volume of 42 ± 10% and 71 ± 19%, respectively. The calibration correction of the optical spectrometer probe for stratospheric aerosol is validated by independent and simultaneous sampling of the particles with impactors. Sizing and counting of particles on microphotographs of scanning electron microscope images give results on total particle surface areas and volumes. After the calibration correction, the optical spectrometer data (averaged over four size distributions) agree with the impactor results (similarly averaged) to within a few percent. We conclude that the optical properties, or chemical makeup, of the sample aerosol must be known for accurate size analysis by optical aerosol spectrometers.     

Highly filled thermoplastic composites. II: Effects of particle size distribution on some properties

The effect of irregularly shaped glass particle size and size distribution on the packing density and flexural mechanical properties of highly-filled composites with a rubbery thermoplastic matrix was studied. Increasing the particle's median size and size distribution width significantly increases the packing density of the composites. Compression molding causes the glass particles to fracture at a decreasing level with an increasing distribution width. Particle median size, rather than size distribution, affects the mechanical properties; The flexural modulus and strength increase and the ultimate deflection in flexure decreases with a decreasing median size. A “glass network” is formed in the compression molded composites because of the mechanical interlocking of particles. The nature of this continuous glass phase predominates the composites mechanical behavior. The particle's size and shape determine the nature of the glass network and, thus, have a dominating effect on the mechanical properties. The latter are significantly affected by the particle's surface properties. A specific silane treatment of the glass particles acts to reduce the particle/particle friction, resulting in a higher packing density. The treatment also acts as a cohesive liquid to increase the strength of the glass network, and to increase the particle/polymer adhesion, increasing the composites' strength and ductility.     

A multi-channel electrical mobility spectrometer with wedge geometry—Design and first evaluation

This paper presents a new design for a multi-channel electrical mobility spectrometer which measures the lognormal size distribution and number concentration of aerosol particles in the size range 5–300 nm with a short response time. The spectrometer charges particles in the test sample by unipolar corona discharge, they are then classified into 16 channels by electrical mobility. Charged particles are detected in the channels by individual aerosol electrometers, giving an electrical mobility spectrum for the sample.The main aspect of the spectrometer design is a wedge-shaped classifier with flat electrodes. This allows a flow to be drawn from the classifier at 16 different levels/channels with minimal disturbance to the remaining flow, hence filter based aerosol electrometers can be used for detection. The varying field within the classifier caused by the wedge shape is advantageous to the classification and optimised through the selection of the wedge angle.Also presented is an alternative technique for inferring the lognormal size distribution of an aerosol from a measured electrical mobility spectrum. This involves using a theoretical model of the instrument to simulate the output mobility spectra for a large number of aerosol samples with lognormal size distributions. The resulting data library can be searched against a measured electrical mobility spectrum to find the corresponding size distribution.The experimental work presented in this paper is a first evaluation of this spectrometer and includes measurement of the classifier transfer functions, basic calibration of the charger, and finally testing the spectrometer's performance on some simple unimodal lognormal aerosol samples.     

Particle size analysis of dioctyl phthalate aerosols using the stöber aerosol spectrometer

Particle size distributions of nearly monodisperse dioctyl phthalate aerosols (dia. between 0–5 and 1–4 μm) have been determined using the Stöber aerosol spectrometer. The particle size distributions can be approximated very well by bimodal distribution functions. From a statistical analysis it turned out that the accuracy of the approximation is limited in case of small particles (dia. ～ 0·5 μm). This is due to evaporation of the particles during the analysis.The mean of the particle size distribution determined with the Stöber aerosol spectrometer was in fair agreement with the particle diameter determined with the higher order Tyndall spectrometer.     

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.     

Development of an Aerosol Mass Spectrometer for Size and Composition Analysis of Submicron Particles

The importance of atmospheric aerosols in regulating the Earth's climate and their potential detrimental impact on air quality and human health has stimulated the need for instrumentation which can provide real-time analysis of size resolved aerosol, mass, and chemical composition. We describe here an aerosol mass spectrometer (AMS) which has been developed in response to these aerosol sampling needs and present results which demonstrate quantitative mea surement capability for a laboratory-generated pure component NH₄ NO₃ aerosol. The instrument combines standard vacuum and mass spectrometric technologies with recently developed aerosol sampling techniques. A unique aerodynamic aerosol inlet (developed at the University of Minnesota) focuses particles into a narrow beam and efficiently transports them into vacuum where aerodynamic particle size is determined via a particle time-of-flight (TOF) measurement. Time-resolved particle mass detection is performed mass spectrometrically following particle flash vaporization on a resistively heated surface. Calibration data are presented for aerodynamic particle velocity and particle collection efficiency measurements. The capability to measure aerosol size and mass distributions is compared to simultaneous measurements using a differential mobility analyzer (DMA) and condensation particle counter (CPC). Quantitative size classification is demonstrated for pure component NH₄ NO₃ aerosols having mass concentrations 0.25_mu g m -3. Results of fluid dynamics calculations illustrating the performance of the aerodynamic lens are also presented and compared to the measured performance. The utility of this AMS as both a laboratory and field portable instrument is discussed.     

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.     

Computational Modeling of Silicon Nanoparticle Synthesis: I. A General Two-Dimensional Model

A two-dimensional model has been developed for silicon nanoparticle synthesis by silane thermal decomposition driven by laser heating in a tubular reactor. This fully coupled model includes fluid dynamics, laser heating, gas phase and surface phase chemical reactions, and aerosol dynamics which includes particle transport and evolution by convection, diffusion, thermophoresis, nucleation, surface growth, and coagulation processes. A moment method, based upon a lognormal particle size distribution, and a sectional method are used to model the aerosol dynamics. The simulation results obtained by the two methods are compared. The sectional method is capable of capturing the bimodal behavior that occurs locally during the process, while the moment method is computationally more efficient. The effect of operating parameters, such as precursor concentration, gas phase composition, inlet gas velocity and laser power input, on the characteristics of the particles produced are investigated. Higher temperature generates more large particles with higher precursor conversion. Shorter residence time, from high inlet velocity, produces more small particles at the cost of lower precursor conversion. Increasing H₂ concentration suppresses particle formation by reducing the rates of gas phase and surface reactions, leading to fewer and smaller particles. In addition, the relative importance of the interconnected mechanisms involved in the particle formation is considered. The results make clear that spatial variations in reaction conditions are the primary source of size polydispersity and generation of non lognormal overall size distributions in a laser-driven process like that considered here.     

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.     

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.     

Using ATOFMS to Determine OC/EC Mass Fractions in Particles

Historically, obtaining quantitative chemical information using laser desorption ionization mass spectrometry for analyzing individual aerosol particles has been quite challenging. This is due in large part to fluctuations in the absolute ion signals resulting from inhomogeneities in the laser beam profile, as well as chemical matrix effects. Progress has been made in quantifying atomic species using high laser powers, but very few studies have been performed quantifying molecular species. In this study, promising results are obtained using a new approach to measure the fraction of organic carbon (OC) associated with elemental carbon (EC) in aerosol particles using single particle laser desorption ionization. A tandem differential mobility analyzer (TDMA) is used to generate OC/EC particles by size selecting EC particles of a given mobility diameter and then coating them with known thicknesses of OC measured using a second DMA. The mass spectra of the OC/EC particles exiting the second DMA are measured using an ultrafine aerosol time-of-flight mass spectrometer (UF-ATOFMS). A calibration curve is produced with a linear correlation (R² = 0.98) over the range of OC/EC ion intensity ratios observed in source and ambient studies. Importantly, the OC/EC values measured in ambient field tests with the UF-ATOFMS show a linear correlation (R² = 0.69) with OC/EC mass ratios obtained using semi-continuous filter based thermo-optical measurements. The calibration procedure established herein represents a significant step toward quantification of OC and EC in sub-micron ambient particles using laser desorption ionization mass spectrometry.     

Laboratory Verification of PH-CPC's Ability to Monitor Atmospheric Sub-3 nm Clusters

Gas-to-particle conversion takes readily place in the atmosphere. Detecting the initial clusters, which act as embryos for the newly formed particles, is beyond traditional aerosol instrumentation. Charged atmospheric clusters can be measured with air ion spectrometers, but typical state-of-the-art condensation particle counters, which detect both neutral and charged clusters, only see particles larger than 2.5 nm in diameter. In this study we present a modified pulse-height condensation particle counter (PH-CPC) and confirm by laboratory verification that it is capable of detecting charged clusters with electrical mobility equivalent diameter down to ～1 nm. We show how the detection efficiency and the pulse heights depend on the calibration particle size, polarity and composition. The effect of butanol supersaturation on the PH-CPC counting efficiency is also discussed. Furthermore, we developed an inversion method for the data to obtain true particle size distribution from the measurement signal.     

In-situ characterization of ultrafine particles by laser-induced incandescence: sizing and particle structure determination

The laser-induced incandescence (LII) method is applied to the in situ size analysis of aerosol particles of different origin at room temperature. A detailed theoretical model of the particle heating and cooling for the different size fractions incorporating a solution of a Fredholm integral equation of the first kind is used to retrieve the particle size distribution from the time-dependent aerosol thermal emission detected after a ns laser pulse. The results are compared with TEM data of deposited aerosol particles along with online measurements employing a differential mobility analyzer (DMA). Besides the size distribution, the LII signal contains information on the internal structure of particle agglomerates, which can be obtained by analyzing the changes in the measured size distribution with the laser pulse energy. The objective of the paper is an evaluation of LII for its capability to measure the size distributions of various types of aerosols in the size range about 5–200 nm and to determine the primary particle sizes in the case of agglomerated particles.     

Development of Variable Flow Rate Isokinetic Sampling System for 0.5–15-μm Aerodynamic Diameter Particles

Isokinetic sampling is required when evaluating the aerodynamic sizes of particles released from dry powder inhalers (DPI) under simulated breathing condition since anisokinetic sampling may lead to significant sampling error for coarse particles. We propose an isokinetic measuring system for aerosol particles from a stream in a narrow conduit of variable flow rates (variable flow rate aerosol sampler, VFAS) combined with Aerodynamic Particle sizer® APS^TM spectrometer (model 3321, TSI Inc.). The VFAS was capable of generating variable sampling flow rates by adjusting the flow resistance of makeup air to produce constant flow rate of aerosol to the APS. The penetrations through the VFAS-APS system were measured using monodisperse particles with a size range of 0.7–15 μm by applying a rectangular flow rate–time pattern of sampling air, and we found that the VFAS-APS system can measure the number concentration of particles with the particle detection efficiency (particle penetration through the system) of almost unity. The VFAS-APS system may be a powerful tool to measure the size and concentration of powder released by the DPI in the size range of 0.5–15 μm.

Copyright 2012 American Association for Aerosol Research