Kensei Ehara

Novel method to classify aerosol particles according to their mass-to-charge ratio—Aerosol particle mass analyser

A new method to classify aerosol particles according to their mass-to-charge ratio is proposed. This method works by balancing the electrostatic and centrifugal forces which act on particles introduced into a thin annular space formed between rotating cylindrical electrodes. Particles having a mass-to-charge ratio lying in a certain narrow range are taken out continuously as an aerosol suspension. A theoretical framework has been developed to calculate the transfer function which is defined as the ratio of the exiting particle flux to the entering particle flux. A similarity rule has been derived which states that a single nondimensional constant determines the shape of the transfer function. To examine the feasibility of the proposed principle, a prototype classifier was constructed, and the mass distribution of monodisperse particles nominally 0.309 μm in diameter was measured. The peak structures corresponding to singly, doubly, and triply charged particles were identified in the experimental spectra. The difference between theory and experiment in the peak location for the singly charged particles was about 6.5% in terms of mass, or 2.3% in terms of diameter.

Nanoscale reference materials for environmental, health and safety measurements: needs, gaps and opportunities

Abstract The authors critically reviewed published lists of nano-objects and their physico-chemical properties deemed important for risk assessment and discussed metrological challenges associated with the development of nanoscale reference materials (RMs). Five lists were identified that contained 25 (classes of) nano-objects; only four (gold, silicon dioxide, silver, titanium dioxide) appeared on all lists. Twenty-three properties were identified for characterisation; only (specific) surface area appeared on all lists. The key themes that emerged from this review were: 1) various groups have prioritised nano-objects for development as “candidate RMs” with limited consensus; 2) a lack of harmonised terminology hinders accurate description of many nano-object properties; 3) many properties identified for characterisation are ill-defined or qualitative and hence are not metrologically traceable; 4) standardised protocols are critically needed for characterisation of nano-objects as delivered in relevant media and as administered to toxicological models; 5) the measurement processes being used to characterise a nano-object must be understood because instruments may measure a given sample in a different way; 6) appropriate RMs should be used for both accurate instrument calibration and for more general testing purposes (e.g., protocol validation); 7) there is a need to clarify that where RMs are not available, if “(representative) test materials” that lack reference or certified values may be useful for toxicology testing and 8) there is a need for consensus building within the nanotechnology and environmental, health and safety communities to prioritise RM needs and better define the required properties and (physical or chemical) forms of the candidate materials.

Mass Range and Optimized Operation of the Aerosol Particle Mass Analyzer

We investigated, theoretically, the mass range in which an aerosol particle mass analyzer (APM) can be used for classification, and how the APM classification performance can be optimized. We listed factors that set limits to the APM, which were constraints of the rotation speed and the voltage, as well as requirements on the APM classification performance parameter, λ, that guarantee at least minimal performance in both resolution and penetration. We introduced the APM operation diagram, which is a tool to visualize the limits and mass range. We proposed to operate the APM that was considered in this study with the λ value set within the range from 0.25 to 0.5 for optimum classification performance by balancing both resolution and penetration. The mass range for the APM, with the λ value maintained between 0.25 and 0.5, was calculated to be from 0.003 to 2000 fg, which corresponds to the diameter range from 20 to 1600 nm for the density of 1 g/cm3. To verify the validity of the mass range and the idea of the optimized operation, we carried out experiments on an APM with polystyrene and sodium-chloride particles that were classified by electrical mobility. We found that the APM was able to provide bell-shaped spectra down to 12 nm, and was able to perform mass classification with an accuracy better than 5% down to 50 nm. Underestimation of mass and reduction of resolution and penetration were observed at sizes smaller than about 30 nm.

Comparison of Three Particle Number Concentration Calibration Standards Through Calibration of a Single CPC in a Wide Particle Size Range

We carried out a set of experiments to compare three particle number concentration standards (NCSs) by calibrating the same condensation particle counter (CPC) unit (Model 3772, TSI Inc., Shoreview, MN, USA). The standards were, in the order of operation size range, the primary NCS of the National Institute of Advanced Industrial Science and Technology (AIST, Japan), the Single Charged Aerosol Reference (SCAR) (Finland), and the Inkjet Aerosol Generator (IAG) of AIST. The results obtained with the 3 standards were found to agree at all overlapping particle sizes within the uncertainty limits. The relative expanded uncertainties varied between 0.6% and 2.6%, depending on the size and standard, while the overall agreement between the standards was within 0.5%. The observed consistency of the results is an important step toward establishing internationally coherent particle NCSs. As a result, the CPC 3772 was successfully calibrated in a particularly wide size range, approximately from 10 nm to 10 μm. The results indicate that the CPC can be considered as a practical tool for calibrating particle number concentration up to 1 μm. In general, the particle number concentration can be measured up to 2.5 μm without a significant decrease of the detection efficiency. By attaching an appropriate size-classifying inlet, the CPC could be used even for measuring the total number concentration for particles smaller than 2.5 μm, in parallel with the PM2.5 mass measurement. Above this particle diameter, the detection efficiency gradually decreased and reached 50% at about 10 μm. Copyright 2012 American Association for Aerosol Research

Design Considerations and Performance Evaluation of a Compact Aerosol Particle Mass Analyzer

A compact aerosol particle mass analyzer (APM) of which the size of the classifier was significantly reduced than that of the first commercial model (Kanomax Model 3600) was developed. Firstly, requirements for desired performance in classifying particle mass were set forth. Secondly, a theoretical framework for the design parameters of an APM that satisfies the requirements was formulated. Thirdly, the design parameters were determined that satisfies the requirements while reducing the instrument size. The requirements include the condition that the classification range covers from 0.001 to 1000 fg (approximately 12 to 1200 nm in size for spherical particles having the density of 1 g/cm3), and the condition that both the classification resolution and particle penetration in this mass range are higher than certain specified values. A prototype having the design parameters determined according to this theoretical framework was constructed, and its performance was evaluated experimentally. The external dimensions of the electrodes of the compact APM are approximately 140 mm in length and 60 mm in diameter. It was confirmed that the performance of the compact APM operated at the aerosol flow rate of 0.3 L/min was comparable to that of the Model 3600 APM operated at 1 L/min. Because of the reduced size and of the resultant improved portability, it is expected that the compact APM is readily applicable to field measurements. Copyright 2013 American Association for Aerosol Research

Metrology of airborne and liquid-borne nanoparticles: current status and future needs

The current status and future needs of nanoparticle metrology are discussed, particularly with respect to measurements of size, size distribution and number concentration of airborne and liquid-borne nanoparticles. Possible classification of types of measurement standards is proposed, and the role of each type of standard, including the feasibility of its establishment, is examined. A desirable interplay between measurement standards and documentary standards in establishing the traceability chain in particle measurements is suggested. Particle-related calibration services currently provided by our laboratory at the National Institute of Advanced Industrial Science and Technology are also described.

Inkjet Aerosol Generator as Monodisperse Particle Number Standard

The AIST-inkjet aerosol generator (IAG) can generate highly monodisperse solid or liquid aerosol particles in the particle diameter range from 0.3 to 20 μm at precisely known particle generation rates. The device has been developed for evaluating the counting efficiencies of optical and condensation particle counters. Particle generation efficiency of the IAG is defined as the number of aerosol particles generated by one voltage pulse sent to an inkjet head. The 95% confidence interval of the efficiency were 0.998 ± 0.006 within the 0.4 to 10 μm particle diameter range. The efficiencies remained close to unity when the droplet generation rates were within 20–500 s−1 and 100–900 s−1 using ultrapure-water and isopropyl alcohol (IPA) as the solvent of the inkjet solution, respectively. The operating aerosol flowrate range of the IAG is currently 0.5 and 1.0 L/min. The coefficients of variations (C.V.) of the size distributions were 2 to 3% indicating the generated particles were highly monodisperse. The generated particle sizes were defined as the volume equivalent diameter, Dve. The uncertainty analysis on the factors affecting Dve indicated that 95% confidence interval of the Dve is expected to be ±5%. The uncertainty of Dve was entirely caused by the uncertainty of the average mass of a droplet. The reproducibility of particle sizes within 0.5 to 10 μm was evaluated using an aerodynamic particle sizer. The C.V. of the measured particle sizes were less than 6% and 4% when NaCl particles and ionic liquid droplets were generated, respectively. Copyright 2014 American Association for Aerosol Research

Absolute Mass and Size Measurement of Monodisperse Particles Using a Modified Millikan's Method: Part II—Application of Electro-Gravitational Aerosol Balance to Polystyrene Latex Particles of 100 nm to 1 μm in Average Diameter

The masses and diameters of monodisperse polystyrene latex particles ranging from 100 nm to 1 μ m in average diameter were determined using the electro-gravitational aerosol balance, a method that we proposed in a previous paper. Charged latex particles were introduced between parallel plate electrodes similar to a Millikan cell, and the particle survival rate after a certain holding time had elapsed was measured as a function of the applied voltage. The number average diameter and the corresponding particle mass were determined by fitting a theoretical survival rate spectrum to the experimentally observed spectrum using the least squares method. In this fitting, the value of the particle density obtained in a separate experiment was used. Detailed uncertainty analysis of the number average diameters thus determined was carried out. The expanded uncertainty with a coverage factor of 2, which corresponds approximately to a 95% confidence interval, was 0.66 nm, or 0.66%, for 100 nm particles and 1.8 nm, or 0.18%, for 1 μ m particles. The diameter values of 200 nm particles obtained under different operating conditions were found to agree within 0.3%. These results demonstrate that the present method is appropriate for use as the primary sizing method to develop particle size standards in the size range of 100 nm to 1 μ m.

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.

Bayesian statistics for determination of the reference value and degree of equivalence of inconsistent comparison data

Three methods for the determination of the key comparison reference value (KCRV) and degree of equivalence of inconsistent comparison data are proposed in this study. These methods are, respectively, based on the premises of (1) unknown biases of individual measurement values, (2) underestimated uncertainties of individual participants or (3) additional and common uncertainty. Bayesian statistics were employed for the analysis using locally uniform priors. In the case of the first premise, Procedure B in the CIPM guidelines (2002 Metrologia 39 589?95) can be derived in the Bayesian context. In the case of the second and the third premises, the weighted mean is a possible candidate for the KCRV. These methods are exemplified using the key comparison data of CIPM CCM.FF-K3 and APMP.L-K1. Markov chain Monte Carlo simulations were conducted for calculations based on the latter two premises. From the results obtained, it is considered that, in addition to Procedure B in the CIPM guidelines, the method based on the second premise is also a robust method for the estimation of the KCRV.