Nanoparticle Size Distribution Measurement

3.5.1 Measuring Size Distribution using Particle Mobility Analysis. The most common instrument used for measuring size distributions of aerosols of nano-particles is the Scanning Mobility Particle Sizer (SMPS). The SMPS is capable of measuring aerosol size distribution in terms of particle mobility diameter from approximately 3 nm up to around 800 nm, although multiple instruments typically need to be operated in parallel to span this range. A schematic diagram is given in Figure 6.

It comprises an electrical aerosol analyser (EAA) to separate the particles according to the electrical mobility diameter followed by a CPC or an electrometer to count the particles. Particles enter a pre-selector with a cut-point at 1 mm into a region where they are then charged to Boltzmann equilibrium by passing them through a bipolar ion cloud formed from a radioactive source. They then pass through a well-defined electrostatic field in the EAA. Electrostatic forces lead to charged particles moving between the electrodes, and particles with a specific mobility are sampled from a small outlet at the exit of the electrodes, form where they are passed to a CPC or electrometer for counting. By scanning the voltage between the electrodes, particles with electrical mobilities corresponding to a range of particle diameters can be counted sequentially, allowing the aerosol size distribution to be determined. In an alternative configuration, the voltage between the electrodes may be stepped rather than continuously scanned.

The sequential scanning or stepping of the voltage takes a significant time with the fastest conventional scan speeds being about 3 minutes, which is suitable provided that the process being monitored does not change within this timescale. Fast Mobility Particle Sizers have been developed using a unipolar

High Performance Filter

High Performance Filter

Corona Particles Counter
Figure 6 Schematic of Scanning Mobility Particle Sizer (SMPS).

particle charger, and a parallel array of electrometer-based sensors to count the size segregated particles. Measurements may be made with a time resolution of one second or less, and operation at ambient pressures reduces evaporation of volatile particles. The instruments are limited to measurements at relatively high aerosol number concentrations, although the lack of a radioactive source may make them a viable alternative to the SMPS in many workplaces. Research is currently being carried out to develop more compact, and therefore cheaper, aerosol mobility classifiers relying on particle migration across an opposing air flow,36 and it is hoped that these will be available soon. Currently SMPS instruments are available from three main companies TSI (USA), Grimm (Austria) and Cambustion (UK).

The SMPS is limited in its widespread application in the workplace due to its size, expense, complexity of operation, the need for two or even three instruments operating in parallel to measure wide aerosol size distributions, and the use of a radioactive source to bring the aerosol to charge equilibrium.

3.5.2 Measuring Particle Size Distribution using Inertial Impaction. Cascade impactors are widely available in a number of configurations, allowing either personal or static sampling with a range of particle size cut points. Personal cascade impactors are available with cut-points of 250 nm and above, and thus are only able to provide very limited information on size distribution in the nanometre size range. Static cascade impactors are available with lower cut points in the nanometre size region, as well as low-pressure impactors or multi-orifice impactors.

There is number of low-pressure cascade impactors available and a recently developed one is the Dekati DLPI, which has 13 impactor stages with stage D50 cut-off sizes between 10 mm and 30 nm and a high-efficiency filter to collect the <30 nm particles. It can operate at flowrates of either 10 or 30 l min-1, depending upon requirements. The stages are greased to prevent particle bounce causing contamination of the lower stages. The MOUDI Model 110 microorifice impactor consists of ten impactor stages, with D50 cut-off sizes between 18 mm and 56 nm; it also has a high-efficiency filter to collect the <56 nm particles. In the MOUDI, the collection substrates can be rotated relative to the impaction plates. This is to prevent the upper stages from becoming overloaded, resulting in coarse particle contamination of the lower stages, and enables the device to be operated for long periods of time. The MOUDI is operated at a flowrate of 30 l min-1. The particles of aerodynamic diameter <56 nm can be further separated by feeding the output to an additional 3 stage Nano-MOUDI with D50 cut-off sizes of 32, 20 and 10 nm. Both impactors require high pressure, mains-powered pumps to provide the necessary flowrates and so are not suitable for personal sampling.

Determination of aerosol size distribution from cascade impactor data requires the application of data inversion routines. The simplest approach is to calculate cumulative mass concentration with particle diameter, and use the data to estimate the mass median aerodynamic diameter (MMAD) and the Geometric Standard deviation (GSD) of the size distribution. This approach assumes no losses between collection stages, ideal impactor behaviour, and a unimodal aerosol with a log-normal size distribution. Cascade impactors are usually used to measure the mass-weighted aerosol size distribution, and so assumptions of particle shape and density need to be made in order to estimate the number or surface-area weighted distribution. As these parameters are rarely quantified, great care needs to be taken in interpreting cascade impactor data in terms of aerosol number or surface area.

3.5.3 Electrical Low Pressure Impactor (ELPI) Measurements. The Electrical Low Pressure Impactor (ELPI) combines inertial collection with electrical particle detection to provide near-real-time aerosol size distributions for particles larger than 7 nm in diameter.37 Aerosol particles are charged in a unipolar ion charger before being sampled by a low pressure cascade impactor discussed in Section 3.4.2. Each impactor stage is electrically isolated, and connected to a multi-channel electrometer, allowing a measurement of charge accumulation with time. As in the case of the diffusion charger (Section 3.3.3), particle charge is directly related to active surface area. Thus the integrated electrometer signal from all stages is directly related to aerosol active surface area.

The electrometer signal from a single stage is related to the active surface area of particles within a narrow range of aerodynamic diameters, allowing limited interpretation of the shape of sampled particles. If the particle charging efficiency as a function of aerodynamic diameter is known or can be assumed, real-time data from the ELPI can be interpreted in terms of the aerosol number-weighted size distribution. In practice, particle-charging efficiency is determined experimentally. Interpretation of measurements in terms of particle mass concentration or mass-weighted size distribution can also be carried out, although it requires the effective particle density as a function of size to be known.

As well as allowing on-line measurements of particle concentration and size distribution, aerosol samples collected by the ELPI are available for off-line analysis, including electron microscopy and chemical speciation. A diagram of the operating principle of the ELPI is shown in Figure 7.

3.5.4 Calculations of Nanoparticle Concentrations from Size Distribution Measurements. As well as providing information about the particle size

Sampling Inlet

Corona Charger Operated with Low Voltage

Impactor Stages

Back-up Filter Stage

Flow Control

Clean Flush Air for Zeroing Filter

High Voltage and Ion Trap Source

Sampling Inlet

Clean Flush Air for Zeroing Filter

Impactor Stages

Back-up Filter Stage

High Voltage and Ion Trap Source

Flow Control

Pressure Gauge

Exhaust Air

Figure 7 Diagram of operating principle of Electrical Low Pressure Impactor (ELPI).

Pressure Gauge

Exhaust Air

Figure 7 Diagram of operating principle of Electrical Low Pressure Impactor (ELPI).

characteristics of the aerosols in workplaces where nanoparticles are being produced or handled, size distribution measurements can be used to calculate integrated nanoparticle exposure levels. For example, number-weighted size distributions can be used simply to calculate number concentrations, or with the assumption that the particles are nearly spherical and that their physical diameters were equivalent to their mobility diameters (for SMPS, see below) or aerodynamic diameters (for LPI, see below), the aerosol surface concentration can be calculated. Similarly, with knowledge of particle density, the aerosol mass concentrations can be determined. However, the accuracy of these estimations is dependent upon the assumptions made about the physical characteristics of the particles. Ku and Maynard33 showed that for monodisperse aerosol particles smaller than 100 nm, particle geometric surface areas calculated by SMPS size distributions agree to within ± 20% of those given by a diffusion charger surface area instrument. However, for larger particles, the relationship diverged with the DC instrument underestimating compared to the SMPS because of the change in response of the DC instrument. A similar relationship was found by Shi et al.38 for polydisperse aerosols found in the ambient atmosphere. From comparative measurements at two outdoor sites they found good agreement between geometric surface area measurements using the epiphaniometer (see Section 3.3.3) and the SMPS. It is therefore reasonable to suggest that, provided that suitable pre-selectors are used with DC instruments, reliable measurements of geometric surface area can be obtained.

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