The benefits of sizing aerosolized submicrometer particles using an electrical mobility sizing technique have been well documented. The technique is highly accurate and has been shown to size 60 nm and 100nm Standard Reference Material (SRM) with an uncertainty of only 1%. The National Institute of Standards and Technology (NIST) have been using electrical mobility to measure its 0.1μm Standard Reference Material (SRM) Particles for well over a decade.
Electrical Mobility Technique
Lately, the electrical mobility technique is finding increased use in the in-situ near real-time sizing of engineered nanoparticles synthesized by a variety of aerosol-based processes like diffusion flame synthesis, spray pyrolysis, thermal plasma etc.
Nanoparticles in Colloids
When combined with electrospray and other dispersion methods, the electrical mobility technique has been shown to accurately size nanoparticles suspended in colloids as well.
With the commercialization of nanotechnology, occupational health risks associated with manufacturing and handling of nanoparticles is a growing concern. Workers may be exposed to nanoparticles through means of inhalation, at levels that greatly exceed ambient concentrations; and no workplace standards exist to limit exposure to nanoparticles. Nanoparticle size governs their deposition pattern in various parts of the lung and their ultimate fate within a human body. Thus, ambient measurements of nanoparticle size distributions provided by the electrical mobility technique is a powerful tool in understanding adverse health effects associated with nanoparticle related exposure.
Scanning Mobility Particle Sizer
This article provides a brief overview of the electrical mobility technology as integrated in TSI Scanning Mobility Particle Sizer (SMPS) spectrometer followed by a discussion on applications in nanoparticle synthesis and exposure research.
Scanning Mobility Particle Sizer Spectrometer
The Scanning Mobility Particle Sizer (SMPS) spectrometer consists of a sample preconditioner, a bipolar charger, a nanoparticle size classifier and a nanoparticle detector. Figure 1 depicts a schematic of the entire system. The pre-conditioner (typically an impactor or a cyclone) eliminates large micrometer sized particles. The bipolar charger establishes bipolar charge equilibrium on the particles. This defined charge condition is necessary for the size classification using electrical mobility. Particles are size classified in a Differential Mobility Analyzer (DMA). The charged aerosol passes from the neutralizer into the main portion of the DMA. Figure 2 shows the schematic of a nano DMA.
Figure 1. Schematic of an SMPS
Figure 2. Schematic of Nano DMA
The nano DMA is specifically designed for sizing nanoaerosols in the size range of 2 nm to 150nm. The nano-DMA contains an outer, grounded cylinder and an inner cylindrical electrode that is connected to a negative power supply (0 to 10 kVDC). The electric field between the two concentric cylinders separates the particles according to their electrical mobility which is inversely related to the particle size. Particles with negative charge(s) are repelled towards and deposited on the outer wall. Particles with neutral charge exit with the excess air. Particles with positive charge(s) move rapidly towards the negatively-charged center electrode. Only particles within a narrow range of electrical mobility have the correct trajectory to pass through an open slit near the DMA exit. The electrical mobility of these selected particles is a function of flow rates, geometric parameters and the voltage of the center electrode.
Condensation Particle Counter (CPC)
The monodisperse particle stream exiting the DMA is counted by a Condensation Particle Counter (CPC). In the CPC, single particles larger than 2 nm are grown to micrometer size by means of condensation of a working fluid (alcohol or water) on the particles. The CPC then optically counts these particles. Particle size distributions are measured by changing the applied high voltage in the DMA, which changes the electrical field, thus scanning the whole size distribution.
Typical Applications in Nanotechnology
Sizing of Nanoparticles in Reactors
The electrical mobility technique is finding increasing use in the in-situ near real-time sizing of engineered nanoparticles synthesized by a variety of aerosol-based processes. The near real-time measurement offered by electrical mobility technique accelerates the research and development process of nanoparticle synthesis since it enhances the understanding of the mechanisms of particle formation and growth. An in-situ measurement eliminates the need for sample collection for off-line methods thus minimizing operator error and providing more consistent repeatable results. Figure 3 gives an overview of important steps in synthesis of nanomaterials in an aerosol based reactor. Real-time sizing of nanoaerosols in these reactors permits to follow dynamics of particle formation and growth in highly reacting flows. A precise control of particle size is key; real-time measurement of particle size distributions in the reactor provides the necessary feedback to control reactor conditions to achieve high quality control.
Figure 3. Aerosol processes for synthesis of nanomaterials
SMPS in Nanotechnology Research
SMPS has been increasingly employed in nanotechnology research. In 1991, Akhtar et al. used an SMPS system to study vapor synthesis of Titania powder, specifically, the effect of process variables (reactor residence time, temperature, and reactant concentration) on powder size and phase characteristics. The SMPS measured particle size distributions were used to validate particle coagulation model. Somer et al. (1994) used SMPS to study agglomeration of Titanium dioxide aerosol in high intensity field. Ahn et al. (2001) studied silica particle growth characteristics in H2/O2/TEOS diffusion flame. They found close agreement of SMPS measured size as compared to the Transmission Electron Microscope (TEM) image processed size data. Ullman et al. (2002) studied properties of nanoparticle aerosols of size 4.9-13 nm, generated by laser ablation. Measurements of eight materials including Silica, Carbon, Titania, Iron oxide, Tungsten oxide, Niobium oxide, Carbon and Gold were successfully achieved.
Other SMPS assisted studies of nanoparticle reactors include liquid flame spray (silver-titania deposit nanoparticles), ethylene flame (soot nanoparticles) and thermal plasma reactors (Si, Ti particles) to name a few. Recently, Zhang et al. (2007) studied temperature effects on Tellurium dioxide synthesis by spray pyrolysis. SMPS data from this reactor (figure 4) clearly shows transition of precursor droplets to product droplets as the temperature increases.
Figure 4. Particle size distributions as measured by SMPS at different furnace temperatures.
Sizing of Nanoparticles Suspended in Colloids
A majority of nanoparticles are produced via a colloidal chemistry route. When combined with electrospray dispersion, the electrical mobility technique has been shown to accurately size nanoparticles suspended in colloids. Figure 5 demonstrates the high size resolution of SMPS. The size distributions of a mixture of nine different proteins and of electrosprayed bovine serum albumin (BSA) nanoparticles were measured with an SMPS (TSI Model 3936-N25). Lengegoro et al. have demonstrated successful use of electrospray and SMPS to determine size distribution of different types of colloids (oxides, metals, and polymers) such as silica, gold, palladium, and polystyrene latex particles, with different nominal sizes below 100 nm. The measured values of particle sizes in their study were found comparable to results obtained by electron microscopy and dynamic light scattering.
Figure 5. Size distributions of electrosprayed nanoparticles measured with SMPS. (a) Mixture of 9 different proteins; (b) BSA nanoparticles.
Naonoparticle Exposure Analysis
Besides the near real-time analysis related to nanoparticle processes discussed above, electrical mobility analysis with SMPS can also be used to monitor process related nanoparticle exposure.
The high size resolution allows the calculation of particle surface and particle volume distributions. Figure 6 shows an example: the measurements were taken during emptying an ultrafine Titanium dioxide baghouse into a powder collection bucket. Besides the SMPS data (number median diameter), the figure 6 a shows total number concentration measured with a CPC and total alveolar deposited surface area concentration measured with a Nanoparticle Surface Area Monitor (TSI NSAM Model 3550). Figure 6 b depicts SMPS measured particle size distribution in the ambient air close to the material handling operation. A peak in concentrations during middle of process coincided with dumping of a drum of Titania powder in a reservoir.
Figure 6. Titanium dioxide concentrations (a) counts, deposited surface area and number median diameter; (b) SMPS size distributions.