Aerosol Instruments: Manual, Size Resolved


Hi, my name is Tom Peters. I am from the University of Iowa. In this video, we are going to talk about aerosol instruments that are manual and size resolved. There are many different categories of sampling instruments. They include instruments that are manual or direct-reading and those that are size integrated or size resolved. Here, we will focus on manual, size resolved instruments. This category includes impactors and samplers designed to collect particles for microscopy. Manual means that a sample must be physically collected and then analyzed to measure concentration. Sampling must be conducted long enough to
to collect sufficient material for analysis, typically 8 hours for occupational and 24 hours for environmental sampling. Resolved by size means that concentration is measured for many size ranges, or size bins. These size-resolved data can be used to estimate the size distribution by other metrics, such as the use of size-resolved mass
concentration data to estimate particle number concentration. Cascade impactors provide a way to collect particles in different size ranges. They pull air through a series of impactor ‘stages’ that collect smaller and smaller particles. An example pictured here is the nanoMOUDI, which has stages with 50% collection efficiencies that range from 10 nanometers to 18 micrometers in diameter. The large size and weight of many cascade impactors limits their use to area sampling. However, small versions are available for personal sampling that can be operated with belt-mounted sampling pumps. These personal versions can be used for particles larger than 500 nanometers and have fewer stages — less sizing resolution — than other cascade impactors such as the nanoMOUDI. Each stage of a cascade impactor must be analyzed separately to obtain size resolved data. Stages can be pre- and post-weighed to directly measure the mass of all the particles or analyzed chemically to directly measure the mass of certain chemical species. to directly measure the mass of certain chemical species. Here I show measurements of welding fume sampled with a nanoMOUDI with each stage analyzed chemically by inductively coupled plasma-mass spectrometry, or ICP-MS. On the left, we have a plot of the mass concentration of iron by particle diameter. On the left, we have a plot of the mass concentration of iron by particle diameter. The error bars represent the standard deviation for three replicate measurements. These size-resolved data allow us to see modes in the size distribution. Most of the mass is associated with fine particles but there are also coarse particles. With ICP-MS, we can also obtain information on other elements at the same time. The plot on the right shows the size distribution for chromium. Although these data provide a wealth of information, they are time-intensive and costly to obtain. There are special collection devices for microscopy. Filters often have complicated structure, making it difficult to identify particles apart from the filter media. Instead, we need flat, featureless surfaces to maximize our ability to “see” particles with microscopy. One way to achieve this is by electrostatic precipitation. The ESPnano is a specific instrument that is available for this purpose. In this device, particles in an incoming air stream are electrically charged and then attracted to and deposit on an oppositely charged substrate suitable for electron microscopy. An alternative way to collect particles is to use a thermal precipitator. As an aerosol is drawn through an intense thermal gradient, particles move from a region of high to low temperature, depositing on a substrate suitable for electron microscopy. A commercial version of this device is available from RJ Lee Group. After collection, particles can be analyzed by microscopy. Light microscopy is limited to particles roughly larger than the wavelength of light, or about 500 nanometers. Scanning electron and transmission electron microscopy allow detection to much smaller particles, Scanning electron and transmission electron microscopy allow detection to much smaller particles, Scanning electron and transmission electron microscopy allow detection to much smaller particles, around 50 nanometers for scanning and 10 nanometers for transmission electron microscopy. From analysis of pictures, the number concentration of particles can be binned by particle size. The state-of-the-art in microscopy is computer-control of the microscope with energy dispersive spectroscopy. The computer drives the microscope until a particle is found. Then, the particle size and morphology of the particle are assessed by image analysis. Finally, the composition of the particle is determined from emitted x-rays by energy dispersive spectroscopy. Such technology is exciting because then we can then figure out what portion of the size distribution is due to certain types of particles, such as engineered particles. With electron microscopy, detailed characterization of particles can also be accomplished. Here is a transmission electron microscope image of a particle collected when sanding epoxy strengthened with multi-walled carbon nanotubes. These images can be used to show that carbon nanotubes protrude from the larger epoxy particles.
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