Hi, my name is Tom Peters of the University of Iowa. In this video, we are going to talk about
aerosol instruments that are manual and integrated by size.
There are many different categories of aerosol sampling instruments. They include instruments that are manual or
direct-reading, and those that are size integrated or size resolved. Here, I'll focus on manual and size integrated. This category
includes filter samplers and size-selective samplers.
Manual means that a sample must be physically collected and then analyzed to
measure concentration, usually mass concentration. Sampling must be conducted for long enough to collect sufficient material for
analysis, typically 8 hours for occupational and 24 hours for environmental sampling. Integrated by size means that a single, average
concentration will be used to represent many sizes. Most of these methods provide a measurement of particle mass concentration.
Thus, I depict average mass concentration integrated over many sizes on the concentration by size plot at right.
Often, particles are collected onto filters for subsequent analysis. Filters collect a range of particles with varying efficiency depending on
how the filter is made. The combined effect of diffusion and impaction results in a collection efficiency curve that is typical of filters. Here, I
show particle collection efficiency by particle size. The purple curve is typical of a low-efficiency filter, like a low-cost furnace filter.
High collection efficiency is achieved for very small particles, say 10 nanometers, due to diffusion,
and for large particles, larger than 5 micrometers, due to impaction. However, the lowest collection efficiencies occur
for particles in the middle size range, about 300 nanometers, because diffusion and impaction
have the least affect on these size particles. Air sampling filters are designed to have high collection efficiency for all size particles. Thus,
particles are collected if they reach the filter.
Typically, filters are sold for air sampling already installed in filter cassettes. Here, we
see a filter in a filter cassette, which is then clipped to the lapel of a worker. Airflow is pulled from the breathing zone through the filter by a
belt-mounted air pump. The filter cassette is composed of a bottom section, which holds a support pad and the air sampling filter. These
components are held in place with a ring that is press fit into the bottom section. Filter cassettes can be operated with a top inlet
(closed faced sampling) or without the top inlet (open faced sampling).
Often certain size particles are excluded from reaching the filter in
an effort to mimic some aspect of particle deposition in the human respiratory system. This type of sampling is called size-selective
sampling. Particles deposit in the human respiratory tract with different efficiencies, resulting in the plot shown here.
300-nanometer-sized particles have the lowest deposition efficiency --
only about 15% of them deposit when inhaled, whereas 85% are breathed back out without depositing.
The fraction of particles depositing in the respiratory system increases for particles progressively smaller than 300 nanometers
due to diffusion. For particles progressively larger than 300 nanometers,
deposition increases due to inertial deposition, primarily in the head airways,
although they also deposit by gravitational settling if they pass to the alveolar region. Gravitational settling of particles larger than
approximately 5 micrometers makes them increasingly difficult to aspirate into the human body.
Many occupational regulations require that particles
are collected with specific sampling efficiencies, called "sampling criteria". Inhalable samplers collect those particles that can be aspirated
into the human respiratory tract. So, the inhalable criterion, shown in blue, follows the respiratory deposition fraction for particles
larger than about 5 micrometers. Shown in yellow, the thoracic criterion is designed to collect only
those particles that pass into the head airways, nominally those particles smaller than 10 micrometers.
And then, shown in green, samplers based on the respirable criterion collect only those particles that can pass
through to the deep lung, nominally smaller than 4 micrometers.
Inhalable samplers follow the inhalable criterion that was shown in
blue in the previous slide. They are designed to aspirate particles like the human mouth and nose. Examples of inhalable samplers are the
IOM sampler that has an opening a bit like a mouth with the filter partially exposed. Another example is the button sampler that has a
perforated metal sphere that provides more protection to the filter. Various vendors sell size-selective cyclones, which are typically designed
to collect particles following the respirable criterion, the green curve that was presented earlier.
Air entering a cyclone is forced to spin several times
with larger particles hitting the wall and being forced to collect in a grit pot in the tip of the cyclone. The smaller (respirable) particles
remain airborne and pass to a filter often housed in a filter cassette, like that shown here. There are also size-selective impactors, such
as the Personal Exposure Monitor (PEM) sampler or the Parallel Particle Impactor. In these samplers, larger particles collect on an
impaction plate and the smaller particles that remain airborne are collected on a filter.
The Parallel Particle Impactor is unique in that it has different diameter
impactor nozzles to produce an efficiency curve closely matching either a thoracic or respirable criteria at different airflow rates. There are
different ways to analyze collected particles. One of the most common is gravimetric analysis. Pre- and post-sampling, the filter is
conditioned in a room with controlled temperature and relative humidity. Any charge on the filter is neutralized, and then the filter is
weighed on a specialized balance. The weight gained during the sampling is interpreted as the mass of collected particles.
Mass concentration is then calculated as the mass of collected particles divided by airflow rate and sampling time.
Alternatively, particles collected on a filter can be analyzed chemically. First, the filter with the collected particles are transferred into a phase
suitable for analysis. This process is called digestion. Second, the digestate is analyzed by one of many different analysis techniques with
output being related to the mass of particles of a particular composition deposited on the filter.
Lastly, the concentration is calculated in the same way as gravimetric
analysis, except that the mass is determined chemically rather than on a balance.