Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 37

2.4 SIZE-RESOLVED AND TIME-INTEGRATED MEASUREMENTS
Size-resolved and time-integrated measurements provide particle size-classified
information such as the mass or number concentration or the chemical composition
as a mean value over the sampling time, which is often a full working shift, that is,
eight hours. Usually, the airborne particles are sampled onto a substrate, which is
then used for downstream analyses. Size discrimination may be done already during
sampling by a size classifier or by size determination during the subsequent analysis.
2.4.1 i inertial P artiCle s amPlers
Size-resolved and time-integrated measurements can be achieved by using size-
classified particle sampling along with subsequent analyses of the particles sampled
on the different stages. Typically, cascade impactors are used for this purpose, that
is, a cascade of impactor stages (see Figure 2.6) with cut-off diameters decreasing
from stage to stage. Depending on the model, cascade impactors can sample par-
ticles between approximately 10 nm and 10 µm. While some cascade impactors use
only three stages for sampling particles >10 µm, 2.5-10 µm, and 1-2.5 µm, others
use many more stages to provide samples with higher size distribution. In a cascade
impactor, the particles are sampled commonly onto flat substrates, which can later
be used for weighing to determine the mass size distribution or electron microscopic
analysis coupled with electron dispersive X-ray (EDX) spectroscopy to determine the
morphology and chemical composition of the particles. Other cascade impactors have
been designed to sample onto glassy carbon substrates for analyses by total X-ray
reflection fluorescence spectroscopy (John et al. 2001) or to homogenously deposit
the particles across the substrate by using a large number of very small parallel ori-
fices per impactor stage (Micro Orifice Uniform Deposit Impactor, MOUDI, MSP
Corp., USA; Marple et al. 1986). To further homogenize the sample, the substrates
may rotate during sampling. MOUDI is available in different models. The stationary
models operate at a flow rate of 30 L/min and can have between 3 and 14 stages to
sample particles in an overall size range from 10 nm to 18 µm. Figure 2.10 shows
the collection efficiency of the different stages of a MOUDI-II. Mini-MOUDI is a
miniaturized MOUDI, which operates at a flow rate of only 2 L/min and can host six,
seven, eight, or ten impaction stages, where the sixth and eighth stage versions can be
used as personal samplers.
2.4.2 t Thermal P artiCle s amPlers
Thermal precipitators have been used as personal samplers for many decades to assess
exposure to airborne dust. The sampling principle makes use of the thermophoretic
motion of particles, which arises from inhomogeneous momentum transfer from col-
liding molecules in a temperature gradient. The molecules on the “warm side” of the
particles transfer a higher momentum than their counterparts on the “cold side,” result-
ing in a net particle motion from warm to cold regions. If a temperature gradient is
established near a cold plate, particles are deposited onto this plate and are available for
further analyses, for example, of the particle size distribution by (electron) microscopy.