Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 193

5.6.4 Chemical Composition

As for total particulate matter, the measurement of nanoparticle chemical com-
position may be conducted using a sampling device such as an impactor for the
collection of NPs, with subsequent extraction and chemical analysis in the labora-
tory, or may be achieved using new on-line techniques such as aerosol mass spec-
trometry. However, at the moment of writing, few aerosol mass spectrometers are
able to measure the chemical composition of NPs, although latest results show
promise.

5.6.4.1 Methods for Bulk Analysis

Size-segregated particles can be collected using impactors such as the MOUDI, the
Berner or the ELPI (see description above) that fi rst classify particles according to
their aerodynamic diameters on a series of stages with decreasing cut size. The
lowest cut-point ranges from 30 to 50 nm depending on the impactor type and
particles smaller than the lowest cut-point are collected downstream on a back-up
fi lter. This means that the last stages of impactors generally permit the collection
of NPs. Impaction substrates are used in order to avoid particle bounce during
collection. After collection, samples are transferred to a laboratory for solvent
extraction and analysis using various analytical techniques, such as high perfor-
mance liquid chromatography (HPLC), gas chromatography (GC) or inductively
coupled plasma mass spectrometry (ICPMS) depending on the nature of com-
pounds measured. The impaction substrate for collection and the solvent used for
extraction also depend on the compounds measured. Non-destructive analytical
methods (i.e. no solvent extraction) such as X-ray techniques may also be used.
These methods require long sampling periods to acquire enough matter to be
quantitatively measured and can be the subject of sampling artefacts, such as
particle bounce, volatilization of semi-volatile compounds and adsorption of gases
during particle collection, losses and contamination during transport and storage,
and so on. Excellent comprehensive reviews on particulate measurement are
available (Chow, 1995; McMurry, 2000) and the reader may refer to them for more
details.

5.6.4.2 Aerosol Mass Spectrometry

Overview of a erosol m ass s pectrometry
The two past decades have seen the emergence of aerosol mass spectrometry
instruments for on-line chemical analysis of aerosols, but further efforts are still
required for the quantitative chemical analysis of NPs, especially for the smallest
ones (below 50 nm).
Aerosol mass spectrometers generally comprise an inlet to introduce the parti-
cles into vacuum and to concentrate them from the gas phase, a possible particle
sizing unit, a vaporization unit and a mass spectrometry unit for the ionization of
vaporized neutral molecules and the mass analysis. The principle of a mass spec-
trometer is to separate and count ions according to their mass to charge ratios
(m/z). Mass spectrometry allows the measurement of a large range of inorganic and