aerosol mass spectrometer (AMS), scanning electron microscopy (SEM), and transmission electron
microscopy (TEM).
An AMS designed and developed by Aerodyne Research, Inc. (ARI), is used worldwide to
determine the size-resolved mass concentrations
of non-refractory aerosol. At first, the Aerodyne
AMS was useful for measuring the ensemble
average data of the composition of the mass size
distributions of fine particles with aerodynamic
diameters of ~50–1000 nm, based on a quadrupole mass spectrometer [155]. With time the sensitivity and time resolution of the AMS were
developed along with its ability to analyze single
particles, along with a shift to time-of-flight
(ToF)-AMS which provide higher mass resolution. The instrument has three main sections: the
aerosol inlet (particle beam generation), the particle size discrimination chamber, and the composition measurement chamber. Fine particles are
focused into a particle beam by an aerodynamic
lens in the entrance section and then pass through
a sizing chamber. After that, the beam enters the
particle composition detection chamber where
particles reach a hot surface (~600
C) causing
them to vaporize. The chemical composition of
the vapor is determined by electron impact ionization (EI) and mass spectrometry [156].
Previous studies have shown the technique can
be used to measure the size-resolved mass compositions of particles [115, 155–157]. Jimenez at
al. [155] used a ToF-AMS for the study of atmospheric fine particles. This study determined the
size-resolved mass composition ratios and chemically resolved mass distribution for atmospheric
fine particles. Also, they observed that mass concentrations of sulfate and nitrate measured with
the AMS agree with those measured by ion
chromatography-based instruments. TOF-AMS
has been developed by Su et al. [157] to measure
fine and ultrafine particles, specifically particles
measuring less than 100–300 nm in diameter. The
aerodynamic size of the particles is determined
using light scattered as the particle passes through
two continuous-wave laser beams. Onasch et al.
[115] used a soot particle AMS (SP-AMS) for the
physical and chemical characterization of black
carbon. The SP-AMS is equipped with an
intracavity laser vaporizer (1064 nm), along with
a common resistively heated tungsten vaporizer.
They showed that the laser vaporizer could measure both the refractory and non-refractory
components.
SEM is a microscopic method that probes a
sample with an electron beam and can be used to
characterize the morphology and to some extent
the composition of nanoparticles deposited on a
conductive substrate. A typical SEM includes the
electron column, scanning system, and detectors.
The electron column operates in vacuum and consists of an electron gun and electromagnetic
lenses. A sample is introduced into the column
and is irradiated by a beam of electrons generating
secondary electrons (SEs), backscattered electrons (BSEs), and X-rays [158]. Generally, the
secondary electrons are produced by inelastic
scattering, while backscattered electrons are
caused by elastic scattering of the electrons by
the sample. Normally secondary electrons have a
low energy, ca. 50 eV. These electrons are analyzed, for example, using an Everhart-Thornley
detector with a scintillator/photomultiplier system, yielding topographic information about the
sample. For SE images, the edges of the sample’s
elements are usually brighter, due to the intensity
of electron emission. The generation of backscattered electrons is related to the atomic number
of the elements in the sample with higher atomic
number giving a brighter image, providing a
rough elemental analysis especially for heavier
elements. Backscattered electrons are generated
at depths of 0.5–1 mm, and thus the spatial resolution of the images is less than for secondary
electrons. The backscattered electrons can be
detected by scintillator and solid-state detectors.
When secondary electrons are produced by the
interaction of the electron beams and the sample
atoms, holes are left in the inner shells of the atom.
X-rays are emitted when outer shell electrons
relax into the vacancy. The frequencies of light
are characteristic for specific elements and even
contain information about their chemical binding.
For technical reasons, these characteristic X-rays
are analyzed for all elements except hydrogen and
helium. The X-rays are detected by either EDS or
wavelength dispersive spectroscopy. In an SEM,
126
Airborne Nanoparticles: Control and Detection
microscopy (TEM).
An AMS designed and developed by Aerodyne Research, Inc. (ARI), is used worldwide to
determine the size-resolved mass concentrations
of non-refractory aerosol. At first, the Aerodyne
AMS was useful for measuring the ensemble
average data of the composition of the mass size
distributions of fine particles with aerodynamic
diameters of ~50–1000 nm, based on a quadrupole mass spectrometer [155]. With time the sensitivity and time resolution of the AMS were
developed along with its ability to analyze single
particles, along with a shift to time-of-flight
(ToF)-AMS which provide higher mass resolution. The instrument has three main sections: the
aerosol inlet (particle beam generation), the particle size discrimination chamber, and the composition measurement chamber. Fine particles are
focused into a particle beam by an aerodynamic
lens in the entrance section and then pass through
a sizing chamber. After that, the beam enters the
particle composition detection chamber where
particles reach a hot surface (~600
C) causing
them to vaporize. The chemical composition of
the vapor is determined by electron impact ionization (EI) and mass spectrometry [156].
Previous studies have shown the technique can
be used to measure the size-resolved mass compositions of particles [115, 155–157]. Jimenez at
al. [155] used a ToF-AMS for the study of atmospheric fine particles. This study determined the
size-resolved mass composition ratios and chemically resolved mass distribution for atmospheric
fine particles. Also, they observed that mass concentrations of sulfate and nitrate measured with
the AMS agree with those measured by ion
chromatography-based instruments. TOF-AMS
has been developed by Su et al. [157] to measure
fine and ultrafine particles, specifically particles
measuring less than 100–300 nm in diameter. The
aerodynamic size of the particles is determined
using light scattered as the particle passes through
two continuous-wave laser beams. Onasch et al.
[115] used a soot particle AMS (SP-AMS) for the
physical and chemical characterization of black
carbon. The SP-AMS is equipped with an
intracavity laser vaporizer (1064 nm), along with
a common resistively heated tungsten vaporizer.
They showed that the laser vaporizer could measure both the refractory and non-refractory
components.
SEM is a microscopic method that probes a
sample with an electron beam and can be used to
characterize the morphology and to some extent
the composition of nanoparticles deposited on a
conductive substrate. A typical SEM includes the
electron column, scanning system, and detectors.
The electron column operates in vacuum and consists of an electron gun and electromagnetic
lenses. A sample is introduced into the column
and is irradiated by a beam of electrons generating
secondary electrons (SEs), backscattered electrons (BSEs), and X-rays [158]. Generally, the
secondary electrons are produced by inelastic
scattering, while backscattered electrons are
caused by elastic scattering of the electrons by
the sample. Normally secondary electrons have a
low energy, ca. 50 eV. These electrons are analyzed, for example, using an Everhart-Thornley
detector with a scintillator/photomultiplier system, yielding topographic information about the
sample. For SE images, the edges of the sample’s
elements are usually brighter, due to the intensity
of electron emission. The generation of backscattered electrons is related to the atomic number
of the elements in the sample with higher atomic
number giving a brighter image, providing a
rough elemental analysis especially for heavier
elements. Backscattered electrons are generated
at depths of 0.5–1 mm, and thus the spatial resolution of the images is less than for secondary
electrons. The backscattered electrons can be
detected by scintillator and solid-state detectors.
When secondary electrons are produced by the
interaction of the electron beams and the sample
atoms, holes are left in the inner shells of the atom.
X-rays are emitted when outer shell electrons
relax into the vacancy. The frequencies of light
are characteristic for specific elements and even
contain information about their chemical binding.
For technical reasons, these characteristic X-rays
are analyzed for all elements except hydrogen and
helium. The X-rays are detected by either EDS or
wavelength dispersive spectroscopy. In an SEM,
126
Airborne Nanoparticles: Control and Detection