The scanning mobility particle sizer (SMPS),
now the most common type of mobility analyzer,
was developed by Wang and Flagan [140] and
commercialized by TSI in 1993. The SMPS system
has three key components: the bipolar particle
charging chamber, the DMA, and the CPC. Bipolar
charging is applied in the charging chamber to
create a Boltzmann distribution of charges on the
particles. The charged particles then enter the
DMA from near the outer electrode. Figure 26b
shows a DMA used in commercial SMPS systems.
The particles, depending on their charge, are
attracted or repelled by the central electrode; the
voltage on the electrode may be varied. Particles
will thus experience a force depending on their
electrical mobility. Particles with a high mobility
are quickly deposited on one of the electrodes
(depending on polarity). Only particles within a
narrow, tunable range of mobility reach the extraction port at the end of the chamber. These selected
“monodisperse” particles are then introduced to the
CPC to measure their number concentration.
The DMA applies voltages to tune the mobility
range, and the voltages are scanned to obtain a
size-resolved number concentration. The SMPS is
often selected as a reference method in aerosol
research. In a classic SMPS program, a scan
takes 1 min or longer, introducing a risk that the
particle size distribution of the source could
change during the course of a single scan
[141]. Trostl et al. [142] have obtained a rapid
scan time of 3 s with a precision of Æ3% using a
newer model of SPMS (TSI 3082). However, the
scan range was limited to 2.21–60.4 nm. The
newest model of these instruments (TSI 3839)
provides the scan time of ~15 s.
Nano-sized particle size distributions can also be
measured using more rapid instruments such as the
Engine Exhaust Particle Sizer (EEPS, TSI 3090)
[143], which uses a design based on the electrical
aerosol spectrometer (EAS) [107], and the Fast
Mobility Particle Sizer (FMPS) (TSI 3091)
[110]. The time resolutions for the FMPS and the
EEPS are 1 and 10 Hz, respectively. While the
EEPS has been designed for engine emissions,
both can be used for industrial measurements [143].
Asbach et al. [144] compared the performance of
two TSI SMPS, one TSI FMPS, and one Grimm
SMPS. The FMPS measured a more narrow
distribution and lower concentrations for NaCl aerosols, while for diesel soot, it showed a broader
distribution and higher concentrations, relative to
the TSI instruments. The authors noted that this
difference was probably due to the different particle
morphologies or particle size-dependent effects. The
SMPS recorded consistent results for both particle
sources. In addition, they found that the FMPS
underestimated the nanoparticle size distribution
by approximately 15% compared to the SMPS.
The Grimm SMPS found broader distributions and
higher concentrations than the TSI instruments. In a
similar study comparing the performance of an
EEPS with a CPC and an SMPS, Johnson et al.
[109] found that the particle number measured by
the EEPS was 50% higher than that measured by the
CPC. For size distributions the number concentration measured by the EEPS was less than the SMPS
for particles larger than 80 nm, in agreement with
the results obtained by Asbach et al. [144]. In
another study the determinations of an FMPS and
an SMPS did not have a significant difference in
their size distributions below 200 nm, but for larger
sizes, the FMPS delivered unreliable results [145].
An important disadvantage for fast-sizing
instruments is the low electrometer sensitivity
for high particle concentrations [141]. Also, the
broad unipolar charge distribution used in these
electrometer-based analyzers means that they
have a lower resolution than the SMPS [126].
The development of a new class of device called
fast integrated mobility spectrometry (FIMS)
described by Kulkarni et al. [146] may eliminate
the need for voltage scanning and increase the time
resolution of the classic SMPS. As described by
Kulkarni et al. [146], in a FIMS, particles are
charged and electrically separated into different
streamlines. The separated particles are introduced
into a condenser and grow into larger droplets. The
droplets are subsequently exposed to a pulse of
laser light and their image captured by a CCD
array. The images are processed to determine the
number of particles and the particle distance from
the electric field, which directly depend on particle
mobility. Counting the particles as a function of
their mobility means that the FIMS is able to measure the number size distribution of particles at a
comparatively impressive time resolution, much
faster than a classic SMPS. The ability to detect
124
Airborne Nanoparticles: Control and Detection
now the most common type of mobility analyzer,
was developed by Wang and Flagan [140] and
commercialized by TSI in 1993. The SMPS system
has three key components: the bipolar particle
charging chamber, the DMA, and the CPC. Bipolar
charging is applied in the charging chamber to
create a Boltzmann distribution of charges on the
particles. The charged particles then enter the
DMA from near the outer electrode. Figure 26b
shows a DMA used in commercial SMPS systems.
The particles, depending on their charge, are
attracted or repelled by the central electrode; the
voltage on the electrode may be varied. Particles
will thus experience a force depending on their
electrical mobility. Particles with a high mobility
are quickly deposited on one of the electrodes
(depending on polarity). Only particles within a
narrow, tunable range of mobility reach the extraction port at the end of the chamber. These selected
“monodisperse” particles are then introduced to the
CPC to measure their number concentration.
The DMA applies voltages to tune the mobility
range, and the voltages are scanned to obtain a
size-resolved number concentration. The SMPS is
often selected as a reference method in aerosol
research. In a classic SMPS program, a scan
takes 1 min or longer, introducing a risk that the
particle size distribution of the source could
change during the course of a single scan
[141]. Trostl et al. [142] have obtained a rapid
scan time of 3 s with a precision of Æ3% using a
newer model of SPMS (TSI 3082). However, the
scan range was limited to 2.21–60.4 nm. The
newest model of these instruments (TSI 3839)
provides the scan time of ~15 s.
Nano-sized particle size distributions can also be
measured using more rapid instruments such as the
Engine Exhaust Particle Sizer (EEPS, TSI 3090)
[143], which uses a design based on the electrical
aerosol spectrometer (EAS) [107], and the Fast
Mobility Particle Sizer (FMPS) (TSI 3091)
[110]. The time resolutions for the FMPS and the
EEPS are 1 and 10 Hz, respectively. While the
EEPS has been designed for engine emissions,
both can be used for industrial measurements [143].
Asbach et al. [144] compared the performance of
two TSI SMPS, one TSI FMPS, and one Grimm
SMPS. The FMPS measured a more narrow
distribution and lower concentrations for NaCl aerosols, while for diesel soot, it showed a broader
distribution and higher concentrations, relative to
the TSI instruments. The authors noted that this
difference was probably due to the different particle
morphologies or particle size-dependent effects. The
SMPS recorded consistent results for both particle
sources. In addition, they found that the FMPS
underestimated the nanoparticle size distribution
by approximately 15% compared to the SMPS.
The Grimm SMPS found broader distributions and
higher concentrations than the TSI instruments. In a
similar study comparing the performance of an
EEPS with a CPC and an SMPS, Johnson et al.
[109] found that the particle number measured by
the EEPS was 50% higher than that measured by the
CPC. For size distributions the number concentration measured by the EEPS was less than the SMPS
for particles larger than 80 nm, in agreement with
the results obtained by Asbach et al. [144]. In
another study the determinations of an FMPS and
an SMPS did not have a significant difference in
their size distributions below 200 nm, but for larger
sizes, the FMPS delivered unreliable results [145].
An important disadvantage for fast-sizing
instruments is the low electrometer sensitivity
for high particle concentrations [141]. Also, the
broad unipolar charge distribution used in these
electrometer-based analyzers means that they
have a lower resolution than the SMPS [126].
The development of a new class of device called
fast integrated mobility spectrometry (FIMS)
described by Kulkarni et al. [146] may eliminate
the need for voltage scanning and increase the time
resolution of the classic SMPS. As described by
Kulkarni et al. [146], in a FIMS, particles are
charged and electrically separated into different
streamlines. The separated particles are introduced
into a condenser and grow into larger droplets. The
droplets are subsequently exposed to a pulse of
laser light and their image captured by a CCD
array. The images are processed to determine the
number of particles and the particle distance from
the electric field, which directly depend on particle
mobility. Counting the particles as a function of
their mobility means that the FIMS is able to measure the number size distribution of particles at a
comparatively impressive time resolution, much
faster than a classic SMPS. The ability to detect
124
Airborne Nanoparticles: Control and Detection