Environmental Engineering Reference
In-Depth Information
organic species within aerosols without sample pre-treatment, with fast time resolu-
tion and high sensitivity, leading to reduced artefacts. A large number of techniques
and principles have been implemented in the development of aerosol mass spec-
trometers and the basic concepts of the operation of these instruments are often
classifi ed according to the method by which particles are volatilized and the result-
ing gas phase compounds ionized. At the moment of writing this chapter, there are
two main classes of instruments.
The fi rst class of instruments uses one or two lasers to both vaporize and ionize
individual atmospheric particles into the mass spectrometer source region. This
class of instruments provides analysis at the single particle level and hence gives
information on the mixing state of particles. A few reviews are available (Noble
and Prather, 2000; Sullivan and Prather, 2005).
The second class of instruments uses thermal vaporization of individual or col-
lected particles followed by various ionization techniques. The most commons
ionization techniques are electron impact ionization and chemical ionization. The
separation of the vaporization and ionization steps permits the quantitative detec-
tion of the aerosol chemical composition for an ensemble of particles. Basic infor-
mation on the two classes of instruments is summarized in Table 5.3.
Laser - b ased m ethods
Instruments that use high-powered pulsed lasers to volatilize and ionize the com-
pounds to detect individual particles, hit effi ciently the particle in-fl ight, have a high
duty factor for particle detection and allow the infl uence of the chemical composi-
tion on the effi ciency of particle analysis to be easily assessed. However, particles
smaller than 200-300 nm cannot be detected by most laser-based methods.
Aerodynamic lenses are used to enhance the sampling effi ciencies for NPs down
to about 20 nm since they reduce the divergence of the particle beam. However,
they are not very effective below this size because of beam broadening from
Brownian motion. Also, the hit rates (defi ned as the ratio between the number of
particles hit by the laser that generates detectable ions) are generally low for aero-
sols smaller than 50 nm.
The Rapid Single-particle Mass Spectrometer (RSMS) is a laser ablation time-
of - fl ight mass spectrometer. The most recent generation of the RSMS is described
by Lake
et al.
(2003) and is called RSMS-III. An aerodynamic lens is used to focus
the particles and a freely fi ring laser beam (193 nm) is used to irradiate the particle
beam in order to generate positive and negative ions, which are analysed with dual
drift tubes. Ions exiting the drift tubes are accelerated and detected with multi-
channel plate detectors. Particles between 45 and 1250 nm are measured in nine
size bins determined by nine fl ow - limiting orifi ces. The optimum particle size range
of the RSMS-III was found to be 50-770 nm in diameter (Lake
et al.
, 2003 ).
Recent modifi cations of the Aerosol Time - of - Flight Mass Spectrometer
(ATOFMS) have increased its detection effi ciency for ultrafi ne particles. The instru-
ment is called the Ultrafi ne Aerosol Time - of - Flight Mass Spectrometer (UF -
ATOFMS) (Su
et al.
, 2004). An aerodynamic lens system replaces the converging
nozzle inlet used on conventional ATOFMS instruments. In addition, the light scat-
tering region has been modifi ed to enhance the scattering signals for smaller
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