Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 27

size to be known, constant, and reproducible. It was found that in the case of water-
based CPCs the eventual droplet size is significantly smaller for hydrophobic than
for hydrophilic particles. The photometric mode was therefore dropped in the latest
versions of water CPCs.
Depending on the model, CPCs can measure particles with sizes down to 2.5 nm.
Particularly in the case of water-based CPC, the lower size limit is affected by the
hygroscopicity of the particle material. While Hering et al. (2005) and Petäjä et al.
(2006) reported the cut-off diameter of model 3785 (TSI) water CPC to fluctuate by
only about ±1 nm with solid hydrophilic and hydrophobic particles, Keller, Tritscher
and Burtscher (2013) found that the newer water CPC model 3788 (TSI) under-
counted even 70 nm soot particles by approximately 50%.
An upper particle size limit of 1 µm is often specified by the CPC manufactur-
ers. The actual upper limit is, however, usually unknown and mainly defined by
particle losses inside the CPC. It has been shown in the past that CPCs are capable of
measuring concentrations of particles with sizes up to several microns (unpublished
data). It should be noted that the number concentrations are usually dominated by
particles that are much smaller than 1 µm, whereas the number concentration of
particles >1 µm is commonly negligibly small.
Although most CPCs are rather large and mains operated, a small number of
handheld CPCs are also available. These handheld CPCs are battery driven and oper-
ated with an alcohol cartridge to provide the working fluid. The battery lifetime and
alcohol reservoir allow for an independent operation for approximately 6-8 hours.
It has been shown that handheld CPCs can be accurate to within ±5% if well main-
tained and used within their specification limits (Hämeri et al. 2002; Asbach et al.
2012).
2.2.2 d iffusion C harger -B ased i nstruments
Another group of instruments that determine particle size-integrated and time-
resolved particle concentrations have entered the market in recent years. These
instruments use unipolar diffusion charging followed by particle charge measure-
ment. It was shown that the charge level obtained by particles in unipolar diffusion
charging is proportional to the fraction of the particle surface area that would deposit
in the alveolar region of the human lung (Shin et al. 2007; Fissan et al. 2007). Since
several studies have shown that the adverse biological response of inhaled particles
seems to correlate best with the total surface area dose (Oberdörster, Oberdörster,
and Oberdörster 2005) of the lung-deposited particles, possibility of measuring the
lung-deposited surface area (LDSA) concentration has raised increased attention.
Another study revealed that unipolar diffusion charging can only mimic the LDSA
concentration in the range from 20 to 400 nm (Asbach et al. 2009a). Instruments
that determine the LDSA concentration are the Nanoparticle Surface Area Monitor
(NSAM, model 3550, TSI Inc., USA), Aerotrak 9000 (TSI), miniDiSC (Fierz et al.
2011, identical with DiSCmini, Matter Aerosol, Switzerland), nanoTracer (Marra,
Voetz, and Kiesling 2010; Philips, The Netherlands, discontinued), and Partector
(naneos, Switzerland). The NSAM was the first instrument of this kind and is still
a rather bulky and mains-operated device. Aerotrak 9000 is basically the same