Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 26

1 s. In the lower concentration range, single light scattering events are counted. When
the concentration increases, there is an increasing chance that more than a single
particle is present in the measurement cell such that the signals can no longer be
differentiated. This so-called coincidence error causes too low concentrations to be
reported back by the instrument. Depending on the CPC model, the upper number
concentration level that can still accurately be detected in the single particle count
mode is between 10,000 #/cm³ for simple and/or old CPCs and 1,000,000 #/cm³
for the most sophisticated models. The lower detection limit of common CPCs is
below 1 #/cm³. A review of the history of CPCs can be found in McMurry (2000).
Figure 2.2 shows an example for the comparison of two CPCs that sampled an aero-
sol of varying number concentrations up to approximately 200,000 #/cm³. While
CPC 1 is specified to measure accurately up to 300,000 #/cm³, CPC 2 only measures
accurately up to 50,000 #/cm³. The figure shows that both CPC readings agree well
up to a concentration of approximately 50,000 #/cm³ (range 1). When the number
concentration exceeded 50,000 #/cm³ (range 2), concentrations measured with CPC
2 were increasingly below those measured by CPC 1, because two or more particles
were simultaneously present in the measurement cell of CPC 2 and counted as one.
Once the concentration exceeded approximately 180,000 #/cm³ (range 3), the number
concentration provided by CPC 2 was constant, because the signals could no longer
be differentiated.
Some CPC models switch to a photometric mode, once the coincidence limit is
reached (i.e., in range 2). In the photometric mode, the CPC measures the total light
intensity scattered by the particle ensemble in the measurement cell to calculate the
particle number concentration from it. In the photometric mode, CPCs can com-
monly measure number concentrations up to 10 6 -10 7 #/cm³. Determining the number
concentration from the total intensity of scattered light requires the mean droplet

200000

Range 1 Range 2 Range 3

150000

100000

50000

0 0

50000 100000

CPC 1 [#/cm 3 ]

150000 200000

FIGURE 2.2 Comparison of two condensation particle counters (CPCs); CPC 1 can measure
accurately up to 300,000 #/cm³ whereas CPC 2 only measures accurately up to 50,000 #/cm³
and shows coincidence errors at higher concentrations.