Classification Strategies for Regulatory Nanodefinitions - Safety of Nanomaterials along Their Lifecycle - page 52

situation is promising, but not more: Both Microchannel Resonators (e.g., Archimedes
from Affinity) (Lee et al. 2010) and Scanning Ion Occlusion Spectroscopy (or
Transient Resistive Pulse Sensing [TRPS], e.g., qnano from Izon) (Anderson et al.
2013) need to reduce the lower detection limits from current 70 nm to 10 nm,
which—by their measurement principle—will further limit the upper diameter
range, so that the entire distribution of unknown samples may not be accessible.
Nanoparticle Tracking Analysis (NTA, e.g., from NanoSight or MicroTrac, see also
ASTM E2834-12) is fundamentally based on localization of scattered light, whereas
highly relevant particles such as 10 nm SiO 2 may never be detectable in a realis-
tic environment. Electrospray-Dynamic Mobility Analyzer-Condensation particle
counter (ES-DMA-CPC, several suppliers, see also ISO/DIN 27891:2013) has not
been tested for the purpose and may suffer from salt impurities present in realistic
products. sp-ICP-MS, like Microchannel resonators, measures mass and infers size
assuming a density and shape.
But most fundamentally, all techniques other than VSSA and SEM require a pre-
dispersion via liquid or aerosol.
3.4 SAMPLE PREPARATION FOR FRACTIONATING
AND COUNTING TECHNIQUES
The size distribution of suspensions or aerosols must be dominated in number met-
rics by individualized primary particles. Note that this validity criterion may still
hold if primary particles do not dominate in volume or mass metrics. A content of
agglomerates is acceptable if their presence does not compromise the quantification
of the individualized primary particle by a specific technique. Pilot round robins
with standardized dispersion protocols resulted generally in a high content of large
agglomerates (cf. the LD data in Figure 3.2d) despite high ultrasonication energies.
Some of the labs obtained significant contents of small agglomerates or aggregates
(cf. the CLS data in Figure 3.2d), which allowed superficially correct identification
as nanomaterials, but the disagreement with TEM or VSSA diameters (Figure 3.2c
vs. Figure 3.2e dashed lines) remained significant (Gilliland and Hempelmann 2013).
However, the specific product is marketed by its performance as a “transparent”
pigment, hence devoid of scattering agglomerates, and how to obtain the optimal dis-
persion is well known. DIN 53238-13 and DIN EN ISO 8781-1 specify the dispersion
in low-viscous media by shaking with milling balls to determine the color strength
of pigments. An iron oxide pigment identical to that mentioned previously was added
(about 4.9%) into the EL2 organic coating and shaken for 1 h, then diluted in xylol to
0.05% and measured by CLS according to ISO13318. The size distribution in number
metrics has a D50 of 14 nm (Figure 3.2e solid lines), in excellent agreement with the
TEM value of the smaller axis, and the specific surface of 75 m²/g derived from the
CLS measurement is in excellent agreement with the BET value of 80 m²/g. These
values can only coincide if the size distribution is dominated by individualized pri-
mary particles, and provide a strong validity proof.
Considering that the EC nanodefinition differentiates between aggregates (nano)
and nanoporous materials (non-nano) without a structural delimitation, some
Associations have proposed a delimitation inspired by the lifecycle perspective