Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 247

size and can thus be used to characterize the quantum dots during synthesis and,
also, in risk assessment experiments (Bailey et al. , 2004). For example, agglomera-
tion of quantum dots can be studied by following changes in their fl uorescence
spectra.

6.2.8 Surface Area Measurements with Nitrogen Gas Adsorption
As discussed in Section 6.2.1.7, the specifi c surface area (m 2 g − 1 ) is very important
for understanding nanoparticle reactivity and toxicity. In microscopy, a projected
area can be determined, but that has limitations in terms of three dimensional area
and porosity. The most commonly used specifi c surface area methodology that is
commercially available is gas adsorption (mainly nitrogen) analysis. The particulate
sample fi rst has to be dried or freeze dried to a powder, and further pre-treatment
to remove surface contamination may be necessary. The drying and vacuum treat-
ments induce morphological changes to the sample that may also lead to a less
accessible surface. Nitrogen gas is allowed to equilibrate with the particle powder,
often at sequential incremental additions of gas. After each nitrogen addition the
pressure is allowed to stabilize and the amount of adsorbed gas is calculated. From
the adsorption isotherms different models can be applied to interpret the surface
area, pore size, pore volume and pore area (Gregg and Sing, 1982). The BET model
from Brunauer, Emmet and Teller (1938), is the most common one applied for both
synthetic materials and natural particles (Marsh et al. , 1984 ). The BET theory
extends the Langmuir monolayer adsorption to multilayer thereby enabling the
extraction of the surface area.

6.2.9 Method Validation

The corner stones of all analytical measurements are quality control, measurement
uncertainties and method validation. A new method needs to be validated (bench-
marked) with either reference materials that have been certifi ed for a specifi c
metric, or by comparing a set of unknown samples with known measurements using
validated methods. The ideal and most independent benchmarking method is an
inter-laboratory comparison (round-robin) where unknown samples are distributed
to the participating laboratories. These kinds of exercises are much less common
for nanometrology than in analytical chemistry in general, but the advantage can
not be emphasised enough. In between the method validations, it is considered
good laboratory practise (and a requirement in all accredited analyses) to routinely
run a quality control sample (stable dispersion) and plot the measured QC value
in a control diagram so any deviations can be easily spotted.
A general review on the development of particulate reference materials can be
found in Mitchell (1992). There exist only a few reference materials for NPs. These
are certifi ed reference materials (CRM) of gold NPs at sizes 10, 30 and 60 nm from
NIST, certifi ed for particle size, and a non-certifi ed reference material from IRMM,
a ~40 nm silica nanoparticle in aqueous dispersion. In addition there exists a range
of non-certifi ed size standards of polystyrene latex from 20 nm and upwards. There
is also a reference iron oxide dispersion from NIST, available where positive zeta
potential is certifi ed. The procedure and requirements for development of CRM