Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 242

elemental ratios of iron phosphates sampled in a eutrophic lake. However, it must
be stressed that the Cliff-Lorimer method (thin foil approximation) can only be
applied for ' thin ' particles, which translates to roughly 100 nm and smaller depend-
ing on the sample composition and acceleration voltage.

EELS/EF - TEM analysis: Elemental characterization of specimens in the TEM can
also be performed using electron energy loss spectroscopy (EELS). In the TEM,
electrons can lose energy when they travel trough the specimen due to inelastic
scattering (mostly due to electron-electron interactions). This energy loss of the
primary electrons can be used to gain elemental information on the specimen.
Useful data can be obtained from samples as thick as 100 nm, but for quantifi cation
samples in the order of only a few tens of a nanometre are generally required. A
short introduction into the EELS technique is given in Brydson (2001) and Williams
and Carter (1996). For a rigorous treatment of the EELS technique the textbook
by Egerton (1996) can be consulted.
The EELS spectrum can be divided into three distinct parts: the zero loss peak,
the low loss spectrum ( 50 eV). The zero loss

peak is caused by the unscattered and elastically scattered electrons and is the most
intense feature in the EELS spectrum. It can be used together with the low loss
spectrum to determine the specimen thickness, but carries no useful elemental or
structural information. The low loss spectrum is dominated by electrons that have
set up plasmon oscillations and is, therefore, often referred to as the plasmon peak.
However, electrons that have also generated intra- or interband transitions are
contained in the low loss spectrum. The high loss spectrum is caused by removal of
inner or core shell electrons from an atom leading to ionization of the atom. This
part of the spectrum carries the elemental information of the sample and thus is
most important for environmental analysis. The decay of the ionized atoms can lead
to the production of either x-rays or auger electrons.
Thus, EELS and EDX are caused by the same phenomena but analysed in a
different way, which explains the highly complementary nature of these two tech-
niques. In the energy fi ltered technique (EF-TEM) the energy loss of the electrons
is used for imaging purposes. In this case, only electrons that experience a certain
energy loss, which is determined by the elements present in the specimen, are used
for image formation. Images then represent elemental maps of the specimen, which
can be recorded very fast (seconds to minutes), depending on the concentration of
the elements of interest.

< 50 eV) and the high loss spectrum ( >

Applications and limits. EELS and EF-TEM analysis are frequently used in
material science to characterize engineered NPs. In environmental sciences, however,
only a few studies have been published using these techniques (Perret et al. , 1991 ;
Mavrocordatos et al. , 1994 ; Mavrocordatos et al. , 2000 ). Quantifi cation of iron(hydr)
oxides has been performed in an outstanding study by Mavrocordatos et al. (2002) .

High Angle Annular Dark Field (HAADF) imaging : Although not strictly an ele-
mental analysis, the HAADF technique is briefl y described in this section, since this
technique provides some elemental information and can be extremely useful when