The measurement and characterization of nanoparticles (nanometrology) is therefore critical to all aspects of nanotechnology. In the field of environmental health and safety (EHS), it has become
clear that "complete" characterization of nanomaterials is critical to interpreting the results of toxicological, human health
and environmental fate studies. Metal-containing ENPs form a particularly significant class, as their use in consumer products
and industrial applications make them the fastest growing category of nanoparticles. Several life cycle assessments conclude
that the predicted environmental concentrations (PEC) of some metal-bearing nanoparticles could exceed the predicted no effect
concentration (PNEC), suggesting that ENPs can enter aquatic systems at potentially harmful concentrations. However, in most
cases, these levels are typically at the part-per-trillion level.
Many analytical techniques are available for nanometrology, only some of which can be successfully applied to nano-EHS studies
(3). These methods differ in part by the properties measured: average size, size distribution, surface characteristics, shape
and chemical composition. Methods for assessing particle concentration and particle size distributions include electron microscopy,
chromatography, centrifugation, laser-light scattering, ultrafiltration and spectroscopy. Difficulties generally arise because
of a lack of sensitivity for characterizing and quantifying particles at environmentally relevant concentrations (low micrograms
per litre). Furthermore, the lack of specificity of the technique is problematic for complex environmental matrices that may
contain natural nanoparticles with polydisperse particle distributions, as well as heterogeneous compositions.
Electron microscopy and dynamic light scattering (DLS) are the most commonly applied methods, but they each have advantages
and disadvantages. Electron microscopy gives the most direct information on the size distribution and shapes of the individual
nanoparticles. However, sample preparation steps such as drying or exposure to vacuum can induce an agglomeration (clustering)
of the particles, thus making it difficult to define their size in the original media. In addition, organic coatings are not
visible without staining, which can lead to errors in the measurement of the particle diameter.
DLS measures the diameter of the particle while in motion (hydrodynamic diameter), and thus provides sizing of organically
coated nanoparticles. Limitations of DLS include poor sensitivity at dilute concentrations, nonselective material detection,
inability to distinguish mixtures in complex matrices, and difficulty in resolving the dominant size in polydisperse samples.
The presence of a small number of aggregates can skew the effective diameter toward a larger particle size distribution. However,
despite its limitations, DLS remains a rapid technique to quickly determine average particle hydrodynamic diameter for a wide
range of particle types.
An emerging technique called single-particle inductively coupled plasma–mass spectrometry (SP-ICP–MS) has been developed for
detecting and sizing metallic nanoparticles at environmentally relevant (nanograms per litre) concentrations. Although this
method is still in its infancy, it has shown a great deal of promise in several applications, including determining concentrations
of silver nanoparticles in complex matrices such as wastewater effluent (4). The method involves introducing nanoparticle-containing
samples, at very dilute concentration, into the ICP-MS system and collecting time-resolved data. Integration times on the
order of 10 ms are used to detect individual particles as pulses of ions after they are atomized by the plasma. Observed pulse
number is related to the nanoparticle concentration by the nebulization efficiency and the total number of nanoparticles in
the sample, and the mass, and thus the size of the nanoparticle, is related to the pulse intensity (5). However, it should
be emphasized that for this approach to work effectively at low concentrations, the speed of data acquisition and the response
time of the ICP-MS detector must be fast enough to capture the time-resolved nanoparticle pulses, which typically last only
a few milliseconds.
Field-flow fractionation (FFF) analysis, the sizing technique highlighted in this article, is a powerful tool for sizing and
separating ENPs. FFF, incorporating UV-absorbance detection, is generally limited to particle concentrations in the parts-per-million
(milligramsperlitre) range and lacks particle specificity. Furthermore, UV response is not a direct measure of particle mass
concentration, but rather depends on particle size, shape and optical characteristics. However, coupling FFF with a sensitive
and selective multielement technique such as ICP-MS lowers detection capabilities by approximately three orders of magnitude,
to the partsper-billion (microgramsper-litre) range, and provides direct information about particle mass concentration and
composition (6). For this reason, it is clear that because of its elemental specificity, excellent resolution and low detection
limit, ICPMS is perhaps becoming the most promising detection method for nano-EHS studies.