Field-Flow Fractionation Coupled with ICP-MS for the Analysis of Engineered Nanoparticles in Environmental Samples

Dec 01, 2012
Volume 25, Issue 12

This article provides an overview of the capabilities of field-flow fractionation (FFF) coupled with inductively coupled plasma–mass spectrometry (ICP-MS) and demonstrates that the technique shows a great deal of promise to separate, detect and quantitate nanoparticles in environmental matrices.

The National Nanotechnology Initiative (1) defines nanotechnology as the understanding and control of materials at dimensions of 1–100 nm, where unique properties enable novel applications to be carried out. Gases, liquids and solids can exhibit unusual physical, chemical and biological properties at the nanoscale level, differing in critical ways from the properties of the bulk materials. Nanomaterials can occur in nature, such as clay minerals and humic acids; they can be incidentally produced by human activity such as diesel emissions or welding fumes; or they can be specifically engineered to exhibit unique optical, electrical, physical or chemical characteristics. Depending on their chemical and physical characteristics, these engineered nanomaterials (ENMs) can be made to exhibit greater physical strength, enhanced magnetic properties, conduction of heat or electricity, greater chemical reactivity or size-dependent optical properties.

Engineered Nanomaterials

Most ENMs can be divided into two main classes: carbonbased nanomaterials, such as nanotubes and spherical buckyballs, and metal-containing ones such as Ag, Au, Fe or TiO2 nanoparticles. Of the carbon-based ENMs, many products incorporate carbon nanotubes to improve strength. These ENMs are strongly bound to the matrix of the material and, as a result, are less likely to be released into the environment. Even though many metal-containing ENMs are also incorporated into a product matrix such as solar cells, a significant amount are used in dispersive applications. In these applications, they are intentionally released from the product, although incidental release can also be quite substantial. For example, fabrics containing silver nanoparticles used to kill bacteria release silver at varying rates during the washing cycle, depending on the type of fabric and the washing conditions. Another good example is a novel washing machine that "creates" Ag nanoparticles during the wash cycle by releasing silver nanoparticles and ions (Ag+ ) from a solid piece of Ag inside the machine.

Therefore, it is clear that the use of ENMs in consumer, industrial and agricultural products, as well as in environmental technology is growing rapidly. Often, the benefit of using nanomaterials stems from the increased surface area per unit mass of material, which increases with the inverse of the diameter. This results in faster rates of chemical reactions (for example, oxidative catalysis) occurring on material surfaces. Sometimes the benefit of nanomaterials arises from the quantum nature of energy states at the nanometer scale, as in the wavelength tuning of the fluorescence of quantum dots used in electronics applications. In the health sciences, the ability of surface-functionalized ENMs to bind to cell walls can be used for drug delivery. It has been estimated by the Project on Emerging Nanotechnologies, that there are currently more than 1000 products containing ENMs used for consumer, healthcare and industrial applications (2).

Potential for Environmental Impact

The unique properties of ENMs have also created intense interest in the environmental behaviour of these materials. Because of the increased use of nanotechnology in consumer products, industrial applications and health care technology, nanoparticles are more likely to enter the environment. Therefore, to ensure sustainable development of nanotechnology products, there is clearly a need to evaluate the risks posed by these engineered nanoparticles (ENPs), which will require proper tools to carry out exposure assessment studies. Current approaches to assess exposure include predictions based on computer modelling of ENP life cycles or, alternatively, by direct measurement techniques. Although these are very different approaches, both require instrumentation and analytical methodologies. Prediction of environmental concentrations of ENPs through modelling is based on knowledge of how they are emitted into the environment using production volumes and life cycle assessment data and also by their eventual fate and behaviour in the environment being studied (that is, soil, sediment, water and air). Although we are now starting to understand the life cycles of ENPs during production, use and disposal, there is very little known about their environmental fate and behaviour. Different ENPs will have different properties and, therefore, will behave very differently when they enter the environment.

The approach for prediction of environmental concentrations through life cycle assessment modelling requires validation through measurement of actual environmental concentrations. Extremely sensitive methods are required for ENPs that were only recently introduced into the environment. Although the direct measurement approach is not hampered by the underlying assumptions of exposure modelling, it is very important to ensure that direct observations are representative in time and space for the regional setting in which the observation was made. ENPs differ from most conventional ''dissolved'' chemicals in terms of their heterogeneous distributions in size, shape, surface charge, composition and degree of dispersion. Therefore, it is not only important to determine their concentrations, but also other metrics such as shape, size distribution and chemical composition.

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