Wednesday, 16. June 2010

Detection of nanoaerosols

With a wide range of available measuring methods and years of experience in the area of nanoaerosol measurement technology, Bayer Technology Services is one of the few companies to offer specific services for the detection and characterization of nanoaerosols. This can provide the customer with valuable assistance with safety assessments of the potential emission of nanomaterials, from laboratory scale to production equipment and environments.

Nanoaerosol analysis using integrated methods

It has to date not been definitively demonstrated that nanotechnology has any influence on humans and the environment. In order to be able to recognize and counter potential hazards at an early stage, investigations along the life cycle of nanomaterials represent an increasing focus of research projects. Owing to their small size and mass, the release of nanoobjects into the atmosphere is invisible to the human eye and can only be detected with difficulty, if at all, using conventional measuring instruments. The detection and characterization of nanoobjects and nanoaerosols consequently not only makes high demands of the measurement and analytical techniques used, but also requires corresponding expertise.

Bayer Technology Services has had methods for the detection and identification of airborne nanoobjects, especially nanoaerosols, available since 2005 in the "Surface Characterization" area of the "Materials Characterization & Testing" Center. The integration of suitable measurement and analytical methods in a single working group not only simplifies internal communications and the exchange of experience in dealing with corresponding problems, but the use of high-quality integrated methods additionally offers the customer solutions from a single source, even in the case of complex measurement tasks, which benefits him through more effective and ultimately cheaper work operations.

In the analysis of aerosols, airborne nanoobjects can firstly be detected by the determination of number-size distributions, in which size-selective counting is carried out using suitable measuring instruments. In addition, however, nanoobjects can also be collected in parallel for specific characterization, i.e. they are firstly precipitated and then assessed by electron microscopy in a later working step.

Figure 1: Scanning electron photomicrograph of a typical nanomaterial

What is a nanoobject?

Nanoobjects is the term applied to objects which have a size of about 1 to 100 nm in one, two or three dimensions and in many cases are produced intentionally owing to their special material properties. If they are in the gas-borne state, they are referred to as nanoaerosols. Nanoobjects are a necessary constituent of any nanotechnology.

Figure 2: Ratio of a nanometer to a meter

Nanoobjects occur in nature, for example as a consequence of combustion processes or volcanic eruptions, and in some cases have already been used industrially for more than 40 years as synthetically produced nanomaterials, for example in the form of pigments or additives for paints and coatings. With the advance of nanotechnology, the production of nanomaterials with specific properties and their use in highly specialized applications have attracted considerable attention in recent years and have entered everyday life in many areas, for example in surface coatings, clothing fabrics and innovative materials.

Great independence at the use locations

As the corresponding measuring instruments are all transportable, they can be used at virtually any location. By way of example, the following pictures show external measurement applications in a research laboratory, above an extruder in a pilot plant, and in production in a packaging facility for nanomaterials.

Figure 3: Typical use locations and examples of results of nanoaerosol measurements

Besides the measurements at workplaces in laboratories, pilot plants or production plants, the measuring instruments are mainly used on an "abrasion box" developed by the working group. Test specimens or samples of a wide variety of materials that may containing nanoobjects, for example polycarbonate compounds or concrete specimens, can be subjected to mechanical stresses in a defined manner, for example by grinding, stirring or drilling, in a particle-free atmosphere in this chamber. These operations are monitored using measuring instruments, enabling nanoobjects produced during the treatment of the material to be counted size-selectively and identified substance-specifically.

Figure 4: Material studies in a preparation chamber constructed especially for the purpose

More detailed information on the measuring instruments used

For size-dependent counting of nanoobjects up to a few 100 µm, a sequential mobility particle sizer (SMPS) and, since the end of 2009, also a fast mobility particle sizer (FMPS) are available. In addition, size distributions of larger objects up to about 30 µm can be determined with the aid of scattered-light particle counters (PDMs). Both the SMPS and the FMPS are based on the mass-dependent and thus ultimately size-specific mobility of fine objects in electrical fields. While the measuring time of the SMPS is fixed due to a sequential mode of operation and is at least 3.5 minutes for a distribution from 5.5 to 350 nm in 44 size classes, the FMPS, due to parallel detection, offers a significantly higher time resolution and determines a number distribution from 5.6 to 560 nm in 32 size classes per second.

Figure 5: Internal structure of a fast mobility particle sizer. While airborne objects are classified in an SMPS with the aid of a sequentially changing field and counted as monodisperse aerosols in a condensation nucleus counter, the charges of the objects can be measured in parallel and converted into numbers in the FMPS using 22 electrometers.

Using a nanometer aerosol sampler (NAS), airborne nanoobjects can be deposited electrostatically on suitable substrates and then assessed with respect to their structure and morphology with the aid of transmission electron microscopy (TEM). In addition, the collected particles can be analyzed for elemental composition by X-ray fluorescence analysis (EDX).

For the identification of any emission sources present or for fast preliminary checks, two compact hand-held instruments, known as NanoTracers, have additionally been available since the end of 2009. They are capable of counting nanoobjects between 10 and 300 nm electrometrically and have a time resolution of one number concentration per 3 seconds.