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July 2006 Symposium on Nanotechnology and the Environment: An Overview on Nanotechnology Detection and Analysis Methods: Highlights, Question and Answer Session
July 12, 2:00-3:00 pm
An Overview on Nanotechnology Detection and Analysis Methods
Mr. John Scalera, US EPA Office of Environmental Information, Washington DC
Highlights Physical-chemical characterization information provided by the manufacturers of nanotechnology materials can provide valuable information that can be used in analyzing for these materials in the environment. This information can include chemical composition, solubility, morphology, particle size distribution, and fluorescent and magnetic properties. Although there are many analytical techniques that can be applied to the analysis of nanoparticles in environmental samples, many challenges remain in obtaining accurate analytical results. These challenges include the environmental transformation of nanoparticles, agglomeration, analytical interferences from analytes of non-interest, particle size fractionation and concentration techniques and the lack of standard methods and reference materials. Some of the available methods/technologies for nanoparticles characterization in environmental samples are identified below.
Nanoparticle Characterization Methods/Technologies:
- Aerodynamic mobility collectors are used in the collection and isolation of nanoparticles fractions in aerosol samples based upon particle inertia. The two basic aerodynamic mobility based collectors employ either cyclones or impactor plate technologies to isolate nanoparticles fractions. The use of multiple impactor plates in series (cascade impactors) has reported particle size isolation down to a 6 nanometer limit.
- Differential mobility analyzers (DMAs) take advantage of an electrical force to isolate charged particles from an aerosol sample based upon the electrical mobility of the charges particles in reaction to the charge core lying within the DMA. Particle size fractions can be focused down to approximately 6 nanometers. Desired particle size fractions are focused to the exit of the DMA by varying the charge on the central core within the DMA. The desired particle fractions can be collected on filters for subsequent analysis by other technologies or sent to a particle counting device Condensation particle counters (CPCs) or condensation nuclei counters (CNCs) set up in tandem with the DMA.
- Condensation particle counters or condensation nuclei counters are technologies designed to provide total counts of aerosol particle fractions. In general, these instruments increase the size of nanoparticles 100 to 1000 times by condensing a vapor about individual particles. This is accomplished in CPCs by sending particles into a supersaturated atmosphere (water, isopropyl or butyl alcohol). As the nanoparticles become larger do to the condensing of the liquid onto the surface, they become optically detectable.
- Aerosol time-of-flight mass spectrometry (ATOFMS) is a relatively expensive technology but capable of providing real-time particle size and chemical composition data on aerosol samples. Commercial instruments are available for field use with sensitivities down to approximately 30 nms.
- Field flow fractionation technology is a particle size fractionation technique applied to particle samples in a liquid media: Unlike HPLC, where a stationary phase is required, field flow
- Fractionation uses the diffusion properties of particles in a liquid media to obtain separation of particles across a wide size range (1nm to several microns). There are several types of field fractionation techniques including those that employ magnetic or electrical or gravitational forces to enhance separation. The various sample fractions generated by this technology can be collected for subsequent analysis or analyzed on-line using technologies such as ICP- mass spectroscopy.
- Available electron microscopy techniques include scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The use of electron microscopy techniques combined with energy dispersive X-ray analysis (EDX) or electron energy loss spectroscopy (EELS) allows for the collection of particle size, morphology, and chemical composition data. These electron miscopy techniques are very labor intensive, expensive, require high vacuums environments and a high level of technician skill.
- Atomic Force Microscopy (AFM) is a relatively new technology. Unlike electron microscopy techniques, AFM do not require high vacuum environments and can be performed in air, liquid or gas atmospheres. The technology is based upon the van der Waals force generated between the interaction of a very fine tip (nominal 30 nms diameter) at the end of a cantilever with the surface of the particle being analyzed. Morphology and particle size information is generated with sensitivity down to the sub-nanometer range.
Question-and-Answer Session A questioner asked about the best method for measuring an aerosol of asbestos in air. Mr. Scalera responded that it depends upon the size fraction, because degree of agglomeration differs with size. Mr. Scalera was then asked about development of a device for measuring nanoparticles in the workplace. Mr. Scalera responded that there is a need for methodologies that people are comfortable with, and that methods are standardized for personal protection. A questioner asked about the certainty of knowing what is captured when a cascade impactor is used. Mr. Scalera responded that capture (by impaction) in a cascade impactor takes advantage of diffusion properties of nanoparticles, and the use of a thirteen-stage cascade impactor have the ability to collect various fraction of nanoparticle down to 7 nm. Mr. Scalera further stated that more than one manufacturer have multi-stage impactors capable of this type of particle size collection resolution.