An alternative method to BET method for estimating off-line the specific surface area of nanostructured particles
Congress title :EuroNanOSH 2007 - Nanotechnologies: A Critical Area in Occupational Safety and Health
Congress date :03/12/2007
Although it is not clear which physical exposure metric play the most important role in the evaluation of the potential impact of nanostructured particles (NP), several studies support the concept that the surface-area is the dose measurement that best predicts pulmonary toxicity to a broad range of airborne NP. Our work aims at theoretically and experimentally investigating the different approaches applicable for characterizing airborne NP surface-area.
Different theoretical ways leading to surface-area estimation have been identified. The BET off-line method, used in toxicological studies for characterizing powders, is not well suited for measuring airborne surface-area of NP at low mass concentrations. Electron microscopy analysis provides different information on collected NP (primary particle size, number of primary particles, fractal dimension etc.). However, this off-line analysis is time consuming and has stringent collection and preparation requirements. Another approach, combining independent simultaneous integral measurements by direct-reading instruments, leads to an estimation of surface-area assuming some of the distribution parameters. From size distribution measurements, the surface-area distribution can be calculated, assuming some properties of the NP (morphology, density etc.). Few on-line techniques have been recently designed to measure surface-area of airborne NP, based on attachment rate of airborne markers (unipolar ions or radioactive atoms) to NP. Yet, there is a lack of data on the performances and on the parameters that could affect their responses.
To investigate some of the identified approaches, a specially designed experimental set-up has been built, where properties of airborne NP can be varied (size, morphology, concentration, chemical composition etc.) in ranges thought to be relevant with the ones encountered in the workplaces. The set-up is composed of a spark discharge generator, a bipolar ion charger, and a high-temperature furnace. Two test modes can be explored with this facility: a polydisperse mode where four simultaneous instruments can be performed, and a monodisperse mode where the response of the tested instrument can be explored as a function of particle size.
As a first step, the complete characterization of the set-up has been performed. Size distributions were measured with FMPS (Fast Mobility Particle Sizer) and SMPS (Scanning Mobility Particle Sizer) for different electrodes in the spark discharge generator (C, Cu and Al). Electrical mobility diameters of NP vary in the range 10 up to about 200 nm. Number concentrations vary in the range less than 105 up to about 107 p/cm3. The stability and the reproducibility of the results are very good. The electric charge of the NP was measured with a Radial-Flow DMA. Fractions of neutral NP vary with the diameter of the particles; they are above 90 % for Cu and Al electrodes and less than 50 % for the C electrode.
As a second step, several commercially available instruments or prototypes devoted for characterizing airborne NP in workplace atmospheres are under investigation.