Significance and Use

1 A tiered strategy for characterization of nanoparticle properties is necessary to draw meaningful conclusions concerning dose-response relationships observed during inhalation toxicology experiments. This tiered strategy includes characterization of nanoparticles as produced (that is, measured as the bulk material sold by the supplier) and as administered (that is, measured at the point of delivery to a test subject).

2 Test Methods B922 and C1274 and ISO 9277 and ISO 18757 exist for determination of the as produced surface area of bulk metal and metal oxide powders. During the delivery of nanoparticles in inhalation exposure chambers, the material properties may undergo change and therefore have properties that differ from the material as produced. This test method describes the determination of the as administered surface area of airborne metal oxide nanoparticles in inhalation exposure chambers for inhalation toxicology studies.

Scope

This test method covers determination of surface area of airborne metal oxide nanoparticles in inhalation exposure chambers for inhalation toxicology studies. Surface area may be measured by gas adsorption methods using adsorbates such as nitrogen, krypton, and argon (Brunauer et al. Anderson, Gregg and Sing) or by ion attachment and mobility-based methods (Ku and Maynard). This test method is specific to the measurement of surface area by gas adsorption by krypton gas adsorption. The test method permits the use of any modern commercial krypton adsorption instruments but strictly defines the sample collection, outgassing, and analysis procedures for metal and metal oxide nanoparticles. Use of krypton is required due to the low overall surface area of particle-laden samples and the need to accurately measure the background surface area of the filter used for sample collection. Instrument-reported values of surface area based on the multipoint Brunauer, Emmett and Teller (BET) equation (Brunauer et al., Anderson, Gregg and Sing) are used to calculate surface area of airborne nanoparticles collected on a filter.

Terminology

3.1 Definitions—For additional definitions related to nanotechnology, see Terminology E2456.

3.1.1 nanoparticles, n—in nanotechnology, a subclassification of ultrafine particle with lengths in two or three dimensions greater than 0.001 micrometre (1 nanometre) and smaller than about 0.1 micrometre (100 nanometres) and which may or may not exhibit a size-related intensive property.

3.1.2 adsorbate, n—material that has been retained by the process of adsorption.

3.1.3 adsorbent, n—any solid having the ability to concentrate or collect significant quantities of other substances on its surface.

3.1.4 adsorption, n—a process in which fluid molecules are concentrated or collected on a surface by chemical or physical forces, or both.

3.1.5 BET-constant, n—an indication of the magnitude of the adsorbent/adsorbate interactions in the first adsorbed layer.

3.1.6 outgassing, n—the evolution of gas from a material in a vacuum or inert gas flow, at or above ambient temperature.

3.1.7 physical adsorption (van der Waals adsorption), n—the binding of an adsorbate to the surface of a solid by forces whose energy levels approximate those of condensation 

3.1.8 surface area, n—the total area of the surface of a powder or solid including both external and accessible internal surfaces (from voids, cracks, open porosity, and fissures); the area may be calculated by the BET equation from gas adsorption data obtained under specific conditions; it is useful to express this value as the specific surface area, for example, surface area per unit mass of sample (m2/kg).

3.1.9 surface area (BET), n—the total surface area of a solid calculated by the BET equation, from gas adsorption data obtained under specific conditions.

3.1.10 surface area, specific, n—the area, per unit mass of a granular or powdered or formed porous solid, of all external plus internal surfaces that are accessible to a penetrating gas or liquid.

Summary of Test Method

4.1 An appropriate filter is pre-weighed to the nearest 1 ×10-8 kg (0.01 mg), outgassed, and the background surface area measured prior to nanoparticle collection in an inhalation exposure chamber. A sufficient amount of nanoparticles (to provide at least the minimum surface area required for reliable results for the instrument used) are collected on the filter, the filter with particles is post-weighed, outgassed, and total surface area measured. The surface area concentration of the airborne nanoparticles in the exposure chamber is estimated by subtracting the background filter surface area from the total surface area of the filter with nanoparticles and normalized by the volume of air sampled, with the final result expressed as m2/m3 (LeBouf et al. (5)).

4.2 Multipoint BET Analyses—Volume of gas adsorbed at 77 K (liquid nitrogen temperature) is determined as 10-6 m3 (cm3) corrected to standard temperature and pressure for a minimum of five relative pressures within the linear BET transformation range of the physical adsorption isotherm characteristic of the filter or nanoparticle, or both. The linear range is that which results in a least squares correlation coefficient of 0.999 or greater for the relationship between BET transformation and relative pressure. Typically, the linear range includes relative pressures between 0.05 and 0.30.

4.3 It is important to use an analytical balance to determine the sample mass. The physical adsorption instrument measures the total amount of gas adsorbed onto the sample under analysis. The sample mass is then used to normalize the measured adsorption results. Any error in the sample mass will affect the final BET surface area.

4.4 Calculations are based on the BET equation, as required by the instrument being used for the determination. The instrument pressure tolerance (pressure range that must be maintained within a sample cell to accept a valid data point) is 6.6 Pa. In this standard, the cross-sectional area for the krypton adsorbate is taken to be 2.02 × 10-19 m2 (ISO 9277); however, some instrument software may use a different default value. As such, the cross-sectional area of the krypton adsorbate used in calculations should be reported with the BET surface area results.

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