Measurements of particle surface area have been possible for some time using the BET method.30 However, it requires the collection of relatively large amounts of particles, and measurements are influenced by particle porosity (which may or may not be important) and collection/support substrate -particularly where the quantity of material analysed is small. The first instrument designed specifically to measure aerosol surface area was the epiphaniometer.31 This device measures the Fuchs or active surface area of the aerosols by measuring the attachment rate of radioactive ions to the sampled particles. A particle pre-selector is required to exclude the unwanted large particles. Measurements of active surface area are generally insensitive to particle porosity. Unfortunately, the epiphaniometer is not well suited to widespread use in the workplace because it uses a radioactive source to provide the radioactive ions, for which a transport licence is required.
The same measurement principle is used in the diffusion charger aerosol surface-area monitors that have recently become available. These instruments measure the attachment rate of positive unipolar ions to particles, from which the aerosol active surface area is inferred.32 A schematic diagram of the principle of a typical monitor is given in Figure 5.
The sampled aerosol passes through a weak plasma created by a corona discharge device where it mixes with the unipolar air ions produced by the corona. The air ions diffuse and attach to the exposed surface of the particles. The excess unattached ions are removed by a collecting electrode and the particles with attached charges are collected on a HEPA filter within a Faraday cup electrometer. The current produced by the charged particles is measured by a sensitive electrometer and related to the surface area of the sampled particles. Diffusion charging surface area monitors are available from a number of
Figure 5 Schematic of diffusion charger surface area monitor.
companies (e.g. LQ1-DC from Matter Engineering, Switzerland, and DC2000CE from EcoChem, USA) and typically have quoted ranges of 0-2000 mm2 cm~3 and sensitivities of 1 mm2 cm~3. The latter is portable and battery-powered making it potentially more suitable for use in workplaces.
As yet, it is unknown how relevant active surface area is to health effects following inhalation exposure. Below approximately 100 nm active surface area has been found to correlate well with geometric surface area as measured by Scanning Mobility Particle Sizer (see Section 3.4.1) and with projected surface area as measured by Transmission Electron Microscopy.33 However, above approximately 1 mm active surface area is a function of particle diameter, and so the relationship with actual particle surface area is lost.
A new device, the TSI Model 3550 Nanoparticle Surface Area Monitor,34 uses a particular configuration of an aerosol charger to indicate the human lung-deposited surface area corresponding to the tracheobronchial and alveolar regions of the lung, rather than the total active surface area (i.e. Fuchs surface area) measured by the conventional diffusion charge instruments described above. The aerosol is drawn through a cyclone with a 1 mm cut point and then into the mixing chamber to mix with the ion stream. The voltage on the ion trap is altered such that it acts as a particle size selector to collect both the excess ions and particles that are not of a charge state (surface area size) corresponding to either the tracheobronchial or respirable aerosol fractions. The electric charges on the penetrating particles are then measured by the electrometer. The principle of this new device has been shown to provide a measurement that correlates well with deposited aerosol surface area in the lungs.35 A portable battery-powered version of this instrument, the TSI Aerotrak 9000, has recently been launched.
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