Calibration, validation and evaluation of aerosol instrumentation, including size distribution and number concentration
Instrumentation for Airborne Nanoparticle Measurement
NPL has for many years had an extensive research programme on the measurement of Airborne Nanoparticles. A large proportion of this research has focused on the evaluation, validation and the development traceable, accurate calibration procedures for instrumentation. The wealth of NPL’s expertise in this area is available for assessment and calibration of a wide range of instrumentation designed to measure such quantities as:
- Number concentration
- Mass concentration
- Size distribution
- Surface area concentration
NPL also has extensive capabilities for the evaluation of nanoparticle aerosol generators.
The number of particles present in a unit volume of air is an important basic parameter dominated by the numbers of nanoparticles. Instruments such as the Condensation Particle Counter (CPC) may be used to detect and count particles larger than about 5 nm in size. Such instruments are sensitive over a concentration range from a few particles to a few million particles per cubic centimetre. NPL was the first laboratory in the world to offer a calibration service independently accredited to ISO 17025 for the calibration of instruments for the measurement of number concentration.
Mass concentration measures the total mass of particles in a unit volume of air, and is typically dominated by larger particles. Well-established instruments such as the Tapered Element Oscillating Microbalance (TEOM) can be used to give online measurements. This may be combined with other techniques to provide measurements of particle properties per unit mass of sample. Alternatively, filtration techniques may be employed to allow mass concentration determination and even post-experimental analysis by methods such as ICPMS.
Size distribution measurements are an invaluable extension of number and mass concentration techniques, because they provide information on the relative abundance of different sizes of particles in a sample. The instruments used for this combine differential mobility analysis with particle detectors such as the CPC or aerosol electrometer to provide size distributions in seconds or minutes. The size range covered by these instruments is from approximately 1 nm to 1000 nm over a range of different concentrations.
Surface Area Concentration
The total surface area of particles in a unit volume of air is considered an important measurement for understanding the interaction of nanoparticles with a wide range of environmental and biological matrices. It is again typically dominated by the nanoparticles present. Reliable estimates of airborne particulate surface area in real time are therefore highly desirable.
Test Aerosol Generation
The evaluation and comparison of these various instruments requires a well-controlled and stable source of particles. As such, the airborne nanoparticle metrology laboratory is equipped with a wide range of aerosol generation equipment to provide the most flexible facility possible. The range of particle types that may be generated includes dry powders, soot, salt, polystyrene latex, organic/biomolecules and a wide range of other solutions and suspensions.
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