Wide Range Aerosol Sampler, WRAS

WRAS units use novel technology for particle collection that provides substantial advantages of better performance, higher resolution and lower prices. These samplers enable particles to be collected in bands according to their size and can be extended down to nanometers.

Aerosol particles are drawn into the isokinetic inlet and into the saturator where the humidity required is maintained at a pre-determined level (humidity control is optional). Humidity control enables monitoring to match the conditions of the human lung to better assess any potential health hazards.

Our WRAS systems employ two separate stages for size-selective collecting of aerosol particles in the wide range of sizes from several nanometers to 20 mm:

• Stage 1: Particle size > 0.3 µm - These are collected by inertial deposition using a 'May' impactor (normally from 0.3 to 10 micrometre µm) and are selectively deposited onto microscope slides.
• Stage 2: Particle size 1 nm to 0.3 µm - These are collected by diffusion deposition onto Nylon nets.

The schematic below shows the main functional elements of a WRAS system with humidity control:

Stage 1: Inertial Deposition

An impactor is a device that classifies particles present in a sample of air or gas into known size ranges. It does this by drawing the air sample through a cascade of progressively finer nozzles. The air jets from these impact on plane sampling surfaces and each stage collects finer particles than its predecessor.

In the impactor particles greater than 0.25 µm diameter are collected. Collection efficiency of particles in an inertial cascade impactor increases with size, therefore, the largest particles are removed from the flow at the first stage and smaller particles are deposited at each following stage.

The inertial deposition and interception are not applicable for smaller particles. In the WRAS instrument the flow emerging out of the cascade impactor contains particles smaller than 0.25 µm.

The particles are classified according to their 'aerodynamic size' to which the instrument is calibrated and which is nearly always the parameter of interest in aerosol assessment. This is the diameter of the sphere of unit density that has the same impaction characteristics as the particle.

The impactor used in our WRAS systems have a single door that gives immediate access to all sampling slides. The sampling slides themselves are standard microscope slides. These are by far the most convenient equipment for microscope examination, for applying special detecting or sticky films, for washing off deposited material for bulk assessment, and for general ease of handling. The particulate material on each slide is in the form of a long regular strip, the best form for microscope scanning. It shou1d rarely be necessary to dismantle the whole instrument for internal cleaning, but this can readily be done by removing four long bolts, which clamp everything together.

The samples may be analysed under the microscope or by any method of chemical analysis that may be suitable for estimating the mass of material on each stage. The latter process is attractive because an adequate approximation to the mass distribution curve may be drawn from the mass data, without the tedium of using the microscope.

Stage 2: Diffusion Deposition

The motion of aerosol particles in the diameter range of 0.002 to 0.25-micrometer is strongly influenced by diffusion. An aerosol particle undergoing diffusion travels a random, irregular path. Smaller particles with less mass are more strongly influenced than larger particles with greater mass. Particles larger than 0.25 micrometer are not significantly affected by diffusion at normal temperatures and pressures.

As particles undergoing diffusion pass through a net they can collide with the net fibre. Surface-attractive forces between particle and fibre cause the particle to stick to the net. Because of diffusion, a larger fraction of small particles will collide with the net than will large particles. Thus, the deposition of small particles will be greater than for large particles.

Collection efficiency of particles in a diffusion unit decreases with the size, therefore, the smallest particles are collected at the first net with larger particles deposited onto sebsequent nets. This is a reverse order of deposition than for inertial deposition.

It is important to recognise that the particles are much smaller than the mesh dimensions of the net (10 to 100 micrometers).

The diffusion unit has been designed to collect sequentially particles in the size range from 1 nm to 0.25 µm. In practice it is achieved by selecting nets with certain diameter of fibres, density of the fibres, number of nets per stage and the flow velocity. It is quite similar to the design of a cascade impactor. A WRAS diffusion collection unit has a cylindrical cross-section (ID = 4.7 cm) and contains 5 stages.

To install a WRAS monitoring system or to request a portable monitoring service:

Please call +44(0)1795 890869 or email info@wrastechnology.com