ChemVol Model 2400 - See PUF Sampling System or High Volume Air Samplers

The ChemVol High Volume Cascade Impactor collects highly concentrated samples of particulate matter for toxicological studies and the analysis of organic aerosols. It provides users with a selection of different size ranges of particulate matter (PM) to be sampled.

High Volume Size Selection

The ChemVol sampler operates at a flow rate of approximately 800 l/min, which is nearly as high as the 1100 l/min flow rate of typical high volume PM samplers. Instead of collecting particles on a filter, however, the ChemVol system collects samples of highly-concentrated ambient PM onto flat segments of exchangeable polyurethane foam (PUF). With the exception of the first and last collection stages, a segment of PUF resides in each size-selective collection stage. The PUF has a large loading capacity that allows multi-day and even multi-week sampling.

The system is made up of two major components: the sampler and high-capacity pump. The sampler is a modular unit with interchangeable stages that allow the user to change the particle size ranges sampled. The pump is a heavy-duty blower sized according to the sampler cut points used in the ChemVol system.

Sampler Configuration

The sampler stages are installed in order from the largest to smallest cut point. An indicative configuration is shown below:

The ambient air sample stream enters the sampler from the top, between the rain cap and the PM-10 stage. The sample flow exits the sampler from the bottom, and proceeds through a flow measurement device and tubing on its way to the high-capacity pump.

The configuration above contains a PM-10 stage for collecting particles larger than 10 microns in diameter, a PM-2.5 stage for collecting coarse particles, a PM-0.1 stage for collecting fine particles, and an ultrafine foam filter for collecting ultrafine particles.

The stages available for the ChemVol sampler are listed below. All stages use PUF as the impaction surface with the noted exceptions.

• PM-10 (greased or PUF impaction surface)
• PM-2.5
• PM-1
• PM-0.5
• PM-0.1
• Ultrafine Filter (polypropylene material as filter)

Collection Stages

With the exception of the final ultrafine filtration stage, each ChemVol collection stage consists of an acceleration platform followed by a collection platform. These platforms are located in a cylindrical enclosure that is part of a stack of cylindrical enclosures that make up the sampling section. A rain cap located at the top of the sampler protects against precipitation.

Acceleration Platform

The acceleration platform is a circular slit that increases the velocity of the air stream to impact particles larger than a certain size on the collection platform. The acceleration slit comes in different widths, depending upon the particle size fraction being sampled.

Collection Platform

The collection platform is a circular component that houses a toroid-shaped piece of PUF (or grease in the case of the greased PM-10 stage). It is located directly beneath the acceleration platform at a specific distance that ensures the desired particle size cut point.

Ultrafine Filtration Hardware

If an ultrafine filtration stage is included in the ChemVol system, it is always the final sampling component. This stage relies upon filtration (as opposed to impaction) to collect the particulate matter remaining in the sample stream after the final impaction stage. The ultrafine filtration stage consists of a multi-layered polypropylene filter.

Principle of Operation

PUF is used as an impaction substrate in the ChemVol sampler, and not as a filtration material. The sample stream flows through a circular Harvard University-patented slit concentrator that causes the sample stream to accelerate in preparation for impaction. The following diagram shows the top and side views of the acceleration platform.

The slit geometry varies, depending upon the size of particles to be collected. Following the slit, particles of a certain size and larger are collected on the removable piece of PUF by means of impaction, as depicted below.

The particle size ranges collected on the PUF in the sampler stages depends upon the selection of cut points. 

Concentrated particle collection on an inert PUF substrate provides the following benefits for sample collection and handling:

• The porosity of PUF material decreases the strength of the collision between particles and the collection substrate. This eliminates bounce-off, re-entrainment and breakup losses of particles. PUF has a very high particle collection efficiency over a large range of particle sizes, even under conditions of heavy particle loading.
  
• Polyurethane foam is inert, and is appropriate for a wide variety of physical, chemical and toxicological analyses.
  
• No oil or grease coating is required for particle impaction, so that potential interferences from impurities in such coatings can be avoided in chemical, biological and toxicological analyses.
  
• PUF material allows for significantly longer sampling durations than filers, as well as greater mass loading capability. At the high flow rate of 800 l/min, a large mass of PM can be collected in a short time on the small collection collection substrate, facilitating recovery of particles for analysis. The length of the round slit is approximately 30 cm. Collected samples can easily be extracted using small amounts of water or organic solvents.
  
• A loading capacity of 1.8 g without a degradation in collection efficiency permits sampling periods of several weeks at an average PM-10 concentration of 100 micrograms per cubic meter.