Sample Collection: Airborne Dust, Loose Particulate and Bulk Materials
This document is intended as a guide for the sampling of unknown particulate and solid materials. The following information focuses on the applicability of sampling techniques based upon general sample type and environment.
Sample collection is relatively simple, in most cases following similar sampling schemes as the asbestos, mold and combustible dust industries. Airborne dust can be collected using high volume pumps and filter cassettes. Settled dust can be collected by micro-vacuum, alcohol wipes, adhesive lifts and grab bags. Bulk materials can simply be placed into Zip-lock™ style bags or rigid sealable containers. Each sampling method has both benefits and limitations. There is no single procedure that is best for every site and situation.
Sample collection generally falls into one of three categories, airborne particles, settled dust and bulk materials. As implied, airborne particles are any materials requiring collection while suspended in the air column. Settled dust is particles that have collected on surfaces, often after dropping from the air column. Collection of airborne particles is a good indicator of what is in the air at the time of sampling. Settled dust represents a history of what has been in the air over a longer period of time. Many particles such as mold spores, pollen and skin fragments can remain suspended in the air for an extended duration. Other particles such as fibrous glass, minerals and metal oxides will tend to settle out of the air more rapidly and can be missed by air sampling alone. Bulk samples are simply any solid to semi-solid materials that can be directly collected by hand. Whenever possible, collection of suspect materials is recommended. For example, fibrous glass is suspected of causing itching experienced by employees at a site. A piece of the ceiling tile or insulation can be submitted. By matching the glass fibers the origin of the dust can be determined.
The choice of sampling media varies based upon the particle type, size and media being sampled. Airborne particles are commonly collected by cassettes containing various filters. Settled dust can be collected by micro- vacuum, adhesive lifts or, if there is enough, scraped or brushed into a container. Bulk materials are often collected by glove-bag or placed into a rigid container.
Common sampling media
| Airborne Particle Sampling | Dust and Bulk Sampling |
*Requires Vacuum Pump |
|
Airborne Particle Collection
Airborne particulate is commonly collected using filter cassettes, which are composed of a sample collection filter, placed on a rigid support filter encased in a plastic housing. A vacuum pump is attached to the base and air drawn through the filter. Sampling of rooms or the environment is performed using high volume pumps capable of 5 to 20 liters per minute. The total volume collected is greatly dependent upon the particle loading in the air. An office or home may require a minimum volume of 1,500 liters to collect enough particulate for accurate identification while a machine shop or refinery may require significantly less. A good rule of thumb in dusty locations is to monitor the flow rate. If the flow rate begins to drop the filter is becoming saturated. Stop the pump and store the cassette for analysis.
Figure 1: Exploded views of 25mm (left) and 37mm (center) filter cassettes and the Air-O-Cell cassette commonly used for airborne particulate collection. Filter cassettes are also useful for settled dust sampling by the micro- vacuum technique.
Sampling with cassettes for airborne particles can be performed Open Face or Closed Face. Open Face sampling is the preferred technique for particulate identification and asbestos testing allowing for a greater degree of particle dispersion across the entire filter, which results a uniform particle distribution. Closed Face sampling condenses the particulate into the center of the filter resulting in a localized deposit. The Closed Face technique is more applicable to specific analytes that often require extensive preparation to extract the analyte from the filter. For example, silica nuisance dust and oil mist.
Figure 2: Example of particle dispersion by the Open Face vs. Close Face techniques. Open Face yields a uniform distribution across the filter which is well suited for microscopic analysis while Closed Face condenses the particles in the center of the filter.
Figure 3: Examples of a 25mm cassette (left), closed face sampling (center) where only the plug is removed and open face sampling (right) where the entire end cap is removed.
When choosing your media for sample collection it is important to consider the particles of interest. For example if you are only interested in mold spores and pollen then an Air-O-Cell cassette may meet your needs. If building materials, metal oxides, minerals and plastics are of concern then a filter-based sampling media is recommended. Impact collection cassettes such as Air-O-Cell are limited to analysis by optical microscopy. Filter-based cassettes enable the use of additional techniques such as electron microscopy, Fourier transform infrared spectrometry (FTIR) and x-ray diffraction spectrometry (XRD), which is necessary for the accurate identification of many materials.
The three common filter membranes used for collection are PVC, MCE and PC filters. Here too there are significant differences. PVC filters, commonly used for nuisance dust sampling, have a pore size of 5.0μm, which limits analysis to particles larger than 5μm. In addition, PVC filters require harsh organic solvents to clear the opaque membrane, resulting in loss of organic particles prior to analysis. This includes mold, pollen, plant matter, skin fragments and many plastics. MCE and PC membranes are more easily cleared enabling optical analysis of all particles including organics such as mold, hair and plastics. Here to, there is a difference between MCE and PC membranes. MCE filters achieve their pore size through thick layers of large pores similar to a sponge. PC filters are flat, thin plastic with precise laser formed holes.
Comparison of Mixed Cellulose Ester vs. Polycarbonate Filters
Figure 4: Comparison between the two filters (MCE vs. PC) with a similar pore size (~0.4μm) by scanning electron microscopy (SEM) shows the dramatic difference between the filter membranes. Particles up to 5μm can be lost within the spongelike MCE filter. However, particles less than 0.4μm are still collected on the PC filter. If fine particles are of interest the polycarbonate filter membrane is a better choice for total particle collection.
Comparison of Sample Collection Techniques – Airborne Particulate
| Airborne Sampling Techniques | Benefits | Limitations |
|---|---|---|
| Mixed Cellulose Ester (MCE) Filters (0.8μm or 0.45μm pore size) Collection: Nano-particles – N/A Large particles – Good Identification: Nano-particles – N/A Large particles – Good |
|
|
| Polycarbonate Filters (PC) Filters (0.4μm or 0.1μm pore size) Collection: Nano-particles – Good Large particles – Good Identification: Nano-particles – Good Large particles – Good |
|
|
| Impact Samplers such as Air-O-Cell Cassettes Collection: Nano-particles – N/A Large particles – Good Identification: Nano-particles – N/A Large particles – Good |
|
|
Light Dust Collection
Settled dust particulate is commonly collected using alcohol wipes, adhesive lifts or the micro-vacuum technique. Each collection media has its benefits and limitations.
Forensic Adhesive Lifts
Forensic adhesive lifts are useful for sampling thin layers of particles on solid, non-porous surfaces such as desks, glass and hard floors. Unlike tape, which tends to have thick adhesive, forensic lifts trap the particles on the surface of a thin layer of adhesive. General purpose adhesive tape and packing tape should be avoided since these products have a thick layer of adhesive that can trap particles hindering analysis. Non-transparent and industrial tapes, such as duct tape, should be avoided as these products can render accurate analysis impossible.
Figure 5: Many forensic adhesive lifts (A) are a two-part construction requiring removal of the protective plastic coating (B) exposes the adhesive. Place the adhesive onto the surface to be sampled (C) and apply light pressure. Once complete the adhesive is then adhered to the backing (D) for storage of the collected particles.
Figure 6: The same procedures can be employed when using slide mounted adhesive samplers (A). Remove the protective plastic coating (B), press onto the material to be sampled (C) and replace the protective plastic to preserve the particles during storage and shipping (D).
Alcohol Wipes
Alcohol wipes are a convenient process for collecting light particle loadings on solid, non-porous surfaces. Wipes are also excellent for sampling unusually shaped surfaces such as machine parts or inside pipes. It is suggested to use simple one square inch alcohol prep pads that can be purchased from most pharmacies. Unlike Ghost Wipes, paper towels and commercial products such as Wet-Wipes™, alcohol pads have a small surface area so particles are easily extracted for analysis. Particles tend to become trapped in larger wipes and paper products often interfering with accurate analysis. Avoid using water-based moistening agents as many common particles are water soluble.
Figure 7: Alcohol prep pads (A) are one inch pieces of polyester fabric (B) that are pre-moistened with alcohol. Simply remove the pad from the packaging and wipe across the area to be sampled (C). Once the particles are collected (D) place the entire pad into a zip-lock style plastic bag or rigid container for storage.
Micro-Vacuum Sample Collection
Micro-vacuum (micro-vac) sample collection involves the use of air filter cassettes. Commonly, MCE filter cassettes are attached to high volume pumps in the same manner as air sampling. A Closed Face cassette setup is used in order to attach a short length of tubing to the sampling inlet. The particulate is them collected by crisscrossing the sampling area with the collection tube. Micro-vacuum is an excellent technique for sampling porous materials such as rugs or fabric and collection from difficult to access places such as vents, narrow grills or cracks.
Figure 8: Filter cassette setup for microvacuum sampling.
Comparison of Sample Collection Techniques – Light Particulate / Settled Dust
| Light Dust Sampling Techniques | Benefits | Limitations |
|---|---|---|
| Alcohol Wipe Collection: Nano-particles – Good Large particles – Good Identification: Nano-particles – Good Large particles – Good |
|
|
| Adhesive Lifts/Tapes Collection: Nano-particles – Poor Large particles – Good Identification: Nano-particles – Poor Large particles – Limited DO NOT USE HEAVY ADHESIVES LIKE DUCT OR PACKING TAPE. Particles can easily be enveloped by the adhesive rendering them unidentifiable. |
|
|
| Micro-Vacuum Collection: Nano-particles – Poor Large particles – Good Identification: Nano-particles – Good Large particles – Good |
|
|
Bulk and Heavy Dust Collection
Heavy dust layers are generally treated as bulk when sampling. Whether heavy dust or bulk material, sampling consists of transferring the sample to a zip-lock style bag or rigid container. Sampling may be performed by glove- bag, trowel, scraping the material into a pile or any other technique available.
Comparison of Sample Collection Techniques – Heavy Particulate and Bulk Materials
| Bulk and Heavy Dust Sampling Techniques | Benefits | Limitations |
|---|---|---|
| Bulk Collection: Nano-particles – Poor Large particles – Good Identification: Nano-particles – Good Large particles – Good |
|
|
| Micro-Vacuum Collection: Nano-particles – Poor Large particles – Good Identification: Nano-particles – Good Large particles – Good |
|
|
English 
French 
Spanish 
Portuguese