Particle Identification Analyses for Indoor Air Quality Investigations
Sponsored by EMSL Analytical Inc.
It is estimated that Americans spend 75 to 80 percent of their time indoors. Therefore, the quality of indoor air undoubtedly has a major impact on the comfort and health of a building’s occupants. Very often, indoor and outdoor air quality investigations require the identification of the components of a mixture of particles. The results of such identification analysis show how to alleviate the problem that prompted the investigation.
A successful comprehensive particle identification analysis always depends upon the quality and quantity of the material in the sample. It is important to make educated choices of sampling procedures and sampling areas. The areas selected for sampling should be those most representative of the indoor air quality project under investigation.
For settled dust, the sampling method options are micro-vacuuming, direct sampling by the grab method, tape lifting, and wiping. Depending on the nature of the surface being sampled and the dust itself, each of these methods offers advantages and disadvantages in terms of sampling efficiency, retention and integrity of particles, and the ability to correctly employ certain analytical procedures.
Micro-vacuuming can be conducted with any type of cassette able to accommodate connection to a pump (cassettes with MCE or PVC filters, carpet samplers, and allergen cassettes). Micro-vacuuming is an efficient method for collecting particles from porous surfaces such as textiles, foams, and wood, as well as from smooth surfaces with medium or heavy particle loading. Most analytical techniques for identifying particles can be used to analyze a sample collected in this fashion. The method is not efficient for collecting particles from smooth surfaces with low particle loading. If not applied correctly, sampling via this method may damage brittle particles. The relative position of the particles on the original surface is not preserved; however, this is a limitation only when the population of particles needs to be reported as the number of particles per unit area.
Direct sampling by the grab method is best employed when sufficient material is present and can be collected and inserted directly in a container. Grab sampling can be applied to any type of surface. It offers the most versatile sample since analysis can be performed on any sampling media. It has the same limitations as the micro-vacuuming technique.
Sampling with adhesive tape lifts can be achieved with bio-tapes, clear office tape, or forensic lifts. Packing tape, duct tape, and painter’s tape should not be used due to the presence of a heavy adhesive layer that can engulf the particles. This is an efficient method for collecting particles from smooth surfaces with low and medium particle loading. It is the only method able to preserve the relative position of particles from the original surface if the material was present as a monolayer. This sampling procedure offers limited sampling area and has poor efficiency on porous, uneven, or heavily loaded surfaces, with preferential sampling from the top layer. The media is amenable to identification by optical microcopy.
Wipe sampling can be achieved with dry or wet wipes. Typically, isopropanol prep wipes are used for particle sampling since the majority of particles present in typical indoor and outdoor environments are insoluble in isopropanol; using deionized (DI) water as a wetting agent is applicable for limited analyses such as anions by ion chromograph or metals by ICP/ICP-MS. The method has good collection efficiency on smooth surfaces with any loading, but it is inefficient on porous surfaces. Wiping may damage delicate particles, and the particle extraction efficiency needs to be assessed.
Sampling of airborne dust involves the use of cassettes connected to air sampling pumps. The choice of the cassette and membrane filters is important because it determines the methods that can be successfully applied for identification and, ultimately, the complexity and accuracy of the analysis. Impact samplers or cassettes with MCE filters are good options for capturing larger particles when identification is employed by light microscopy. Collection using PC filters is the best option for particle identification using scanning electron microscopy with an energy dispersive x-ray detector. The PVC or PTFE filters are not recommended, as they do not collapse with solvents and introduce interferences.
Whichever sampling option is selected, it is important to remember that the method determines the representativeness of the relatively small collection of particles in a sample to the larger scale of the indoor environment.
The analysis for the identification of the particles in the samples may involve an array of methods that include light and electron microscopy, elemental analysis by energy dispersive x-rays, x-ray diffraction (for crystalline materials), and infrared and Raman spectroscopy (for identification of organics). Advanced methods using x-ray fluorescence (elemental composition), gas chromatography (for semi-volatile organics), or high-performance liquid chromatography (for targeted organics) could be used if the nature of the sample permits it. The selection of methods depends on the scope and the nature of the sample. Most of the particle identification analytical approaches start with optical and electron microscopy (SEM/EDX).
Indoor environments may contain particulate components in these categories: common building dust (crystalline silica, calcite and dolomite, gypsum, clays and feldspars, cement dust, paint, wood and lumber fragments, and rust and iron oxides); biological components (epithelial cells, insect fragments, nematodes, fungi, diatoms or algae, pollen, and starches); fibrous particulate (asbestiform minerals, synthetic vitreous fibers, and natural and synthetic cellulosic fibers); and combustion byproducts (char, soot or black carbon, and ash). Other target analytes such as zinc whiskers, UFFI foam insulation, coal or coke, fly ash, or welding dust may also be of interest.
Some of the sample types are qualitatively identified. It is often expected that the analytical report will also provide a quantitative assessment of the components in the sample. Depending on the type of sample, the concentrations may be derived by several techniques: visual area estimation (VAE), which is percentage based on relative projected area; point count, which is percentage based on gross counting; or particle count, that is, particles per area or particles per volume.
The overall results of the analysis provide knowledge that can be used to identify the potential sources of indoor and outdoor air pollution and ways to eliminate their emission or entrainment into the building.