Significance and Use
5.1 The following is a non-exclusive list of standards to which this guide applies: Guide D6062; Test Methods D4185, D4532, D6785, D7035, D7439, D7948; and Practices D6061 and D6552.
5.2 The applicability of this guide to other standards under the jurisdiction of ASTM Committee D22, but not the direct responsibility of Subcommittee D22.04, should be considered where analyte entry into the sampler is considered the sample and where analyte adherence to internal sampler surfaces (“walls”) is likely to scavenge analyte from the collection substrate.
5.3 Aerosol samplers typically consist of a filter or other collection substrate, for example an impaction plate or foam, supported in a container or holder. The entire device typically is considered an aerosol sampler. The sampling efficiency of the aerosol sampler, that is, the ratio of the concentration collected by the collection substrate to the undisturbed concentration in the air, has three components: (1) aspiration (or entry) efficiency; (2) transport efficiency (depending on design, both from entry “plane” to internal separator and from any internal separator to collection substrate); and (3) penetration (through the internal separator). For a sampler of a specific design, the three efficiency components are functions of particle (aerodynamic) size and flow rate. The aspiration efficiency also depends on wind speed and direction, while the sampler’s angle to the vertical influences both the aspiration efficiency and the transport efficiency. Ideally, when a sampler is designed and tested for its sampling performance, or both, it should first be established what is considered as the collected sample (that is, the deposit on the collection substrate, but also any deposits on any internal surfaces if these are to be analysed).
5.4 Part of the aerosol entering a sampler will deposit on the internal surfaces of the sampler prior to reaching the collection substrate. There are number of mechanisms by which this can occur, including bounce from the filter, inertial impaction, gravitational settling and electrostatic attraction after entry. In addition, after sample collection, if the collection substrate is transported while mounted in the sampler, it is possible that particles originally deposited on the collection substrate may dislodge during transportation. Such particles can thereby contribute to deposits on the walls, as well as on the base of any cover plate or plug. All particles found elsewhere than on or in the collection substrate are often loosely termed “wall deposits.” If the sample of interest entails the entire aspirated air particulate into the container or holder (sampler), it is necessary to account for these wall deposits, especially if it cannot be shown that they should be disregarded.
5.5 The research underpinning the information in this guide has arisen partly from studies of inert particles (3, 4), but mostly from investigations of methods for the determination of airborne metalliferous particulates (2, 5-15). However, the issues at hand are also important in sampling airborne organic materials, including bacterial endotoxin (16), wood (17), and pharmaceutical dusts (18); another relevant study reported results from investigations in thermosetting plastics, wood, paper, and animal breeding (19). Except in the case of very large wood dust particles, there is no evidence to suggest that wall deposited particles are sufficiently different from those found on the collection substrate to warrant their exclusion (13, 14). Wall deposits are not limited to aerosol samplers for larger airborne particles but may also be found in samplers for finer particles (20, 21). There may be a justification for excluding wall deposits where the performance of an aerosol sampler tested to EN 13205 shows appropriate compliance with the relevant ISO 7708 size-selective convention without their inclusion.
5.6 The findings of studies that have been carried out to assess wall deposits in two commonly used samplers are summarized in Table 1 and Table 2. A commonly used sampler, the 37-mm closed-face polystyrene cassette (CFC), is specified as the sampler of choice in many U.S. National Institute for Occupational Safety and Health (NIOSH) and U.S. Occupational Safety and Health Administration (OSHA) methods (1). While the specific methods may not explicitly call for the recovery and analysis of CFC wall deposits, inclusion of wall deposits is called for by both agencies (22). Another widely used sampler, the Institute of Occupational Medicine (IOM) personal inhalable sampler, was specifically developed for the purpose of collecting the inhalable fraction of aerosol in accordance with ISO 7708 specifications (23). Wall deposits in this sampler were noted during its development and are specifically included as part of the sample (24), although no standard protocol has been published for their inclusion other than for gravimetric analysis. Side-by-side studies have shown little difference between these two samplers when used to collect aerosol in metals industries (12), provided they are analyzed by the same procedure (that is, filter only or filter plus wall deposits). Fewer studies have been carried out in non-metal industries. However, in the study of sewage composting facilities (16), wall deposits of endotoxin exceeded 40 % of the total sample in 34 % of cassettes and exceeded more than half the total sample in some. In the laboratory study of wood dust (17) 85 % of the sample aspirated was found on the cassette walls. In the pharmaceutical industry study (18), averages of 51 %, 62 %, and 72 % of the sample was found on non-filter internal surfaces, depending on compound. Figure 8.2 of Aitken and Donaldson (3) provides a graph of mass faction wall deposits of inert particles in the IOM sampler versus particle size. Although the actual data points are not provided the median is approximately 18 % and the maximum approximately 55 %, in accordance with the data in Table 2. Witschger, et al., (4) provides similar data, with a maximum wall deposit of 50 %. While both these studies were performed in a laboratory, Lidén, et al., (19) presents averages of 24–37 % wall deposits in a range of field samples from non-metal industries, depending on industry.
5.7 The Gesamtsstaubprobenhame (GSP) inhalable sampler, and similar metal or plastic versions referred to as a conical inhalable sampler (CIS), has not been the subject of similar extensive investigations of wall deposits. While the GSP met the inhalable convention in a European study without considering wall deposits for particles up to 25 µm AED (25), for particles up 50 µm AED it under-samples by an average of 21 % with respect to the IOM sampler (when wall deposits are considered in the IOM sampler) (26). A study of wall deposits at a lead mine concentrate mill (5) showed up to 40 % (median 24 %) of total aspiration on the walls, while the laboratory wood study (17) found an average of 42 %, suggesting that wall deposits be considered with this sampler. Other samplers not specified in this practice may also have wall deposits; these should be evaluated on a case-by-case basis.
5.8 No pattern has been discerned that might allow for correction factors to be used in any single sampler without introducing too great an uncertainty into the result (1, 12)). Therefore, it is necessary to account for the wall deposits in all cases where the sample is meant to include the total aspirated aerosol into the sampler. On the other hand, enough data have now been accumulated to allow rough assumptions to be made regarding the effect of wall deposits on a large population of samples, either historically or for predictive purposes, including estimating the proportion of likely overexposures. These estimates become more precise where there is a body of data involving filter-only and filter plus wall deposits from the specific environment of interest.
5.9 Samplers for the ISO 7708 respirable fraction of dust have filters contained in holders downstream of (after) the size-separation device, typically a cyclone. These sample holders, where not electrically conductive, have also been shown to exhibit significant proportions of wall deposits. In a study of field samples (19), up to 32 % of total collected quartz was found on the walls of 2-piece non-conductive styrene cassettes and up to 55 % on the walls of 3-piece styrene non-conductive cassettes, which is similar to what was found in laboratory studies (20).
1.1 Many methods for sampling airborne particulate matter entail aerosol collection on a substrate (typically a filter) housed within a container (or holder), the whole apparatus being referred to as an aerosol sampler. In operation, the sampler allows a vacuum (pressure below ambient or room air pressure) to be applied to the rear of the substrate so that sampled air will pass through the substrate, leaving collected particles on the substrate for subsequent analysis. The sampler may also protect the substrate, while the opening (orifice) of the container may further play some role in determining what size range(s) of particles approach the collection substrate (size-selective sampling).
1.2 All particles entering the container orifice are considered part of the sample, unless stated otherwise in the method, but not all particles are necessarily found on the substrate after sampling (1).2 Particles may be deposited on the inner walls of the sampler during sampling or may be deposited on the inside walls of the sampler or on the orifice plug or cap following transportation (2). These particles are often loosely referred to as wall deposits. This guide presents background on the importance of these wall deposits and offers procedures by which these deposits can be assessed and included in the sample.
1.3 Wall deposits may also occur in multi-stage samplers (for example, cascade impactors), but this guide does not cover such samplers.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.