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CONSIDERATION OF SAMPLER WALL DEPOSITS

Inclusion of material adhering to internal cassette surfaces during sampling and analysis of airborne particles

Kevin Ashley and Martin Harper

Many of the methods in the NIOSH Manual of Analytical Methods (NMAM) specify the collection of workplace aerosol samples using filter samplers such as 37-mm closed-face cassettes. NIOSH considers that all particles entering the sampler should be included as part of the sample whether they deposit on the filter or on the inside surfaces of the sampler. This issue is detailed in the section on ‘Sampler Wall Losses’ of NMAM Chapter O, and is addressed in an Occupational Safety and Health (OSHA) gravimetric method.1 All aerosol particles entering occupational air samplers should comprise the sample for gravimetric analysis and for analytes such as metals/metalloids. Thus during sample preparation and analysis, procedures should be used to account for material adhering to the internal walls of sampling cassettes.

Numerous research studies have shown that material deposited on the inside surfaces of air sampling cassettes may constitute a significant fraction of the aerosol that enters the sampler.2 Investigations on particles containing lead have shown that there is no qualitative difference in the size distribution of aerosols deposited on the filter or on the internal surfaces of the cassette.3 Both therefore would contribute to the inhaled dose that is delivered to the airways of exposed workers. This is a concern not only for sampling and analysis of lead and other elements, but can apply to aerosol sampling in general, and to various targeted aerosol fractions (e.g., inhalable, respirable or thoracic).4

Consideration of internal sampler wall deposits is included in related international voluntary consensus standards that describe the sampling and analysis of airborne metals and metalloids.5,6 The recommendation to account for sampler wall deposits is consistent with the outlook of OSHA, whose sampling and analytical methods for metals and metalloids specify wiping of internal cassette surfaces.7

Suitable techniques for accounting for internal sampler wall deposits include: (1) wiping (with inclusion of the wipe along with the filter during subsequent sample preparation);7 (2) carrying out sample extraction directly within the cassette;8,9 (3) thorough washing of internal cassette surfaces;8 and (4) the use of internal sampling cartridges (sampler inserts).1 These techniques are consistent with NIOSH methodologies which, to reiterate, should include not just the filter catch but also material collected on internal cassette walls.

In summary, owing to the significance of internal sampler wall deposits, it is generally recommended to account for this contribution to worker exposures to airborne chemical agents. This recommendation may apply to methods that collect and analyze both aerosols and vapor where, for example, the sampler consists of a filter cassette and sorbent tube in series; in such cases inclusion of the particulate material adhering to the filter cassette internal surfaces should be considered. Inclusion of sampler wall deposits does not pertain to methods that entail the use of impregnated filters (e.g., isocyanates).

A list of NIOSH methods that are affected by the recommendation to account for cassette wall deposits include (but are not necessarily limited to) the following:

Methods that may be affected, but for which more evaluation is desired, include:

References

1 (a) Baron PA, Factors affecting aerosol sampling, in NIOSH Manual of Analytical Methods (Chapter O), NIOSH, Cincinnati, OH, 2003 (www.cdc.gov/niosh/nmam); (b) Occupational Safety and Health Administration (OSHA) Method PV2121, in OSHA Sampling and Analytical Methods, OSHA Salt Lake Technical Center, Sandy, UT (2003; www.osha.gov/dts/sltc/methods).

2 (a) Demange M, Gendre JC, Hervé-Bazin B, Carton B, Peltier A, Aerosol evaluation difficulties due to particle deposition on filter holder inner walls, Ann Occup Hyg 34: 399-403 (1990); (b) Puskar MA, Harkins JM, Moomey JD, Hecker LH, Internal wall losses of pharmaceutical dusts during closed-face, 37-mm polystyrene cassette sampling, Am Ind Hyg Assoc J 52: 280-286 (1991); (c) Demange M, Görner P, Elcabache J-M, Wrobel R, Field comparison of 37-mm closed-face filter cassettes and IOM samplers, Appl Occup Environ Hyg 17: 200-208 (2002);(d) Dobson L, Reichmann L, Popp D, Evaluation of quartz residue on cassette interiors of AIHA proficiency samples, J ASTM Int 2(4), 6 pp. (2005) [DOI: 10.1520/JAI12229]; (e) Harper M, Demange M, Concerning sampler wall losses in the chemical analysis of airborne metals, J Occup Environ Hyg 4: D81-D86 (2007) [and references therein].

3 (a) Lee T, Chisholm WP, Slaven JE, Harper M, Size distributions of 0.5 to 20 µm aerodynamic diameter lead-containing particles from aerosol sampler walls and filters, Aerosol Sci Technol 43, 1042-1050 (2009); (b) Chisholm WP, Lee T, Slaven JE, Nelson J, Harper M, Comparison of filter and wall deposits from samplers used to collect airborne lead-containing dusts at field sites, Aerosol Sci Technol 46, 411-418 (2012).

4 (a) Harper M, A review of workplace aerosol sampling procedures and their relevance to the assessment of beryllium exposure, J Environ Monit 8: 598-604; (b) Brisson MJ, Archuleta M, The real issue with wall deposits in closed filter cassettes – What’s the sample? J Occup Environ Hyg 6: 783-788 (2009).

5 (a) ASTM D6785, Standard test method for determination of lead in workplace air using flame or graphite furnace atomic absorption spectrometry; (b) ASTM D7035, Standard test method for determination of metals and metalloids in airborne particulate matter by Inductively coupled plasma atomic emission spectrometry (ICP-AES); (c) ASTM D7439, Standard test method for determination of elements in airborne particulate matter by inductively coupled plasma mass spectrometry. ASTM International, West Conshohocken, PA (updated periodically; www.astm.org).

6 (a) ISO 15202, Workplace air – Determination of metals and metalloids in airborne particulate matter by inductively coupled plasma atomic emission spectrometry (3 Parts); (b) ISO 30011, Workplace air – Determination of metals and metalloids in airborne particulate matter by inductively coupled plasma mass spectrometry. International Organization for Standardization, Geneva (updated periodically; www.iso.org).

7 (a) Hendricks W, Stones F, Lillquist D, On wiping the interior walls of 37-mm closed-face cassettes: An OSHA perspective. J. Occup. Environ. Hyg. 6, 732-734 (2009); (b) OSHA Methods ID-121, ID-125g and ID-215, in OSHA Sampling and Analytical Methods, OSHA Salt Lake Technical Center, Sandy, UT (2008).

8 (a) Ashley K, Andrews RN, Cavazos L, Demange M, Ultrasonic extraction as a sample preparation technique for elemental analysis by atomic spectrometry, J Anal At Spectrom 16: 1147-1153 (2001); (b) Ashley K, Applegate GT, Marcy AD, Drake PL, Pierce PA, Carabin N, Demange M, Evaluation of sequential extraction procedures for soluble and insoluble hexavalent chromium compounds in workplace air samples, J Environ Monit 11: 318-325 (2009).

9 Institut National de Recherche et de Sécurité (INRS), Métaux – Métalloïdes (Fiche 003), in Métrologie des Polluants – Évaluation de l’Éxposition Professionelle – Methodes de Prélèvement et d’Analyse de l‘Air, INRS, Paris (updated periodically).