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 at a temperature of 37o C [15] or at room temperature [17]. An American Society for Testing Material (ASTM) procedure [16] is meant to provide an estimate of the bioavailability of several metals in art materials, using ~0.1 M HCl and extraction at body temperature. Quantity of Solvent. Another uncontrolled variable is the quantity of solvent used in laboratory analytical procedures for soluble metals at different laboratories. Different laboratories may (and do) use different amounts of deionized water for extraction. For example, one laboratory might use 10 mL of deionized water to extract the metal from a sample, while another lab may use anywhere from 25 to 100 mL to extract the compound. Depending upon the amount of material present in the sample, the procedure using 25 to 100 mL can dissolve a larger mass of solute than that using 10 mL. A conservative analytical method for metals used 15 mL of deionized water for extraction. This volume was chosen as a convenience and may not bear any relationship to what the body might absorb. Thus, two identical samples could produce two different results depending on the volume of solvent and other analytical parameters. A further factor affecting solubility is the particle size distribution of the sample: smaller particles are ordinarily more easily dissolved than larger ones. Thus for two samples having the same mass, the sample comprised of small particles may be more easily dissolved than a sample having large particles. 6.

ISO PROCEDURE FOR SOLUBLE METALS AND METALLOIDS Various procedures for the extraction of soluble metals and metalloids have been used for years, based on different operational definitions of solubility. The International Organization for Standardization, Technical Committee 146 on Air Quality, Subcommittee 2 on Workplace Atmospheres, Working Group 2 on Inorganic Particulate Matter (ISO/TC 146/SC 2/WG 2) has attempted to standardize extraction procedures for “soluble” metal compounds by offering an operational definition in terms of a sample preparation method for metallic elements in industrial hygiene measurements. Bioavailability. In 1994, Fairfax and Blotzer argued that the solubility in body fluids should be considered in the development of a new definition for soluble TLVs [1]. But since different body fluids have different solubility characteristics (e.g., pH, salts, polypeptides), such an operational and uniform definition for “bioavailable” cannot realistically be decided. Indeed, the meaning of “bioavailability” has been debated nationally and internationally for years, and it was not deemed practicable nor defensible to attempt to operationally define solubility based on biochemical arguments. Hence, it was decided by consensus within the ISO working group (ISO/TC 146/SC 2/WG 2) to describe procedures for soluble metal compounds in terms of strictly chemical, and not biochemical, criteria [19]. Laboratory Consistency. With regard to analytical methods for the extraction of soluble metals and their compounds, Fairfax and Blotzer suggested that the extraction media, temperature, and extraction volume should be consistent among all laboratories [1]. For an operational definition of “soluble” to be offered, delineation of these analytical parameters is necessary in order to fully standardize the extraction procedure for soluble metal species. Moreover, the apparatus used, as well as chemical compatibility issues, must be amply described. Matters that are outside of laboratory control, notably sampling, cannot always be adequately influenced; however, recommendations as to sampling media, sample handling, and transport requirements should be provided to the field industrial hygienist.

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