soluble in water. Thus, low recoveries of “soluble” silver will result unless an alternative filter medium, such as polytetrafluoroethylene (PTFE), is used. Another problem is illustrated by soluble forms of Cr VI, which can react with the filter medium to form insoluble compounds and/or undergo reduction to Cr III and, therefore, be lost to CrVI analysis. Hence, chemical compatibility issues must be investigated before samples meant for “soluble” extraction procedures are taken. Membrane filters are appropriate for sampling aerosols for subsequent determination of soluble metal compounds. Such filters are manufactured from a variety of polymeric materials by a number of different processes. Choice of polymer material comprising the filter (e.g., MCE, PTFE, PVC) will depend on chemical reactivity issues discussed in the preceding paragraph. The metal content of the filters must be as low as possible, since it can make a significant contribution to the blank value. 5.
ANALYTICAL CONSIDERATIONS A number of analytical methods for soluble metal compounds in occupational hygiene samples have been published [10-13]. European Standard. Guidance on sample preparation methods for soluble metals and metalloids in workplace air has been promulgated recently in a European Standard [14]. In these published methods and guidelines [10-13,] two methodologies are generally favored for the extraction of “soluble” metal species: (1) extraction in pure water, or (2) extraction in diluted (~0.1 M) hydrochloric acid (HCl). In the European Standard [12,13], both strategies are given as options for the dissolution of “soluble” metals in workplace air samples. Extraction of metals and metalloids in water is meant to reflect the chemical definition of “soluble” (as mentioned above), while extraction in diluted HCl is designed to mimic the dissolution of “soluble” metal compounds in stomach acid, which is highly acidic (pH .1). A temperature of 37o C is recommended since this is normal body temperature. Extraction Solvent. In a recent paper, [15] Hahn argued for the establishment of 0.1 M HCl in an operational definition for soluble metals in occupational hygiene samples and, also, specified mechanical agitation at 37o C for two hours prior to sample analysis. An exception is made for thallium, which can form insoluble TlCl, in which case HCl is replaced by diluted nitric acid (HNO3 ). (A similar problem would arise if HCl were used as the extraction acid for soluble silver compounds.) The German extraction method for several soluble metal species [13] is consistent with the strategy outlined by Hahn, [15] which attempts to address bioavailability by choosing HCl as the extraction acid. However, in standard methods promulgated by the United States, [10] the United Kingdom, [11] and France, [12] deionized water is the solvent chosen in the operational definition of “soluble” for numerous metallic elements in workplace air samples. An exception to extraction in water is made for nickel, [10] where an ammonium citrate solution is specified as the leachate for soluble compounds of this element. Ammonium citrate provides buffering and chelating properties that are desirable for leaching soluble nickel compounds [11]. Operation Definitions. Operational definitions of “soluble” metal species have been promulgated for consumer products such as toys, paper products, paints and art materials [16-18]. Standard procedures for the extraction of metal compounds from consumer products are based on sample treatments in 0.07 - 0.14 M HCl (depending on the sample) for an hour
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