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SIR: The correspondence by Zatka and Warner referring to our paper (I) shows the complexity in the study of occupational exposures to nickel. In the paper, we report findings of speciation of airborne nickel inside a building used for manufacturing specialized nickel products. This paper was not intended to propose a generally applicable procedure for nickel analysis. Their concern about the alkaline surface of glass fiber filter is based on a special experiment: “We spiked two such filters with 10 pg of Ni as NiC1, [solution], dried them at ambient temperature, and then leached them in water”. They recovered only -65% of the total nickel due to Ni2+ ion reacting with glass alklai at the water-fiber interface. However, this is not the situation with our air sampling, where the Ni2+ particulates were trapped by the filter membrane in the solid state, the filter membranes stored in a desiccator, and then the air particulates sonicated into dionized water of pH 5.7. It is only in the last step that the filter membrane contacts water, and at that point the glass alkaline surface is more likely quenched by H+ in water than by the solid Ni2+ particles being drawn into water. We have found no evidence of loss of Ni2+ in validating the nickel speciation scheme for our air filter samples. It should be noted that this paper does not advocate using glass filters. We have made a statistical comparison of air sample data obtained from two types of air monitoring systems: one with a glass fiber membrane and the other with a cellulose ester membrane; both are in regular use and approved by OSHA for the nickel manufacturing building. Our observations are consistent with significant heterogeneity in airborne nickel concentrations and nickel species, but further study is necessary to develop the proper air sampling technique for determining nickel exposure profiles. The second concern is about separation based on the magnetic property of NiO. They suggest that (a) this approach may have been suitable for the workplace we studied, (b) the magnetic separation is likely to overestimate the amount of metallic nickel in some workplaces, and (c) their leaching method may underestimate the amount of metallic nickel when it is encapsulated by an
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0 1991 American Chemical Society
oxidic phase. Thus, it is important to interpret the analytical data obtained within the framework of calibration. We have described such calibration in our previous paper dealing with development of the new nickel speciation method (2). The third concern is about our use of 18.5% HC1 to dissolve oxidic nickel. They suggest that “sodium peroxide fusion and digestion with fuming perchloric acid are the only ways...to ensure the complete dissolution of any type of oxidic nickel”. Below we have compared the nickel determinations of Nickel Oxide on Silica, a manufactured product, digested with the two acids. From 18.5% HC1, six separate experiments gave Ni of 44.2, 44.4, 44.6, 44.9, 44.1, and 44.5%, mean 44.5%, coefficient of variation 0.7%. From 70% HC104, four determinations of the same nickel product gave Ni of 42.0,42.9,42.3,and 42.9%,mean 42.5%,coefficient of variation 1.1%. These results do not support the contention that perchloric acid dissolution is a better method or that we have underestimated the refractory nature of oxidic nickel by using 18.5% HCl. In sample preparations, our experience is that perchloric acid is necessary for digesting hair samples but not for dissolving air samples. The procedure for evaporation of nonvolatile HCIOI is much more tedious (careful heating to avoid spattering) and time consuming (four times longer) than evaporation of the volatile HC1. Registry No. Ni, 7440-02-0.
Literature Cited (1) Wong, J. L.; Wu, T.-G. Environ. Sci. Technol. 1991, 25,
306-309. (2) Wu, T.-G.; Wong, J. L. Anal. Chim. Acta 1990,235,457-460.
John L. Wong,” ling-Guo Wu Department of Chemistry University of Louisville Louisville, Kentucky 40292
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