1. H. R. Notenboom, W. J. Holland, and R. G. Billinghurst, Mikrochim. Acta [Wien]1973, 467.
2. P. Beaupr� and W. Holland, Mikrochim. Acta [Wien], to be published.
3. E. S. Underwood, Trace Elements in Human and Animal Nutrition, 2nd ed. New York: Academic Press. 1962. Chapter 6.
4. D. C. Burrell, At. Absorption Newsletter4, 309 (1965).
5. R. F. Gould, Advances in Chemistry Series, Trace Inorganics in Water, American Chemical Society Publications, U. S. A., Vol. 73, p. 230.