thicker membranes or even better adhesive or sealing procedure might yield electrodes with longer life. Finally, the elastomeric copolymer applied in this work may be useful as a host matrix for other ion-selective carriers, providing improved stability and lifetime. LITERATURE CITED (1) P. Mueller and D.0. Rudln, Biochem. Biophys. Res. Commun., 26, 398 (1967). (2) J. Stefanacand W. Simon, Microcbem. J., 12, 125 (1967). (3) R. P. Buck, Anal. Chem., 46, 28R (1974). (4) J. F. Brown, 0. H. LeBlanc, and W. T. Giubb, US. Patent 3,767,533(October 23, 1973). (5) H. A. Vaughan, J. Polymer Sci., Part 6, 7, 569 (1969). (6) 0. H. LeBlanc, J. F. Brown, J. F. Klebe, L. W. Niedrach, G. M. J. Slusarczuk, and W. H. Stoddard, J. Appl. Physiol., 40,644 (1976).
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RECEIVEDfor review April 19, 1976. Accepted June 24, 1976.
Determination of Subnanogram Amounts of Fluoride with the Fluoride Electrode Alan S. Hallsworth,* John A. Weatherell, and Dan Deutsch Department of Oral Biology, Dental School, University of Leeds, Leeds LS 1 3EU. England.
An extremely sensltive method of determlnlng fluoride wlth the Orion electrode is descrlbed havlng an absolute detectlon limit of 10 pg (IO-" g) of F-. One-mlcrollter volumes of sample solution are confined as thin layers between the fluoride-Ion sensing element of a standard Orion electrode and the flat sleeve-junctlon of a calomel reference electrode mounted g) of Fimmediately below it. One hundred picograms ( have thus been determined wlth a precislon (relative standard deviatlon) of 5%: the precision improvlng to 2.8% at the 1-ng level. Sample preparation procedures and a fluoride mlcrodiffusion technique of great sensltlvlty and range (
