Clinical Chemistry, Vol 15, 204-215, Copyright © 1969 by the American Association for Clinical Chemistry

Cerate-Arsenite Measurement of Iodine in the Subnanogram Range

¹ Physical Chemistry Laboratory, Veterans Administration Center, Martinsburg, W. Va. 25401; and the National Institute of Arthritis and Metabolic Diseases, National Institutes of Health, Bethesda, Md.

A method for the measurement of subnanogram quantities of iodine is described. Procedural detail and precision of results are presented for application of the method to protein-bound or total iodine in duplicate on 12.3 µl serum. Following wet ashing with chloric reagent, the iodine-catalyzed cerate-arsenite reaction was carried out at temperatures between 23 and 27°, and the reaction rate coefficients were corrected to refer to 25.0° by an empirical equation. The recovery of iodine standards, added to the precipitated protein before ashing, ranged between 96 and 106%. One contributing cause of this variation is the formation of an inhibitor of unknown nature during heating. Samples were brought to a final volume of 180 µl prior to fading. The detection limit was of the order of 0.04 ng iodine. For 62 serum samples having a mean iodine content of 0.558 ng (range 0.15-1.31 ng), the standard deviation calculated from duplicates was 0.019 ng/180 µl. For 31 samples having a mean iodine content of 2.24 ng (range 1.61-2.96 ng), the standard deviation was 0.039 ng/180 µl. Expressed as micrograms of iodine per 100 ml of serum, the mean ± standard deviation for the group of 62 samples was 4.54 ± 0.15.