Comparison of Two Methods for Analysis of Fluoride in Vegetation Standard Reference Material

Abstract

Analysis of fluoride (F−) levels in vegetation by Ion Selective Electrode (ISE) is subject to interference from aluminum (Al3+) and iron (Fe3+). Although these interferences can be removed by distilling the sample from perchloric acid, the procedure is lengthy and complex. Dilute perchloric acid extraction is simpler and more rapid, but may not extract all of the fluoride complexed with iron and aluminum. We compared the accuracy and precision of dilute perchloric acid extraction and alkali fusion methods to determine F− in standard vegetation samples containing known amounts of aluminum, iron and fluoride. The perchloric acid extraction recovered only 9.5 to 18.8% of the F− with RSD of 3–4%. The alkali fusion extraction recovered 101 to 121% of the F− with 7.2 to 9.6% RSD. The source of the high bias in the alkali fusion method is unknown. Both methods have a detection limit of 0.06 μg F−. Unless samples are known to be free of interfering ions, a method, which can remove these interferences, should be used instead of dilute perchloric acid extraction.