Which one of the following elements cannot be detected by "Lassaigne's test"?

The sodium fusion test, or Lassaigne's test, is used in elemental analysis for the qualitative determination of the presence of foreign elements, namely halogens, nitrogen, and sulphur, in an organic compound. It was developed by J. L. Lassaigne.Silver fluoride is soluble in water and does not precipitate and thus this method cannot be used for detection of fluorine.