Tiniest amount (4)
(Other definitions for iota that I've seen before include "'Very small amount, jot (4)'" , "Smidgen" , "Greek letter - small quantity" , "(Not one) little bit" , "Greek letter; tiny amount" .)
(Other definitions for trace that I've seen before include "Very small amount that can only just be detected" , "Minute quantity" , "Suggestion of rain" , "Follow to source" , "Track down; tiny amount" .)