Many natural and synthetic molecules contain aldehyde or ketone carbonyl groups. These carbonyls react with hydrazides with the formation of hydrazones. The reaction is spontaneous at pH values around neutral, and the resulting hydrazones are very stable.
Compounds containing 1,2-diol function, like sugars, can be oxidized with sodium periodate with the formation of carbonyl compounds for the subsequent modification with hydrazides. This is an efficient method for the labeling of glycoproteins (like antibodies), and polysaccharides.
FAM hydrazide is a hydrazide label for the attachment of fluorescein, a bright dye for 488 nm channel, to carbonyl compounds.