Sodium azide is a salt of weak hydrazoic acid with pKa 4.5-4.7.
Sodium azide is very toxic. It is typically used as a stabilizer for aqueous solution to suppress bacteria grows.
Chromatography separation is based on weak ionic interaction of the partially ionized hydrazoic acid with positively charged column surface. Additional polar interaction is a result of high acetonitrile concentration in the mobile phase.