Pentylammonium formate

An aqueous solution of NaCl, NaN03, and Na2S04 was passed through a Cig-silica reversed-phase liquid chromatography column eluted with water. None of the cations or anions is retained by the Cjg stationary phase, so all three salts were eluted in a single, sharp band with a retention time of 0.9 min. Then the column was equilibrated with aqueous 10 mM pentylammonium formate, whose hydrophobic tail is soluble in the C18 stationary phase. 

When the mixture of NaCl, NaN03, and Na2S04 was passed through the column and eluted with 10 mM pentylammonium formate, the cations all came out in a single peak with a retention time of 0.9 min. However, anions were separated, with retention times of 1.9 min (Cl ), 2.1 min (NO3), and 4.1 min (SO4 ). Explain why this column behaves as an anion exchanger. Why is sulfate eluted last ... 

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