Unexpected separate elution of cation and anion of an ammonium salt in supercritical fluid chromatography

Amines are frequently used as additives in supercritical fluid chromatography (SFC). They allow eluting basic analytes with reasonable retention times and less distorted peak shapes. Since amines are chemically active compounds, their introduction into SFC mobile phase always raises a question on whether they can react with analytes or mobile phase constituents and, if so, can it affect chromatography separation. Primary and secondary amines are known to react with carbon dioxide, also all amines, being bases, can interact with CO₂-alcohol mixtures which are known to be slightly acidic. In this work, we report a case of separate elution of an ammonium salt, salbutamol sulfate, anion and cation in SFC. Retention time of a peak which molecular mass registered by mass-spectrometry in ES(-) mode corresponds to HSO₄^- differs substantially from the retention time of a peak corresponding to salbutamol [M+H] registered in ES(+) regime. Moreover, sulfate anion retention time depends both on amine additive type and concentration whereas salbutamol retention time doesn't. Similar effect is observed on other columns as well as with other ammonium salts. We suppose that such behavior is caused by the exchange chemical reaction happening between ammonium salt analyte and amine additive. An additive converts a salt into a free base form and turns into a salt form itself. If this hypothesis is true, it might be important to take the possibility of such interactions into account during preparative SFC work since the compound injected might not be equivalent to a compound eluted.