Selective host-guest chemistry investigated by mass spectrometry: Which of the two, choline or acetylcholine, is the preferred one by the 3iPO triphosphonate-cavitand?

► 3iPO phosphonato cavitand designed as ammonium trap in solution. ► Preferential trap for the acetylcholine ion in comparison to the choline ion. ► Titration-like method adapted for ESI-MS to estimate Ach+-to-Ch+ exchange constant.