Variable Electrospray Ionization and Matrix-Assisted Laser Desorption/ Ionization Mass Spectra of the Bisquaternary Ammonium Salt Ethonium

Abstract

Determination of a single "fingerprint" mass spectrum of a biologically active compound, required for compound identification in mixtures and biological materials in such applied tasks as ecological monitoring and biological imaging, may become a nontrivial problem for labile compounds whose mass spectra depend strongly on the applied experimental conditions. In the present communication, qualitatively different mass spectral patterns obtained for the bisquaternary ammonium salt ethonium Cat 2+ •2Clunder varied conditions of electrospray ionization and matrixassisted laser desorption/ionization (MALDI) are described and systematized. It is shown that qualitative changes in the electrospray mass spectra of ethonium occur upon a cone voltage increase from 10 V to 100 V, which are caused by the subsequent appearance and destruction of several primary ions: the intact dication Cat 2+ , the dication-counterion cluster Cat 2+ •Cland[Cat-H] + and [Cat-CH 3 ] + ions. Novel experimental evidence of survival of the gas-phase dication Cat 2+ with the shortest possible distance of ca 4 Ǻ between the quaternary nitrogen atoms provided by two CH 2 groups and Cat 2+ fragmentation with preservation of the doubly charged state of the fragments are revealed under soft electrospray conditions. The interpretation of the MALDI mass spectrum is made taking into account the formation of a salt of the bisquaternary ammonium base and 2,5-dihydroxybenzoic acid, which results in the absence of any chlorine-containing ions in the mass spectra and a fragment ion distribution that is different from that observed under electrospray ionization conditions. The main mass spectral features revealed for ethonium may aid in the identification of other types of bisquaternary ammonium compounds.