European Journal of Mass Spectrometry - Abstract of Equilibrium conditions in laser-desorbed plumes: thermodynamic properties of α-cyano-4-hydroxycinnamic acid and protonation of amino acids

Gary R. Kinsel,^a,* Daqing Yao,^b Faten H. Yassin^c and Dennis S. Marynick^d
^aSouthern Illinois University Carbondale, Department of Chemistry and Biochemistry, 1245 Lincoln Dr, Carbondale, IL 62901-4409, USA
^bIRIX Pharmaceuticals, Inc., Analytical R&D Laboratory, 101 Technology Place, Florence, SC 29501, USA
^cThe University of Michigan, Department of Chemistry, Ann Arbor, MI 48109-1055, USA
^dThe University of Texas at Arlington, Department of Chemistry and Biochemistry, 502 Yates St, Arlington, TX 76019-0065, USA

The equilibrium nature of a plume of laser desorbed material is explored through the application of a simple equilibrium model to the ion signals observed in 355 nm laser desorption/ionization mass spectra of mixtures of the MALDI matrix α-cyano-4-hydroxycinnamic acid (αCHCA) with the amino acids glycine, alanine, valine, isoleucine, and phenylalanine. In these studies it is found that there are systematic and predictable increases in the relative yield of protonated amino acid with increases in amino acid gas-phase basicity. In addition, the thermodynamic values extracted from the equilibrium plot are shown to be in good agreement with values obtained from computational investigation of plausible αCHCA proton donor species. These results are supportive of a picture wherein the laser-desorbed material is viewed as a dense plume in which facile charge transfer occurs leading, ultimately, to a thermodynamically equilibrated distribution of proton donor and proton acceptor species.