title: Detection of Neutral Species in the MALDI Plume Using Femtosecond Laser Ionization: Quantitative Analysis of MALDI-MS Signals Based on a Semiequilibrium Proton Transfer Model.
abstract: We investigated neutral species in the matrix-assisted laser desorption and ionization (MALDI) plume using femtosecond laser ionization spectrometry with simultaneous measurement of the standard MALDI spectrum of the identical MALDI event induced by pulsed UV laser irradiation. The ratio of neutral species in the plume [A]p/[M]p(A = phenylalanine (Phe) or alanine (Ala), M = 2,5-dihydroxybenzoic acid (DHB)) was confirmed to be the same as that of the sample mixture in the range of [A]0/[M]0= 4 × 10-4-1, indicating the validity of the widely adopted approximation [A]p/[M]p= [A]0/[M]0in the reaction quotient of the proton transfer reaction MH++ A ⇄ M + AH+. An effective parameter representing the extent of thermal equilibrium in the thermal proton transfer model is introduced for the first time. Numerical simulation based on this semiequilibrium model successfully reproduced variations of MALDI signal intensities AH+and MH+with two parameters: the fraction of ionized matrix a ≤ 10-5and an effective temperature T = 1200 and 1100 K for Phe/DHB and Ala/DHB systems, respectively. These values show good agreement with those determined previously by different experimental approaches. The extent of thermal equilibrium was determined to be 95% and 98% for Phe/DHB and Ala/DHB systems, respectively, suggesting that the proton transfer reactions almost proceed to their thermal equilibrium.
authors.affiliation: Faculty of Pharmaceutical Sciences, Niigata University of Pharmacy and Applied Life Sciences , 265-1, Higashijima, Akiha-ku, Niigata 956-8603, Japan.
authors.forename: Tatsuro
authors.initials: T
authors.lastname: Shirota
authors.affiliation: Department of Applied Chemistry, National Chiao Tung University , 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan.
authors.forename: Masashi
authors.initials: M
authors.lastname: Tsuge
authors.affiliation: Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama , 2630 Sugitani, Toyama 930-0194, Japan.
authors.forename: Yasumasa
authors.initials: Y
authors.lastname: Hikosaka
authors.forename: Koichi
authors.initials: K
authors.lastname: Soejima
authors.affiliation: Faculty of Pharmaceutical Sciences, Niigata University of Pharmacy and Applied Life Sciences , 265-1, Higashijima, Akiha-ku, Niigata 956-8603, Japan.
authors.forename: Kennosuke
authors.identifier: http://orcid.org/0000-0002-0613-1920
authors.initials: K
authors.lastname: Hoshina