Browsing by Author "Liao, Chih-Yu"

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Fluorescence spectroscopy of UV-MALDI matrices and implications of ionization mechanisms
(2014-10-28)
Lin, Hou-Yu
;
Hsu, Hsu Chen
;
Lu, I-Chung
;
Hsu, Kuo-Tung
;
Liao, Chih-Yu
Matrix-assisted laser desorption ionization (MALDI) has been widely used in the mass analysis of biomolecules; however, there are a lot of debates about the ionization mechanisms. Previous studies have indicated that S-1-S-1 annihilation might be a key process in the generation of primary ions. This study investigates S-1-S-1 annihilation by examining the time-resolved fluorescence spectra of 12 matrices. No S-1-S-1 annihilation was observed in six of these matrices (3-hydroxy-picolinic acid, 6-aza-2-thiothymine, 2,4-dihydroxy-acetophenone, 2,6-dihydroxy-acetophenone, 2,4,6-trihydroxy-acetophenone, and ferulic acid). We observed two matrix molecules reacting in an electronically excited state (S-1) in five of these matrices (2,5-dihydroxybenzoic acid, alpha-cyano-4-hydroxycinnamic acid, 2,5-dihydroxy-acetophenone, 2,3-dihydroxybenzoic acid, and 2,6-dihydroxybenzoic acid), and S-1-S-1 annihilation was a possible reaction. Among these five matrices, no S-1-S-1 annihilation was observed for 2,3-dihydroxybenzoic acid in typical peak power region of nanosecond laser pulses in MALDI, but a very small value of reaction rate constant was observed only in the high peak power region. The excited-state lifetime of sinapinic acid was too short to determine whether the molecules reacted in an electronically excited state. No correlation was observed between the ion generation efficiency of MALDI and S-1-S-1 annihilation. The results indicate that the proposal of S-1-S-1 annihilation is unnecessary in MALDI and energy pooling model for MALDI ionization mechanism has to be modified. (C) 2014 AIP Publishing LLC.
Ion Intensity and Thermal Proton Transfer in Ultraviolet Matrix-Assisted Laser Desorption/Ionization
(2014-04-17)
Lu, I-Chung
;
Lee, Chuping
;
Chen, Hui-Yuan
;
Lin, Hou-Yu
;
Hung, Sheng-Wei
The ionization mechanism of ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) was investigated by measuring the total cation intensity (not including sodiated and potasiated ions) as a function of analyte concentration (arginine, histidine, and glycine) in a matrix of 2,4,6-trihydroxyacetophenone (THAP). The total ion intensity increased up to SS times near the laser fluence threshold as the arginine concentration increased from 0% to 1%. The increases were small for histidine, and a minimal increase occurred for glycine. Time-resolved fluorescence intensity was employed to investigate how analytes affected the energy pooling of the matrix. No detectable energy pooling was observed for pure THAP and THAP/analyte mixtures. The results can be described by using a thermal proton transfer model, which suggested that thermally induced proton transfer is crucial in the primary ion generation in UV-MALDI.
Is energy pooling necessary in ultraviolet matrix-assisted laser desorption/ionization?
(2014-01-15)
Lin, Hou-Yu
;
Song, Botao
;
Lu, I-Chung
;
Hsu, Kuo-Tung
;
Liao, Chih-Yu
RATIONALEEnergy pooling has been suggested as the key process for generating the primary ions during ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI). In previous studies, decreases in fluorescence quantum yields as laser fluence increased for 2-aminobenzoic acid, 2,5-dihydroxybenzoic acid (2,5-DHB), and 3-hydroxypicolinic acid were used as evidence of energy pooling. This work extends the research to other matrices and addresses whether energy pooling is a universal property in UV-MALDI. METHODSEnergy pooling was investigated in a time-resolved fluorescence experiment by using a short laser pulse (355nm, 20ps pulse width) for excitation and a streak camera (1ps time resolution) for fluorescence detection. RESULTSThe excited-state lifetime of 2,5-DHB decreased with increases in laser fluence. This suggests that a reaction occurs between two excited molecules, and that energy pooling may be one of the possible reactions. However, the excited-state lifetime of 2,4,6-trihydroxyacetophenone (THAP) did not change with increases in laser fluence. The upper limit of the energy pooling rate constant for THAP is estimated to be approximately 100-500 times smaller than that of 2,5-DHB. CONCLUSIONSThe small energy pooling rate constant for THAP indicates that the potential contribution of the energy pooling mechanism to the generation of THAP matrix primary ions should be reconsidered. Copyright (c) 2013 John Wiley & Sons, Ltd.