title: Timeframe Dependent Fragment Ions Observed in In-Source Decay Experiments with β-Casein Using MALDI MS.
abstract: The fragment ions observed with time-of-flight (TOF) and quadrupole ion trap (QIT) TOF mass spectrometers (MS) combined with matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) experiments of phosphorylated analytes β-casein and its model peptide were compared from the standpoint of the residence timeframe of analyte and fragment ions in the MALDI ion source and QIT cell. The QIT-TOF MS gave fragment c-, z'-, z-ANL, y-, and b-ions, and further degraded fragments originating from the loss of neutrals such as H(2)O, NH(3), CH(2)O (from serine), C2H4O (from threonine), and H(3)PO(4), whereas the TOF MS merely showed MALDI source-generated fragment c-, z'-, z-ANL, y-, and w-ions. The fragment ions observed in the QIT-TOF MS could be explained by the injection of the source-generated ions into the QIT cell or a cooperative effect of a little internal energy deposition, a long residence timeframe (140 ms) in the QIT cell, and specific amino acid effects on low-energy CID, whereas the source-generated fragments (c-, z'-, z-ANL, y-, and w-ions) could be a result of prompt radical-initiated fragmentation of hydrogen-abundant radical ions [M + H + H](+) and [M + H - H](-) within the 53 ns timeframe, which corresponds to the delayed extraction time. The further degraded fragment b/y-ions produced in the QIT cell were confirmed by positive- and negative-ion low-energy CID experiments performed on the source-generated ions (c-, z'-, and y-ions). The loss of phosphoric acid (98 u) from analyte and fragment ions can be explained by a slow ergodic fragmentation independent of positive and negative charges.
authors.affiliation: Koichi Tanaka Mass Spectrometry Research Laboratory, Shimadzu Corporation, Nakagyo-ku, Kyoto, 604-8511, Japan.
authors.forename: Sadanori
authors.initials: S
authors.lastname: Sekiya
authors.forename: Keishiro
authors.initials: K
authors.lastname: Nagoshi
authors.forename: Shinichi
authors.initials: S
authors.lastname: Iwamoto
authors.forename: Koichi
authors.initials: K
authors.lastname: Tanaka
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Takayama
mesh_terms: D000595:Amino Acid Sequence; D002364:Caseins / Q000032:analysis / Q000737:chemistry; D007477:Ions / Q000032:analysis* / Q000737:chemistry; D008969:Molecular Sequence Data; D010750:Phosphoproteins / Q000032:analysis / Q000737:chemistry; D010756:Phosphoric Acids; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization / Q000379:methods*
chemical_list: D002364:Caseins; D007477:Ions; D010750:Phosphoproteins; D010756:Phosphoric Acids