Degree of Ionization in MALDI of Peptides: Thermal Explanation for the Gas-Phase Ion Formation

作者：Bae Yong Jin; Shin Young Sik; Moon Jeong Hee; Kim Myung Soo^*

来源：Journal of the American Society for Mass Spectrometry, 2012, 23(8): 1326-1335.

DOI：10.1007/s13361-012-0406-y

• Degree of ionization
• Peptide
• MALDI
• Ion formation mechanism
• Laser fluence dependence
• Preformed ion
• Gas phase proton transfer

Degree of ionization (DI) in matrix-assisted laser desorption ionization (MALDI) was measured for five peptides using alpha-cyano-4-hydroxycinnanmic acid (CHCA) as the matrix. DIs were low 10(-4) for peptides and 10(-7) for CHCA. Total number of ions (i.e., peptide plus matrix) was the same regardless of peptides and their concentration, setting the number of gas-phase ions generated from a pure matrix as the upper limit to that of peptide ions. Positively charged cluster ions were too weak to support the ion formation via such ions. The total number of gas-phase ions generated by MALDI, and that from pure CHCA, was unaffected by the laser pulse energy, invalidating laser-induced ionization of matrix molecules as the mechanism for the primary ion formation. Instead, the excitation of matrix by laser is simply a way of supplying thermal energy to the sample. Accepting strong Coulomb attraction felt by cations in a solid sample, we propose three hypotheses for gas-phase peptide ion formation. In Hypothesis 1, they originate from the dielectrically screened peptide ions in the sample. In Hypothesis 2, the preformed peptide ions are released as part of neutral ion pairs, which generate gas-phase peptide ions via reaction with matrix-derived cations. In Hypothesis 3, neutral peptides released by ablation get protonated via reaction with matrix-derived cations.

• 出版日期2012-8