Effect of laser intensity on the apparent isotope patterns of heme and peptide ions in MALDI-TOF MS

• Isotopic distributions of heme and peptide ions in MALDI-MS analysis were comprehensively investigated as a function of laser intensity.
• For most peptides, the relative abundance of the second isotopic peak was similar to the theoretical relative abundances.
• The experimental abundances of the second isotopic peak of heme ions were consistently higher than the theoretical abundances, due to the overlap of isobaric species.
• Saturation of the MALDI-TOF MS detector was observed in a certain laser intensity range with higher loadings (80 pmol) of tryptically digested cytochrome c using the CHCA matrix.

The apparent isotope patterns of heme ions and tryptically digested peptide ions in matrix-assisted laser desorption/ionization (MALDI)-time-of-flight (TOF) mass spectra (MS) were systematically investigated as a function of laser intensity. Two standard proteins, myoglobin and cytochrome c, were tryptically digested to provide heme b and heme c ions, respectively, along with several peptides. MALDI mass spectra of these materials, co-deposited with one of two matrix materials, 2,5-dihydroxybenzoic acid or α-cyano-4-hydroxycinnamic acid (CHCA), were then generated using various laser intensities. For most peptides, the relative abundance of the second isotopic peak (SIP) to that of the monoisotopic peak was similar to the theoretical relative abundances. Regardless of laser intensity, the experimental SIP abundances of heme ions were consistently higher than the theoretical SIP abundances, which is due to the overlap of isobaric species. A gradual increase in the relative SIP abundance of the heme c ion was observed within a certain laser intensity range when the CHCA matrix was used with higher loadings (80 pmol) of tryptically digested cytochrome c, which is likely due to saturation of the MALDI-TOF MS detector.

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