Principles of Peptide Ionization – ESI & MALDI

Mass spectrometry, MS, is a powerful analytical technique used to identify and quantify peptides based on their mass-to-charge ratio, m/z. One of the critical steps in MS is the ionization of peptides, which enables their detection and analysis. Two primary ionization techniques are used for peptides: Electrospray Ionization, ESI, and Matrix-Assisted Laser Desorption/Ionization, MALDI. Each method has its advantages and applications in peptide science.

Electrospray Ionization – ESI

Electrospray Ionization, ESI, is a soft ionization technique widely used in peptide analysis. In ESI, peptides in solution are sprayed through a fine needle under a strong electric field, resulting in the formation of highly charged droplets. As the solvent evaporates, the droplets become smaller until they release gas-phase ions. ESI is especially useful for analyzing peptides with multiple charge states and is commonly paired with liquid chromatography to separate peptide mixtures before ionization.¹

Matrix-Assisted Laser Desorption/Ionization – MALDI

Matrix-Assisted Laser Desorption/Ionization, MALDI is another soft ionization method, particularly suited for analyzing peptides in solid samples. In MALDI, peptides are embedded in a matrix, which absorbs laser energy and facilitates ionization by transferring energy to the peptides without fragmenting them. MALDI generates mostly singly charged ions, making it advantageous for rapid analysis of complex peptide mixtures, especially in time-of-flight, TOF, mass spectrometers.²

Conclusion

Both ESI and MALDI are indispensable tools in peptide mass spectrometry. While ESI is preferred for liquid-phase samples and high-sensitivity applications, MALDI excels in the rapid analysis of complex peptide mixtures. Together, these ionization techniques provide the versatility needed for a wide range of peptide research applications.

Citations and Links

1. Kebarle, Paul, and Udo H. Verkerk. “Electrospray: From Ions in Solution to Ions in the Gas Phase, What We Know Now.” Mass Spectrometry Reviews, vol. 28, no. 6, 2009, pp. 898–917. doi:10.1002/mas.20247.

2. Hillenkamp, Franz, and Michael Karas. “Matrix-Assisted Laser Desorption/Ionization (MALDI) Mass Spectrometry: A Historical Introduction.” Methods in Enzymology, vol. 193, 1990, pp. 280–295. doi:10.1016/S0076-6879(90)93022-Z.