Two Soft Ionization Techniques - ESI and MALDI
With the development of biochemistry, mass spectrometry technology has continually improved and innovated, leading to the advent of various soft ionization techniques such as Fast Atom Bombardment (FAB), Field Desorption (FD), Secondary Ion Mass Spectrometry (SIMS), Plasma Desorption (PD), Laser Desorption (LD), Electrospray Ionization (ESI), Thermospray Ionization (TSI), and Matrix-Assisted Laser Desorption/Ionization (MALDI), among others. Among these, ESI and MALDI are currently the most widely used soft ionization techniques.
In the 1980s, Fenn initially developed a new sample ionization technique that utilized a metal syringe with a high-voltage electric field (typically 4000 electron volts). The sample solution is passed through this high-voltage syringe to generate charged droplets, which are then introduced into the ionization chamber. In the ionization chamber, hot dry gas evaporates these charged droplets and breaks them down into numerous ions carrying one or more charges, which then proceed to the separation chamber for analysis. The main feature of this ionization technique is that when the sample molecules are ionized, the integrity of the entire molecule is maintained without generating fragment ions. Therefore, it can be used for the analysis of biomolecules such as proteins and nucleic acids. Due to the formation of charged droplets from the sample solution during ionization, this ionization technique is referred to as Electrospray Ionization (ESI) technology.
Electrospray mass spectrometry uses liquid sampling, allowing it to be combined with high-efficiency separation methods such as High-Performance Liquid Chromatography (HPLC) and Capillary Electrophoresis to better analyze complex samples.
In the late 1980s, Hillenkamp and Karas proposed another ionization technique, the MALDI technique. It involves mixing the sample and matrix (usually an organic acid) in a certain ratio and placing the mixture on a probe. Under the irradiation of a pulsed laser, both sample and matrix molecules absorb the laser energy and are vaporized. The vaporized matrix molecules readily release protons, which can then be captured by the sample molecules, thereby making the sample molecules positively charged. The charged sample molecules are then accelerated by a strong electric field into the flight tube for analysis.
The principle behind MALDI technology is to use a pulsed laser to ionize matrix molecules while evaporating sample molecules. In the gas phase, matrix molecules transfer protons to the sample molecules, making them charged and ready for mass spectrometry analysis. The matrix absorbs most of the laser energy, preventing laser damage to the sample molecules and only producing quasi-molecular ions. Additionally, the pulsed laser serves as an ionization method, reducing the interaction time between the laser and sample molecules, lowering the interaction temperature, and preventing thermal decomposition of the sample molecules. Therefore, the MALDI mass spectrometry method can also be used for the analysis of biomolecules.
Compared to the hard ionization methods of early mass spectrometry, soft ionization techniques, including ESI and MALDI, cause less damage to sample molecules and can maintain the integrity of the entire molecule.
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References:
- Fenn J B, Mann M, Meng C K, et al. Electrospray ionization for mass spectrometry of large biomolecules. Science, 1989, 246(4926): 64-71.
- Hillenkamp F, Karas M, Beavis R C, et al. Matrix-assisted laser desorption/ionization mass spectrometry of biopolymers. Analytical Chemistry, 1991, 63(24): 1193A-1203A.
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