Electrospray Ionization


Electrospray ionization (ESI) is a technique to generate ions for mass spectrometry using electrospray by applying a high voltage to a liquid to produce an aerosol. Due to relatively fragile biomacromolecules, their structures are easily destroyed during the process of dissociation and ionization. ESI overcomes the tendency of these molecules to fragment upon ionization. ESI differs from other atmospheric pressure ionization processes, in which it may produce multiple charge ions, effectively extending the analyzer's mass range to accommodate the magnitude of kDa-mDa observed by proteins and their associated peptides.

The Principle of Electrospray Ionization

ESI applies a high voltage at the outlet of the capillary, and the high electric field generated atomizes the liquid flowing out of the capillary into tiny charged droplets. As the solvent evaporates, the charge intensity on the surface of the droplet gradually increases, and finally the droplet splits into one or a plurality of charged ions, allowing the analyte to enter the gas phase in the form of a single charge or multiple charges and become a gas phase ion.

There are two explanations for the mechanism of gas phase ion generation: the ion evaporation model (IEM) proposed by Thomsona and lribarne, and the charged residue model (CRM) advocated by Dole and Rllgen. In the two models, the ions to be analyzed are not excited by external energy and do not generate debris during the process of becoming a gas phase ion.

Electrospray Ionization Figure 1. Mechanism of electrospray ionization (CS, Ho; CWK, Lam. 2003.)

The Process of Electrospray Ionization

The ESI process can be roughly divided into three stages: droplet formation, desolvation, and gas phase ion formation.

Electrospray Ionization Figure 2. The electrospray ionization process. (Markides, K; Gräslund, A. 2002.)

Advantages and Disadvantages of Electrospray Ionization

As a new type of soft ionization technology, ESI has been widely used in many fields. Although this technology makes up for shortcomings of some traditional mass spectrometry techniques, it still has some drawbacks.

  1. Electrospray provides a relatively simple way to ionize non-volatile solutions, allowing the mass spectrometer to provide a sensitive direct detection. Electrospray mass spectrometry can be used not only for the detection and analysis of inorganic substances, but also for the analysis of organometallic ion complexes and biomacromolecules.
  2. In electrospray mass spectrometry, high molecular weight molecules typically carry multiple charges, and the distribution of charge states accurately quantifies molecular weight, providing both accurate molecular mass and structural information.
  3. Multiple ionization modes to choose from: positive ion mode and negative ion mode.
  4. Effectively combined with a variety of chromatographs for complex system analysis.
  1. The experimental parameters or technical conditions must be carefully selected based on the problem to be solved.
  2. There is a limit to the choice of solvent and the range of solutions that can be used. At the same time, the response of the mass spectrometer to different complexes varies widely, which prevents accurate quantitative analysis.
  3. Since the solution parameters control the spray process, there is fluctuation in the ion signal even under good conditions.

Applications of ESI

ESI, a soft ionization technology, solves the problem of high polarity, ionization of thermally unstable proteins, and determination of the molecular mass of macromolecular organisms. Moreover, ESI has significantly improved sensitivity, accuracy, and complexity of analytical mixtures over previous mass spectrometry techniques, broadening the application of mass spectrometry in the protein field.

ESI is a soft ionization method, which promotes the widespread use of the ESI source in mass spectrometry. Even compounds with large molecular weight and poor stability will not decompose during ionization.

We have briefly introduced the ESI, a type of ionization methods, which can help you understand more about mass spectrometry. At Creative Proteomics, we have the professional mass spectrometry platform, which contains different instruments. By using mass spectrometry, Creative Proteomics can provide different services to meet various requirements, including:

References

  1. Ho, CS; et al. Electrospray Ionisation Mass Spectrometry: Principles and Clinical Applications. Clin Biochem Rev. 2003,24 (1): 3–12.
  2. Pitt; James, J. Principles and Applications of Liquid Chromatography-Mass Spectrometry in Clinical Biochemistry. Clin Biochem Rev. 2009, 30 (1): 19–34.
  3. Markides, K; Gräslund, A. Mass spectrometry (MS) and nuclear magnetic resonance (NMR) applied to biological macromolecules. Advanced information on the Nobel Prize in Chemistry. 2002.
  4. Teo, CA; Donald, WA. Solution additives for supercharging proteins beyond the theoretical maximum proton-transfer limit in electrospray ionization mass spectrometry. Anal. Chem. 2014, 86 (9): 4455–62.
  5. Monge; et al. Mass Spectrometry: Recent Advances in Direct Open Air Surface Sampling/Ionization. Chemical Reviews. 2013, 113 (4): 2269–2308.
  6. Li, Anyin; et al. Synthesis and Catalytic Reactions of Nanoparticles formed by Electrospray Ionization of Coinage Metals. Angewandte Chemie International Edition. 2013, 53 (12): 3147–3150.

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