Electrospray and Related Ionization Techniques

Fenn and co-workers described electrospray ionization for mass spectrometry analyzing large biomolecules. ESI greatly enhanced MS ability for analysis of proteins or peptides. The mechanism of the ESI source is relatively simple. In an electrospray ionization source, the solution of analytes is nebulized into fine droplets via a capillary tube under a high electric field. The positive charges are accumulated on the surface of the droplets in this field. Later, because of evaporation of droplets, the surface coulombic forces exceed the surface tension and the droplets are dissociated into smaller droplets. This process continues until nanometer-sized droplets are formed. In this way, the ions pass from the source into the mass analyzer, whereas the bulk solvent is pumped away by a vacuum system. The stability limit (Rayleigh limit) of droplets is determined by the Coulomb forces of the accumulated positive 

In comparison with other ionization sources, ESI represents an even softer ionization technique and causes no fragmentation of analyte ions. A major benefit of the generation of multiply charged ions from polypeptides is that they can be readily analyzed with a less sophisticated instrument with limited mass range, such as a quadrupole. A serious problem is its poor tolerance to matrix, such as mobile-phase buffers, when ESI-MS is used for metallomics and metalloproteomics studies. Since the MS response significantly depends on solvent and sample composition, ion signal intensities of a given analyte do not necessarily correlate with its concentration in samples. Therefore, the internal standards are essential for the quantitative analysis. Another solution is the use of ICP-MS as complement.The detailed methods will be described later. 

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