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Analytical Chemistry , 83 21 , Schwarzenbach , and Kathrin Fenner. Journal of the American Society for Mass Spectrometry , 22 3 , Journal of Proteome Research , 10 2 , Analytical Chemistry , 83 3 , Journal of the American Society for Mass Spectrometry , 22 2 , Abboud , Juan Z. In , further advances in instrumentation and column technology achieved significant increases in resolution, speed, and sensitivity in liquid chromatography.
Columns with smaller particles [1. Much of what is embodied in this current technology was predicted by investigators such as John Knox in the s. Knox predicted the optimum particle diameters would be 1- 2m and chromatography would be thermally sensitive to frictional heat.
Technology capable of developing robust, uniform small particles was necessarily encountered and resolved on the path to developing UPLC for widespread use. The general term "atmospheric pressure ionization" API includes the most notable technique, electrospray ionization ESI , which itself provides the basis for various, related techniques capable of creating ions at atmospheric pressure rather than in a vacuum. The sample is dissolved in a polar solvent typically less volatile than that used with GC and pumped through a stainless steel capillary which carries between and V.
The liquid aerosolizes as it exits the capillary at atmospheric pressure, the desolvating droplets shedding ions that flow into the mass spectrometer, induced by the combined effects of electrostatic attraction and vacuum. Simplified schematic showing an ESI probein a typical position in front and orthogonal to the MS ion inlet. A cone or counter-current gas is often applied to aid desolvation of liquid droplets as they enter the rarified gas vacuum region of the analyzer.
The mechanism by which potential transfers from the liquid to the analyte, creating ions, remains a topic of controversy. In , Malcolm Dole first proposed the charge residue mechanism in which he hypothesized that as a droplet evaporates, its charge remains unchanged.
The droplet's surface tension, ultimately unable to oppose the repulsive forces from the imposed charge, explodes into many smaller droplets.
These Coulombic fissions occur until droplets containing a single analyte ion remain. When the solvent evaporates from the last droplet, a gas-phase ion forms. In , Iribarne and Thomson proposed a different model, the ion evaporation mechanism, in which small droplets form by Coulombic fission, similar to the way they form in Dole's model.
However, according to ion evaporation theory, the electric field strength at the surface of the droplet is high enough to make leaving the droplet surface and transferring directly into the gas phase energetically favorable for solvated ions. It is possible that the two mechanisms may actually work in concert: the charge residue mechanism dominant for masses higher than Da while ion evaporation dominant for lower masses see R Cole, Some tenets pertaining to Electrospray ionization mass spectrometry, J of Mass Spec, 35, The liquid from the liquid chromatograph enters the ESI probe in a state of charge balance.
So when the solvent leaves the ESI probe it carries a net ionic charge. To ensure that ESI is a continuous technique, the solution must be charged by electrochemical reactions whereby electrons transfer to a conductive surface acting as an electrode. Among other effects, this process can lead to pH changes. It is assumed that, in positive mode, positive-charged droplets leave the spray, and electrons are accepted by the electrode oxidation.
The reverse would be true in negative mode. The surface area of the electro-active electrode, the magnitude of the current, and the nature of the chemical species and their electrode potentials all exert an effect. Over all, ESI is an efficient process. However, the activation energy and energy difference for the reaction, in total, for individual species varies.
The flow rate of the solution and the applied current define limits for each droplet. Competition between molecules occurs, and suppression of analytes of interest is not uncommon. Figure adapted from Andreas Dahlin showing positive ion mode. The adjunctive capability of APCI permits analytes that resist conversion to gas-phase ions by ESI, the less polar and more volatile ones introduced into a mass spectrometer from a condensed phase, or liquid, stream.
Climate Change. Climate Feedback. Ocean Acidification. Rising Sea Level. Ionization Figure 1. Simplified ionization process where an electron is removed from an atom. May 14, Compendium of Chemical Terminology [Online], 2nd ed. July 31, Electron Affinity [Online]. Ionization Energy [Online]. May 19,
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