Development of a Novel Mass Spectrometric ... - Jacobs University
Development of a Novel Mass Spectrometric ... - Jacobs University
Development of a Novel Mass Spectrometric ... - Jacobs University
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Introduction<br />
an ionisation technique <strong>of</strong> choice for the analysis <strong>of</strong> medium to less polar, small<br />
and thermally relative stable analytes. It is a form <strong>of</strong> CI which takes place at<br />
atmospheric pressure. APCI was first introduced by Horning in 1973 for the<br />
analysis <strong>of</strong> volatile compounds. 43 However APCI wasn’t spread until the<br />
commercialisation <strong>of</strong> ESI after Fenn’s work in 1985. 98 Contrary to ESI, APCI has<br />
the capability to vaporise higher boiling point analytes which resist volatilisation.<br />
Figure 1-5 Schematic description <strong>of</strong> the atmospheric pressure chemical ionisation<br />
(APCI) interface and the mechanism <strong>of</strong> ion formation in the corona discharge<br />
region<br />
Ionisation inside APCI is separated from solvent evaporation. After the mobile<br />
phase is introduced into a pneumatic nebuliser, it is heated to high temperatures<br />
(400-450 °C) inside a heated quartz tube and sprayed with high flow <strong>of</strong> nebulizer<br />
gas (nitrogen gas). Ionisation occurs in gas phase, in contrast to ESI, by subjecting<br />
the vaporised neutral analytes and reagent gas molecules (N 2 , H 2 O, O 2 ) to corona<br />
discharge needle that creates the ions. It is emphasized that the corona discharge<br />
needle is used as an electron source to ionise gas phase molecules such as<br />
molecules <strong>of</strong> N 2 (commonly used as sheath gas) and molecules <strong>of</strong> the solvent<br />
(commonly used methanol/water) forming radical cation in positive ion-mode.<br />
These ions collide with the neutral analytes resulting in the creation <strong>of</strong> ions. The<br />
high frequency <strong>of</strong> collisions results in high ionisation efficiency and thermalisation<br />
<strong>of</strong> the analyte ions. These ions enter pumping and focusing stages within mass<br />
21