Aliphatic Hydrocarbon Spectra by Helium Ionization Mass Spectrometry (HIMS) on a Modified Atmospheric-Pressure Source Designed for Electrospray Ionization |
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Authors: | Zhihua Yang Athula B Attygalle |
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Institution: | (1) Center for Mass Spectrometry, Department of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, USA; |
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Abstract: | Chemical-ionization techniques that use metastable species to ionize analytes traditionally use a flat pin or a sharp solid
needle onto which the high potential needed to generate the discharge plasma is applied. We report here that direct analysis
of samples containing volatile and semivolatile compounds, including saturated and unsaturated aliphatic hydrocarbons, can
be achieved on any electrospray-ionization mass spectrometer by passing helium though the sample delivery metal capillary
held at a high potential. In the helium plasma ionization source (HPIS) described here, the typical helium flow required (about
20–30 mL/min), was significantly lower than that needed for other helium-ionization sources. By this procedure, positive ions
were generated by nominal hydride ion removal from molecules emanating from heated saturated hydrocarbons as large as tetratetracontane
(C44H90), at capillary voltages ranging from 2.0 to 4.0 kV. Unsaturated hydrocarbons, on the other hand, underwent facile protonation
under much lower capillary voltages (0.9 to 2.0 kV). Although saturated and monounsaturated hydrocarbons bearing the same
number of carbon atoms generate ions of the same m/z ratio, a gas-phase deuterium exchange method is described to ascertain the identity of these isomeric ions originating from
either protonation or hydride abstraction mechanisms. Moreover, mass spectrometric results obtained by exposing unsaturated
hydrocarbons to D2O vapor in an HPIS-MS instrument confirmed that the proton donor for ionization of unsaturated hydrocarbons is protonated
water. |
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