2 research outputs found
Improved Resolution of Hydrocarbon Structures and Constitutional Isomers in Complex Mixtures Using Gas Chromatography-Vacuum Ultraviolet-Mass Spectrometry
Understanding the composition of complex hydrocarbon
mixtures is
important for environmental studies in a variety of fields, but many
prevalent compounds cannot be confidently identified using traditional
gas chromatography/mass spectrometry (GC/MS) techniques. This work
uses vacuum-ultraviolet (VUV) ionization to elucidate the structures
of a traditionally “unresolved complex mixture” by separating
components by GC retention time, <i>t</i><sub>R</sub>, and
mass-to-charge ratio, <i>m</i>/<i>z</i>, which
are used to determine carbon number, <i>N</i><sub>C</sub>, and the number of rings and double bonds, <i>N</i><sub>DBE</sub>. Constitutional isomers are resolved on the basis of <i>t</i><sub>R</sub>, enabling the most complete quantitative analysis
to date of structural isomers in an environmentally relevant hydrocarbon
mixture. Unknown compounds are classified in this work by carbon number,
degree of saturation, presence of rings, and degree of branching,
providing structural constraints. The capabilities of this analysis
are explored using diesel fuel, in which constitutional isomer distribution
patterns are shown to be reproducible between carbon numbers and follow
predictable rules. Nearly half of the aliphatic hydrocarbon mass is
shown to be branched, suggesting branching is more important in diesel
fuel than previously shown. The classification of unknown hydrocarbons
and the resolution of constitutional isomers significantly improves
resolution capabilities for any complex hydrocarbon mixture
Evaluation of a New Vocus Reagent-Ion Source and Focusing Ion-Molecule Reactor for use in Proton-Transfer-Reaction Mass Spectrometry
<div>We evaluate the performance of a new chemical ionization source called Vocus, consisting of a discharge reagent-ion source and focusing ion-molecule reactor (FIMR) for use in protontransfer-reaction time-of-flight mass spectrometry (PTR-TOF) measurements of volatile organic compounds (VOCs) in air. The reagent ion source uses a low-pressure discharge. The FIMR consists of a glass tube with a resistive coating, mounted inside a radio-frequency (RF) quadrupole. The axial electric field is used to enhance ion collision energies and limit cluster ion formation. The RF field focuses ions to the central axis of the reactor and improves the detection efficiency of product ions. Ion trajectory calculations demonstrate the mass-dependent focusing of ions and enhancement of the ion collision energy by the RF field, in particular for the lighter ions. Product ion signals are increased by a factor of 10 when the RF field is applied (5,000-18,000 cps ppbv<sup>-1</sup>), improving measurement precision and detection limits while operating at very similar reaction conditions as traditional PTR instruments. Due to the high water mixing ratio in the FIMR, we observe no dependence of the sensitivity on ambient sample humidity. In this work, the Vocus is interfaced to a TOF mass analyzer with a mass resolving power up to 14,000, which allows clear separation of isobaric ions, observed at nearly every nominal mass when measuring ambient air. Measurement response times are determined for a range of ketones with saturation vapor concentrations down to 5Ă—10<sup>4</sup> ÎĽg m<sup>-3</sup> and compare favorably with previously published results for a PTR-MS instrument.</div