Using Variable Ionization Energy Time-of-Flight Mass Spectrometry with Comprehensive GC×GC To Identify Isomeric Species

Abstract

Although GC×GC-ToF-MS allows the separation of thousands of peaks, many of these peaks are not positively identified owing to the lack of mass spectral library data and/or standard materials, leading to a substantial amount of information being inaccessible. The fragmentation patterns of molecules in mass spectrometers using electron impact ionization at 70 eV can be useful for molecule identification, provided a match is available in a published EI MS library, but are indistinguishable for many isomeric organic compounds (for example, linear and branched alkanes). Lower ionization energies have been exploited leading to organic compounds being ionized with lower excess internal energy and less fragmentation, retaining the molecular ion and maximizing its relative signal. This has enabled the identification of a large number of isomeric organic compounds, both aliphatic and aromatic, between C12-C36, in the previously unresolved complex mixture (UCM) of two motor oil samples. This technique also demonstrates problems associated with separation of coeluting isomers, particularly for the n-alkanes, which are routinely measured by 1D GC/MS and may be overestimated, due to coelution. As a consequence retention times in 2 dimensions and mass spectra at variable ionization energies are shown to give unparalleled power to identify specific isomers.

Mapping and quantifying isomer sets of hydrocarbons ( ≥ C12) in diesel exhaust, lubricating oil and diesel fuel samples using GC × GC-ToF-MS

“…Previous work from this group identified a large number of isomeric species in base oil using 14 eV EI ionization energy mass spectrometry (Alam et al, 2016a). Although we were able to identify a large number of compounds, there still existed a small amount of fragmentation at 14 eV, particularly for alkyl-methyl-, alkyl-dimethyl-and alkyl-trimethylcyclohexanes.…”

“…Our recent work exploited soft ionization (14 eV) to identify a large number of isomers, demonstrating the ability to separate and identify individual alkanes (normal, branched and cyclic) with specific carbon numbers, based on their volatility and polarity (Alam et al, 2016a). In this study we expand our previous qualitative analysis and separate the alkane series (as well as other homologous series) into isomer sets containing the same carbon number.…”

“…The mass/charge range was 30 to 525 m/z, and quantification was conducted on a nominal unit mass resolution. Electron impact ionization energies on this ToF-MS can be tuned between 10 and 70 eV, the former retaining the molecular ion and the latter causing extensive fragmentation, but it allows comparison with standard library spectra (Alam et al, 2016a). Data were processed by using GC Image v2.5 (Zoex Corporation, Houston, USA).…”

“…For example, the isomeric alkanes all exhibit the same m/z 43, 57, 71, 85, 99 patterns, thus obscuring the match with the NIST library and making identification from the mass spectrum very difficult. To address this issue, a lower ionization energy (10-14 eV) was also employed in our study so that the organic compounds are ionized with minimal excess internal energy and thus less fragmentation, hence retaining the distinct identity of the molecule with a much larger fraction of the molecular ion (Alam et al, 2016a). Running samples on the GC × GC with both low and high ionization energy mass spectrometry results in a wealth of data for identification of compounds; where 14 and 70 eV mass spectra can be compared for a given species owing to the identical retention times of the repeat runs.…”

“…Use of a flame ionization detector has the advantage of allowing generic quantification of any part of the chromatogram in terms of the carbon mass contained within it, but identification of specific chemical constituents with this detector can only be achieved on the basis of retention times which, in a very complex two-dimensional chromatogram and set-up-dependent chromatogram, are laborious to assign objectively. Mass spectrometric detection, especially when employing both low and high ionization energies, adds a third analytical dimension with the ability to overcome the problem of compound identification (Alam et al, 2016a) but has not generally been applied to generic quantification of compound groups within complex samples. In this study, we show that time-of-flight mass spectrometric detection can be used not only to identify and quantify individual chemical constituents within the chromatogram but can also be used to quantify generic groups of compounds.…”