Molecular Characterization of Volatiles and Petrochemical Base Oils by Photo-Ionization GC×GC-TOF-MS

Abstract

The characterization of organic mixtures by comprehensive two-dimensional gas chromatography (GC×GC) coupled to electron impact (EI) ionization time-of-flight mass spectrometry (TOF-MS) allows the detection of thousands of compounds. However, owing to the exhaustive fragmentation following EI ionization, despite the use of mass spectral libraries, a majority of the compounds remains unidentified because of the lack of parent ion preservation. Thus, soft-ionization energies leading to organic compounds being ionized with limited or no fragmentation, retaining the molecular ion, has been of interest for many years. In this study, photoionization (PI) was evaluated as the ion source for GC×GC-TOF-MS measurements. First, capabilities and limitations of PI were tested using an authentic mixture of compounds of several chemical classes. Ionization energy exhibited by PI, equivalent to 10.8 eV, resulted in significant retention of molecular ion information; [M] for alkanes, ketones, FAMEs, aromatics, [M-H] for chloroalkanes, and [M-HO] for alcohols. Second, considering the potential of PI for hydrocarbons, base oils, complex mixtures of saturated and unsaturated hydrocarbons blended for finished lubricant formulations, were extensively evaluated. Several chemical classes of hydrocarbons were positively identified including a large number of isomeric compounds, both aliphatics and cyclics. Interestingly, branched-alkanes were ionized with lower excess internal energy, not only retaining the molecular ions but also exhibiting unique fragmentation patterns. The results presented herein offer a unique perspective into the detailed molecular characterization of base oils. Such unprecedented identification power of PI coupled with GC×GC-TOF-MS is the first report covering volatiles to low-volatile organic mixtures.

Compositional elucidation of heavy petroleum base oil by GC × GC‐EI/PI/CI/FI‐TOFMS

“…In depth observation of contour plots of base oil 1 (see Figure S5) clearly revealed the high selectivity of CI towards two chemical classes of base oil 1, esters, and benzothiophenes. Interestingly, elution zones of esters were very close to alkyl benzenes and its methyl/ethyl‐substituted analogues at 2 t R , which is in agreement with previous reports . Therefore, in other sources, particularly in EI, although the presence of esters were confirmed by extracted ion chromatograms, mass spectra were heavily shadowed by coeluting alkyl benzene fragments, rendering difficulties to successfully detect and identify esters in base oil 1.…”

“…The systematic elution pattern of branched alkanes in GC × GC structured chromatogram revealed several mono‐, di‐, and heavily branched alkanes for base oil 2 as demonstrated in Figure for C 20 isomeric species with fragmentation hypothesis. Similar observation was reported earlier where 14 isomeric species of alkanes were identified in a base oil sample using PI . In positive CI (methane with 5% ammonia), alkanes were characterized by abundant (M‐H) + ions, which originated from hydride abstraction.…”

“…Among different chromatographic techniques, gas chromatography (GC), and in particular two‐dimensional GC (GC × GC) is trending since its invention two decades ago . It is due to superior compound separation and production of structural elution patterns leading to improved characterization of complex petroleum oils, as reported elsewhere . Several detectors, including flame ionization (FID), electron capture (ECD), sulphur chemiluminescence (SCD), nitrogen chemiluminescence (NCD), and most importantly time‐of‐flight (TOF) mass spectrometric detectors were successfully applied to GC × GC applications.…”

“…However, FI generates little or no fragmentation at all, providing no structural information (fragments) anymore. Other ion sources include chemical ionization (CI), photo ionization (PI), variable ionization, atmospheric pressure photo ionization (APPI), and atmospheric pressure laser ionization (APLI) . In general, CI, performed under vacuum or at atmospheric pressure, reveals molecular ions but is highly condition‐dependent (reagent gases, flow, humidity, temperature, source geometry, etc) .…”

“…Next to these techniques, PI is (re)emerging as a more universal soft‐ionization technique due to its limited dependency only towards the type of lamp used as the photon source. A recent study using PI for the consideration of a wide range of chemical classes revealed that both the retention of molecular ions and the minimal production of structurally significant fragments enables unambiguous identification of a vast majority of analyzed compounds . Despite these significant differences between the soft ionization techniques available for GC‐MS, very limited comparative assessments are available, particularly for lubricating oils that exclusively contain hydrocarbons that are highly susceptible to hard ionization.…”