Comparison of Thermal and Flow-Based Modulation in Comprehensive Two-Dimensional Gas Chromatography-Time-of-Flight Mass Spectrometry (GC x GC-TOFMS) for the Analysis of Base Oils

Base oils are produced by refining crude oil or through chemical synthesis. They are a key component of engine oils. With an immense range of carbon numbers and boiling points, analyzing such complex mixtures is very difficult. The need to monitor industrial petroleum processing steps, as well as to identify petrochemical environmental pollutants, drives the search for improved characterization methods. Comprehensive two-dimensional gas chromatography (GC x GC) is one of the best tools for that. The modulator used in GC x GC is responsible for trapping/sampling the first dimension (D-1) column analytes, then reinjecting them in the form of narrow bands onto the second dimension (D-2) column for further separation. Modulators used today generally fall into two categories, thermal and flow ones. Heater-based thermal modulators trap the D-1 column effluent at or above ambient temperatures. Flow-based modulators utilize storage loop(s) to collect the D-1 effluent, which is subsequently flushed into the second-dimension column for further separation. A single-stage, consumable-free thermal modulator and a reverse fill/flush flow modulator were compared for the characterization of base oils. Both were evaluated on their ability to achieve separation of several conventional and synthetic engine oils components. A reverse column set, polar D-1 and nonpolar D-2, allowed group-type analysis of all classes, including linear, branched, and aromatic species. The results show the ability to achieve a comprehensive separation of specific compound classes and the differentiation of engine oil types and manufacturers. Soft ionization assisted in tentative identification of two alkylated diphenylamines in each sample. The advantages and limitations of both thermal and flow modulation are presented.