Abstract
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 × GC) is one of the best tools for that. The modulator used in GC × GC is responsible for trapping/sampling the first dimension (1D) column analytes, then reinjecting them in the form of narrow bands onto the second dimension (2D) column for further separation. Modulators used today generally fall into two categories, thermal and flow ones. Heater-based thermal modulators trap the 1D column effluent at or above ambient temperatures. Flow-based modulators utilize storage loop(s) to collect the 1D 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 1D and nonpolar 2D, 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.
Highlights
As a major component of lubricating greases, engine oils and metal processing fluids, base oils are encountered in both industry and in everyday life
A reverse fill/flush flow modulator and single-stage, consumable-free thermal modulator were compared for the separation of several conventional and synthetic engine oils of various viscosities and brands with dual detection
The closed design of the select ionization Time-of-flight mass spectrometry (TOF-MS) was detrimental in obtaining baseline resolution of the heavier-molecular-weight compounds and unresolved complex mixture (UCM), in comparison to the FID
Summary
As a major component of lubricating greases, engine oils and metal processing fluids, base oils are encountered in both industry and in everyday life. They are produced by refining crude oil using typical processes of distillation, cracking, hydrogenation, and dewaxing [1,2], or by chemical synthesis. To differentiate the levels of refinement, the American Petroleum Institute (API) classifies these oils into five groups. Group 1 contains the least refined, conventional petroleum base oils. Group 2 has a better grade of petroleum when compared to group 1, partially produced by hydrocracking. Group 3 contains the best grade of petroleum that
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