Elucidation of Low Molecular Weight Polymers in Vehicular Engine Deposits by Multidimensional Mass Spectrometry

Abstract

Engine oil is mainly comprised of base oils and various additives, which are low molecular weight polymers mixed into the oil resulting in a polymer blend. Dispersants and detergents, the two most abundant additives, are intended to keep the engine free of particulate but are not always successful. At high temperatures, species not combusted may undergo oxidation and degradation or create other byproducts, generating particulate deposition. Knowledge of the molecular makeup of these byproducts is essential, as particulate accumulation can cause serious issues to the engine and ultimately to the operator. In this study, unknown deposits from the air-intake valve of a vehicular engine have been analyzed via a palette of mass spectrometry (MS) methods, including matrix-assisted laser desorption/ionization MS with postacquisition data processing, atmospheric solids analysis probe MS, and electrospray ionization MS interfaced with 2D separation via reversed-phase liquid chromatography and ion mobility spectrometry. Low molecular weight, aminated poly­(propylene glycol) and polyisobutylene detergents and a poly­(methyl methacrylate) viscosity modifier were conclusively identified in the deposit, along with oxidized polyethylene chains leaked into the oil/additives blend from vehicular tubing and tanks. The use of different methods was essential for the confident elucidation of the low molecular weight macromolecules giving rise to the vehicular engine particulates.

Poly(ethylene glycol) Hydrogel Crosslinking Chemistries Identified via Atmospheric Solids Analysis Probe Mass Spectrometry

“…In ASAP experiments of hydrocarbon chains, homolytic bond cleavage may be accompanied by radical oxidation to yield −OH groups. 56,67 Such reactions in the crosslinked PMA− PEG gel would generate degradants with a comb polymer architecture; predicted structures that agree well with the measured m/z values (Table S3) are shown above the corresponding C A−D peaks in Figure 3b. Trimeric or tetrameric PMA chains predominate within the m/z range ionized via ASAP due to their stability; these products appear in distributions with the 44 Da repeating, resulting from homolytic bond cleavages in the PEG ester chains of the gel.…”

“…A de novo approach was taken again to reconcile the pyrolysis pattern of the PMA–PEG network (Figure ) based on the known thermal degradation behavior of PMA and PEG backbones (Schemes S3 and S1, respectively). , Homolytic bond cleavages within the PMA and PEG backbones, followed by hydrogen abstraction or loss, lead to shorter chains terminating with saturated or unsaturated substituents. In ASAP experiments of hydrocarbon chains, homolytic bond cleavage may be accompanied by radical oxidation to yield −OH groups. , Such reactions in the crosslinked PMA–PEG gel would generate degradants with a comb polymer architecture; predicted structures that agree well with the measured m / z values (Table S3) are shown above the corresponding C A–D peaks in Figure b. Trimeric or tetrameric PMA chains predominate within the m / z range ionized via ASAP due to their stability; these products appear in distributions with the 44 Da repeating, resulting from homolytic bond cleavages in the PEG ester chains of the gel.…”

Atmospheric pressure matrix‐assisted laser desorption/ionization mass spectrometry of engine oil additive components

“…31 Another analytical technique employed for direct analysis of base oils and additives is the atmospheric solids analysis probe in conjunction with ion mobility mass spectrometry (ASAP-IM-MS). 32,33 Atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) is complimentary to the previously mentioned analytical methods and already a powerful tool to analyse proteomes and lipidomes. 34,35 Moreover, this technique has also been already successfully adopted for characterization of lubricant oils to detect additives such as anti-oxidants 36 and frictions modifiers 37 directly from base oil solutions as well as from tribologically stressed steel surfaces.…”

“…Direct surface analysis techniques such as desorption electrospray ionization (DESI‐MS) 29 oil analysis and direct analysis in real time coupled with mass spectrometry (DART‐MS) 30 have been applied for quantitative analysis of a commercial lubricant antioxidant additive in a base oil, while mass spectrometric imaging (MSI) by means of laser desorption/ionization reflectron time‐of‐flight mass spectrometry (LDI‐RTOF‐MS) was used to analyse ex situ oil components applied as lubricant additives in a tribological layer of a tribologically stressed surface 31 . Another analytical technique employed for direct analysis of base oils and additives is the atmospheric solids analysis probe in conjunction with ion mobility mass spectrometry (ASAP‐IM‐MS) 32,33 …”

“… 31 Another analytical technique employed for direct analysis of base oils and additives is the atmospheric solids analysis probe in conjunction with ion mobility mass spectrometry (ASAP‐IM‐MS). 32 , 33 …”