Electrospray ionization and atmospheric pressure matrix-assisted laser desorption/ionization mass spectrometry of antioxidants applied in lubricants

Amino antioxidants: A review of their environmental behavior, human exposure, and aquatic toxicity

Amino antioxidants (AAOs), a suite of emerging organic contaminants, have been widely used in numerous industrial and commercial products to inhibit oxidation and corrosion. Recently, their environmental ubiquity, health risks, bioaccumulative and toxic potential have led to mounting public concern. This review summarizes the current state of knowledge on the production and usage, environmental occurrence, bioavailability, human exposure, and aquatic toxicity of representative AAOs, and provides suggestions for future research directions. Previous studies have revealed widespread distribution of many AAOs in various environmental matrixes, including air, water, sediment, dust, and biota. In addition to parent compounds, their degradation products, such as 2-anilino-5-(1,3-dimethylbutylamino)-1,4-benzoquinone (6PPD-Q) and 4-nitrodiphenylamine (4-NO₂-DPA), have also been detected at high levels in multiple compartments. Dust ingestion and air inhalation are the two most well-investigated routes for human exposure to AAOs and their transformation products, while studies on other pathways (e.g., skin contact and dietary intake) still remain extremely limited. Moreover, AAO burdens in human tissue have been poorly documented. Toxicological data have shown that a few AAOs may cause teratogenic, developmental, reproductive, endocrinic, neuronic, and genetic toxicity to aquatic organisms, and the toxic capacities of degradation products differ from their precursors. Future studies should focus on elucidating AAO exposure for humans and associated health risks. Additionally, more attention should be given to AAO transformation products (particularly those quinoid derivatives possessing substantial affinity with DNA) and to the effects of complex mixtures of these chemicals.

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

RATIONALE

The efficiency of lubricants strongly depends on the content of functional additives. In order to assess the chemical and structural changes taking place in the lubricating oil and its additives during operation, it is essential to develop a method for simple and prompt analysis.

METHODS

Two single additives as well as a fully formulated engine oil were analysed using an atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) source coupled to a linear trap quadrupole Orbitrap XL hybrid tandem mass spectrometer and compared with results obtained by means of electrospray ionization (ESI) including additional low-energy collision-induced dissociation (LE-CID). The identification of additives directly from technical surfaces was simulated by using steel substrates as AP-MALDI targets with varying roughness.

RESULTS

After assessment and selection of the most suited AP-MALDI matrix it was found that pure additives such as calcium sulfonate and zinc dialkyldithiophosphates (ZDDPs) could well be identified with abundant signal intensity based on their elemental composition. Molecular identification was corroborated by LE-CID in ESI mode. Additionally, additives present in the fully formulated commercial oil such as ZDDPs and salicylates could be reliably identified based on the elemental composition of the deprotonated molecules by means of the Orbitrap unit on different substrates including steel surfaces with high roughness.

CONCLUSIONS

AP-MALDI is an efficient technique for determination of lubricant additives directly from commercial oil blends. Identification of additive components was also achieved on steel surfaces with high roughness as applied in tribological systems and thus it is expected that it will be possible to assess additive degradation in real applications, enabling more effective and timely maintenance measures.

Direct Dielectric Barrier Discharge Ionization Promotes Rapid and Simple Lubricant Oil Fingerprinting

Petroleomics, which is the characterization, separation, and quantification of the components of petroleum and crude oil, is an emerging area of study. However, the repertoire of analytical methods available to understand commercial automotive lubricant oils (ALOs) is very limited. Ambient mass spectrometry is one of the most sensitive analytical methods for real-time and in situ chemical analysis. With this technique, the chemical fingerprinting of ALOs can be performed quickly and simply using dielectric barrier discharge ionization time-of-flight mass spectrometry. In this study, the mass spectra of 35 samples were obtained without any sample preparation in positive-ion mode, and no carryover was observed. To elucidate the similarities and differences between the ALO samples, the data generated from these spectra were analyzed using four chemometric techniques: principal component analysis, multivariate curve resolution, hierarchical cluster analysis, and pattern recognition entropy. The ALO samples were readily differentiated according to their American Petroleum Institute classification and base oil types: mineral, semisynthetic, and synthetic. The development of this new methodology will aid in the semiquantitative control analysis of ALOs and offers an improved ability to identify the components therein.

Development of an accelerated artificial ageing method for the characterization of degradation products of antioxidants in lubricants by mass spectrometry

The understanding of ageing mechanisms of antioxidants in base oils is indispensable for the development of improved lubricants. In this study, a novel artificial ageing method based on the application of peroxide as oxidant is presented for improved monitoring of thermo-oxidative degradation processes in combination with mass spectrometry. Model oils containing aminic and phenolic antioxidants were aged and chemical structures of their oxidation products were elucidated by ultrahigh performance liquid chromatography and electrospray ionization high resolution (Orbitrap) mass spectrometry. Additionally, synergistic mixtures of four antioxidants were investigated, because the formation of condensed molecules from amines and phenols would have a major influence on the antioxidant potential but could not be detected in the bulk lubricant.

Modified-Atmospheric Pressure-Matrix Assisted Laser Desorption/Ionization Identification of Friction Modifier Additives Oleamide and Ethoxylated Tallow Amines on Varied Metal Target Materials and Tribologically Stressed Steel Surfaces

For many tasks in failure and damage analysis of surfaces deteriorated in heavy tribological contact, the detailed characterization of used lubricants and their additives is essential. The objective of the presented work is to establish accessibility of tribostressed surfaces for direct characterization via modified atmospheric pressure-matrix assisted laser desorption/ionization-mass spectrometry (m-AP-MALDI-MS). Special target holders were constructed to allow target samples of differing shape and form to fit into the desorption/ionization chamber. The best results of desorption and ionization on different target materials and varying roughnesses were achieved on smooth surfaces with low matrix/substrate interaction. M-AP-MALDI characterization of tribologically stressed steel surfaces after pin-on-disc sliding wear tests (SRV-tribotests) yielded positive identification of used friction modifier additives. Further structure elucidation by electrospray ionization mass spectrometry (ESI-MS) and measurements of worn surfaces by time-of-flight-secondary ion mass spectrometry (TOF-SIMS) accompanied findings about additive behavior and deterioration during tribological contact. Using m-AP-MALDI for direct offline examinations of worn surfaces may set up a quick method for determination of additives used for lubrication and general characterization of a tribological system.

Rapid analysis of lubricants by atmospheric solid analysis probe-ion mobility mass spectrometry

Formulated lubricants are complex mixtures composed of base oil(s) and additives with various functions (detergents, corrosion inhibiter, antioxidant, viscosity modifiers, etc.). Because of the aliphatic nature of base oil and the chemical diversity of additives, the characterization of lubricant is currently a long and complex process. The comprehensive analysis of lubricant samples involves several techniques such as nuclear magnetic resonance, mass spectrometry, chromatography and infrared spectroscopy. The coupling of atmospheric solid analysis probe (ASAP) with ion mobility-mass spectrometry (IM-MS) has been shown to be an efficient tool for the characterization of complex mixture containing vaporizable polar to non-polar compounds. This approach affords the coupling of a direct ionization technique that does not require sample preparation, with a bi-dimensional separation method with high peak capacity. In this work, we show that ASAP-IM-MS is a suitable method for rapid and direct characterization of lubricant samples. Indeed, base oil and additives yielded, by ASAP, ions series which could be separated by IM-MS. Molecular additives such as Zn-dithiocarbamate, phosphite, thiophosphate and Alkyl diphenylamine were ionized as molecular ions [M](+•) or protonated molecules [M + H](+), depending of their polarity. In some cases, fragment ions were observed, confirming the additive identification. In addition, high molecular weight polymeric additives such as poly(alkyl methacrylate) (PAM) were pyrolized in the ASAP source leading to characteristic fragment ions. ASAP-IM-MS is shown to be a powerful tool for studying complex mixtures, allowing the first comprehensive analysis of lubricants in just a few minutes.

Ultrahigh-performance liquid chromatography/electrospray ionization linear ion trap Orbitrap mass spectrometry of antioxidants (amines and phenols) applied in lubricant engineering

RATIONALE

For the qualification and quantification of antioxidants (aromatic amines and sterically hindered phenols), most of them applied as lubricant additives, two ultrahigh-performance liquid chromatography (UHPLC) electrospray ionization mass spectrometric methods applying the positive and negative ion mode have been developed for lubricant design and engineering thus allowing e.g. the study of the degradation of lubricants.

METHODS

Based on the different chemical properties of the two groups of antioxidants, two methods offering a fast separation (10 min) without prior derivatization were developed. In order to reach these requirements, UHPLC was coupled with an LTQ Orbitrap hybrid tandem mass spectrometer with positive and negative ion electrospray ionization for simultaneous detection of spectra from UHPLC-high-resolution (HR)-MS (full scan mode) and UHPLC-low-resolution linear ion trap MS(2) (LITMS(2)), which we term UHPLC/HRMS-LITMS(2).

RESULTS

All 20 analytes investigated could be qualified by an UHPLC/HRMS-LITMS(2) approach consisting of simultaneous UHPLC/HRMS (elemental composition) and UHPLC/LITMS(2) (diagnostic product ions) according to EC guidelines. Quantification was based on an UHPLC/LITMS(2) approach due to increased sensitivity and selectivity compared to UHPLC/HRMS. Absolute quantification was only feasible for seven analytes with well-specified purity of references whereas relative quantification was obtainable for another nine antioxidants. All of them showed good standard deviation and repeatability.

CONCLUSIONS

The combined methods allow qualitative and quantitative determination of a wide variety of different antioxidants including aminic/phenolic compounds applied in lubricant engineering. These data show that the developed methods will be versatile tools for further research on identification and characterization of the thermo-oxidative degradation products of antioxidants in lubricants.

Dehydrogenation and dehalogenation of amines in MALDI-TOF MS investigated by isotopic labeling

Secondary and tertiary amines have been reported to form [M-H](+) that correspond to dehydrogenation in matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS). In this investigation, we studied the dehydrogenation of amines in MALDI-TOF MS by isotopic labeling. Aliphatic amines were labeled with deuterium on the methylene of an N-benzyl group, which resulted in the formation of [M-D](+) and [M-H](+) ions by dedeuteration and dehydrogenation, respectively. This method revealed the proton that was removed. The spectra of most tertiary amines with an N-benzyl group showed high-intensity [M-D](+) and [M-H](+) ion peaks, whereas those of secondary amines showed low-intensity ion peaks. Ratios between the peak intensities of [M-D](+) and [M-H](+) greater than 1 suggested chemoselective dehydrogenation at the N-benzyl groups. The presence of an electron donor group on the N-benzyl groups enhanced the selectivity. The dehalogenation of amines with an N-(4-halobenzyl) group was also observed alongside dehydrogenation. The amino ions from dehalogenation can undergo second dehydrogenation. These results provide the first direct evidence about the position at which dehydrogenation of an amine occurs and the first example of dehalogenation of haloaromatic compounds in MALDI-TOF MS. These results should be helpful in the structural identification and elucidation of synthetic and natural molecules.

Free and immobilized matrix molecules: impairing ionization by quenching secondary ion formation in laser desorption MS

Within the last 25 years, matrix-assisted laser desorption ionization (MALDI) has become a powerful analytical tool in mass spectrometry (MS). While the method has been successfully applied to characterize large organic molecules such as proteins, sugars and polymers, its utilization for small molecules (≤ 600 Da) is significantly impaired by the coformation of matrix ions. Reducing or eliminating matrix-related signals has been subject of many studies. Some of which propose the enhancement of so-called matrix suppression effects, while others suggest the replacement of matrix molecules by materials such as microporous silicon. Alternatively, the immobilization of matrix molecules by utilizing them as self-assembled monolayers (SAMs) has been discussed. In continuation of this research, the current manuscript focuses on the elucidation of ion formation processes occurring on the surface of light absorbing SAMs. Ion yields obtained by free and immobilized matrix molecules as well as those generated by matrix-free gold film-assisted laser desorption ionization (GF-LDI) were compared. Experiments showed that the formation of strong analyte signals essentially required the presence of free matrix molecules, while the immobilization of the latter severely impaired ionization. The observed effect inversely correlated with the surface coverage of SAMs determined by cyclic voltammetry (CV). Based on these findings, the MS signal generated on light absorbing SAMs could be used supplementary to CV for determining the surface coverage of light absorbing SAMs.

CID of singly charged antioxidants applied in lubricants by means of a 3D ion trap and a linear ion trap-Orbitrap mass spectrometer

The aim of this study was to investigate the fragmentation behavior induced by low-energy collision-induced dissociation (LE-CID) of four selected antioxidants applied in lubricants, by two different types of ion trap mass spectrometers: a three-dimensional ion trap (3D-IT) and a linear IT (LIT) Orbitrap MS. Two sterically hindered phenols and two aromatic amines were selected as model compounds representing different antioxidant classes and were characterized by positive-ion electrospray ionization (ESI) and LE-CID. Various types of molecular ions (e.g. [M](+•) , [M + H](+) , [M + NH(4) ](+) or [M + Na](+) ) were used as precursor ions generating a significant number of structurally relevant product ions. Furthermore, the phenolic compounds were analyzed by negative-ion ESI. For both IT types applied for fragmentation, the antioxidants exhibited the same unusual LE-CID behavior: (1) they formed stable radical product ions and (2) CC bond cleavages of aliphatic substituents were observed and their respective cleavage sites depended on the precursor ion selected. This fragmentation provided information on the type of structural isomer usually not obtainable for branched aliphatic substituents utilizing LE-CID. Comparing the two instruments, the main benefit of applying the LIT-Orbitrap was direct access to elemental composition of product ions enabling unambiguous interpretation of fragmentation trees not obtainable by the 3D-IT device (e.g. loss of isobaric neutrals). It should be emphasized that the types of product ions formed do not depend on the type of IT analyzer applied. For characterizing degradation products of antioxidants, the LIT-Orbitrap hybrid system, allowing the determination of accurate m/z values for product ions, is the method of choice.