The quantitative surface analysis of an antioxidant additive in a lubricant oil matrix by desorption electrospray ionization mass spectrometry.
Bottom Line: The quantitative surface analysis of the antioxidant additive octyl (4-hydroxy-3,5-di-tert-butylphenyl)propionate in an oil lubricant matrix was carried out by DESI-MS in the presence of 2-(pentyloxy)ethyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate as an internal standard.A quadrupole/time-of-flight mass spectrometer fitted with an in-house modified ion source enabling non-proximal DESI-MS was used for the analyses.The application of DESI-MS to the direct, quantitative surface analysis of a commercial lubricant additive in a native oil lubricant matrix is demonstrated.
Affiliation: Centre for Analytical Science, Department of Chemistry, Loughborough University, Loughborough, LE11 3TU, UK.Show MeSH
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Mentions: The characterization of base oils and the detection of additives have been reported using both polar and non-polar electrospray phases with differing detection capabilities.21 The use of a polar electrospray phase, 95:5 MeOH/H2O, and negative mode analysis for the detection of the additive I in a base oil matrix using DESI-MS was found to generate a mass spectrum that had little chemical noise resulting from the base oil matrix. Figure 3 shows the DESI-MS spectrum obtained from the desorption of a spot containing I (10 µg) and the internal standard II (27 µg) in a base oil matrix deposited on a filter paper surface. Desorption from glass, PTFE and metal surfaces is also possible (data not shown). The deprotonated molecule of the additive at m/z 389 (observed 389.3062, calculated 389.3056, 1.5 ppm error) and the internal standard (m/z 391) can be clearly distinguished from the chemical noise resulting from the oil matrix. The insert in Fig. 3 shows the selected ion responses for the deprotonated molecules of I, m/z 389.3, and II, m/z 391.3, with the lubricant spot positioned under the electrospray to generate a DESI response for the sample. Blank areas of the surface were measured before and after the sample analysis, demonstrating the absence of background interference and sample-to-sample carry over. The analyte and internal standard responses began to decrease as a result of depletion of the sample from the surface and the sample spot was moved under the electrospray so that a new area of the spot could be analyzed. This process was repeated for the 1.5-min acquisition time, covering a cross-section of the spot, which was a representation of the whole sample deposited.
Affiliation: Centre for Analytical Science, Department of Chemistry, Loughborough University, Loughborough, LE11 3TU, UK.