The use of mass spectrometry for the analysis of petrochemical products and crude oils enables the generation of detailed molecular data essential for chemical characterisation and product development. However, the need for multistage sample preparation techniques can be time consuming and may result in the loss of information. Ambient ionisation in combination with mass spectrometry enables the direct analysis of compounds present on a surface with minimal or no sample preparation. The work presented in this thesis evaluates the application of mass spectrometry (MS) hyphenated with ambient ionisation and ion mobility for the analysis of chemical additives used in lubricant and petrochemical products and also crude oil.
A technique called desorption electrospray ionisation (DESI) pioneered the ambient ionisation field. An in-house designed and constructed DESI source has been developed to enable hyphenation of DESI with MS and ion-mobility mass spectrometry (IM-MS) for the interrogation of chemical additives used in lubricant and petrochemical oils directly from multiple surface substrates. The approach has been successfully applied to the analysis of a range of chemical additives as standards and when present in a lubricating oil matrix. Data has also shown that DESI-MS can be used to map additive deposition on a surface.
The quantitative capabilities of DESI-MS have been assessed using a lubricant antioxidant additive present in a lubricant oil matrix and deposited on a surface. The DESI-MS method showed good linearity with a limit of detection (LOD) for the antioxidant additive below that used in typical commercial formulations. The use of a suitable internal standard in the DESI-MS analysis has been shown to significantly improve the repeatability of the approach.
Hyphenation of DESI with post ionisation separation methods, such as high field asymmetric waveform ion mobility spectrometry (FAIMS), can improve mass spectral response for targeted analytes through selective transmission. The analysis of a series of corrosion inhibitor additives in a base oil matrix has been carried out using electrospray (ESI) and DESI hyphenated with FAIMS-MS. FAIMS selection of target ions improved the sensitivity of ESI and DESI through enhanced analyte transmission and a reduction in the chemical noise resulting from the oil matrix. DESI-FAIMS-MS was shown to improve target analyte response compared to DESI-MS alone using the corrosion inhibitors as model compounds, showing how the combined technique can be used for the rapid analysis of analytes directly from surfaces with no sample preparation or pre concentration.
Direct analysis in real time (DART) is an alternative ambient ionisation approach to DESI. The use of DART-MS for the direct analysis of lubricant and oil additives has been evaluated. All selected additives were successfully detected by DART-MS as standards and in an oil matrix. The surface material, DART helium gas temperature and the presence of an oil matrix were all shown to effect the desorption and ionisation of target analytes. The quantitative capabilities of DART-MS were assessed using the antioxidant additive in a lubricant oil matrix and in the presence of an internal standard. The technique showed good linearity and repeatability. The untargeted analysis of chemical additives present in a fully formulated lubricant oil has been carried out by DESI and DART ionisation techniques. The effect of DESI electrospray solvent and DART helium temperature were both shown to impact the observed mass spectral response for the sample.
The analysis of crude oil is particularly problematic due to the high complexity of the sample. A crude oil sample has been analysed using ESI combined with high resolution MS, ESI-FAIMS-MS and DESI-MS. High resolution mass spectrometry enabled the identification of molecular ions that could be characterised using specialist software. The use of FAIMS resulted in shift in the observed chemical profile for the crude oil sample showing selective transmission of molecular species based upon the differential mobility of ions rather than factors such as polarity or solubility that are typically used for sample fractionation. Molecular species from within the crude oil sample were successfully desorbed and ionised by DESI-MS using a DESI solvent composition of 6:4 toluene:methanol.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.