Hyphenated mass spectrometry methods for the direct characterisation and quantification of polar molecules in crude oil or modified crude oils

Abstract

Crude oil is arguably one of the most complex organic mixture in nature. Therefore, crude oil characterisation requires the use of high resolution and high mass accuracy mass spectrometer such as FT-ICR MS; needed to resolve thousands of ions and assign their elemental formulae. Different heteroatom containing compounds classes are present in crude oil such as N1, S1 and O2 containing compounds. These compounds cause variety of different problems such as N containing compounds cause catalyst deactivation for processes such as hydrodesulfurisation. The choice of the ionisation technique and its polarity is critical to the type of compounds that are observed in a crude oil mass spectrum. Two main studies for crude oil samples are covered in this thesis. The first is crude oil-1, 2 and 3 characterisation using ESI and APPI. The second is structural elucidation of nitrogen containing compounds in crude oil-2.

Positive ion ESI FT-ICR MS ionises basic molecules in crude oils. In the literature crude oil samples are dissolved using different ratios of toluene:methanol. The effect of using different solvent composition is often not regarded as important factor to consider. One of the reasons is the addition of ionisation enhancing additive such as formic acid, thought to normalise the ionisation response across different sample solvent composition. However, the solvent composition study data show that different toluene:methanol ratios play a critical role on the ionisation response of nitrogen containing compounds in different crude oils even with the addition of formic acid. Three different ratios were used which are toluene:methanol solvent ratios of 1:9, 3:7 and 6:4 with and without the addition of 0.1% formic acid for the analysis of crude oil-1, 2 and 3. The highest ionisation for N1 containing compounds are achieved through using toluene:methanol ratio of 1:9 with 0.1% formic acid. Further to this the increase of toluene content in the sample solvent decreased the ionisation of N1 containing compounds in the analysed crude oils even with the addition of formic acid. However, the rate of decrease in the ionisation of N1 containing compounds is more significant for crude oil-1 and 2 compared to crude oil-3. Thus, comparing nitrogen containing compounds among different crude oils should be undertaken using the solvent composition, toluene:methanol ratio of 1:9 with 0.1% formic acid. Another aspect for the solvent composition study is that multimer formation is not only concentration driven but as well sample solvent composition dependent. The data showed that multimer formation in N1 DBE versus carbon number plots are reduced with acid addition and methanol content increase in the sample solvent.

The use of positive ion ESI allowed the ionisation of basic compounds in crude oil-1, 2 and 3. To ionise non-polar classes such as aromatics and thiophene containing compounds in crude oil-1, 2 and 3 positive ion APPI is used. Aromatics and thiophene were the most abundant ions in crude oil-1, 2 and 3 mass spectra. No significant difference in ion intensities for these ions were observed for crude oil-1, 2 and 3 mass spectra. However, the use of negative ion APPI showed major differences in the ions intensities of crude oil-1, 2 and 3 regarding HC, HC-R, N1, S1 and S1-R classes. Further, comparable data for the O2 class were obtained using negative ion ESI and APPI Orbitrap MS for crude oil-1, 2 and 3. Thus, negative ion APPI Orbitrap MS can be used to compare the O2 class relative abundance among different crude oils.

Further to the characterisation study, structural elucidation of nitrogen containing compounds in crude oil-2 using positive ion ESI FT-ICR MS/MS was undertaken. Understanding the chemical structure might have applications in designing more effective catalysts for HDN process. At first a method development approach was undertaken to reduce the analysis time to 4.5 min and increase detection of low m/z low intensity fragment ions. This aim was achieved through increasing the ion accumulation time from 0.05 s to 5 s with averaging 40 spectra. Different N1 precursor ions were isolated at different DBE values and degree of alkylation. A collision energy of 60 V was required to observe characteristic fragment ions such as N expulsion for N1 precursor ion with DBE value of 13.5. While for N1 precursor ion at DBE value of 6.5 a CE 40 V was enough to observe characteristic fragment ions. However, different approach was used for N1 precursor ion with low DBE values, isolating the precursor ion with the lowest degree of alkylation. This approach was essential to observe N expulsion fragment ions for N1 precursor ion with DBE value of 6.5. The core aromatic structure for N1 precursor ions from DBE
values of 3.5 to 10.5 were suggested. This suggestion was based on N expulsion fragment ion and the dealkylated fragment ion. Furthermore, N expulsion from the aromatic ring supports the postulation that the nitrogen in the various precursor ions discussed is pyridinic. This was further confirmed for N1 precursor ion with DBE value of 6.5 using a model compound, 2-butlyquinoline.

More information

Identifiers

ORCID for G. John Langley: orcid.org/0000-0002-8323-7235

Catalogue record

Export record