Coupling Thermal Analysis with APCI MRMS to Characterize Petroleum Samples

The in-depth analysis and determination of petroleum fractions remain challenging. Evolved gas analysis (EGA) is a complementary method to conventional spray-based and ionization techniques such as APCI, APPI and ESI.

Thermal analysis involves measuring a specific thermal property, e.g. mass or heat flux, whilst applying a defined temperature program. Heating of the sample results in the vaporization and decomposition of the sample, and the gas mixture that is evolved can then be analyzed and characterized using a technique such as MS.

Thermal analysis with a thermo-balance allows analysis to take place at high temperatures, including the pyrolysis regime above 300-350 °C. If analysis takes place at lower temperatures (the desorption phase) any volatile components of the sample will be vaporized and remain intact during analysis.

Experiments in the pyrolysis phase involve the analysis of fragments and decomposition products, which can be used to elucidate structural information about the sample components. The ultra-high resolving power, large dynamic range and high mass accuracy afforded by Fourier Transform Mass Spectrometry (MRMS) means individual components of low concentration can be detected in complex samples.

Using an atmospheric pressure chemical ionization (APCI) method allows polar species to be targeted, such as polycyclic aromatic hydrocarbons (PAH) and their nitrogen and sulfur-containing derivatives, that cannot be accessed using ESI.

This article will explore a case study that demonstrates what can be achieved using MRMS alongside thermal analysis and a gas phase APCI source for petroleum sample analysis.

Method and Material

A GC-APCI II source (Bruker) was coupled to a thermo balance (TG 209, Netzsch Gerätebau GmbH, Selb, Germany) using a heated transfer line, as shown in Figure 1.¹ An MS spectrum was captured using an Apex Qe 7 T MRMS (Bruker) set to positive ion mode. The sample was heated from 20 to 600 °C over the course of an hour using a temperature gradient of 10 K/minute.

Figure 1. Schematic representation of a TG coupled to a 7 T apex II ultra MRMS system using a GC-APCI II source

The samples, which were diluted in DCM, were held in an aluminum crucible and the mass loss over the course of the experiment was recorded to give a curve. Around 1 mg of sample was used per experiment. Spectra were captured between 100 – 1000 m/z with five spectra and 2 MW data points giving a resolving power of 260,000 (m/z 200) and an acquisition frequency around 0.4 Hz.

Analysis was carried out on a diesel fuel (DIN EN 590), a fatty acid methyl ester standard mixture (FAME Mix, Sigma-Aldrich), a crude oil (Greek crude) and a heavy fuel oil (HFO). Data was pre-processed, using m/z calibration and peak picking, and exported using an analysis program (DA 4.0, Bruker) alongside custom Matlab (MATLAB R2016a) scripts. The assignment of elemental composition was carried out within 2 ppm of error and the restriction of C_{6 -100} H_{6 -200} N_{0 -2} O_0-10 S_{0 -2}. A similar method can be found in the literature for GC-APCI.^2-3

Results and Discussion

The coupling of MS with thermal analysis for the analysis of evolved gases is useful for the characterization of complex mixtures. Here, the use of TG-MRMS, using a GC-APCI II ionization source, for the analysis of petroleum analysis is explored. The technique’s unparalleled mass accuracy and resolving power allows sample components to be characterized at the molecular level.

Figure 2, a survey view of the results that plots the temperature against m/z with the intensity indicated by the color, illustrates how complex the petroleum samples analyzed were.

Figure 2. Survey view of a) FAME standard, b) diesel fuel, c) heavy fuel oil and d) Greek crude oil measured by TG-APCI FT-MS. The high complexity and mass range of the petroleum samples is shown.

Several hundred features (elemental compositions with an individual temperature evolution profile) were detected for the diesel fuel and thousands for the HFO and crude oil, respectively. The evolved mixture spans a mass range from the lower acquired m/z boundary at 100 (for the diesel fuel) and reached up to 750 m/z (crude oil, HFO).

The use of high-resolution MS allowed elemental compositions to be determined. The diesel fuel was rich in CH-, CHO₁- and CHO₂ species, in contrast the HFO contained mostly CH and CHS-class species.

The use of the double bond equivalent (DBE) can be used to help understand the results. Figure 3 shows a temperature resolved visualization of the diesel sample’s DBE pattern.

Figure 3. Three dimensional visualization of the double bond distribution evolved over temperature for a diesel TG-MRMS measurement. Intensity is color coded and a selection of potential structures is given.

Thermal analysis showed the desorption and pyrolysis phases (Figure 4) that was to be expected for a high bp fuel that contains some non-volatile components. During the pyrolysis phase there is a shift towards analytes of a smaller m/z, which are the result of fragmentation of the compounds present. The pattern in Figure 4 shows this shift in the averaged mass spectra between 300 – 500 °C. Pyrolysis results in the detection of a higher proportion of CHO_x compounds.

Figure 4. Total ion count chromatogram with marked area of the desorption and desorption/pyrolysis phase for the heavy fuel oil TG-MRMS measurement. Averaged mass spectra of the marked areas are given below

The pattern resulting from analyte pyrolysis is overlaid with signals from the desorption of heavier analytes, which extend into the upper m/z. Due to the complex nature of the sample the desorption and pyrolysis phases show some overlap.

Conclusion

Coupling thermal analysis to ultra-high-resolution MS facilitates the in-depth, temperature resolved chemical determination of evolved gas mixtures.

This method can be used alongside direct infusion methods (e.g. ESI) or GC-APCI. The use of a thermal balance allows more information to be collected at higher temperatures, e.g. structural information can be elucidated from pyrolysis fragments. APCI is a powerful method of detecting any species with polar groups that are released during heating.

This article has demonstrated that TG-APCI MRMS can be used for the analysis of different petroleum samples over a variety of distillation ranges.

References

[1] Rüger, C. P.; Miersch, T.; Schwemer, T.; Sklorz, M.; Zimmermann, R. Hyphenation of Thermal Analysis to Ultrahigh-Resolution Mass Spectrometry (Fourier Transform Ion Cyclotron Resonance Mass Spectrometry) Using Atmospheric Pressure Chemical Ionization For Studying Composition and Thermal Degradation of Complex Materials. Analytical chemistry 2015, 87 (13), 6493–6499. DOI: 10.1021/acs.analchem.5b00785.

[2] Schwemer, T.; Rüger, C. P.; Sklorz, M.; Zimmermann, R. Gas Chromatography Coupled to Atmospheric Pressure Chemical Ionization FT-ICR Mass Spectrometry for Improvement of Data Reliability. Analytical chemistry 2015. DOI: 10.1021/acs.analchem.5b02114.

[3] Smit, E.; Rüger, C. P.; Sklorz, M.; Goede, S. de; Zimmermann, R.; Rohwer, E. R. Investigating the trace polar species present in diesel using high resolution mass spectrometry and selective ionization techniques. Energy Fuels 2015, 150827122347008. DOI: 10.1021/acs.energyfuels.5b00831.

This information has been sourced, reviewed and adapted from materials provided by Bruker Life Sciences Mass Spectrometry.

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