Q&A: Applying Gas Chromatography to Environmental Geochemistry
LCGCspoke to Paul A. Sutton, a research fellow in the Petroleum and Environmental Geochemistry Group (PEGG) at Plymouth University, about the analysis of crude oil and how high temperature gas chromatography can be used to save millions of dollars for the oil industry.
Q. What are your current research interests and what led you to these areas of research?
In contrast HTGC, with oven temperatures up to around 430 °C, can be used for compounds with up to in excess of 100–120 carbon atoms, allowing an extended analytical window. Although not all compounds are stable under HTGC conditions, it has broad applicability and relatively low discrimination when used with flame ionization detection (FID). Developments in column technology have meant that HTGC is now a robust and routine technique that can be operated in the same way as conventional GC. So it is worthwhile screening organic extracts from sediments, for example, using HTGC and comparing the data to that obtained using “normal” GC.
Q. Why are you interested in the analysis of crude oil specifically?
Q. Why do polycyclic C80 tetracarboxylic (“ARN”) acids represent a concern to the oil industry?
During oil production, it is often necessary to force oil to the surface using seawater. This can result in a pressure drop towards the surface platform leading to an outgassing of CO2 and a rise in the pH of the fluid. Tetraacids present in crude oil under these conditions congregate at the interface of the aqueous and oil phases. This is because the C80 part of the molecule is extremely hydrophobic, whereas the terminal acid groups are hydrophilic. The rise in fluid pH causes dissociation of the tetraacids and saponification with metal ions in the seawater, with calcium of special importance.
Calcium is divalent and so can link with more than one tetraacid, leading to a cross-linked polymeric-type structure that forms a solid deposit in topside equipment, particularly in the oil/water separator. This calcium salt is termed calcium naphthenate. These deposits can build-up until equipment becomes clogged and production has to be halted costing millions of dollars. While calcium naphthenate tends to be rich in tetraacids (around 30 wt% of a cleaned deposit), the parent crude oil typically contains low ppm levels of individual or total tetraacids (up to 20 ppm total).
However, if C80 tetracarboxylic acids are detected early enough, then mitigation strategies can be formulated at an early stage before flow assurance issues arise.
Q. Are there other analogs of C80 tetracarboxylic acids?
Q. What techniques are available to analysts when detecting the presence of C80 tetracarboxylic acids? What are the challenges associated?
Analysis has been conducted in positive and negative modes. In positive mode the pseudomolecular [M+H]+ ion at m/z 1232 for C80:6 is observed or the sodium or potassium adducts. For the C80:6 acid in negative mode the pseudomolecular [M-H]- ion at m/z 1230 and the doubly deprotonated [M-2H]2- ion at m/z 614.5 have been reported. Obviously, the tetra-protic nature of these compounds suggests that [M-3H]3- and [M-4H]4- ions also have the potential to be formed. The presence of such ions in the mass spectrum provides additional confidence in the assignment. Where a liquid chromatographic separation is used prior to mass spectrometry the eluent conditions need to be carefully controlled to prevent peak splitting which is not good for quantitation. Not only is peak splitting through ionization an issue but the reactivity of these compounds can lead to metal salt formation during analysis. I have observed complex mass spectra that included ions from [M-H]-, [M-2H+Na]-, [M-3H+2Na]- and [M-4H+3Na]- and equivalent potassium adducts. Obviously this implies the potential for tetra-salt formation which is incompatible with ESI-MS, and I have experienced issues with intermittent blockages when analyzing the free acids. Another challenge with the free acids is their interfacial activity, which in practical terms means that they are quite “sticky” in the chromatography system or mass spectrometer.
Whilst infusion ESI-MS techniques have been most widely used for detection of C80 tetracarboxylic acids, liquid chromatography (LC) has been coupled with ultraviolet spectroscopy for detection of the acids as their naphthacyl derivatives or with an evaporative light scattering detector for analysis of the acid per-methyl esters. Although useful for analyzing tetraacids from deposits these techniques probably do not offer the specificity or sensitivity required to analyze tetraacid content in crude oil unless you start with a large mass of oil. LC–ESI-MS has been used for the analysis of free acids but peak splitting was shown to be problematic. To overcome many of the issues highlighted above I prefer to analyze tetraacids as their per-methyl esters (or per-trimethylsilyl esters) using HTGC and LC-ESI-MS. HTGC is useful for screening samples and tetraacid esters elute across the thermal inflexion at an oven temperature around 430°C. The advent of steel coated columns has overcome many of the problems associated with aluminium coated or high temperature silica columns. Our system has a cool-on-column inlet and autosampler so it can be treated like a traditional GC for these compounds. As part of a joint industry project, I developed a method for the selective isolation and semi-quantitative determination of tetraacids from crude oil. Because representative tetraacids are not commercially available, I spent a couple of months isolating individual ring number compounds by preparative LC from a calcium naphthenate deposit for use as an internal standard. We now have a semi-quantitative method that involves spiking 1 g of oil with 1 μg of tetraacid and isolating the tetraacid fraction before LC–ESI–MS measurement of the tetraacids as the ammoniated adducts of their per-methyl esters. Our limit of quantitation is about 0.1 ppm of individual tetraacids.
Q: Anything else you would like to add?
Paul Sutton has been at Plymouth University since graduating as a mature student with a B.Sc. (Hons) degree in Environmental Science in 1995. After completing a Ph.D in Organic Geochemistry in 2000, he undertook a post-doctoral post investigating the nutritional status of soils in the Shimba Hills National Reserve, Kenya. This was followed by a three-year post-doctoral project characterizing chromatographically “unresolved complex mixtures” (UCMs) from crude oils using preparative-gas chromatography. Until 2011 I was employed as a Scientific Officer and was seconded onto a two-year Joint Industry Project to develop a method for the quantification of C80 ('ARN') tetraacids in crude oils. In 2011 my role changed to a Senior Research Fellow in the School of Geography, Earth & Environmental Sciences. My current research interests include developing separation techniques for high molecular weight petroleum organic compounds and development of applications for high temperature gas chromatography (HTGC) and HTGC coupled with mass spectrometry.