Applying Gas Chromatography to Environmental Geochemistry

In the first of a two-part Q&A The Column spoke to Paul A. Sutton, a research fellow in the Petroleum and Environmental Geochemistry Group (PEGG) at Plymouth University (Plymouth, UK), 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?

A. My current research interests fall into two main categories: The application of high temperature gas chromatography (HTGC) techniques to the characterization of organic extracts, and the fundamental analysis of crude oil. Fortunately these research areas have much crossover. GC is amenable to a wide range of compounds in their natural state or derivatized to improve their volatility or demote their interaction with the stationary phase. However, conventional GC is somewhat limited by the thermal stability of the stationary phase, which can readily breakdown at temperatures above 300 °C. This effectively means that GC is limited to the analysis of compounds with <35–40 carbon atoms.

(PHOTO CREDIT: CONEYL JAY/GETTY IMAGES)

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 "conventional" GC.

Q: Why are you interested in the analysis of crude oil specifically?

A. Crude oil is an intriguingly complex mixture of hydrocarbons, polar (N, S, O) compounds, metals, and particulates. Its properties vary geographically, geologically, and throughout its production and processing. The petroleum industry often classifies oils for quality and flow assurance issues based on bulk properties such as total acid number (TAN), total base number (TBN), and specific gravity (API), by their distribution into operationally defined fractions based on solvent solubility such as asphaltenes, or by their separation into bulk chromatographic fractions such as saturates, aromatics, resins, and asphaltenes (SARA). Sometimes these measurements are not sufficiently adequate to identify particular flow assurance issues. The issue of calcium naphthenate (calcium salts of tetracarboxylic acids) deposition is a case in point because it does not appear to be related to TAN. The analysis of crude oil can be academically challenging and I am interested in investigating the development of techniques that can be used to comprehensively characterize crude oils in a more consistent physico-chemical manner.

Q: Why do polycyclic C₈₀ tetracarboxylic ("ARN") acids represent a concern to the oil industry?

A. The term "ARN" comes from the Norwegian for eagle and reflects their discovery in 2005 by Baugh et al.¹ C₈₀ tetracarboxylic acids represent a family of isoprenoid compounds with 80 carbon atoms arranged in an H-shape with terminal carboxylic acid groups at the end of each arm and between 0–8 cyclopentyl rings, which are particularly associated with immature, biodegraded, and heavy crude oils. They represent a problem because they are a pre-indicator of calcium naphthenate deposition.

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). If C₈₀ tetracarboxylic acids are detected early enough, then mitigation strategies can be formulated at an early stage before flow assurance issues arise.

Quantification of the tetraacid content of crude oil is therefore of huge benefit to the oil industry. It should also be noted that the presence of C₈₀ tetraacids in a crude oil does not necessarily mean that there will be a flow assurance issue, but calcium naphthenate formation does not proceed without them.

Q: Are there other analogues of C₈₀ tetracarboxylic acids?

A. Besides the C₈₀ tetraacids, C₈₁ and C₈₂ methylated analogues are usually present in deposits and parent crude oils. However, other carbon number tetraacids (C_60–77) have also been reported based on mass spectrometry analysis. C₈₀ tetraacids have been reported to occur in crude oils from across the globe, for example in the North Sea, West Africa, offshore South America, South-East Asia, China, Australia, and the Gulf of Mexico. What is interesting is that we see different distributions of cyclopentyl ring numbers and different C_80/81/82 ratios from different locations. Typically, the tetraacid distributions in crude oils and deposits examined so far have been dominated by the C₈₀ compounds with between 4–8 cyclopentyl rings. Usually the 6-ring compound dominates but this is not always the case and we have seen distributions dominated by 7- or 8-ring compounds and by C₈₁ compounds. I expect that alternative distributions will be identified in the future as we examine more samples. So, while the petroleum industry primarily wants to know the tetraacid content of crude oils for potential flow assurance issues, the same information may be useful regarding the depositional setting and thermal history of the oil reservoir. As methods for isolating and measuring tetraacids develop and more crude oils and deposits are tested we may also find additional related compounds. This also suggests that our analytical strategy needs to be specific enough for tetraacids but sufficiently broad to cope with a relatively wide range of carbon numbers.

Q: What techniques are available to analysts when detecting the presence of C₈₀ tetracarboxylic acids? What are the challenges associated?

A. Much of what has been learned about petroleum tetraacids so far has been due to their high concentration in calcium naphthenate deposits. They can be extracted from the deposit by first removing entrained oil and then acidifying the deposit to liberate the free acids. The free acids can then be analyzed directly using mass spectrometric techniques: Infusion electrospray ionization–mass spectrometry (ESI–MS) is commonly used, for example using low resolution instruments or higher resolution instruments such as Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). Low resolution infusion analysis may be complicated by the fact that 0- and 1-ring compounds of a particular carbon number are isobaric with the 7- and 8-ring compounds of the next higher carbon number. For example, the nominal mass of C_80:1 and C_81:8 are the same but can be differentiated at higher resolution (calculated monoisotopic masses for C_80:1, C₈₀H₁₅₂O_8, 1241.1487 Da and C_81:8, C₈₁H₁₄₀O_8, 1241.0548 Da) or through the use of chromatographic separation in low resolution mass spectral studies.

Analysis has been conducted in positive and negative modes. In positive mode the pseudomolecular [M+H]⁺ ion at m/z 1232 for C_80:6 is observed or the sodium or potassium adducts. For the C_80:6 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 MS the eluent conditions need to be carefully controlled to prevent peak splitting, which is not satisfactory for quantitation. Not only is peak splitting through ionization an issue but the reactivity of these compounds can also 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 conventional 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?

A. Yes, whilst I have performed most of the practical work covered in this interview, none of this would have been possible without research and industrial sponsorship and support from the School of Geography, Earth and Environmental Sciences, and members of the PEGG research team within which I work.

Reference

1. T.D. Baugh et al., "The discovery of high-molecular-weight naphthenic acids (ARN acid) responsible for calcium naphthenate deposits." SPE 93011 presented at 2005 SPE International Symposium on Oilfield Scale, Aberdeen, UK (2005).

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 PhD 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 Paul 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 his role changed to a senior research fellow in the School of Geography, Earth & Environmental Sciences. His 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 (MS).

Paul Sutton

E-mail:P.A.Sutton@plymouth.ac.uk

Website: http://www.research.plymouth.ac.uk/pegg/staff/sutton_pa/pasutton.htm

This article is from The Column. The full issue can be found here:http://images2.advanstar.com/PixelMags/lctc/digitaledition/July24-2014-uk.html#2