Analysis of Naphthenic Acids and Derivatization Agents Using Two-Dimensional Gas Chromatography and Mass Spectrometry: Impact on Flow Assurance Predictions

Abstract

In this work, a sensitivity study was conducted on naphthenic acid derivatization agents. Four silylation chemistries and one methylation chemical were initially evaluated on 10 model naphthenic acids using gas chromatography. An experimental design procedure was setup to look at a number of effects, including contact time, catalyst presence, and reagent concentration. Overall, the silylation agents resulted in higher derivatization yields compared to the methylation agent. Moreover, the silylation agents did not show major evidence of selective derivatization as a function of the naphthenic acid structure. A silylation agent [(N,O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA)] was then used to test commercial naphthenic acid mixtures with two-dimensional gas chromatography coupled to time-of-flight mass spectrometry using the electron-impact source (GC×GC−TOFMS). The results enabled identification of many different naphthenic acid species. To test the derivatization agents on realistic samples, naphthenic acid extracts obtained from two crude oils of flow assurance significance were separated with a liquid-phase extraction procedure. The naphthenic acids were then treated with a silylation agent (BSTFA) and a methylation agent (BF3/methanol). The derivatized naphthenic acids together with the non-derivatized naphthenic acids from both crude oils were further examined using medium-resolution time-of-flight mass spectrometry with an electrospray source (TOFMS). Differences were observed in the TOFMS spectra for the naphthenic acid extracts. Extracts that did not contain ARN naphthenic acid species did not show major differences between non-derivatized and derivatized spectra in the negative mode. Extracts that contained ARN did show differences between derivatized and non-derivatized samples in the negative mode. Use of BSTFA resulted in enhanced signals for ARN, particularly the second ionization. Use of BF3/methanol resulted in a poor ARN response compared to the non-derivatized spectra. ARN species were also observed in the positive mode after treatment with both BSTFA and BF3/methanol, but signals were very poor. Moreover, use of BF3/methanol resulted in poor solubility of the naphthenic acid extracts from the crude oil containing ARN species. No solubility issues were observed with the use of BSTFA. Overall, the results point to the shortcomings of the application of methylation chemicals as derivatization agents, particularly for naphthenic acids extracted from crude oil samples containing high-molecular-weight acids of flow assurance significance (e.g., ARN species).

Advances in mass spectrometric characterization of naphthenic acids fraction compounds in oil sands environmental samples and crude oil—A review

“…For GC analyses, the amenable NAFCs are generally derivatized to their methyl or tert‐butyldimethylsilyl analogs. Early research with GC‐MS (Headley et al, , ; Young, Coy, & Fedorak ; Young, Michel, & Fedorak ) paved the way for the extensive applications (Shepherd et al, ) of two‐dimensional GC‐MS, sometimes known as GCxGC MS or 2D‐GC‐MS, which had its first application to oil sands naphthenic acids in 2005 (Hao et al, ). Rowland et al have published a number of papers on the application of 2D‐GC‐MS to the characterization of NAFCs in OSPW (Rowland et al, 2011a–c; Jones et al, ).…”

“…These would include the increased activity in oil sands environmental monitoring programs in countries such as Canada, the exponential increase in research studies on the isolation and identification of toxicants in oil sands process water, and analytical programs in support of remediation strategies for treatment of OSPW. The advancements have also occurred in part due to the quest to predict and control corrosion from naphthenates in the crude oil industry (Mohammed & Sorbie, 2009; Hur et al, ; Shepherd et al, ; Mapolelo et al, ). Of particular significance are the developments of combined methods that couple chromatographic or ion mobility separation with mass spectrometry, and improved visualization methods for oil sands environmental forensics.…”

Equilibrium Partitioning of Naphthenic Acid Mixture, Part 1: Commercial Naphthenic Acid Mixture

“…Through these characteristic ions, individual components eluting within the usually large, unresolved hump of naphthenic acids can be classified, providing the composition of the naphthenic acids based on Z and carbon numbers 26 . This method has later been applied by several authors 25,26,36,[38][39][40] . On testing multiple derivatization agents on different model acid structures Shepherd, van Mispelaar, Nowlin, Genuit and Grutters 40 found MTBSTFA to give one of the best yields overall although recent research claim pentafluorobenzyl bromide (PFBBr) to be superior to MTBSTFA 41 .…”

“…This method has later been applied by several authors 25,26,36,[38][39][40] . On testing multiple derivatization agents on different model acid structures Shepherd, van Mispelaar, Nowlin, Genuit and Grutters 40 found MTBSTFA to give one of the best yields overall although recent research claim pentafluorobenzyl bromide (PFBBr) to be superior to MTBSTFA 41 . Mass spectrums of commercially available NA mixtures and OSPW or crude oil extracts have been studied by several authors 8,10,13,14,36,[42][43][44][45][46][47][48] and Saab, et al 49 discusses various naphthenic acid extraction methods.…”

Interpretation of comprehensive two-dimensional gas chromatography data using advanced chemometrics

“…The whole picture of experimental variables clearly shows the difficulty in deducing optimal conditions. In practice, design of experiment (DoE) [180][181][182][183][184], multi-objective and variable optimization [185][186][187] and employment of separation peak capacity and orthogonality [155,162,163,[188][189][190] should be three pillars to achieve this purpose. For example, Omar et al [181] optimized the conditions for GC×GC-MS analysis of essential oils of plants by using the DoE method and Multisimplex.…”