J. Sep. Sci. 2007, 30, 375 – 380 B. E. Smith et al. 375 Ben E. Smith 1, 2 Paul A. Sutton 1 C. Anthony Lewis 1 Braden Dunsmore 2 Geoffrey Fowler 3 Jostein Krane 4 Bjart F. Lutnaes 4 Øystein Brandal 4 Johan Sjöblom 4 Steven J. Rowland 1 1 Petroleum & Environmental Geochemistry Group, University of Plymouth, Plymouth, UK 2 Oil Plus Limited, Newbury, Berkshire, UK 3 BG Group plc, Reading, Berkshire, UK 4 Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Trondheim, Norway Short Communication Analysis of , ARN’ naphthenic acids by high temperature gas chromatography and high performance liquid chromatography Examination by high temperature GC (HTGC) of the methyl esters of the so-called , ARN’ naphthenic acids from crude oils of North Sea UK, Norwegian Sea and West African oilfields revealed the distributions of resolved 4–8 ring C 80 tetra acids and trace amounts of other acids. Whilst all three oils contained apparently the same major acids, the proportions of each differed, possibly reflecting the growth tempe- ratures of the archaebacteria from which the acids are assumed to have originated. The structures of the 4, 5, 7 and 8 ring acids are tentatively assigned by comparison with the known 6 ring acid and related natural products and an HPLC method for the isolation of the individual acids is described. ESI-MS of individual acids isolated by preparative HPLC established the elution order of the 4–8 ring acids on the HPLC and HTGC systems and revealed the presence of previously unreported acids tentati- vely identified as C 81 and C 82 7 and 8 ring analogues. Keywords: , ARN’ / Calcium naphthenate / High temperature GC / Naphthenic acids / Tetraprotic acids / Received: July 7, 2006; revised: October 4, 2006; accepted: October 5, 2006 DOI 10.1002/jssc.200600266 1 Introduction The deposition of salts of high molecular weight organic acids (naphthenates) in oil pipelines and equipment causes major engineering problems for the petroleum industry (reviewed by [1–5]). The phenomenon is becom- ing more common as more oilfields containing imma- ture, heavy oils with higher naphthenic acid contents are becoming economically viable [1–3]. Until recently, the characterisation of these acids had proved difficult, but a significant advance was made when the acids liber- ated from calcium salt deposits of the Heidrun oilfield (Norwegian Sea) were characterised by a multimethod approach [4]. Thus, the acids mixture liberated from deposits by acidification and fractionated by an ion exchange resin method was infused directly into a mass spectrometer with atmospheric pressure photoionisa- tion (APPI) operated in the negative ion mode. This estab- lished the presence of compounds with molecular weights in the range 1227–1235 (attributed from obser- vations of the deprotonated molecular ions). This was further confirmed by APPI mass spectrometric analysis of both permethyl esters and perbenzyl esters in the posi- tive ion mode, where sodiated molecular ions were observed. Finally, Fourier Transform Ion Cyclotron MS of the methyl esters of the acids revealed the accurate mole- cular weight of the major 6-ring acid. In all, the findings were consistent with a mixture of C 80 tetraprotic acids with 4–8 alicyclic rings (double bond equivalents). These assignments were supported by NMR spectroscopy and pyrolysis-GC (py-GC). However, the only other structural feature to be assigned by these methods was the presence of a methyl substituent on an alicyclic side chain-b to the carboxyl groups [4]. Surprisingly, high temperature GC (HTGC) revealed one poorly resolved peak for the 4–8 ring acid mixture with a retention index (RI) of about n- C 80 [4]. The distribution of these so-called , ARN’ acids was said to be similar in oils from several other oilfields [4]. Further detailed NMR and MS studies of the mixture of acids liberated from the calcium naphthenates (CaNAs) of an oil from West Africa revealed the likely structure of the major acid (1), which bore a crosslinking feature simi- lar to that proposed for a constituent of the , H’-shaped membrane lipids of some species of hyperthermophilic archaebacteria [5]. An estimate of the proportions of the Correspondence: Professor Steven Rowland, Petroleum and En- vironmental Geochemistry Group, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK E-mail: srowland@plymouth.ac.uk Fax: +44-1752-23035 Abbreviations: APPI, atmospheric pressure photoionisation; CaNA, calcium naphthenate; ELSD, evaporative light scattering detector; HTGC, high temperature GC; Py-GC, pyrolysis-GC; RI, retention index; RT, retention time i 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com