Characterization of naphthenic acids in crude oils and naphthenates by electrospray ionization FT-ICR mass spectrometry

Abstract

We present the selective ionization of acidic components of crude oils and naphthenates by negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We further characterize isolated naphthenic acids from a calcium napthenate deposit by negative-ion ESI tandem mass spectrometry (MS n): collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD). Selective ionization by electrospray affords direct characterization of neutral nitrogen species and naphthenic acids in petroleum without derivatization or preconcentration of the sample, and with minimal sample consumption. Acids isolated from a calcium naphthenate deposit are tetraprotic with a C 80 hydrocarbon skeleton; commonly known as “ARN” acids, whereas sodium naphthenate consists of low molecular weight (C 15 to C 35) linear saturated monoprotic carboxylic acids. IRMPD and CID fragmentation of ARN acids result in both dehydration and decarboxylation of the carboxylic acid groups without dealkylation. However, CID produced more extensive fragmentation leading to dealkylation of the hydrocarbon skeleton. The ultrahigh resolution and mass accuracy of FT-ICR MS and MS/MS provide for detailed identification and compositional differences of acidic species in crude oils and naphthenates, and also afford structural characterization of acids isolated from naphthenate deposits.