Microbial alteration of the acidic and neutral polar NSO compounds revealed by Fourier transform ion cyclotron resonance mass spectrometry

A suite of six genetically related oils that had experienced varying degrees of subsurface, anaerobic biodegradation was analyzed by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. By use of electrospray ionization of whole oil samples, all neutral nitrogen compounds and acid NSO compounds, from 300 to 900 Da, are selectively characterized and assigned unambiguous molecular formulae. Several methods of data visualization reveal changes in the relative abundances of these compounds with increasing degradation. Evidence for selective biodegradation is observed in all compound classes. NSO compounds associated with long alkyl side chains are removed, regardless of the NSO core, under conditions associated with moderate (saturated biomarkers unaffected) to severe biodegradation. Some compound series, such as O1, O3, and SO3–9, are mineralized under conditions of mild biodegradation (n-alkanes, isoprenoids altered but still present). Changes in the Z-series (hydrogen deficiency) and alkyl distributions of the O2 species result from simultaneous microbial degradation and generation. Acyclic fatty acids decrease, whereas Z = −10 O2 species, which correspond to five-ring naphthenic (hopanoic) acids, increase in relative abundance during early stages of biodegradation. Monocyclic (Z = −2) O2 are enriched initially, and then decrease during advanced stages of biodegradation. O2 species corresponding to di-, tri-, and tetra-cyclic naphthenic acids (Z = −4, −6 and −8) are preferentially produced during these advanced stages. The ratio of acyclic to 2–4 ring cyclic O2-species provides a new parameter to define the degree of biodegradation.