Resumo:
Nitrogen compounds (N-compounds) are a small fraction chemical composition of oil (0,1 to 2,0%) and are present mainly as aromatic and heterocyclic structures, classified in two groups: basic (pyridines, quinolines and benzoquinolines) and neutral (indoles, carbazoles and benzocarbazoles). Carbazoles their alkylated analogues and benzocarbazoles are used as geochemical markers (N-markers) provinging information about origin, migration, depositional environment and oil maturity. The determination of N-markers is conventionally performed by gas chromatography coupled to mass spectrometry (GC-MS) after sample pre-fractionation steps associated with a directed fractionation to separate the neutral from the basic nitrogen compounds. In this work, an analytical method was developed, validated and applied for the first time using gas chromatography coupled to triple quadrupole mass spectrometry (GC-MS/MS) for N-markers. Petroleum samples collected at eight points in the Potiguar Basin were analyzed without fractionation steps being the following Nmarkers determined: carbazole, 1-methylcarbazole, 2-methylcarbazole, 9-methylcarbazole, 1,8-dimethylcarbazole, 2,7-dimethylcarbazole, 3,6-dimethylcarbazole, 3-ethylcarbazole, 1,4,8-trimethylcarbazole, benzo[a]carbazole, benzo[b]carbazole, benzo[c]carbazole and dibenzocarbazole. The GC-MS/MS method in MRM mode, presented LOQ from 7,71 μg L-1 to 389,03 μg L-1 and LOD from 2,54 μg L-1 to 95,38 μg L-1 with determination coefficient values greater than 0,98. The precision tests resulted in relative standard deviation less than 20%. Recoveries between 78,63% and 117,34% were obtained. In the samples, the results obtained for the Nmarkers ranged from 0,29 ± 0,01 ug g-1 to benzo[c]carbazole in P6 and 59,74 ± 0,35ug g-1for 1,8-dimethylcarbazole in P1. The use of Principal Component Analysis (PCA) made it possible to differentiate samples from the lake environment and samples from the marine environment. Finally, diagnostic reasons based on correlation graphs according evaluation data of the quantitative assessment allowed to assess the environment of deposition and maturity.
