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A contribution to the determination of group type distribution of aromatic in petroleum middle distillates by NPHPLC

The knowledge of the group-type distribution of aromatic and polycyclic aromatic compounds in petroleum middle distillates is a very important parameter for final properties from the aspects of application and environmental requirements. The determination of group-type distribution by NP HPLC is regulated by the standard methods of EN 12916 or ASTM-6591[1]. The determination of aromatics in middle distillates is generally carried out by means of normal phase high-performance liquid chromatography (NP-HPLC) with refractive index detection (RID), NH2 column, n-heptane as the eluent and column backflush. The content of particular species ranges from trace (tri+aromatics, TA) to percentage level of mono-aromatics and as a method is not absolute, the accuracy being calibration-dependent. Use of RI detector leaves no doubt in regard to mono-aromatics and di-aromatics determination, but the limit of quantification for tri+aromatics is 0.1%, due to a very low refractive index difference at the required concentration. As environmental regulations force to reduce the amount of polyciclic aromatics, especially tri+aromatics, there is a need for a more sensitive detection. The UV detector is particulary suitable for the determination of very low contents of polycyclic aromatics [2]. The EN 12916 method requires a mass % of standard solutions and results are delivered also in mass %. Some uncertainty is involved in preparation of standard solution due to a very low amount of tri+aromatics. To overcome this possibility in this work a different approach has been proposed. Only the highest concentration of aromatic standard (solution A from EN 12916 method) is prepared by weighing, while all other solutions (B, C, D) just by diluting the A solution. Picture 1. Kalibration curve for di-aromatics, DA with RI Picture 2. Kalibration curve for di-aromatics, DA with UV detection Picture 3. Kalibration curve for tri+aromatics, TA with RI Picture 4. Kalibration curve for tri+aromatics, TA with UV detection A second contribution to the calibration curve accuracy is based on a different fitting procedure in the obtained calibration curves (ignore zero, force zero and include zero) Detection mode Equitation of calibration curves Correlation coefficient Fitting RI detection MA y=4, 802x105+5, 849x103 0, 9999 Linear, include DA y=8, 270x105+2, 716x103 0, 9999 Linear, include TA y=1, 053x106+2, 406x103 0, 9997 Linear, include UV detection MA y=1, 586x106-1, 370x103 0, 9998 Linear, include DA y=1, 439x107+3, 548x104 0, 9995 Linear, include TA y=1, 935x108+6, 840x104 0, 9992 Linear, include Table 1. Calibration curves equitations The difference in the determined content of polyciclic aromatics by UV-DAD and by RI detection is not negligible and depends on the type of aromatic and source of calibration curves. The differences are more pronounced in case of mono-aromatics. In this work numerous middle distillate samples were analyzed and contents of aromatic species were determined. The applied method precision was checked by determination of repeatability, by standard and relative standard deviations (SD, RSD). The limits of detection and quantification were also determined. Sample MA (m/m)% DA (m/m)% TA (m/m)% polyA (m/m)% total A (m/m)% Kerosene 17, 3 1, 5 - 1, 5 18, 8 Light gas oil 15, 4 6, 4 0, 9 7, 3 19, 7 Eurodiesel fuel 20, 8 1, 4 0, 1 1, 5 22, 3 Diesel fuel 17, 7 6, 4 1, 0 7, 4 25, 1 Table 2. Aromatic group type composition (wt%) of different middle distilates (RI detection) Sample MA (m/m)% DA (m/m)% TA (m/m)% poli-A (m/m)% total A (m/m)% Kerosene 14, 8226 2, 1585 0, 0001 2, 1586 16, 9812 Light gas oil 10, 7897 8, 0458 0, 7918 8, 8376 19, 6273 Eurodiesel fuel 11, 2272 2, 3560 0, 0988 2, 4548 13, 6820 Diesel fuel 10, 4617 7, 1144 0, 6728 7, 7872 18, 2489 Table 3. Aromatic group type composition (wt%) of different middle distilates (UV detection) Lit: [1] Petroleum products- Determination of aromatic hydrocarbon types in middle distillates- High performance liquid chromatography refractive index method, European Standard EN 12916-00 [2] M. Kaminski, E.Gilgenast, A. Przyjazny, G. Romanik, J. Chromatogr. A, 1122(2006)153

aromatic hydrocarbons; petroleum; NPHPLC

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