Content Estimation of Trace Elements in Gasoline and Diesel Oil by Neutron Activation Analysis

Abstract

Concentrations of 12 elements (Mg, Mn, Cu, As, Cd, Sb, Hg, Ni, Zn, Na, Cl, Br) in Diesel oil, regular gasoline and high octane gasoline were determined by instrumental neutron activation analysis (INAA).
The total of about 300 samples was directly activated in an irradiation pit of the Musashi Institute of Technology Research Reactor (MITRR). Liquid oil samples (30-50 ml) in polyethylene bottles were irradiated with a thermal neutron flux of 7.5 × 10¹¹n·cm^-2·s^-1 for 10min (short time method), 2 h (midle time method) and 10 h (long time method). The irradiated liquid samples were transferred to new polyethylene bottles and then bubbled with oxygen gas for 2---95 min in order to decrease activity from dissolved "Ar. The measurement was performed by use of a Ge (Li) detector coupled to an 8192 channel multichannel analyzer.
The concentrations of the analysed trace elements except Cl and Br showed logarithmic normal distribution for each of Diesel oil, regular gasoline and high octane gasoline. The geometric means for all the samples were evaluated on the basis of the cumulative frequency distribution curves. The evaluated geometric means were 0.033 ppb, 11 ppb and 0.60 ppb for Mn, Cu and As in Diesel oil, 0.031 ppb and 0.25 ppb for Mn and Hg in regular gasoline and 1.2 ppb for Hg in high octane gasoline, respectively. As a result, we found that the concentrations of trace elements in gasoline were lower than those in Diesel oil. This means that the gasoline is more refined. And the concentrations of Cl and Br in gasoline were higher than those in Diesel oil. From the correlation of concentrations of Na, Cl and Br in Diesel oil, it was found that the concentrations of Br were much lower than those of Na and Cl and that Na and Cl might dissolve as NaC1 compound. The ratio of Cl to Na concentration was increased in the order of Diesel oil, regular gasoline and high octane gasoline. So Br in gasoline was nearly the same concentration as Cl. The high concentrations of Cl and Br in gasoline might be mixed in refined process.