Determination of rare earth elements in Niger Delta crude oils by inductively coupled plasma-mass spectrometry

Abstract

A profile for rare earth elements (REE) of crude oils from the offshore – shallow water and onshore fields in the Niger Delta, analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) is reported. The oil samples were prepared for ICP-MS measurement by acid digestion into colourless aqueous solution. The analysis method was validated using standard reference materials SLRS-4 and NIST-1640. Results showed percentage recovery values that ranged from 81.8% to 115.4% for Co, Cu, Fe, Mn, Ni, Sb and U and from 98.8% to 104.7% for Co, Cu, Fe, Mn, Ni and Sb. The magnitude of deviation recorded in SLRS-4 for Co and Fe suggests that it may not be a suitable standard for these elements using the ICP-MS method outlined in this study. The concentrations of the detected REE; La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er and Yb ranged from 0.01 to 1.58 ppb with an average of 0.98 ppb (%RSD < 5) for the oil samples analyzed. Light REEs (LREE) were identified in all the oil samples while heavy rare earth elements (HREE) were identified in offshore oil samples only. LREE patterns constructed from chondrite-normalized values for the oils show some similarities among the oils, which suggest common origin of the oils and that the REE got into the oils from similar source. While those with different chondrite-normalized REE patterns suggest different source input of the REE. This indicates that REE will be a useful tool in oil–oil correlation. Statistical evaluation of these oils by cluster analysis using the REE as variables clearly discriminated according to their geographic sources. Biodegradation has pronounced effect on the concentration of REE in oils. Therefore, REE contents of oils will be useful in oil classification. ICP-MS proved to be a versatile method for the determination of rare earth elements in Niger Delta oils.