Metals determination in crude oil by inductively coupled plasma optical emission spectrometry using nanoemulsification as sample preparation

Abstract

In this work, a metastable emulsion classified as a nanoemulsion (NANO) is suggested as a new preparation procedure for the determination of Ca, Mg, Sr and Na in crude oil samples by inductively coupled plasma optical emission spectrometry (ICP OES). The NANO was prepared by using 0.20 ± 0.05 g of crude oil, 1.0 mL of HNO3, 0.6 mL of o-xylene, 0.80 ± 0.01 g of Triton X-100 and ultrapure water up to 20 mL. An experimental design was performed for both the NANO preparation and ICP OES conditions applying the Doehlert matrix. The NANO showed physical stability with just 4% surfactant (w v−1) and the concentrations of all four elements were still stable after 8 days, which improves between-day precision. The accuracy of the procedure was evaluated by standard reference materials (NIST 1085b and NIST 1634c) and by addition/recovery tests. The values obtained with NANO were in accordance with the values certified for Ca, Mg and Na for both standard reference materials. The recoveries were between 92.8 ± 3.8 and 102.2 ± 7.2% for all elements, including Sr, when spikes were made in crude oil samples. A comparative method (ASTM D5863-00a) based on total decomposition was also performed and no differences were obtained between NANO and the comparative method. Thus, NANO exhibited good stability, no phase separation for several days, good cost-effectiveness, proper detection limits and accuracy, being a feasible alternative for Ca, Mg, Sr and Na determination in routine analysis of oil samples.