Multielemental determination in oil matrices diluted in xylene by ICP-MS with a dynamic reaction cell employing methane as reaction gas for solving specific interferences

Abstract

The analysis of organic samples by ICP-MS is still a challenge due to the occurrence of non-spectral and spectral interferences, which usually requires time-consuming sample preparation procedures for their minimization. In this work, results are presented on the direct multielemental determination in oil samples after simple dilution in xylene. The solutions were introduced into the plasma by a sample introduction system especially suited for organics, and the operational conditions of the plasma and the dynamic reaction cell were optimized. A central composite design showed that the nebulizer and the auxiliary gas flow rates had a significant influence on interference formation, monitored by Ba⁺⁺ and LaO⁺ intensities. The compromised plasma conditions were: RF power of 1300 W and gas flow rates of 0.42 L min⁻¹ for the nebulizer (Ar), 1.1 L min⁻¹ for the auxiliary gas (Ar), and 0.1 L min⁻¹ of O₂ used as an additional gas. In order to minimize spectral interferences on Ca, Cr, Fe and Mg, the operational conditions of the dynamic reaction cell, working with methane as reaction gas, were optimized for ⁵⁶Fe, ⁵²Cr, ⁴⁰Ca and ²⁴Mg. Two certified reference materials, NIST 1634c (used fuel oil) and NIST 1085b (lubricating oil), were analyzed and the recoveries were in the range of 90% to 110% for most elements, being the sample detection limits between 0.2 μg kg⁻¹ (Cr) and 40 μg kg⁻¹ (Zn). Reliability of the data was also tested by comparing the results for a set of biodiesel samples with those determined by ICP OES and good agreement was obtained for most elements. Differences in the elemental compositions of biodiesel samples produced from different raw materials were observed, suggesting that elemental fingerprinting may be used for source identification.