Direct determination of Cr and Ni in oil samples by isotope dilution and external standard calibration using inductively coupled plasma mass spectrometry

In the present work, a method for the determination of Ni and Cr in crude oil samples and derivatives (petroleum asphalt cement (CAP) and malthene) through direct analysis of samples diluted in a mixture of organic solvents (xylene/butanol), by isotope dilution inductively coupled plasma mass spectrometry (ICP-MS), was developed. An exciting advantage of the proposed method was the addition of a polar solvent (butanol) to xylene, which has allowed the dilution of the samples, making possible the use of aqueous standards for the isotope dilution, without phase separation. For comparison, external standard calibration was carried out with organic standards and dilution in the same solvent. The spectrometer was set up and the instrumental parameters were optimized for the introduction of organic samples. Oxygen was introduced into the plasma to avoid carbon build-up at the interface. For interference correction from (ArC+)-Ar-40-C-12 over Cr-52(+), the dynamic reaction cell was employed with CH4 as reaction gas. Accuracy evaluation was performed by analyzing the standard reference material NIST SRM 1084a (wear-metals in lubricant oil). The agreement between the obtained results and the SRM values were within the range of 95% to 106%. The RSDs were lower than 12% for the isotope dilution, and 10% for external standard calibration. The results obtained for Ni and Cr in crude oil and derivatives have shown that both calibration methods are statistically in agreement. Although isotope dilution is not commonly used for routine analysis, it has the potential to be. The time necessary for sample preparation could be reduced with the employment of an autodiluter, and the cost of enriched isotopes is a relative issue. The ID can also be employed as a primary method to verify the accuracy of external standard calibration methods.